Independent summary for policymakers of the IPCC Fourth Assessment Report

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Editorial Advisory Board

Prof. Armen Alchian

Prof. Terry Anderson

Prof. Robert Barro

Prof. Michael Bliss

Prof. J.M. Buchanan

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Prof. Erwin Diewert

Prof. Stephen Easton

Prof. J.L. Granatstein

Prof. Herbert G. Grubel

Prof. James Gwartney

Prof. Ronald W. Jones

Dr. Jerry Jordan

Prof. Ross McKitrick

Prof. Michael Parkin

Prof. Friedrich Schneider

Prof. L.B. Smith

Dr. Vito Tanzi

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Copyright© 2007 The Fraser Institute. Text revised March 2, 2007. All rights reserved. No part of this
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Editing: Serena Howlett. Cover design: Kim Forrest. Design and typesetting: Irma Rodriguez
Date of issue: February 2007

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Contents

List of Authors / 4
Preface / 5
Executive Summary / 7

Observed changes in factors that may influence the climate / 9

1.1 “Radiative Forcing” as a conceptual tool for comparing climatic effects / 9
1.2 Greenhouse Gases / 10
1.3 Aerosols / 12
1.4 Changes in the Sun and Solar-Climate connections / 13
1.5 Changes to the land surface / 16

Observed changes in weather and climate / 18

2.1 Large-scale temperature averages / 18
2.2 Precipitation and snow cover / 24
2.3 Storms and extreme weather / 25
2.4 Ocean temperatures and sea levels / 27
2.5 Glaciers, sea ice and ice caps / 30
2.6 Humidity and radiation flux / 33

Climatic changes in paleoclimate perspective / 34

3.1 Geological evidence of warming and cooling episodes / 34
3.2 Global climate reconstructions over the past 2,000 years / 36

Climate models and their evaluation / 39

4.1 Fundamental limitations of climate models / 39
4.2 Significant known model problems / 40

Global and regional climate projections / 42

5.1 Reproduction of the present climate / 42
5.2 Forecasts for the coming century are inherently uncertain / 42
5.3 Model-generated global warming forecasts / 43

Attributing the causes of climate change / 47

6.1 Measuring and analyzing climate change / 47
6.2 Difficulties in attributing observed climate change to specific causes / 48
6.3 Assumptions needed to attribute climate change to anthropogenic causes / 50

Overall conclusions / 52

References / 53
Appendix 1 – Expert Review / 55
About the Authors / 57
Glossary / 60

List of Authors

Coordinator

Ross McKitrick,

Ph.D.

Associate Professor, Department of Economics, University of Guelph

and Senior Fellow, Fraser Institute, Vancouver BC.

Writing Team

Joseph D’Aleo,

M.Sc.

Chief Meteorologist (Ret’d) WSI Corporation. Past Chairman, Ameri-

can Meteorological Society Committee on Weather Analysis and Forecasting. Member,
American Meteorological Society Council. Fellow, American Meteorological Society. Certified
Consulting Meteorologist.

Madhav Khandekar,

Ph.D.

Research Scientist (ret’d), Environment Canada. Editor,

Climate

Research

2003-2005. Member, Editorial Board,

Natural Hazards

since 1999. Previously,

Lecturer in Meteorology, Barbados (West Indies); International Civil Aviation Organization
Expert in Aeronautical Meteorology, Qatar.

William Kininmonth,

M.Sc.

M.Admin. Head (ret’d) National Climate Centre, Australian

Bureau of Meteorology. Previously: Consultant to the World Meteorological Organization
Commission for Climatology; Scientific and Technical Review Coordinator, United Nations
Task Force on El Niño.

Christopher Essex,

Ph.D.

Professor of Applied Mathematics, University of Western Ontario,

and Associate Director, Program in Theoretical Physics. Formerly, NSERC Postdoctoral
Fellow, Canadian Climate Centre.

Wibjörn Karlén,

Ph.D.

Professor emeritus, Dept. of Physical Geography and Quaternary

Geology, Stockholm University, Sweden

Olavi Kärner,

Ph.D.

Senior Research Associate, Atmospheric Sensing Group, Tartu

Astrophysical Observatory, Tõravere, Estonia.

Ian Clark,

Ph.D.

Professor of Arctic Paleohydrology and Geology, University of Ottawa.

Tad Murty,

Ph.D.

Adjunct Professor, Departments of Earth Sciences and Civil Engineering,

University of Ottawa; Editor,

Natural Hazards

; Associate Editor

Marine Geodesy

; Leader,

World Meteorological Organization group to prepare a manual on storm surges from
hurricanes and extra-tropical cyclones. Formerly: Senior Research Scientist, Canadian
Department of Fisheries and Oceans; Professor of Earth Sciences, Flinders University,
Adelaide, Australia; Director of Australia’s National Tidal Facility.

James J. O’Brien,

Ph.D.

Robert O. Lawton Distinguished Professor, Meteorology & Ocean-

ography and Director Emeritus of the Center for Ocean-Atmospheric Prediction Studies,
Florida State University. Florida State Climatologist. Fellow of the American Meteorological
Society, Fellow of the American Geophysical Union, Fellow of the Royal Meteorological
Society, Fellow of the American Association for the Advancement of Science.

Preface

This is an

Independent Summary for Policymakers

(ISPM) of the Fourth Assessment

Report (AR4), Working Group 1, of the Intergovernmental Panel on Climate Change (IPCC).
In producing this Summary we have worked independently of the IPCC, using the Second
Order Draft of the IPCC report, as circulated after revisions were made in response to the
first expert review period in the winter and spring of 2006. Section references will be checked
against the final IPCC version, as soon as copies are available following the release later in
2007. If, in preparing the final draft of the Fourth Assessment Report, the IPCC substan-
tially rewrites the Assessment text, such that the key summary materials presented herein
need to be re-worded, we will do so and publish an Appendix to that effect.

The IPCC was established in 1988 by the World Meteorological Organization (WMO)

and the United Nations Environment Program (UNEP) to assess the risk of human induced
climate change. The IPCC is open to all members of the WMO and UNEP. The IPCC has
three working groups. Working Group 1 assesses the scientific aspects of climate change;
Working Group 2 assesses the positive and negative impacts of climate change and the
options for adaptation; and Working Group 3 assesses policy options to mitigate climate
change.

The Fraser Institute’s
Rationale for the ISPM

The IPCC involves numerous experts in the preparation of its reports. However, chapter
authors are frequently asked to summarize current controversies and disputes in which
they themselves are professionally involved, which invites bias. Related to this is the prob-
lem that chapter authors may tend to favor their own published work by presenting it in a
prominent or flattering light. Nonetheless the resulting reports tend to be reasonably
comprehensive and informative. Some research that contradicts the hypothesis of green-
house gas-induced warming is under-represented, and some controversies are treated in a
one-sided way, but the reports still merit close attention.

A more compelling problem is that the

Summary for Policymakers

, attached to the

IPCC Report, is produced, not by the scientific writers and reviewers, but by a process of
negotiation among unnamed bureaucratic delegates from sponsoring governments. Their
selection of material need not and may not reflect the priorities and intentions of the
scientific community itself. Consequently it is useful to have independent experts read the
underlying report and produce a summary of the most pertinent elements of the report.

Finally, while the IPCC enlists many expert reviewers, no indication is given as to

whether they disagreed with some or all of the material they reviewed. In previous IPCC
reports many expert reviewers have lodged serious objections only to find that, while their
objections are ignored, they are acknowledged in the final document, giving the impression
that they endorsed the views expressed therein.

The ISPM addresses these concerns as follows.

The ISPM was prepared by experts who are fully qualified and experienced in their
fields, but who are not themselves IPCC chapter authors, nor are they authors of the
IPCC

Summary for Policymakers

The ISPM summarizes the most important elements of the science, regardless of whether
it is given the same level of focus in the IPCC’s Summary documents. There is no attempt
to downplay or re-word uncertainties and limitations in the underlying science, hence the
summary paragraphs in the ISPM may not be identical to those of the Summary produced
by the IPCC.

If a chapter of the Fourth Assessment Report introduces its topic by briefly elaborating
on deep uncertainties, then presents results at length as if the uncertainties were not
there, the ISPM may devote proportionally more attention to understanding the uncer-
tainties than summarizing all the results, where this is deemed a more pertinent way to
characterize the underlying state of knowledge.

Independent Summary for Policy Makers of the Text of the IPCC Fourth Assessment Report

In a number of places the writing team felt the treatment of a topic was inadequate in
the Fourth Assessment Report, or some additional comments were needed for perspec-
tive. These are noted in separate sidebars. Also, the Fraser Institute will publish a series
of short supplementary papers to provide more detailed critical discussion of some
technical subjects. These are noted at various points in the ISPM as well.

The ISPM was subject to expert review by the reviewers listed at the end. Their responses
to review questions are tabulated so readers can see to what extent the reviewers agree
with the contents of this Summary.

Format Notes

Third Assessment Report

refers to the Third Assessment Report (TAR) of the IPCC,

Working Group I, published in 2001

Fourth Assessment Report

refers to the Fourth Assessment Report (AR4) of the IPCC,

Second Order Draft, Working Group 1

Section references in brackets, e.g., [3.4.3.1], refer to the Fourth Assessment Report of
the IPCC, Second Order Draft, Working Group 1. Some references are to Summation
Questions included in the Fourth Assessment Report chapters, e.g., [Question 5.1].

Acknowledgments

Stephen McIntyre assisted in collation of data, preparation of many graphs and technical
editing of some sections. Nicholas Schneider was involved in this project from inception
and acted as the key Fraser Institute staff person. Their contributions are gratefully
acknowledged.

Disclaimer

The text presented herein uses our best estimate of the wording of the final version of the
Working Group I contribution to the Fourth Assessment Report. Much of the text herein
follows wording as set by IPCC Lead and Contributing Authors in the Second Order Draft
as of the close of the scientific review period on June 2, 2006, on the assumption that this
will also be the wording in the final draft. However a check against the final wording will
take place after the IPCC releases the underlying report. The IPCC has indicated that,
although they are publishing the Summary for Policymakers on February 2, 2007, they will
not release the underlying report until some time in May 2007. Until that time, readers
should note that the IPCC has not officially accepted the wording of the underlying report
or of drafts on which it is based.

Executive Summary

Observed Changes in
Factors That May
Influence the Climate

The climate is subject to potential influence by both natural and human forces, including
greenhouse gas concentrations, aerosols, solar activity, land surface processes, ocean
circulations and water vapor. Carbon dioxide is a greenhouse gas, and its atmospheric
concentration is increasing due mainly to human emissions.

The IPCC gives limited consideration to aerosols, solar activity and land-use change for

explaining 20th century climate changes. Aerosols have a large potential impact on climate
but their influence is poorly understood. Some evidence suggests that solar activity has
increased over the 20th century to historically high levels. Land use changes are assumed
by the IPCC to have only a minor role in explaining observed climate change.

Observed Changes in
Weather and Climate

Globally-averaged measurements of lower atmospheric temperatures from satellite data
since 1979 show an increase of 0.12°C to 0.19°C per decade over this period, at the low
end of the IPCC estimate of future warming. Globally-averaged temperature data collected
at the surface show an increase from 1900 to 1940 and again from 1979 to the present.

There is no globally-consistent pattern in long-term precipitation trends, snow-covered

area, or snow depth. Many places have observed a slight increase in rain and/or snow cover.
There is insufficient data to draw conclusions about increases in extreme temperature and
precipitation. Current data suggest a global mean sea-level rise of 2 mm to 3 mm per year
over the past several decades. In the tropics, there is evidence of increased cyclone intensity
but a decrease in total tropical storms, and no clear global pattern since 1970.

Arctic sea ice showed an abrupt loss in thickness prior to the 1990s, and the loss

stopped shortly thereafter. There is insufficient data to conclude that there are any trends
in Antarctic sea ice thickness. Glaciers have retreated in most places and the loss acceler-
ated in the 1990s.

Climatic Changes
in a Paleoclimate
Perspective

Paleoclimate refers to the Earth’s climate prior to the start of modern instrumental data
sets. There are historical examples of large, natural global warming and cooling in the
distant past. The Earth is currently within a warm interglacial period, and temperatures
during the last interglacial period were warmer than present.

Natural climate variability and the uncertainty associated with paleoclimate studies are

now believed to be larger than previously estimated. In general, data are sparse and uncer-
tain, and many records have been questioned for their ability to show historical temperature
variability. These uncertainties matter for assessing the ability of climate models to simulate
realistic climate changes over historical intervals.

Climate Models and
Their Evaluation

Some broad modeling predictions made 30 years ago are consistent with recent data, but
there remain fundamental limitations of climate models that have not improved since the
Third Assessment Report. Many models are incapable of simulating important aspects of the
current climate, and models differ substantially in their projections. It is not possible to say
which, if any, of today’s climate models are reliable for climate prediction and forecasting.

Global and Regional
Climate Projections

Models project a range of forecasts, and uncertainty enters at many steps in the process.
Forecasts for the 21st century are inherently uncertain, especially at the regional level.

Current models predict: an increase in average surface temperature; an increased risk

of drought, heat waves, intense precipitation and flooding; longer growing seasons; and an
average sea levels rise of about 20 cm over the next 100 years.

Glacier mass is projected to decrease. An abrupt change in ocean circulation is very

unlikely. Tropical cyclone intensity may increase or decrease.

Independent Summary for Policy Makers of the Text of the IPCC Fourth Assessment Report

Attributing the Causes
of Climate Change

Attributing an observed climate change to a specific cause like greenhouse gas emissions
is not formally possible, and therefore relies on computer model simulations. As of yet,
attribution studies do not take into account the basic uncertainty about climate models, or
all potentially important influences.

Increased confidence that a human influence on the global climate can be identified is

based the proliferation of attribution studies since the Third Assessment Report. Models
used for attributing recent climate change estimate that natural causes alone would not
result in the climate that is currently observable.

ISPM Overall
Conclusions

The following concluding statement is not in the Fourth Assessment Report, but was
agreed upon by the ISPM writing team based on their review of the current evidence.

The Earth’s climate is an extremely complex system and we must not understate the
difficulties involved in analyzing it. Despite the many data limitations and uncertainties,
knowledge of the climate system continues to advance based on improved and expand-
ing data sets and improved understanding of meteorological and oceanographic
mechanisms.

The climate in most places has undergone minor changes over the past 200 years,

and the land-based surface temperature record of the past 100 years exhibits warming
trends in many places. Measurement problems, including uneven sampling, missing
data and local land-use changes, make interpretation of these trends difficult. Other,
more stable data sets, such as satellite, radiosonde and ocean temperatures yield
smaller warming trends. The actual climate change in many locations has been rela-
tively small and within the range of known natural variability. There is no compelling
evidence that dangerous or unprecedented changes are underway.

The available data over the past century can be interpreted within the framework of

a variety of hypotheses as to cause and mechanisms for the measured changes. The
hypothesis that greenhouse gas emissions have produced or are capable of producing
a significant warming of the Earth’s climate since the start of the industrial era is
credible, and merits continued attention. However, the hypothesis cannot be proven by
formal theoretical arguments, and the available data allow the hypothesis to be credibly
disputed.

Arguments for the hypothesis rely on computer simulations, which can never be

decisive as supporting evidence. The computer models in use are not, by necessity,
direct calculations of all basic physics but rely upon empirical approximations for many
of the smaller scale processes of the oceans and atmosphere. They are tuned to
produce a credible simulation of current global climate statistics, but this does not
guarantee reliability in future climate regimes. And there are enough degrees of
freedom in tunable models that simulations cannot serve as supporting evidence for
any one tuning scheme, such as that associated with a strong effect from greenhouse
gases.

There is no evidence provided by the IPCC in its Fourth Assessment Report that the

uncertainty can be formally resolved from first principles, statistical hypothesis testing
or modeling exercises. Consequently, there will remain an unavoidable element of
uncertainty as to the extent that humans are contributing to future climate change, and
indeed whether or not such change is a good or bad thing.

SUPPLEMENT

ARY INFORMA

TION

Observed changes in factors that may
influence the climate

1.1 “Radiative Forcing”
as a conceptual tool for
comparing climatic
effects

1.1a

“Radiative Forcing” (RF) is a modeling concept that attempts to summarize the

climatic effect of diverse changes in the environment. It is not directly measured, nor is it
related to the “greenhouse effect,” and overall remains poorly quantified.

RF is a concept that arose from early climate studies using simple radiative-convective
models. It is not directly measured. Instead it is calculated by simplified climate models
under the assumption that a comparison can be made between equilibrium states of the
climate. The climate does not reach equilibrium, but reflects transient responses to
external and internal changes. The RF relationship to transient climate change is not
straightforward. To evaluate the overall climate response associated with a forcing agent
its time evolution and its spatial and vertical structure need to be taken into account.
Further, RF alone cannot be used to assess the potential climate change associated with
emissions, as it does not take into account the different atmospheric lifetimes of the
forcing agents. [2.2]

RF itself is not directly related to the “greenhouse” effect as associated with greenhouse
gases. [2.3.8]

Measurement of RF in Watts/square meter is a convention, but RF itself is not a meas-
ured physical quantity. Instead it is computed by assuming a linear relationship between
certain climatic forcing agents and particular averages of temperature data. The various
processes that it attempts to approximate are themselves poorly quantified. [2.2]

An observed decrease in radiative flux at the characteristic radiation bands of CO

and

methane between 1970 and 1997 has been associated with changing concentrations.
This change is what is meant by the term “enhanced greenhouse effect”, but it is not
directly related to the “Radiative Forcing” concept. [2.3.8]

Greenhouses and ‘Greenhouse Gases’

While use of the term ‘greenhouse’ is nowadays unavoidable, the term ‘greenhouse effect’ is an in-
appropriate metaphor since it suggests a parallel between the mechanism that causes warming in an
actual greenhouse and the influence of infrared-active gases, like water vapour and carbon dioxide, on the
Earth’s climate system. The two mechanisms are quite distinct, and the metaphor is misleading. It leaves out
the complexities arising from the nonlinear, dynamic processes of our climate system, namely evaporation,
convection, turbulence and other forms of atmospheric fluid dynamics, by which energy is removed from the
Earth’s surface. Simplistic metaphors are no basis for projecting substantial surface warming due to increases
of human-caused carbon dioxide concentration in the atmosphere.

This problem is explored in the forthcoming Fraser Institute Supplementary Analysis Series report, “Why the
‘Greenhouse’ Metaphor is Misleading.”

Independent Summary for Policy Makers of the Text of the IPCC Fourth Assessment Report

1.2 Greenhouse Gases

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

1959

1964

1969

1974

1979

1984

1989

1994

1999

Year

280

300

320

340

360

380

400

1959

1964

1969

1974

1979

1984

1989

1994

1999

2004

Year

ion

(ppm

)

FIGURE ISPM-1: CARBON DIOXIDE CONCENTRATIONS

TOP: Annual average atmospheric carbon dioxide concentration since 1958.
BOTTOM: annual percentage rate of change of carbon dioxide concentration.

Source:

Marland

et al

., 2006.

1.2a

Carbon dioxide (CO

) levels in the atmosphere are rising at approximately

0.5% per year.

Figure ISPM-1(top) shows the atmospheric CO

concentration since the late 1950s. The

rate has no overall trend but fluctuates around a mean of 0.5% since the early 1990s, up
from 0.4% in the 1970s and 1980s (Figure ISPM-1(bottom)).

The main causes of this accumulation are fossil fuel burning, cement production, gas
flaring, and, to a lesser extent, land-use changes such as deforestation. [2.3.1]

Human activities contribute about 7 Gigatonnes carbon equivalent to the atmosphere
each year, up from around 6 Gigatonnes in 1990. [2.3.1]

Independent Summary for Policy Makers of the Text of the IPCC Fourth Assessment Report

SUPPLEMENT

ARY INFORMA

TION

Per capita carbon emissions have not increased for 30 years

The growth rate of CO

emissions (in carbon equivalent) is equal to or slightly below the growth rate of

world population (see Figure ISPM-2). Global per capita carbon emissions peaked at 1.23 tonnes per person
in 1979 and the per-person average has declined slightly since then. As of 2003 the global average is 1.14
tonnes per capita, an average that has not changed since the early 1980s. If this trend continues, global
emissions growth in the future will be constrained by total population growth.

ISPM-2: GLOBAL PER
CAPITA CARBON
EMISSIONS, 1950-2003

Source:

Marland

et al.

2006.

0.4

0.6

0.8

1.0

1.2

1.4

1950 1955 1960 1965 1970 1975 1980 1985 1990 1995 2000

Year

1.2b

Ice core records indicate that the atmospheric CO

levels were constant at about 280

parts per million (ppm) for at least several thousand years prior to the mid-1800s.

This implies a post-industrial accumulation in the atmosphere of about 100 ppm,
yielding the current level of nearly 380 ppm. [2.3.1]

variations over the last 420,000 years broadly followed Antarctic temperature,

typically with a time lag of several centuries to a millennium (i.e., atmospheric carbon
dioxide levels rise several centuries after the air temperature rises). [6.4.1]

200

400

600

800

1000

1200

1008

1098

1198

1329

1448

1549

1649

1751

1847

1950

Year

(ppb)

1620

1650

1680

1710

1740

1770

1800

1830

1982

1984

1987

1990

1993

1995

1998

2001

2004

2006

Year

(ppb)

FIGURE ISPM-3: METHANE CONCENTRATIONS

LEFT: Long-term atmospheric methane levels, 1008 to 1950.

Source:

Etheridge

et al

., 2002.

RIGHT: Mauna Loa, Hawaii methane record, 1983-2005.

Source:

World Data Center for Greenhouse Gases, 2006.

Independent Summary for Policy Makers of the Text of the IPCC Fourth Assessment Report

1.2c

Atmospheric methane (CH

) levels stopped growing in the late 1990s and have

declined somewhat in recent years. Sources of methane emissions are poorly understood,
but the total appears to be declining. It is not understood how this could be happening
despite ongoing atmospheric temperature increases.

Ice core records indicate pre-industrial methane levels were about 700 parts per billion
(ppb), prior to the 18th century. The methane level increased over the next three centu-
ries, and at the global level currently averages about 1,780 ppb (see Figure ISPM-3).

Overall sources of methane emissions to the atmosphere are poorly known, but are
thought to include wetlands, rice agriculture, biomass burning and ruminant animals.

Emissions from anthropogenic sources remain the major contributor to atmospheric
methane budgets. [7.4.1.2]

Atmospheric methane concentrations peaked several years ago and have been flat or
declining since then [Fig 2.5, see Figure ISPM-3, Bottom]. The reason for the recent
decline is not understood. [2.3.2]

The atmospheric concentration of methane is tied to atmospheric temperature, as total
emissions increase with atmospheric warming. Total emissions from sources are sug-
gested to have decreased since the time of the Third Assessment Report, as nearly zero
growth rates in atmospheric methane concentrations have been observed with no
change in the sink strengths. It is not well understood why emissions have decreased
despite continued warming of the Earth’s surface and the atmosphere. [7.4.1.2]

1.2d

Hydrochlorofluorocarbons (HCFCs) and Chlorofluorocarbons (CFCs) are presently

covered by other emission control legislation, and are declining.

HCFCs and CFCs are covered by the Montreal Protocol on ozone-depleting substances.
Global emissions have fallen radically since 1990 and their atmospheric levels are slowly
declining. [2.3.4]

1.2e

Other infrared active gases (Nitrous Oxide (N

O) and Hydrofluorocarbons (HFCs)) are

accumulating slowly in the atmosphere, or are at levels that imply very low climatic effects.
[2.3.3; Table 2.1]

1.3 Aerosols

1.3a

Aerosols play a key role in the Earth’s climate, with a potential impact more than

three times that of anthropogenic carbon dioxide emissions, but their influence remains
subject to low or very low scientific understanding.

Aerosols have a significant presence in the global atmosphere. The combined Direct
Radiative Effect of natural and anthropogenic sources on climate, is estimated to be
about -5.3 Watts/m

, more than three times the magnitude of the estimated Radiative

Forcing of anthropogenic CO

(1.63 Watts/m

) [2.4.2.1.2]

It is very challenging to distinguish natural and anthropogenic aerosols in satellite data.
Validation programs for these advanced satellite-data products have yet to be developed
and initial assessments indicate some systematic errors. [2.4.2.1]

The climatic effect of each type of aerosol consists of both direct and indirect effects, the
latter including influences on cloud formation. Overall direct and indirect effects are
subject to wide uncertainties, and some important semi-direct effects were not included
in the Third Assessment Report. [2.4]

Independent Summary for Policy Makers of the Text of the IPCC Fourth Assessment Report

Effects on cloud formation are not well understood and the magnitude of the effects are
not reliably estimated at this time, in part because of the lack of satellite data to support
model development and testing. [2.4.6]

Modelling the cloud albedo indirect effect from first principles has proven difficult
because the representation of aerosol-cloud interactions and of clouds themselves in
climate models are still crude. [2.4.6.5]

Although there is agreement about the quality of the basic evidence (data), there is no
consensus about the direct climatic (radiative forcing) effect of aerosols on climate, and
the overall state of knowledge is categorized as

Low Scientific Understanding

. [Table

2.11]

All categories of indirect aerosol effect on climate, are characterized by: no consensus;
varying confidence in the basic empirical evidence, and

Low

Very Low Scientific

Understanding

. [Table 2.11]

1.3b

Aerosols can affect both cloud lifetime and cloud albedo (reflectivity), though models

contradict one another on which effect is larger.

Whereas some models show that the cloud albedo effect is four times as important as
the cloud lifetime effect, other models simulate a cloud lifetime effect that is larger than
the cloud albedo effect [7.5.2.4].

1.3c

It is generally assumed that aerosols exert an overall cooling effect on the climate.

Quantitative estimates of the overall effect vary by a factor of 10.

The global mean total anthropogenic aerosol effect (direct, semi-direct and indirect
cloud albedo and cloud lifetime effect) is defined as the change in net radiation at the
top of the atmosphere from pre-industrial times to present-day, and ranges from
–0.2 Wm

–2

to –2.3 Wm

–2

. This implies that aerosol emissions exert an overall cooling

effect, but the magnitude of this effect is unknown. [7.5.2.4]

1.3d

Studies that attribute observed global warming to greenhouse gases are based on

models that assume that aerosols exert a large cooling effect.

The models used for the Fourth Assessment Report assume a large cooling effect from
aerosols. [Table 2.12]

The effect is assumed to be strongest in the Northern Hemisphere. [Figure 9.2.1e]

1.4 Changes in the
Sun and Solar-Climate
connections

1.4a

New studies since the Third Assessment Report have improved empirical knowledge

of climate responses to forcing by solar variability on annual to decadal time scales.

Overall the troposphere is warmer and moister during solar maxima, and thickens in
response to solar variability with a distinct zonal signature. [2.7.1.1.2]

Independent Summary for Policy Makers of the Text of the IPCC Fourth Assessment Report

1.4b

The Third Assessment Report reported that solar activity was exceptionally high in

the 20th century in the context of the last 400 years. Since then, new reconstructions of
solar activity have indicated modern solar output levels are high, and possibly exceptionally
high, compared to the past 8,000 years.

Solar activity is estimated by historical information on sunspot counts and, prior to that,
by cosmogenic isotopes (residual C14 and Be-10). [2.7.1.2.1]

One reconstruction shows modern solar levels to be exceptional within the past 8,000
years while another shows few comparable episodes. [2.7.1.2.1; see Figure ISPM-4]

Several reconstructions of solar activity show a strong upward trend from 1700 to the
present. [see Figure ISPM-4]

The minimum in solar activity around 1700 AD (the Maunder Minimum) has been
associated with contemporary cold temperatures. [see Figure ISPM-9]

FIGURE ISPM-4: SOLAR ACTIVITY

LEFT: reconstruction for past 8,000 years (Usoskin

et al.

2006); RIGHT: reconstructions for the past millennium;

Blue – reconstruction of sunspot numbers from residual C14 (Usoskin

et al.

, 2006); Red – group sunspot

number (Hoyt and Schatten, 1993); Green – reconstruction of solar modulation from residual C14 (meV)
(Muscheler

et al.

, 2005).

1.4c

Scientific understanding of solar variability remains low.

Estimates of the change in solar forcing between the Maunder Minimum and the late
20th century range over almost an entire order of magnitude. [2.7, 2.7.1.2.1, Table 2.10;
see Figure ISPM-5]

A new estimate of solar irradiance increase since the Maunder Minimum (0.037%
according to Wang

et al.

, 2005) is nearly an order of magnitude lower than another

recent estimate of 0.3% by Fligge and Solanki, 2000. [2.71, Table 2.10]

100

-4995 -3995 -2995 -1995 -995

1005 1995

Year

uns

100

1000

1200

1400

1600

1800

2000

Year

uns

200

400

600

800

1000

1200

Sol

)

Independent Summary for Policy Makers of the Text of the IPCC Fourth Assessment Report

FIGURE ISPM-5: SOLAR FORCING ANOMALY (WATTS/ M

) FOR THE PAST MILLENNIUM

Forcing anomaly calculated as irradiance divided by 4 and multiplied by 0.7 (albedo) following Table 2.10
of the Fourth Assessment Report. Anomaly centered on 1850-1960.
NOTE: There are four variations from Bard et al., 2000 Be-10 isotopes, and two from Crowley, 2000 using C14.

1.4d

Total solar irradiance measurements are subject to important uncertainties due to

instrumentation.

Total Solar Irradiance has been measured only since 1978 and even then only with
different instruments, none of which cover the entire interval. ACRIM instruments show an
increase in excess of 0.04% between 1989 and 1992. This apparent increase may merely
be a result of instrumental changes. [2.7.1]

A continuous record can be constructed only by combining records from different satellites
with different instruments. If the measured change of 0.04% proves accurate, this increase
is as large as the increase since the Maunder Minimum. [2.7.1.1.2, Figure 2.19]

1.4e

New evidence has emerged of indirect solar effects on climate.

Although solar UV radiation represents only a small fraction of the energy from total
irradiance, UV radiation is more variable by at least an order of magnitude. Since the Third
Assessment Report, new studies have confirmed and advanced the plausibility of indirect
effects on the climate system involving the modification of the stratosphere by solar UV
irradiance variations (and possibly by solar-induced variations in the overlying mesosphere
and lower thermosphere), with subsequent dynamical and radiative coupling to the
troposphere. [2.7.1.3]

It is now well established from both empirical and model studies that solar cycle changes
in UV radiation alter middle atmospheric ozone concentrations, temperatures and winds.
[2.7.1.3]

-2.0

-1.5

-1.0

-0.5

0.0

0.5

1.0

1000

1100

1200

1300

1400

1500

1600

1700

1800

1900

2000

Year

Sol

Lean et al., 1995

Bard et al., 2000 (1) Be-10 isotopes

Bard et al., 2000 (2) Be-10 isotopes

Bard et al., 2000 (3) Be-10 isotopes

Bard et al., 2000 (4) Be-10 isotopes

Lean, 2000

Crowley, 2000 using C14

Crowley, 2000 using C14 (2)

Crowley, 2000 using Be-10, and Mann et al., 2005

Wang et al., 2005

Independent Summary for Policy Makers of the Text of the IPCC Fourth Assessment Report

When solar activity is high, the more complex magnetic configuration of the heliosphere
reduces the flux of galactic cosmic rays in the Earth’s atmosphere. Various scenarios have
been proposed whereby solar-induced galactic cosmic ray fluctuations might influence
climate, possibly through low cloud formation. [2.7.1.3]

An unequivocal determination of specific mechanisms – whether direct or indirect - that
involve solar variability and climate has yet to be accomplished. [2.7.1.3]

The sun and climate change

Solar and greenhouse forcings have both increased through the 20th century, making it extremely difficult
to conclusively identify the influence of the sun on the recent climate.

New IPCC estimates of solar forcing are much lower than those used in millennial simulations (e.g.,

Crowley, 2000; Gonzalez-Rauco

et al

., 2003; Mann

et al

., 2005). If the new estimates prove reliable, many

explanations of past climate variations relying on former estimates of solar forcing will need to be
re-considered.

If the sun does have a strong effect on climate, this adds importance to recent projections that solar

output is likely to decline over the next several decades (e.g., Zhen-Shan, 2007)

This topic is explored in the forthcoming Fraser Institute Supplementary Analysis Series report, “Solar
Changes and the Climate.”

SUPPLEMENT

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1.5 Changes to the
land surface

1.5a

Changes in the land surface over the 20th century have likely had large regional and

possibly global effects on the climate, but the effects do not fit into the conceptual model
used for assessing anthropogenic climate change.

Changes to the land surface act as anthropogenic perturbations to the climate system
and fall at least partly within the “forcing” component of the

forcing-feedback-response

conceptual model. But it is difficult to quantify the pure forcing component of such
changes as distinct from feedbacks and responses. A quantitative metric separating
forcing from feedback and response has not yet been implemented for climatic
perturbation processes which do not act directly on the radiation budget. [2.5.1]

Attempts to use climate models to convert land use changes into RF measures have
produced a wide range of results. Some estimated magnitudes of the local RF effects of
agricultural change in North America and Eurasia are considerably larger than that from
CO

in the atmosphere [2.5.3]. However the data for parameterizing basic RF effects are

not consistent and the uncertainties remain large. [2.5.3.1]

1.5b

Many studies have found that urban areas are warmer than the surrounding country-

side, introducing a “non-climatic” warm bias into local long term weather records. If true,
this would imply IPCC climate data overstate the recent global warming trend. Some
studies have asserted, however, that urbanisation is adequately corrected in the globally-
averaged data. All IPCC analysis assumes the latter to be the case.

The urban heat island effect is real, and causes temperature records from urban and
suburban areas to have an upward trend unrelated to climatic changes. [3.2.2.2]

Independent Summary for Policy Makers of the Text of the IPCC Fourth Assessment Report

Some studies argue that the global climate data sets, which compile urban, suburban
and rural records into regional averages, are not contaminated by such upward biases.
[3.2.2.2]

All IPCC usage of climatic data operates on the assumption of no contamination.
However many studies have shown that changes in land use and land cover can have
large regional effects on the climate that are comparable in magnitude to temperature
and precipitation changes observed over the last several decades, and the large numbers
of such studies collectively demonstrate a potentially important impact of human
activities on climate, especially local climate, through land use modification. [7.2.4.4]

Detection and attribution studies do not account for urbanization, data quality problems
or other non-climatic effects in the temperature data. All observed changes in the data
are assumed to be due to climatic changes. [9.4.1.2]

Problems with the surface temperature record

Research on the nature of the surface thermometer network has cast some doubt on the claim of the IPCC
that the surface temperature record is free of biases related to non-climatic effects, such as land-use change,
urbanization and changes in the number of stations worldwide. For example, studies have shown that the
spatial pattern of warming trends over land correlate strongly with the distribution of industrial activity, even
though such a correlation is not predicted by climate models (e.g., de Laat and Maurellis 2004, 2006).

These and related issues are explored in the forthcoming Fraser Institute Supplementary Analysis Series
report, “Problems in the Surface Thermometer Network.”

SUPPLEMENT

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Observed changes in weather and climate

2.1 Large-scale
temperature averages

2.1a

Weather satellites collect daily data throughout the atmosphere and are used to

measure average atmospheric temperatures. Different teams produce slightly different
results based on different assumptions about the way to interpret the data.

Satellites measure atmospheric radiation from two layers of the atmosphere, denoted
T2 and T4.

T2 radiation mostly comes from the surface and lower troposphere, whereas T4 mostly
emanates from the stratosphere. From these radiation readings, temperature averages
can be inferred based on an assumed set of weights. [3.4.1.2.2]

The “true” weights cannot be known with certainty. The weights that yield results most
closely matching data from weather balloons shows the least amount of tropospheric
warming. [Figure 3.4.3]

FIGURE ISPM-6:
SATELLITE-MEASURED
MEAN GLOBAL
TEMPERATURE
ANOMALIES SINCE
1979 (

TOP: Global average;
MIDDLE: Northern
Hemisphere;
BOTTOM: Southern
Hemisphere.

Sources:

Top: Red -Mears and
Wentz (2006); Blue -
Global Hydrology and
Climate Centre – University
of Alabama in Huntsville
(GHCC-UAH);
Middle and Bottom:
GHCC-UAH.

-0.6

-0.4

-0.2

0.0

0.2

0.4

0.6

0.8

1978

1982

1986

1990

1994

1998

2002

Year

tur

-0.8

-0.4

0.0

0.4

0.8

1.2

1978

1982

1986

1990

1994

1998

2002

Year

tur

-0.6

-0.4

-0.2

0.0

0.2

0.4

0.6

0.8

1.0

1978

1982

1986

1990

1994

1998

2002

Year

tur

Independent Summary for Policy Makers of the Text of the IPCC Fourth Assessment Report

2.1b

Satellite data from the lower atmosphere (T2) yield trends of about 0.04

C to 0.2

C /

decade over 1979-2004, implying estimated trend coefficients in averaged tropospheric
temperatures of about 0.12

C to 0.19

C/decade.

Three different teams of analysts have examined satellite-measured microwave radiation
data spanning 1979 to the present. [3.4.1.2.2]

The channel T2 data imply global warming of the troposphere of 0.04 to 0.20°C /decade
for the period 1979-2004, depending on assumptions about instrument calibration,
orbital drift and diurnal cycle corrections and merging across data sets. [3.4.1.2.2; Figure
ISPM-6]

Adjusting T2 data to remove an estimated contribution from the stratosphere yields
tropospheric trend coefficients ranging from about 0.12

C to 0.19

C per decade,

depending on the method. [3.4.1.2.2; Figure 3.4.3]

Tropospheric trends computed from NRA (model-based) reanalysis data are lower and
statistically insignificant, but may be unreliable. [3.4.1.5]

Extrapolated to a century scale these trends compare to the low end of past IPCC
warming projections (0.14

C to 0.58

C/decade) as presented in the TAR.

2.1c

There is no significant warming in the tropical troposphere, which accounts for half

the world’s lower atmosphere. This is where models that assume a strong influence of
greenhouse gases forecast some of the most rapid warming should occur.

The tropics account for half the world’s atmosphere. In none of the available data sets is
significant warming observed in the tropical troposphere [Figure 3.4.3]. One of the
available satellite data sets shows trends consistent with increased warming at higher
altitude in the tropics [3.4.1.2.2], while others do not.

Climate models based on the assumption that greenhouse gases drive climate change
predict some of the strongest warming should be observed in the upper troposphere
over the tropics [Figure 10.3.4]. This pattern is predicted to be evident early in the
forecast period and the pattern is simulated consistently among the models. [10.3.2.1]

-0.8

-0.6

-0.4

-0.2

0.0

0.2

0.4

0.6

0.8

1850

1870

1890

1910

1930

1950

1970

1990

Year

tur

Hadley Centre for Climate Change

Goddard Institute for Space Studies
and National Climate Data Center

FIGURE ISPM-7: ANNUAL AVERAGE MEAN TEMPERATURE ANOMALIES
MEASURED AT THE EARTH’S SURFACE OVER THE LAST 120-150 YEARS (

Sources:

Goddard Institute for Space Studies (GISS), National Climate Data

Center (NCDC), and Hadley Centre for Climate Change.

Independent Summary for Policy Makers of the Text of the IPCC Fourth Assessment Report

2.1d

A global average of temperature data collected over land, combined with ocean

surface measurements from ships and buoys, with local means removed and some adjust-
ments applied to control for uneven sampling, loss of half the land-based weather stations
in the early 1990s, changes in measurement techniques and other potential problems,
exhibits an upward trend from 1900 to 1940, and again from 1979 to the present.

The statistic is commonly called the global mean temperature anomaly or “global
temperature” for short.

The global temperature statistic produced by the Goddard Institute for Space Studies
(GISS) and the National Climate Data Center (NCDC) was slightly higher in 2005 than at
any time since 1998, while that produced by the Hadley Center peaked in 1998 and has
been slightly lower ever since. (see Figure ISPM-7) [3.2.2]

See also Section 2.1e below.

1850–2005

1901–2005

1910–1945

1946–1978

1979–2005

Land: Northern Hemisphere

CRU (Brohan et al., 2006)

0.063

0.089

0.142

–0.038

0.330

± 0.018

± 0.030

± 0.057

± 0.064

± 0.108

GHCN (Smith and Reynolds, 2005)

0.072

0.127

–0.040

0.344

± 0.031

± 0.065

± 0.074

± 0.121

GISS

0.083

0.166

–0.053

0.294

± 0.030

± 0.061

± 0.062

± 0.090

Lugina et al. (2005) up to 2004

0.074

0.144

–0.051

0.278

± 0.032

± 0.074

± 0.061

± 0.096

Land: Southern Hemisphere

CRU (Brohan et al., 2006)

0.034

0.078

0.091

0.031

0.135

± 0.033

± 0.054

±0.076

±0.063

± 0.087

GHCN (Smith and Reynolds, 2005)

0.057

0.091

0.054

0.220

± 0.020

± 0.069

± 0.072

± 0.114

GISS

0.056

0.033

0.060

0.085

± 0.015

± 0.042

± 0.052

± 0.067

Lugina et al. (2005) up to 2004

0.056

0.064

0.014

0.074

± 0.013

± 0.046

± 0.052

± 0.062

Land: Globe

CRU (Brohan et al., 2006)

0.054

0.084

0.125

-0.016

0.268

± 0.020

± 0.026

±0.042

±0.055

± 0.084

GHCN (Smith and Reynolds, 2005)

0.068

0.116

–0.013

0.315

± 0.029

±0.057

±0.061

± 0.108

GISS

0.069

0.102

0.003

0.188

± 0.020

± 0.041

± 0.046

± 0.084

Lugina et al. (2005) up to 2004

0.065

0.108

- 0.021

0.183

± 0.024

± 0.043

± 0.059

± 0.075

TABLE ISPM-1: LINEAR TRENDS OF TEMPERATURE (

C/DECADE)

Reproduction of Table 3.2 from the Fourth Assessment Report. Each cell shows the IPCC-estimated trend and 2-standard error confidence
interval. ‘CRU’ denotes Climatic Research Unit; ‘GHCN’ denotes Global Historical Climatology Network; ‘GISS’ denotes Goddard Institute for
Space Studies. Bold denotes a statistically significant (1%) trend in IPCC methodology; italics denotes significant (1-5%): but see Section 2.1g
below.

Independent Summary for Policy Makers of the Text of the IPCC Fourth Assessment Report

2.1e

Post-1979 trends in temperature data averaged over land areas in the Southern

Hemisphere are small compared to those from the Northern Hemisphere, and statistically
less significant.

Temperature trends in land-based data for the Northern and Southern Hemispheres
from 1979-2005 are shown in Table ISPM-1. In all cases the Southern Hemisphere trend
is small compared to the Northern Hemisphere trend.

In two of the four surface data sets the Southern Hemisphere trend is less than one-third
as large as the Northern Hemisphere trend and is statistically less significant. [Table 3.2]

Both data sets that merge land-based data with relatively sparse and uncertain sea
surface temperature data show Southern Hemisphere trends less than half those in the
Northern Hemisphere. [Table 3.2]

2.1f

The Third Assessment Report drew attention to the declining Diurnal Temperature

Range (DTR) as evidence of global warming (Working Group 1 Summary for Policymakers,
page 1). The decline in the DTR has now ceased, and appears to be growing in most places.

The DTR declined after 1950, but stabilized as of the mid-1990s. [3.2.2.7, Figure 3.2.2]

From 1979 to 2004, data from many locations on all continents show an increasing
DTR, especially in North America, Europe, Australia and South America. [Figure 3.2.11]

2.1g

The significance of trends in temperature and precipitation data is likely to have

been overstated in previous analyses.

The climate system responds to change slowly over time, and past changes accumulate
through long term persistence to influence ongoing trends. As a result the trend estima-
tion techniques used in recent IPCC Assessments likely overstate the statistical signifi-
cance of observed changes, and the results of trend analysis often depend on the
statistical model used. [3.2.2.1]

Long Term Persistence and Trend Analysis

Methods for estimating trends, and assessing their statistical significance, have undergone considerable
advance in the past decade. Technical issues being raised include

nonstationarity

and

Long Term Persistence

While the literature on these issues originated in hydrology, econometrics, finance and statistics, it has
begun to be applied to climate data sets as well. The main findings are that proper treatment of long term
processes in climate data often require a major reinterpretation of the significance of recent trends, as the
new methods attribute more of the observed changes in climate data to natural variance.

This is explored in the forthcoming Fraser Institute Supplementary Analysis Series report, “Long Term
Persistence in Geophysical Data.”

SUPPLEMENT

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TION

Independent Summary for Policy Makers of the Text of the IPCC Fourth Assessment Report

2.1h

There are differences in linear trends of tropospheric temperatures between the high

latitudes of the Northern and Southern Hemispheres that are not consistent with computer
model projections.

Geographical patterns of the linear trend in tropical temperature show coherent warm-
ing over the Northern Hemisphere but areas of cooling over the Southern Hemisphere.
[3.4.1.2.2, Figure 3.4.4]

The North Pole exhibits a sudden upward trend in mean temperature after 1990, but
not the South Pole. (see Figure ISPM-8)

Model projections suggest greenhouse gas-induced warming patterns at the north and
south poles will be nearly symmetrical. [Figure 10.3.5]

FIGURE ISPM-8. AVERAGE SURFACE TEMPERATURE ANOMALY
POLEWARD OF 64 DEGREES LATITUDE (GRAY LINE) AND
CORRESPONDING SATELLITE-MEASURED DATA (BLUE LINE)

TOP: South Pole; BOTTOM: North Pole.

Source:

Gray-Goddard Institute for Space Studies (2006); Blue- Mears and

Wentz (2006).

-2.0

-1.5

-1.0

-0.5

0.0

0.5

1.0

1.5

2.0

2.5

1880 1890 1900 1910 1920 1930 1940 1950 1960 1970 1980 1990 2000

Year

tur

Goddard Institute for Space Studies (2006)

Mears & Wentz, 2006

North Pole

-3.5

-3.0

-2.5

-2.0

-1.5

-1.0

-0.5

0.0

0.5

1.0

1.5

2.0

1880 1890 1900 1910 1920 1930 1940 1950 1960 1970 1980 1990 2000

Year

tur

Goddard Institute for
Space Studies (2006)

Mears & Wentz, 2006

South Pole

Independent Summary for Policy Makers of the Text of the IPCC Fourth Assessment Report

2.1i

The Earth’s climate is characterized by many modes of variability, involving both the

atmosphere and the oceans, and also by the cryosphere and the biosphere [1.4.6]. There
is an increasing recognition that changes in the oceans may be playing a role in climate
change.

Our understanding of the variability and trends in different oceans is still developing, but
it is already apparent they are quite different. The Pacific is dominated by the El-Niño/
Southern Oscillation (ENSO) cycle and is modulated by the Pacific Decadal Oscillation
(PDO), which may provide ways of transporting heat from the tropical oceans to higher
latitudes and from the ocean to the atmosphere. [3.6.3]

Since 1900, North Pacific Sea Surface Temperatures (SST) show warm mode phases
from 1925-1946 and 1977 to 2005. [3.6.3]

Since the 1850s, North Atlantic SSTs show a 65–75 year variation, with apparent warm
phases at roughly 1860–1880 and 1930–1960 and cool phases during 1905–1925 and
1970–1990. This feature has been termed the Atlantic Multidecadal Oscillation (AMO).
The cycle appears to have returned to a warm phase beginning in the mid-1990s and
tropical Atlantic SSTs were at record high levels in 2005. [3.6.6.1]

The AMO has been linked to multi-year precipitation anomalies over North America, as
well as Atlantic hurricane formation, African drought frequency, winter temperatures in
Europe, sea ice concentration in the Greenland Sea and sea level pressure over high
northern latitudes. [3.6.6.1]

The multidecadal variability in the Atlantic is much longer than the Pacific but it is
noteworthy that all oceans exhibit a warm period around the early 1940s. [3.2.2.3]

SUPPLEMENT

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TION

Major Ocean-Atmosphere Climate Oscillations

An important theme in recent meteorological research is the identification of some large-scale atmospheric
cycles that operate on time spans of 30 years or more. These oscillations arise from the interaction of the
oceans and atmosphere, and are typically measured using pressure gradients across large regions of the
Earth’s surface. Representation of the oceans in climate models as truly dynamic systems (as opposed to the
earlier “slab” ocean models) is only beginning. A comprehensive description of the atmospheric and ocean
circulations has been delayed by lack of observations from the high atmosphere and deep oceans.

Major oscillation systems have been shown to have significant explanatory power for recent climatic

changes, including trends in temperature and precipitation. The El Niño-Southern Oscillation (ENSO) is a
coupled air-sea phenomenon that has its origins in the Pacific Ocean but affects climate globally. The
mechanisms and predictive skill of ENSO are still under development. The North Atlantic Oscillation (NOA,
first discovered by Sir Gilbert Walker in the 1930s) is a phenomenon that affects weather and climate and is
associated with variability and latitudinal shifts of the westerly winds and jet streams. Despite a long history
of observation and research the NAO and its low-frequency variability remains poorly understood.

The IPCC discusses some of these issues, but does not provide adequate detail about the connection

between these systems and recent weather changes.

This topic is explored in the forthcoming Fraser Institute Supplementary Analysis Series report, “Major
Climatic Oscillations and Recent Weather Changes.”

Independent Summary for Policy Makers of the Text of the IPCC Fourth Assessment Report

2.2 Precipitation and
snow cover

2.2a

There is no globally-consistent pattern in long-term precipitation trends.

At the global level, slight decline was observed in total precipitation from 1950 to the
early 1990s, which has since reversed. [3.3.2.1; Figure 3.3.1]

Precipitation in North and South America has risen slightly over the past century in
many places, though in some regions it has fallen. [3.3.2.2]

The drying trend noted in the 1980s in the Sahel (the coastal region in Africa bordering
the Sahara desert) has since reversed considerably. [3.3.2.2]

Rainfall in India increased from 1901 to 1979 then declined through to the present
[3.3.2.2], and there is no overall trend. [3.3.2.2]

Australian precipitation trends vary by region and are closely linked to the El Niño cycle.
[3.3.2.2]

2.2b

There is no globally-consistent pattern in snow-covered area or snow depth.

In the Northern Hemisphere, mean observed snow cover in April declined somewhat
from the 1950s to the 1970s, declined rapidly in the 1980s and has increased slightly
since 1990. [Figure 4.2.1]

Over the 1966 to 2004 interval, mean Northern Hemisphere snow cover in October
showed a statistically insignificant decline. But over the entire span of available data
(1922 to 2004) the mean Northern Hemisphere snow cover in October shows a statisti-
cally significant increase. [Table 4.2.1]

Over the 1966 to 2004 interval, mean Northern Hemisphere snow cover trended down-
ward in spring and summer, but not substantially in winter. [4.2.2.2; Table 4.2.1]

In North America the trend in November-January snow-covered area over the 20th
century is upward overall, with a recent downward trend especially in Western North
America. [4.2.2.2.1]

Snow-covered area in mountainous areas of Switzerland and Slovakia has declined
since 1931, but not in Bulgaria. [4.2.2.2.2]

Lowland areas of central Europe have exhibited decreased snow-covered area, while
increased maximum snow depth has been recorded in the former Soviet Union, Tibet
and China. [4.2.2.2.2]

In South America a long term increasing trend in snow days has been observed in the
eastern central Andes. [4.2.2.3.1]

In Southeastern Australia, late-winter snow depth has declined considerably, though
winter precipitation has decreased only slightly. [4.2.2.3.2]

2.2c

In areas north of 55N latitude, snowfall has increased over the past 50 years. Trends

in the frequency of heavy snowfall events vary by region.

At high latitudes, winter precipitation has increased in the past 50 years [3.3.2.3] and
there has been little change in the fraction falling as snow rather than rain. [3.3.2.3]

In North America, the incidence of heavy snowfall events has increased in Northern
Canada and in the lee of the Great Lakes, but decreased in the lower Missouri river
basin. [3.3.2.3]

In some areas, namely Southern Canada and western Russia, the earlier onset of the
spring season over the past 50 years has meant an increasing fraction of precipitation
falls as rainfall [3.3.2.3]. However other data have shown an overall increase in snowfall
in parts of southern Canada. [3.3.2.3]

Independent Summary for Policy Makers of the Text of the IPCC Fourth Assessment Report

Recent North American snowfall records

“Record-breaking” local hot weather events are sometimes promoted as evidence of global warming. What
can we infer if record-breaking cold weather events begin to accumulate in some local data?

New York City’s Central Park has a January (their coldest month) average temperature of 0.1°C and

winter average of 1.0°C. For the first time since records began in the 1860s, Central Park reported four
successive years of 100 centimetres of snow or more ending in the winter of 2005/06. On February 11-12,
2006, Central Park broke the all-time single snowstorm record with 68.3 centimetres of snow. Also in 1995/
96, Central Park and most other cities in the central and eastern US had all-time record seasonal snowfall.
In Central Park, that winter brought 192 centimetres of snow.

Not far to the north in Boston, MA where the winter temperature averages -0.1°C, the 12 year average

snowfall in the winter ending 2004/05 was 130.3 centimetres, the highest in their entire record dating back
into the 1800s. A new all-time single snowstorm record was set on February 17-18, 2003 with 70 centime-
tres and a new all-time seasonal snowfall record of 273 centimetres was set in 1995/96. In the last dozen
years, Boston has recorded their 1st, 3rd, 5th, 7th and 12th snowiest winters.

In the Canadian Atlantic provinces winter snow accumulation has increased in recent years. The city of

St. John’s (Newfoundland) recorded its highest ever snow accumulation in one season, ~650 cm, from
November 2000 through May 2001. This is the highest snow accumulation at a sea-level location any
where in the world. In February 2004 the city of Halifax (Nova Scotia) received a record-breaking 100 cm of
snow in a 24-hour period.

Data Source: US National Weather Service and Environment Canada

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2.3 Storms and
extreme weather

2.3a

Perceptions of increased extreme weather events are potentially due to increased

reporting. There is too little data to reliably confirm these perceptions.

People tend to hear about extreme events more now because of technology. Pictures
shot by camcorders on the news may foster a belief that weather-related extremes are
increasing in frequency. [3.8.1]

Global studies of temperature and precipitation extremes over land suffer from a scarcity
of data. In various parts of the globe, there is a lack of homogeneous (i.e., subject to
consistent quality control and constant sampling conditions) daily observational records.
The lack of homogeneous data has been attributed to, among other things, changes in
observing practices or urban heat island effects. [3.8.1]

Identification of changes in extremes is also dependent on the statistical analysis
technique employed. [3.8.1]

Global studies of daily temperature and precipitation extremes over land suffer from
both a scarcity of data and regions with missing data. [3.8.1]

Analyses of trends in extremes are also sensitive to the analysis period; e.g., the inclusion
of the exceptionally hot European summer of 2003 may have a marked influence on
results if the period is short. [3.8.1]

Independent Summary for Policy Makers of the Text of the IPCC Fourth Assessment Report

2.3b

Since 1970, there is some evidence of increased tropical cyclone intensity in both

hemispheres, but a decrease in total tropical storm numbers, and no clear global pattern.

A number of recent studies suggest that cyclone activity over both hemispheres has
changed over the second half of the 20th century. General features include a poleward
shift in storm track location and increased storm intensity, but a decrease in total storm
numbers. [3.5.3]

Station pressure data over the Atlantic-European sector (which has long and consistent
records) show a decline of storminess from high levels during the late-19th century to a
minimum around 1960 and then a quite rapid increase to a maximum around 1990,
followed again by a slight decline. [3.5.3]

Data suggest that cyclone activity in the Northern Hemisphere mid-latitudes has in-
creased during the past 40 years, whereas there have been significant decreases in
cyclone numbers, and increases in mean cyclone radius and depth, over the southern
extratropics over the last two or three decades. [3.5.3]

With respect to storm data generally, data uncertainties compromise evidence for trends.
[3.8.1]

The considerable inter-decadal variability reduces the significance of any long-term
trends. Careful interpretation of observational records is therefore required. [3.8.3]

The overall power of cyclones has been characterized using the Accumulated Cyclone
Energy (ACE) index and the Power Dissipation Index (PDI). The ACE is proportional to
the square of the wind speed and the PDI is proportional to the wind speed cubed. The
PDI for the world as a whole shows an upward trend since the 1970s, but because of its
cubic exponent it is very sensitive to data quality. Pre-1970 data are particularly uncer-
tain [3.8.3]. The ACE index is available in some regions back to 1948 and shows no
overall trend over the entire interval. The ACE shows an upward trend after 1980 only in
the North Atlantic, but a downward trend post-1980 in the West North Pacific, East
North Pacific, Australian-South Pacific, North Indian and South Indian regions [Figure
3.8.4]. At the global level, the ACE Index values for 2004 and 2005 are about average for
the whole post-1980 interval. [3.8.3]

2.3c

Data are too sparse, and trends inconsistent, to identify a pattern in extratropical

cyclones.

As with tropical cyclones, detection of long-term changes in extratropical cyclone meas-
ures is hampered by incomplete and changing observing systems. Some earlier results
have been questioned because of changes in the observation system. [3.8.4.1]

An increase in the number of deep cyclones is apparent over the North Pacific and
North Atlantic, but only the North Pacific trend is statistically significant. Significant
decreases have been noted in cyclone numbers over the southern extratropics over the
last two or three decades, along with increases in mean cyclone radius and depth.
[3.8.4.1]

2.3d

Evidence for changes in temperature variability is sparse and insignificant.

Evidence for changes in observed interannual variability is still sparse. Seasonal mean
temperature in central Europe showed a weak increase in summer and decrease in
winter, for the time period 1961 to 2004. These changes are not statistically significant
at the 10% level. [3.8.2.1]

Independent Summary for Policy Makers of the Text of the IPCC Fourth Assessment Report

Regional studies from several continents show patterns of changes in extremes consist-
ent with a general warming, although the observed changes of the tails of the tempera-
ture distributions are not consistent with a simple increase in the entire temperature
distribution. [3.8.2.1]

For the period 1951–2003, three-quarters of the global land area sampled showed a
significant decrease in the annual occurrence of cold nights; while a significant increase
in the annual occurrence of warm nights took place over 72% of the area. This implies a
positive shift in the distribution of daily minimum temperature throughout the globe.
Changes in the occurrence of cold days and warm days show warming as well, but
generally less marked. This is consistent with the increase in minimum as opposed to
maximum temperature. [3.8.2.1]

2.4 Ocean
temperatures and
sea levels

2.4a

Regarding the Gulf Stream and the global Meridional Overturning Circulation (MOC),

it is very likely that the MOC has changed on annual and decadal time scales, but evidence
for overall weakening is mixed and uncertain, and the connection to surface climate is not
well understood.

The global Meridional Overturning Circulation (MOC) consists primarily of dense waters
that sink to the seafloor at high-latitudes in the North Atlantic Ocean and near Antarc-
tica. This influences global ocean currents and may influence wind patterns, including
the Gulf Stream. [Box 5.1]

Only indirect estimates of the MOC strength and variability exist, and the best evidence
for observational changes in the overturning circulation comes from the North Atlantic.
[Box 5.1]

There is evidence for a link between MOC and abrupt changes in surface climate during
the past 120,000 years, although the exact mechanism is not clear. [Box 5.1]

One recent study concluded that the MOC transport in the North Atlantic at 25°N has
decreased by 30% between 1957 and 2004, indicating a stronger mid-ocean return flow
in the upper kilometre, though not a decrease in Gulf Stream strength. Note however
that this result is based on 5 snapshots in time, and it is not clear whether the trend
estimate can be viewed as robust in the presence of considerable variability. [Box 5.1]

Two other studies examined a model-based relation of MOC transport with interdecadal
sea surface temperature patterns and concluded that the MOC has increased since the
1970s. [Box 5.1]

There is only a low level of confidence that the strength of deep limb of the MOC in the
North Atlantic MOC has actually decreased. [Box 5.1]

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Questions about the MOC Mechanism

It has not been formally established that deep-water formation drives the MOC. Others have argued (e.g.,
Wunsch, 2002) that deep-water formation does not provide sufficient energy to drive the MOC, and that it
is a largely wind-driven circulation, where the wind field provides the mechanical energy necessary to
overcome the natural stratification of the ocean.

A recent paper (Latif et al, 2006) concludes that multi-decadal MOC variations can be understood as the

lagged response to the multi-decadal variations in the NAO, and further does not provide any evidence for a
sustained weakening of the MOC during the last few decades.

Independent Summary for Policy Makers of the Text of the IPCC Fourth Assessment Report

2.4b

Regarding sea levels, a critical issue concerns how the records are adjusted for

vertical movements of the land upon which the tide gauges are located. Current data
suggest a global mean sea level rise of between 2 and 3 millimeters per year.

Tide gauges provide data about sea level variations with respect to the land on which
they lie. However, the Earth’s crust is subject to various vertical motions due to geological
factors such as tectonics and local subsidences. To extract an accurate sea level signal,
tide gauge readings need to be adjusted to compensate for vertical motions. [5.5.1]

Sea level change based on satellite altimetry measurements is measured with respect to
the earth’s center of mass, and thus is not distorted by land motions, except for a small
component due to large scale deformation of ocean basins from Glacial Isostatic Adjust-
ment (GIA). [5.5.1]

Models are used to correct recent global tide gauge estimates for Glacial Isostatic
Rebound (GIR), but not for other land motions. Adjusted rates could be underestimated
by several tenths of millimeters per year in analyses which employ extrapolations of
geological data obtained near the gauges. [5.5.2.1]

Tide gauge data suggests a rise in mean sea level over 1961-2003 of about 1.8 mm/year,
±0.5 mm. [5.5.2.1]

Satellite estimates of mean sea level yield an accuracy of ±5 mm. Satellite data show a
rate of sea level rise of +3.1 ± 0.8 mm per year over 1993–2005. The accuracy of this
estimate is partly dependent on the calibration against vertical land motions as measured
by tide gauges. [5.5.2.1]

By comparison, satellite observations show a 15 mm rise and fall of mean sea level and a
0.4

C rise and fall of global mean sea surface temperature accompanying the 1997–1998

El Niño-Southern Oscillation (ENSO) event. [5.5.2.1]

2.4c

Regional trends in sea level are quite varied and some regions are experiencing

declining sea levels. Changes in air pressure and wind account for some observed sea level
increase.

While global sea level rose by approximately 120 metres during the several millennia
that followed the end of the last glacial maximum, the level stabilized between 3000 and
2000 years ago. Since then, paleo sea level indicators suggest that global sea level did
not change significantly: the average rate of change from 2000 years ago to about 100
years ago is near zero. [Question 5.1]

Although regional variability in coastal sea level change had been reported from tide
gauge analyses, the global coverage of satellite altimetry provides unambiguous evidence
of non-uniform sea level change in open oceans. [5.5.2.2]

For the past decade, the western Pacific Ocean and eastern Indian Oceans show the
highest magnitude of sea level rise, however, sea level has been dropping in the eastern
Pacific and western Indian Oceans. [5.5.2.2]

Except for the Gulf Stream region, most of the Atlantic Ocean shows sea level rise
during the past decade. [5.5.2.2]

Northeast Atlantic sea level records are notable for their 20th century trends that are
lower than the global average. Explanations include Glacial Isostatic Adjustment, and air
pressure and wind changes associated with North Atlantic Oscillation (NAO). [5.5.2.6.1]

Arctic Ocean sea level time series have well pronounced decadal variability which
corresponds to the variability of the North Atlantic Oscillation Index. In this particular
region, wind stress and atmospheric pressure loading contribute to nearly half of the
observed Arctic sea level rise. [5.5.2.6.2]

Independent Summary for Policy Makers of the Text of the IPCC Fourth Assessment Report

2.4d

There is very little sea-level data from Pacific Ocean islands. The available series

appear to indicate less than one millimeter sea level rise per year.

There are only four Pacific island stations with more than 50 years of data. Data from
these stations show an average rate of sea-level rise (relative to the Earth’s crust) of 1.6
mm/year. Twenty-two Pacific island stations have more than 25 years of data and they
indicate an average sea level rise less than half as great, at 0.7 mm/year. However, these
data suffer from poorly quantified vertical land motions. [5.5.2.6.3]

2.4e

Changes in extreme sea level are due to changes in sea level and storminess. 20th

century trends differ by location.

The annual maximum high water surge at Liverpool since 1768 was larger in the late-
18th, late-19th and late-20th centuries than for most of the 20th century. [5.5.2.7]

The tide gauge record at Brest from 1860 to 1994 shows an increasing trend in storm
surges (as measured by maxima and top-1% groups), but shows a decreasing trend
during the period 1953–1994. [5.5.2.7]

Extreme winter surges at San Francisco have exhibited a significant increasing trend
since about 1950. [5.5.2.7]

The rise in extreme sea level along the US east coast is closely correlated to the rise in
mean sea level. [5.5.2.7]

A long term increase in the number and height of extreme daily sea level readings has
been noted at Honolulu, but no evidence indicates an increase relative to the underlying
upward mean sea level trend. [5.5.2.7]

2.4f

Sea level increases over the past decade are not uniform, and it is presently unclear

whether they are attributable to natural variability.

The instrumentally-based estimates of modern sea level change provide evidence for an
onset of acceleration at the end of the 19th century. Recent estimates for the last half of
the 20th century (1950–2000) give approximately 2 mm/year global mean sea level rise.
New satellite observations show that since 1993 sea level has been rising at a rate of 3.1
mm/year. [Question 5.1]

Satellite data also confirm that sea level is not rising uniformly over the world.
[Question 5.1]

It is presently unclear whether the higher rate of sea level rise in the 1990’s indicates an
acceleration due to global warming, or a result of natural climate variability, or a combi-
nation of both effects. [Question 5.1]

Historical Storm Surges

The greatest storm surge in historical time was 13.6 meters and occurred in 1876 in the Bay of Bengal. The
second highest on record was 13 meters in the Bathurst Bay in Australia in 1899. Since 1876, the maximum
surge in the Bay of Bengal was about 9 meters in 1970 and 1999. By comparison, the maximum surge by
Hurricane Katrina of August 2005 was 8.5 meters.

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Independent Summary for Policy Makers of the Text of the IPCC Fourth Assessment Report

2.5 Glaciers, sea ice
and ice caps

2.5a

Glacier archives indicate that most of the Earth’s alpine glaciers receded or

disappeared between 9,000 and 6,000 years ago.

Most archives from the Northern Hemisphere and the tropics show small or absent
glaciers between 9,000 and 6,000 years ago. [Box 6.3]

Glaciers began growing thereafter, up to the 1800s. [Box 6.3]

This tendency is primarily related to changes in the Earth’s orbit, however shorter,
decadal-scale, regionally diverse glacier responses must have been driven by other
factors which are complex and poorly understood. [Box 6.3]

2.5b

Glaciers in most places have retreated since the 1800s

General retreat of glacier termini started after 1800, with considerable mean retreat
rates in all regions after 1850 lasting throughout the 20th century. A slowdown of
retreats between about 1970 and 1990 is evident in the raw data. Retreats were again
generally rapid in the 1990s; though advances of glaciers have been observed in western
Scandinavia and New Zealand. [4.5.2]

There are few records of directly measured glacier mass balances, and they stretch back
only to the mid 20th century. [4.5.2] When areal weighting and spatial interpolation are
used to estimate large-scale patterns from the available data, the 1990s trend towards
glacier retreat appears to have leveled off or reversed after 1998. [Figure 4.5.2]

2.5c

Over the last half century, global mean winter accumulation and summer melting of

glacier ice have both increased.

At least in the Northern Hemisphere, winter accumulation and summer melting of
glacial ice correlates positively with hemispheric air temperature, whereas the net
balance correlates negatively with hemispheric air temperature. An analysis of 21
Northern Hemisphere glaciers found a rather uniformly increased mass-turnover rate,
qualitatively consistent with moderately increased precipitation and substantially in-
creased low-altitude melting. [4.5.2]

2.5d

While the loss of Northern Hemisphere glacier mass accelerated in the 1990s, loss of

Arctic sea ice thickness slowed or stopped during the 1990s

In the Northern Hemisphere, the rate of glacier mass loss was twice as rapid in the
1990s compared to the period from the 1960s to1990. [4.5.2]

An early study of Arctic ice found that ice draft in the mid 1990s was less than that
measured between 1958 and 1977 at every available location (including the North Pole).
The decline averaged about 42% of the average 1958–1977 thickness. Subsequent
studies indicate that the reduction in ice thickness was not gradual, but occurred
abruptly before 1991, with no evidence of thinning along 150°W from six springtime
cruises during 1991–1996. Springtime observations from 1976 to 1994 along the same
meridian indicated a decrease in ice draft sometime between the mid 1980s and early
1990s, with little subsequent change. [4.4.3.2]

Independent Summary for Policy Makers of the Text of the IPCC Fourth Assessment Report

2.5e

On a regional basis the pattern of glacier regimes remains complex. Precipitation and

solar changes appear to be important factors, especially in the tropics, including Kilimanjaro.

Although reports on individual glaciers or limited glacier areas support the global picture
of ongoing strong ice shrinkage in almost all regions, some exceptional results indicate
the complexity of both regional to local scale climate and respective glacier regimes.
[4.5.3]

Whereas Himalayan

glaciers have generally shrunk at varying rates, several high glaciers

in the central

Karakoram

are reported to have advanced and/or thickened at their

tongues, probably due to enhanced transport of moisture to high altitudes. [4.5.3]

Norwegian coastal glaciers advanced in the 1990s and started to shrink around 2000 as
a result of almost simultaneous reduced winter accumulation and greater summer
melting. Norwegian glacier termini farther inland have retreated continuously at a more
moderate rate. [4.5.3]

Glaciers in the New Zealand Alps

advanced during the 1990s, possibly due to increased

precipitation, but since 2000 they have started to shrink. [4.5.3]

Tropical glaciers, being in principle very sensitive to changes in both temperature and
atmospheric moisture, have shrunk mostly in response to regional changes in atmos-
pheric moisture content and related energy and mass balance variables such as solar
radiation, precipitation, albedo, and sublimation during the 20th century. Inter-annual
variation in the seasonal pattern of moisture strongly dominates the behaviour of
tropical glaciers. [4.5.3]

Glaciers on Kilimanjaro behave exceptionally. Even though the thickness of the tabular
ice on the summit plateau has not changed dramatically over the 20th century, the ice
has shown an incessant retreat of the vertical ice walls at its margins, for which solar
radiation is identified as the main driver. The mass balance on the horizontal top ice
surfaces is governed by precipitation amount and frequency and associated albedo, and
has sporadically reached positive annual values even in recent years. In contrast to the
plateau ice, the shrinkage of the glaciers on Kilimanjaro’s slopes is constantly decelerat-
ing. [4.5.3]

2.5f

Sea ice thickness is one of the most difficult geophysical parameters to measure on

large-scales.

Because of the large variability inherent in the sea-ice-climate system, evaluation of ice
thickness trends from the available observational data is difficult. [4.4.3.7]

Recent changes have occurred within the context of longer term decadal variability due
to both dynamic and thermodynamic forcing of the ice by circulation changes associated
with low-frequency modes of atmospheric variability. [4.4.3.7]

Ice thickness varies considerably from year to year at a given location and so the rather
sparse temporal sampling provided by submarine data makes inferences regarding long-
term change difficult. [4.4.3.2]

There are insufficient data to draw any conclusions about trends in the thickness of
Antarctic sea ice. [4.4.3.7]

Independent Summary for Policy Makers of the Text of the IPCC Fourth Assessment Report

2.5g

It is not possible to attribute the abrupt decrease in sea ice thickness inferred from

submarine observations entirely to the (rather slow) observed warming in the Arctic.

Some of the dramatic decrease may be a consequence of wind-driven redistribution of
ice volume over time. [4.4.3.4]

Low-frequency, large-scale modes of atmospheric variability (such as interannual changes
in circulation connected to the Northern Annular Mode) affect both wind-driving of sea
ice and heat transport in the atmosphere, and therefore contribute to interannual
variations in ice formation, growth and melt. [4.4.3.4]

2.5h

Estimates of Greenland ice cap changes indicate near coastal thinning and inland

thickening.

Many recent studies have addressed Greenland mass balance. They yield a broad picture
of slight inland thickening and strong near-coastal thinning, primarily in the south along
fast-moving outlet glaciers. [4.6.2.2]

Assessment of the data and techniques suggests overall mass balance of the Greenland
Ice Sheet ranging between growth by 25 Gigatonnes per year (Gt/year) and shrinkage by
60 Gt/year

for 1961–2003. [4.6.2.2]

This range changes to shrinkage by 50 to 100 Gt/year

for 1993–2003 (which translates

to 0.1-0.2 mm per year sea level rise: [10.3.4]) and by even higher rates between 2003
and 2005. However, interannual variability is very large, driven mainly by variability in
summer melting and sudden glacier accelerations. Consequently, the short time interval
covered by instrumental data is of concern in separating fluctuations from trends.
[4.6.2.2]

2.5i

The ice sheet in Eastern Antarctica appears to have grown while that in Western

Antarctica appears to have shrunk. The overall change may be positive or negative depend-
ing on assumptions about ice dynamics.

Assessment of the data and techniques suggests overall Antarctic ice-sheet mass balance
ranging from growth by 50 Gt/year to shrinkage by 200 Gt/year from 1993–2003.
[4.6.2.2]

There is no implication that the midpoint of this range provides the best estimate. Lack
of older data complicates a similar estimate for the period 1961–2003. [4.6.2.2]

A pattern of East Antarctic thickening and West Antarctic thinning was observed across
several independent studies. [4.6.2.2]

Considering the lack of estimated strong trends in accumulation rate, assessment of the
possible acceleration and of the slow time scales affecting central regions of the ice
sheets, it is reasonable to estimate that the behavior from 1961–2003 falls between ice-
sheet growth by 100 Gt/year and shrinkage by 200 Gt/year. [4.6.2.2]

Independent Summary for Policy Makers of the Text of the IPCC Fourth Assessment Report

2.5j

Summing changes in Greenland and Antarctic indicates either a gain or a loss of ice

mass over the 1961-2003 interval.

Simply summing the 1993–2003 contributions from Greenland and Antarctica produces
a range from balance (0 Gt/year) to shrinkage by 300 Gt/year, or contribution to sea-
level rise of 0 to 0.8 mm per year. [4.6.2.2]

· For 1961–2003, the same calculation spans growth by 125 Gt/year

to shrinkage by 260

Gt/year. [4.6.2.2]

2.6 Humidity and
radiation flux

2.6a

Changes in mid and upper tropospheric water vapour are proposed as an important

potential amplifier of climate change. There is evidence of increased specific humidity, but
not relative humidity, over the past two decades.

Water vapour in the mid and upper troposphere accounts for a large part of the atmos-
pheric greenhouse effect and is believed to be an important amplifier of climate change.
[3.4.2.2]

Due to instrumental limitations, long-term changes of water vapour in the upper tropo-
sphere are difficult to assess. [3.4.2.2]

Satellite data indicate that specific humidity in the upper troposphere increased over the
period 1982-2004, but changes in relative humidity were negligible. [3.4.2.2]

This signature is generally consistent with increased tropospheric temperatures, though
the increase in specific humidity is strongest over the tropics [Figure 3.4.6] where
temperature trends are insignificant. (see Section 2.1g)

2.6b

Observed changes in radiation flux at the top of the atmosphere are small and

equivocal, and may simply reflect natural variability.

Although there is independent evidence for decadal changes in top-of-atmosphere
(TOA) radiative fluxes over the last two decades, the evidence is equivocal. [3.4.4]

Changes in the planetary and tropical TOA radiative fluxes are consistent with inde-
pendent global ocean heat storage data, and are expected to be dominated by changes
in cloud radiative forcing. To the extent that the evidence is valid, these changes may
simply reflect natural low-frequency variability of the climate system. [3.4.4]

Climatic changes in paleoclimate perspective

3.1 Geological evidence
of warming and cooling
episodes

3.1a

On a time scale of millions of years, current temperatures are not unprecedented.

Through much, if not most, of the last 100 million years, temperatures were warmer than
at present, including a super-warm interval approximately 50 million years ago.

The earth was ice-free during most of its history [6.3.1]

The Pliocene (about 3 million years ago) was the most recent time in Earth’s history when
mean global temperatures were substantially warmer (about 2°C to 3°C warmer) [6.3.2]

The Paleocene-Eocene Thermal Maximum was several degrees warmer still. [6.3.3]

3.1b

On the other hand, temperatures during most of the most recent 1 million years

(the Pleistocene) have been colder than at present. Long glacial periods have alternated
with short (10 to 30,000 year long) interglacials.

Continental glaciers covered much of North America, Europe and Asia during the
Pleistocene. [6.4.1]

Ice cores and ocean sediment cores have enhanced our understanding of both glacial
and interglacials. [6.4.1]

Glacials and interglacials are attributed to changes in the earth’s orbit: precession,
obliquity and eccentricity. [Box 6,1]

3.1c

The last interglacial (LIG, 129,000–116,000 years ago) was warmer than the present.

Globally, there was less glacial ice and higher sea level on Earth during the Last Inter-
glacial than now. This suggests significant meltback of the Greenland and possibly
Antarctica ice sheets occurred. The climate of the LIG has been inferred to be warmer
than present, although the evidence is regional and not neccessarily synchronous
globally. Proxy data indicate warmer-than-present coastal waters in the Pacific, Atlantic,
and Indian Oceans and in the Mediterranean Sea, greatly reduced sea ice in the coastal
waters around Alaska, and extension of boreal forest into areas now occupied by tundra
in interior Alaska and Siberia, during the early LIG. Ice core data indicate Greenland and
Antarctic temperatures were 4–5°C warmer than present. [6.4.1.6]

The length and amplitude of interglacials varied. The shortest lasted only a few thousand
years, while the longest (Stage 11) lasted nearly 30,000 years. [6.4.1.5]

3.1d

The current interglacial (the Holocene) began about 11,600 years ago and is already

longer than some interglacials. Some features are comparable to the unusually long Stage
11 interglacial. [6.5, 6.4.1.5]

The most recent ice age began about 116,000 years ago. Glaciation reached a maximum
about 21,000 years ago. Deglaciation, or the transition to a warm interval, took place
between 20,000 and 10,000 years ago. [6.4.1.2, 6.5]

The present orbital configuration has been compared to the Stage 11 configuration
(420,000 -395,000 years ago), when there was a long interglacial. [6.4.1.5]

Independent Summary for Policy Makers of the Text of the IPCC Fourth Assessment Report

3.1e

Large, widespread, abrupt climate changes have occurred repeatedly throughout the

ice age/post-glacial interval.

Abrupt climate change refers to events of large amplitude

regionally, typically a few

degrees Celsius, and that occur on time scales significantly shorter than 1,000 years.
[1.4.2]

Abrupt temperature events were larger and more widespread during the ice age then
during the warm Holocene. The most dramatic of these abrupt climate changes are
characterised by a warming in Greenland by 8 to 16°C within a few decades, followed by
much slower cooling over centuries. Another type of abrupt change is the Heinrich event,
involving sea surface cooling that lasts several thousands of years, followed by abrupt
warming over several decades. At the end of the last ice age, as the climate warmed and
ice sheets melted, climate went through a number of abrupt cold phases, notably the
Younger Dryas and the 8.2 kyr event. [6.4.2.1]

Abrupt temperature changes were first detected in deep ice cores from Greenland. By
the end of the 1990s it became clear that abrupt climate changes, as found in the
Greenland ice cores during the last ice age, were numerous, indeed abrupt, and of large
amplitude. [1.4.2]

The importance of internal variability and processes was reinforced in the early 1990s
with the analysis of records with high temporal resolution: new ice cores, ocean cores
with high sedimentation rate, lacustrine sediments, and also cave stalagmites. Recon-
struction of the thermohaline circulation of deep and surface water shows the participa-
tion of the ocean in these abrupt changes. [1.4.2]

There are many examples of abrupt changes that are regional rather than global in
extent. [1.4.2]

Abrupt climate change during both ice age and warm epochs alters the notion of relative
climate stability, as previously suggested. Rather there is a coherent picture of an unsta-
ble ocean-atmosphere system of global extent. [1.4.2]

3.1f

The causes of large-scale climate variations on the century and longer time scales are

not well-understood.

Based on the correlations between changes in climate proxy records and production of
cosmogenic isotopes – assumed to relate to solar activity changes – some authors argue
that solar activity, through cosmic radiation and cloud nucleation, may be the driver for
centennial to millennial variability. Correlations between climate proxy records and
geomagnetic field variations suggest further influence on climate by cosmic radiation on
millennial and greater time scales. The possible importance of internal climate variabil-
ity, for instance related to the deep ocean circulation, has also been highlighted. [6.5.1.6]

However, in many records, there is no apparent consistent pacing at specific centennial
to millennial frequencies through the Holocene period, but rather shifts between differ-
ent frequencies. [6.5.1.6]

The current lack of consistency between various data sets makes it difficult, based on
current knowledge, to attribute the century and longer time scale large-scale climate
variations solely to solar activity, episodes of intense volcanism, or variability internal to
the climate system. [6.5.1.6]

Independent Summary for Policy Makers of the Text of the IPCC Fourth Assessment Report

3.2a

Natural climatic variability is now believed to be substantially larger than was esti-

mated in the Third Assessment Report, as is the uncertainty associated with paleoclimate
studies.

The Third Assessment Report placed considerable emphasis on the “hockey stick”
climate reconstruction, which suggested the late 20th century climate was unusual in the
context of the past 1,000 years. This graph has subsequently been subject to considerable
criticism [6.6.1.1]

When viewed together (Fig ISPM-9), the currently available reconstructions indicate
generally greater variability in centennial time scale trends over the last 1000 years than
was apparent in the Third Assessment Report.

Proxy evidence cannot characterize the mean Northern Hemisphere temperature to
within at least ±0.5

C, and over significant stretches of time the available reconstructions

differ by 0.7-1.0

C [Figure 6.10; Figure ISPM-9].

3.2 Global climate
reconstructions over
the past 2,000 years

SUPPLEMENT

ARY INFORMA

TION

Recent refutations of hockey stick millennial paleoclimatic methods and conclusions.

Two recent, detailed reviews of the methodology of paleoclimatic reconstructions (National Research Council
2006, Wegman et al. 2006) both concluded that there were methodological errors in the “hockey stick” graph
of Mann et al. which was prominently promoted in the Third Assessment Report (Summary for Policymakers
Fig 1). Both reports concluded that the data and methods did not support the assertions that the 1990s
were the “warmest decade of the millennium” and 1998 the “warmest year” of the millennium (NRC p. 109;
Wegman et al. p.49). The National Research Council Report also concluded that uncertainties of published
paleoclimate reconstructions have been generally underestimated (NRC p. 91).

The National Research Council recommended that proxies sensitive to precipitation be avoided in tempera-

ture reconstructions and, in particular, that strip-bark bristlecones and foxtails be avoided. However, none of
the IPCC reconstructions for the past millennium observe the National Research Council recommendations.

Independent Summary for Policy Makers of the Text of the IPCC Fourth Assessment Report

The Divergence Problem

A number of recent studies (Briffa et al. 1998, Briffa et al. 2001, d’Arrigo et al 2006, National Research
Council 2006) have observed that proxies, especially tree ring proxies, and reconstructions relying on them
diverge from instrumental temperature series as temperatures increased in the 1980s and 1990s. This
creates a fundamental uncertainty over whether such reconstructions could have detected warming trends
in the past (the “Divergence Problem”).

The Divergence Problem is a major unresolved problem in millennial reconstructions. Until it is

resolved, it is statistically invalid to splice an instrumental series onto a proxy-based series as if the two
are interchangeable.

For this reason, Figure ISPM-9 reproduces IPCC Figure 6-10b with the black instrumental series removed.

-1

-0.8

-0.6

-0.4

-0.2

0.2

700

800

900

1000

1100

1200

1300

1400

1500

1600

1700

1800

1900

2000

Year

ture

(°

res

1961-

1990

)

JBB1998

MBH1999

BOS2001

CED2004

B2000

MJ2003

RMO2005

MSH2005

DWJ2006

HCA2006

FIGURE ISPM-9: SOME RECENT PALEOCLIMATE TEMPERATURE RECONSTRUCTIONS OF PAST 1300
YEARS FOR THE NORTHERN HEMISPHERE, ALL CALIBRATED OVER THE 1902-1980 INTERVAL

NOTE: Instrumental splice post-1850 has been removed (see Supplementary Information Box below).

Source:

Reproduction of Fourth Assessment Report Figure 6-10b. (JBB1998) = Jones et al., 1998 calibrated by

Jones et al., 2001; (MBH1999) = Mann et al, 1999; (BOS2001) = Briffa et al., 2001; (CED2004) = Cook et al.,
2004; (B2000) = Briffa, 2000 calibrated by Briffa et al., 2004; (MJ2003) = Mann and Jones, 2003; (RMO2005) =
Rutherford et al., 2005; (MSH2005) = Moberg et al., 2005; (DWJ2006) = D’Arrigo et al., 2006; and (HCA2006) =
Hegerl et al., in press.

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Independent Summary for Policy Makers of the Text of the IPCC Fourth Assessment Report

3.2b

Paleoclimatic proxy data are sparse and uncertain, and many appear to be sensitive

only to summer temperature, or to precipitation.

In the Northern Hemisphere as a whole there are relatively few long and well-dated
climate proxies, particularly for the period prior to the 17th century. Those that do exist
are concentrated in extra-tropical, terrestrial locations, and many have greatest sensitivity
to summer rather than winter (or annual) conditions or to precipitation. [6.6.1.1]

There are markedly fewer well-dated proxy records for the Southern Hemisphere
compared to the Northern Hemisphere, and consequently little evidence of how large-
scale average surface temperatures have changed over the past few thousand years.
[6.6.2]

There are very few strongly temperature-sensitive proxies from tropical latitudes. Stable
isotope data from high-elevation ice cores provide long records and have been inter-
preted in terms of past temperature variability, but recent studies indicate a dominant
sensitivity to precipitation changes, at least on seasonal to decadal timescales, in these
regions. [6.6.1.1]

Melting of tropical glaciers has been observed in recent decades. [6.6.1.1; see Supple-
mentary Information box below]

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TION

Regional paleoclimatic indicators

The Fourth Assessment Report provides a very small survey of regional paleoclimatic evidence from the
Southern Hemisphere [6.6.2]. The available literature on location-specific paleoclimatology is very large, and
in many locations in both the Northern and Southern Hemispheres indicates periods of anomalous warmth
exceeding that in the late 20th century. Little of this information is surveyed in the IPCC Report .

There is evidence that several tropical glaciers (Quelccaya, Puruogangri, Dasuopu) formed after the

Holocene Optimum. Radiocarbon dating on fossils disgorged from receding glaciers often yields evidence
that tree lines and vegetation were higher in the past and were engulfed by past glacier advances during
the past few thousand years and/or the Little Ice Age. This evidence shows that modern recession is not
unprecedented even within the Holocene.

The literature is explored in the forthcoming Fraser Institute Supplementary Analysis Series report,
“Paleoclimatic Indicators of Medieval Climate Conditions.”

3.2c

Uncertainties in paleoclimate reconstructions affect climate modeling work since

models are tested against results from paleoclimate reconstructions.

Testing models with paleoclimatic data is important, as not all aspects of climate models
can be tested against modern instrumental climate data. Good performance for present
climate is not a conclusive test for a realistic sensitivity to carbon dioxide. To test this,
simulation of a climate with very different CO

levels can be used. [6.2.2]

Also, many empirical parameterizations describing sub-grid scale processes (e.g., cloud
parameters, turbulent mixing) have been developed using present-day observations;
hence climate states not used in model development provide an independent bench-
mark for testing models. [6.2.2]

Paleoclimate data are therefore key to evaluating the ability of climate models to simu-
late realistic climate change. [6.2.2]

Climate models and their evaluation

4.1 Fundamental
limitations of climate
models

4.1a

Early climate models provided some qualitative conjectures at the global scale that

are consistent with some observed changes.

At the global scale, some broad predictions made 30 years ago about the possible
response to increased CO

concentration in the atmosphere, namely increased average

tropospheric temperature, decreased average stratospheric temperature and a more
rapid hydrological cycle, are consistent with data that have emerged since then. [8.1.2]

Even when specific predictions are shown to be correct, models should be viewed
critically. [8.1.1]

4.1b

The fundamental limitations of climate modeling have not changed since the Third

Assessment Report.

Climate models employ approximations to basic physical processes, some of which are
controlled approximations (e.g., those based on large scaled Newtonian mechanics) and
some of which are empirically based (e.g., fundamental convection processes). [8.1.3]

“Parameterization” is the process of constructing empirically-based procedures that
account for the significant large-scale effects of processes that cannot be resolved (i.e.,
represented within the computational scheme) because of basic limits in computational
power. These limits are induced by the scope of the climate modeling problem. Empirical
parameterizations are not unique. Because empirical parameterizations can be invented
to force a model to match observations, the ability of a model to represent observed
conditions cannot be cited as grounds for confidence in the model’s physical realism.
[8.1.3]

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TION

Basic Modeling uncertainties

The following observation, made in the Third Assessment Report, remains just as true today. The Fourth
Assessment Report had nothing to add to it:

In climate research and modeling, we should recognize that we are dealing with a coupled non-linear

chaotic system, and therefore that the long-term prediction of future climate states is not possible. The most
we can expect to achieve is the prediction of the probability distribution of the system’s future possible states
by the generation of ensembles of model solutions. (Third Assessment Report, Section 14.2.2.2)

An extended discussion of this is provided in the forthcoming Fraser Institute Supplementary Analysis Series
report “Fundamental Uncertainties in Climate Modeling.”

4.1c

A model’s ability to accurately simulate the current mean climate state does not imply

it is reliable for projecting future climate changes.

Multimodel evaluations have shown that even though a group of climate models of
intermediate complexity can all replicate observed mean ocean temperature and salinity,
and mean atmospheric temperature and humidity, they are not strongly constrained in
their future predictions [8.1.2].

Figure 8.4.2 of the Fourth Assessment Report shows that different models can produce
results spreading over more than a factor of 10 for long (climate) time scales exceeding
100 months.

Independent Summary for Policy Makers of the Text of the IPCC Fourth Assessment Report

Models tuned to “perfectly” reproduce an observed mean climate state have nonetheless
shown only a weak ability to predict subsequent climatic conditions. It is not possible to
say which, if any, of today’s climate models are reliable for climate prediction and fore-
casting. [8.3]

4.1d

It is not formally known if today’s climate models are a suitable basis for projecting

climate.

A model that has been “tuned” to give a good representation of certain key observations
may have a greater likelihood of giving a better prediction than a similar model which is
less closely tuned. If the number of tunable parameters of a General Circulation Model
(GCM) exceeds the number of degrees of freedom in the observational testing scheme
for the GCMs, then the use of GCMs to forecast climate change is not justifiable. There
has been no formal evaluation of the extent to which current GCMs satisfy this require-
ment. [8.1.3.1]

4.1e

Some climate models now obey the law of conservation of mass, but it is not known if

this is an improvement.

Numerical advection schemes have been introduced in some cases that do not violate
conservation of mass—a fundamental law of nature. However there is no consensus on
whether they are better than the alternatives. [8.2.1.1]

In some cases new schemes do not permit negative concentrations of water vapor.
[8.2.1.1]

4.2 Significant

known

model problems

4.2a

The strength of the coupling between land processes and the atmosphere is not

known.

Models strongly disagree on this important feedback. There is insufficient data at the
global level to evaluate this feature of GCMs. [8.2.3.2]

4.2b

Cryosphere

Simulation of high latitude processes in models is still enough of a problem that their
projections of sea ice extent remain highly uncertain. Northern Hemisphere winter is the
best-simulated case, and even here the range of simulated sea ice extent exceeds 50% of
the mean, and ice thickness also varies considerably. This is particularly troubling because
the model sea ice biases may influence estimated global climate sensitivity. [8.3.3]

On the continental scale, the peak monthly amount of water in snow integrated over
the North American continent in models varies within ±50% of the observed value of
~1500 km

. The magnitude of these model errors is large enough to affect continental

water balances. [8.3.4.1]

Glaciers are not modeled. [8.2.4.1]

Global and regional climate projections

5.1 Reproduction of
the present climate

5.1a

Quantitatively, individual climate models are typically unable to reproduce the

observed mean surface temperature to better than +/- 3 kelvin, with worse performance
near the poles. They are also unable to reproduce the onset of ice ages. The margin of
present-day error is similar to the size of the projected global warming trend over a century.

Errors in polar regions average between 3 and 5 kelvin (K), and on average all climate
models overestimate mean Antarctic temperatures by at least 5 K. [Figure 8.3.1]

The extent to which these errors detract from the models’ ability to accurately simulate
climate change in response to external perturbation (e.g., GHG emissions) is unknown,
but may be significant. [8.3.1]

Climate models are not able to successfully simulate the onset of an ice age [6.4.1.7],
although they are able to reproduce some features of the end of an ice age. [6.4.2.3]

Models are used to evaluate greenhouse-induced changes that are about 0.3 K per
decade, a tenth the size of the annual margin of error for estimates in most regions.

5.2 Forecasts for the
coming century are
inherently uncertain

5.2a

The spread of model outcomes shown in the Fourth Assessment Report forecast

ensembles does not span the full range of uncertainty.

For future climate change in the 21st century, a subset of three scenario simulations
have been selected from the six commonly used ones. This subset constitutes a “low”,
“medium”, and “high” scenario among the marker scenarios, and this choice is solely
made by the constraints of available computer resources that did not allow for the
calculation of all six scenarios. This choice, therefore, does not imply a qualification of or
preference over the six marker scenarios. By the same argument, it is not within the
remit of this report to assess the realism and likelihood of emission scenarios. [10.1]

Even though the ability to simulate present day mean climate and variability, as well as
observed trends, differs across models, all submitted models are weighted equally in the
mean. Since the ensemble is strictly an ‘ensemble of opportunity’, without sampling
protocol, the spread of models is unable to span the full possible range of uncertainty,
and a statistical interpretation of the model spread is therefore problematic. [10.1]

5.2b

Uncertainties enter model projections at every step in the process.

There are multiple emission scenarios for the 21st century, and even at this first stage
there is uncertainty with regard to the evolution over time of emissions of various forcing
agents, such as greenhouse gases. Then these emissions must be converted to concen-
trations of constituents in the atmosphere. Gas cycle models must be employed, and
these models include their own set of parameterisations, assumptions and caveats. Then
the concentrations in the atmospheric models produce radiative forcing that acts on the
climate system within the atmospheric model components, each with their own radiation
schemes and other formulations that affect radiative forcing. Finally, the modelled
coupled climate system takes those radiative forcings and produces a future simulated
climate. The components of the atmosphere, ocean, sea ice and land surface in each
model interact with their sets of strengths and weaknesses to produce a spread of
outcomes for future climate. [10.1]

Thus at every step in this process there are uncertainties and assumptions that must be
made to proceed from emissions, to concentrations, to radiative forcing, and eventually
to simulated climate changes and impacts. [10.1]

Independent Summary for Policy Makers of the Text of the IPCC Fourth Assessment Report

5.2c

Few of the climate models used for the Fourth Assessment Report forecasts account

for solar changes, land-use changes and indirect aerosol effects.

Only two out of 23 models account for the effects of time-varying solar changes.
[Table 10.2.1]

Only two out of 23 models account for effects of time-varying land-use changes.
[Table 10.2.1]

Only nine out of 23 models include the first indirect effect of aerosols, only six include
the second indirect effect and only four include both. [Table 10.2.1]

SUPPLEMENT

ARY INFORMA

TION

Defining ‘Climate Change’

The IPCC assumes that climate change can be defined as a change in the mean state of the climate. This
assumes that means of climatic variables are stationary and well-defined, something recent research has
put into question. If the climate is nonstationary, a change in the mean is consistent with an ‘unchanged’
climate since the observed mean is dependent on the time period over which the observations are collected.
Also the concept of variability is problematic since the variance of a nonstationary process is, in some cases,
mathematically undefined.

For more on this topic see the forthcoming Fraser Institute Supplementary Analysis Series report “Long Term
Persistence in Geophysical Data.”

5.3 Model-generated
global warming
forecasts

5.3a

Climate models predict warming is occurring everywhere on Earth.

The average across models implies a forecast that, over all land areas on Earth, a
warming of 0.5 to 1

C will be noticeable in a comparison of the two decades beginning

at 2011 relative to the 1980 -1999 interval. [Figure 10.3.5]

The North and South polar regions are forecast to warm relatively faster, and land
areas are forecast to warm faster than adjacent ocean areas. [Figure 10.3.5]

1979-2005 trends as measured by weather satellites show temperature trends are
0 -0.5

C/decade over land and are not systematically stronger than over adjacent ocean

areas. [Figure 3.4.4]

1979-2005 trends as measured by weather satellites show Southern Hemisphere
warming trends get weaker towards the South Pole, which exhibits zero or negative
temperature trends in many surrounding areas.

5.3b

On average, models that assume strong greenhouse warming project the tropical

troposphere to warm faster than the surface. Current data do not support these forecasts.

The tropical troposphere is forecast to warm faster than the surface. [Figure 10.3.4]

This conflicts with current data. (see Section 2.1c)

Independent Summary for Policy Makers of the Text of the IPCC Fourth Assessment Report

5.3c

All climate models used for the Fourth Assessment Report are tuned so that the

average surface temperature will increase between about 2.0°C and 4.5°C in response to a
doubling of the atmospheric carbon dioxide concentration.

The “equilibrium climate sensitivity” refers to a model’s assumed increase in global
surface temperature following a doubling of the atmospheric equivalent CO

concentra-

tion. [10.5.2.1]

The suite of models used for the Fourth Assessment Report simulations apply an
equilibrium climate sensitivity between approximately 2.0

C and 4.5°C. [Figure 10.5.1]

5.3d

Models generate many specific global forecasts based on assumptions of significant

greenhouse warming.

Global Mean Temperature:

Climate models based on the assumption that atmos-

pheric carbon dioxide levels will double over the next century predict that global average
surface temperature will increase by between about 2.0

C and 4.5°C. [Figure 10.5.2]

Sea Ice:

Models show a range of responses in Northern Hemisphere sea ice areal extent

ranging from very little change to a dramatic change, and accelerating reduction over the
21st century. Seasonal ice cover is rather robust and persists to some extent throughout
the 21st century in most (if not all) models. In 20th and 21st century simulations,
Antarctic sea ice cover decreases more slowly than in the Arctic. Overall models have
poor agreement on the amount of thinning of sea ice and the overall climate change in
the polar regions. [10.3.3.1; Figure 10.3.10a,b Figure 10.3.11]

Ocean Circulation:

Models initialized at the year 1850 have difficulty producing late

20th century values of the Meridional Overturning Circulation (MOC) in the observed
range. Of the model simulations consistent with the late 20th century observational
estimates, no simulation predicts an increase of MOC during the 21st century; reduc-
tions range from indistinguishable within the simulated natural variability to 60% relative
to the 1960 –1990 mean; none of the models projects an abrupt transition to an off state
of the MOC [Figure 10.3.13]. The best estimate of sea level increase from 1993 –2003,
associated with the slight net negative mass balance from Greenland, is 0.1–0.2 mm per
year. The corresponding amount of sea water, even when added directly and exclusively
to the North Atlantic, has been suggested to be too small to affect the North Atlantic
MOC. Taken together, it is likely that the MOC will reduce, but very unlikely that the
MOC will undergo an abrupt transition during the course of the 21st century. [10.3.4,
Figure 10.3.13]

Temperature Variability:

Climate models predict a decrease in temperature variability

during the cold season in the extratropical Northern Hemisphere and a slight increase
of temperature variability in low latitudes and in the warm season in northern mid
latitudes. [10.3.5.1]

Monsoons:

Climate models runs predict that pronounced warming over the tropics in

the middle-to-upper troposphere would result in a weakening of monsoon circulations.
Also, atmospheric moisture buildup due to increased GHGs and consequent tempera-
ture increase is predicted to result in a larger moisture flux and more precipitation for
the Indian monsoon. [10.3.5.2]

Precipitation:

Climate models predict an increased chance of summer drying in most

parts of the northern subtropics and midlatitudes and an associated increased risk of
drought. Associated with the risk of drying is also an increased chance of intense precipi-
tation and flooding. Though somewhat counter-intuitive, this is because precipitation is

Independent Summary for Policy Makers of the Text of the IPCC Fourth Assessment Report

concentrated into more intense events, with longer periods of little precipitation in
between. Increases in the frequency of dry days does not necessarily mean a decrease in
the frequency of extreme high rainfall events depending on the threshold used to define
such events. The change in the frequency of extreme precipitation at an individual
location can be difficult to estimate definitively due to model parameterization uncer-
tainty. Climate models continue to confirm the earlier predictions that in a future climate
warmed by increasing GHGs, precipitation intensity would increase over most regions.
[10.3.6.1]

Temperature Extremes:

The Third Assessment Report concluded that models project

that there is very likely a risk of increased temperature extremes, with more extreme
heat episodes in a future climate. This result has been confirmed in subsequent climate
model simulations. Several recent studies have found that climate models predict that in
a future climate there is an increased risk of more intense, longer-lasting and more
frequent heat waves [10.3.6.2] though the change does not become strong until after
2020. [Figure 10.3.17]

Cyclones:

There have been a number of climate change experiments with global models

that can begin to simulate some characteristics of individual tropical cyclones, though
studies with classes of models with 100 km resolution or higher cannot simulate observed
tropical cyclone intensities. Global climate models with 100 km resolution or higher
predict a decrease in tropical cyclone frequency globally, and no change or slight de-
creases in intensity of cyclones, but some regions may differ. Studies performed with
models that use a high resolution (down to 9 km) mesoscale hurricane model predict
that future tropical cyclones will be more intense. [10.3.6.3]

Growing Season:

Globally, models project an increase in the average growing season

length by three to five standard deviations by mid-century. [Figure 10.3.17]

Ocean Surface Acidity:

Increasing atmospheric CO

concentrations lowers oceanic pH

and carbonate ion concentrations, thereby increasing acidity. Surface ocean pH today is
already 0.1 unit lower than preindustrial values. By the end of the century, models predict
it may decline by another 0.13 to 0.34 pH units. Experimental evidence suggests that if
these trends continue, key marine organisms – such as corals and some plankton – will
have difficulty maintaining their external calcium carbonate skeletons. [10.4.2, Figure
10.4.5]

Sea Levels:

Models project that a doubling of CO

levels in the atmosphere (A1B

scenario), if accompanied by a warming of 2—4.5°C, will cause a sea level increase of
about 20 centimeters, plus or minus 10 cm over the next 100 years [10.6.5; Fig 10.6.1].
However the spatial pattern in projections is not uniform. The geographical patterns of
sea level change from different models are not generally similar in detail, but the differ-
ences are not as large as they were in the Third Assessment Report. Still, the largest
spatial correlation coefficient between any pair is 0.76, and only 20% of correlation
coefficients exceed 0.5. [10.6.2]

Glaciers:

Since their mass balance depends strongly on their altitude and aspect, use of

data from climate models to make projections requires a method of downscaling, because
individual glaciers are too small to be handled in typical GCMs. Statistical relations can
be developed between GCM and local meteorology but they may not continue to hold
in future climates [10.6.3]. Models predict overall loss of glacier volume, but there is
uncertainty about how to estimate the dynamics. [10.6.3.3]

Independent Summary for Policy Makers of the Text of the IPCC Fourth Assessment Report

5.3e

Models have also been used to generate regional forecasts, though the uncertainties

are substantial.

Important details about climate change pertain to geographical details too small to be
resolved in global models. Hence regional models have been developed, which involve
schemes for downscaling the information from a global model. [11.1.1]

Downscaling can be done two ways. “Dynamical downscaling” involves feeding informa-
tion from a global model into a regional climate model, using the data from the global
model to impose boundary conditions on the regional model. However this does not
necessarily yield a better match to observations. [11.2.1.1.1]

“Statistical downscaling” involves applying empirical estimates between local variables
and global variables to estimate changes in the local variables based on global model
forecasts. This requires the assumption that the relationships are stationary — i.e., that
the empirical relationship is steady over time and under different climatic conditions.
Stationarity remains a concern with statistical downscaling. It is not know whether the
cross scale relationships are valid under future climate regimes. This limitation is only
weakly assessed through cross-validation tests. [11.2.1.1.2]

Most sources of uncertainty on regional scales are similar to those on the global scale,
but there are both changes in emphasis and new issues that arise in the regional
context. Of the climate forcing agents, uncertainty in aerosol forcing adds especially to
regional uncertainty because of the spatial inhomogeneity of the forcing and the re-
sponse. Changes in land-use and cover have an inherently regional scope as well. When
analyzing studies involving further layers of models to add local detail, the cascade of
uncertainty through the chain of models used to generate regional or local information
has to be considered. The degree to which these uncertainties influence the projections
of different climate variables is not uniform. Also, the climate may itself be poorly known
on regional scales in many data-sparse regions. Thus, evaluation of model performance
as a component of an analysis of uncertainty can itself be problematic. [11.2.2.1]

Attributing the causes of climate change

6.1 Measuring and
analyzing climate
change

6.1a

There is reliance on computer models both to identify what might be the scales of

internal variability and the magnitude of natural forcing, as well as the form of the anthro-
pogenic-forcing signal. It is against these basic shortcomings that attribution studies must
be assessed [1.3.3].

Detection and attribution of climate change are separate processes. [1.3.3]

Detection of climate change is the process of demonstrating that climate has changed in
some statistical sense, without providing a reason for that change. Attribution of causes
of climate change is the process of establishing the most likely causes for the detected
change with some defined level of confidence. [1.3.3]

Both detection and attribution rely on observational data as well as model output. [1.3.3]

In practical terms, attribution of anthropogenic climate change is understood to mean:

detection;

demonstration that the detected change is consistent with computer model predictions
of the climate change “signal” that is calculated to occur in response to anthropogenic
forcing; and

demonstration that the detected change is not consistent with alternative physically
plausible explanations that exclude anthropogenic forcing. [1.3.3]

Estimates of century-scale natural climate fluctuations are difficult to obtain from the
observations because of the relatively short length of records. [1.3.3]

6.1b

The definition of climate change assumes stationarity of the climate system.

Climate change

“refers to a change in the state of the climate that can be identified (e.g.,

using statistical tests) by changes in the mean and/or the variability of its properties, and
that persists for an extended period, typically decades or longer”. [9.1.1]

6.1c

The climate is subject to natural variability on all time scales, from days up to centuries.

Natural climate variability results from internal climate processes and the climate’s
response to natural external forcing. Internal variability is present on all time scales from
virtually instantaneous (e.g., the triggering of convection) up to years (e.g., tropospheric-
stratospheric or inter-hemispheric exchange). Other components of the climate system,
such as the ocean and the large ice-sheets tend to operate on longer time scales of
decades to centuries. These components produce internal variability directly and by
integrating variability from the rapidly varying atmosphere. In addition, internal variability
is also produced by coupled interactions between components, such as is the case with
the El-Niño Southern Oscillation. [9.1.1.]

6.1d

Internal variability and climate change are inherently difficult to estimate, and usually

require the use of climate models.

The climate’s internal variability is difficult to estimate because all climate observations
are influenced, at least to some extent, by variations in external forcing. However esti-
mates can be obtained from observations or models under certain conditions. [9.1.1.]

The methods used to identify change in observations are based on the expected
responses to external forcing, either from physical understanding or as simulated by
climate models. An identified change is

detected

in observations if its likelihood of

occurrence by random chance or by internal variability alone is determined to be small.

Independent Summary for Policy Makers of the Text of the IPCC Fourth Assessment Report

A failure to detect a particular response might occur for a number of reasons, including
the possibility that the response is weak relative to internal variability, or that the metric
used to measure change is insensitive to the expected change. [9.1.2]

The detection of an effect of external forcing on the climate does not necessarily imply
that it has an important impact on the environment, biota, or human society. [9.1.3]

6.2 Difficulties in
attributing observed
climate change to
specific causes.

6.2a

Detection of climate change relies on model-generated predictions of the response

of the climate to external forcing, such as greenhouse gas emissions, and as such can
never be absolutely certain.

Many studies use climate models to predict the expected responses to external forcing,
and these predictions are usually represented as patterns of variation in space, time, or
both. Such patterns, which are commonly referred to as

fingerprints

, are usually derived

from changes simulated by a climate model in response to forcing. [9.1.2]

The spatial and temporal scales used to analyze climate change are carefully chosen so
as filter out internal variability and enable the separation of the responses to different
forcings. The choice of filter criteria is based on prior expectations about the time and
spatial scales to be analyzed. [9.1.2]

Because detection studies are necessarily statistical in nature, inferences about whether
an external influence has been detected can never be absolutely certain. It is always
possible that a significant result at, say, the 5% level, could simply reflect a rare event
that would have occurred in any case with less than 1 chance in 20 in an unchanged
climate. Corroborating lines of evidence providing a physically consistent view of the
likely cause for the change reduces the risk of spurious detection. [9.1.2]

6.2b

Investigation of the causes of observed individual climate events can be biased due

to “self-selection” phenomena.

For many decision-makers, the most pertinent detection questions involve a particular
observed phenomenon, (for example, whether the drying in the Sahel region can be
attributed to greenhouse gases). It is difficult to respond to such questions because of a
statistical phenomenon known as “selection bias”. Only large observed climate anoma-
lies in a historical context would be likely to be the subject of such a question. Decision-
makers are unlikely to ask about small or non-existent events. Hence the selection of
events to analyze is biased towards large, anomalous observations. The fact that the
questions are “self selected” from the observations makes it difficult to assess their
statistical significance from the same observations. [9.1.2]

6.2c

Attribution of the cause in climate change is not formally possible.

Detection does not imply attribution of the detected change to the assumed cause.

Attribution

“of causes of climate change is the process of establishing the most likely

causes for the detected change with some defined level of confidence”. As noted in the
Second Assessment Report (published in 1996) and the Third Assessment Report
(published in 2001), unequivocal attribution would require controlled experimentation
with our climate system. That, of course, is not possible. [9.1.2]

Independent Summary for Policy Makers of the Text of the IPCC Fourth Assessment Report

6.2d

The term “attribution” means consistency with a climate model-generated scenario,

rather than formal proof of causality. The same data could be consistent with contradictory
hypotheses, including large or small greenhouse warming.

From a practical perspective, attribution of anthropogenic climate change is under-
stood to mean the detected change is “consistent with the estimated responses to the
given combination of anthropogenic and natural forcing”. [9.1.2]

Any assessment of observed climate change that compares simulated and observed
responses will be affected by errors and uncertainties in the forcings prescribed to a
climate model and its corresponding responses. [9.2.3]

Assessment of the consistency between an observed change and the estimated response
to a hypothesized forcing is often achieved by determining whether the amplitude of
the hypothesized pattern of change estimated from observations is statistically consist-
ent with expectations based on climate model predictions, as measured by statistical
tests. [9.1.2]

Attribution also requires evaluating the possibility that the observed change is consist-
ent with alternative explanations that exclude important elements of a given combina-
tion of forcings that are hypothesized to have influenced the climate. Statistical analysis
requires that the separate influences on climate are properly accounted for. For instance,
the attribution of recent warming to greenhouse gas emissions becomes more reliable
if the influences of other external forcings, for example solar forcing, are explicitly
accounted for in the analysis. [9.1.2]

This is an area of research with considerable challenges because different forcing
factors may lead to similar large-scale spatial patterns of response. [9.1.2]

If it is not possible to distinguish the spatial pattern of greenhouse warming from that
of fossil-fuel related aerosol cooling, then the observed warming over the last century
could be explained by large greenhouse warming balanced by large aerosol cooling or
alternatively by small greenhouse warming with very little or no aerosol cooling. [9.2.3]

6.2e

Attribution studies rely on the validity of model-generated estimates of the climatic

response to forcing, and model-generated estimates of natural variability.

All three aspects of attribution require knowledge of the internal climate variability on
the timescales considered, usually decades or longer. The residual variability that remains
in instrumental observations after the estimated effects of external forcing have been
removed is sometimes used to estimate internal variability. However, these estimates
are uncertain because the instrumental record is short relative to the timescales of
interest, and because of uncertainties in the forcings and the estimated responses.
Thus internal climate variability is also estimated from long control simulations from
coupled climate models. [9.1.2]

Subsequently, an assessment is usually made of the consistency between the residual
variability referred to above and the model based estimates of internal variability.
Confidence depends on the ability of models to simulate the various modes of observed
variability, comparisons between variability in observations and climate model data and
by comparison between proxy reconstructions and climate simulations of the last
millennium. [9.1.2]

Independent Summary for Policy Makers of the Text of the IPCC Fourth Assessment Report

6.2f

The reported uncertainties in attribution studies do not take into account basic

uncertainty about climate model parameters. These uncertainties can be considerable.

Model and forcing uncertainties are important considerations in attribution research.
Ideally, the assessment of model uncertainty should include uncertainties in model
parameters, and in the representation of physical processes in models (structural
uncertainty). Such an assessment is not yet available, although research with that goal in
mind is underway. [9.1.2]

The effects of forcing uncertainties, which can be considerable for some forcing agents,
such as solar and aerosol forcing, also remain difficult to evaluate, despite advances in
research. [9.1.2]

There are also very large uncertainties in the temporal forcing associated with solar
radiation changes, particularly on timescales longer than the 11-year cycle. Previous
estimates have used sun spot numbers to determine these slow changes in solar irradi-
ance over the last few centuries, but are not necessarily supported by current under-
standing. In addition, the magnitude of radiative forcing associated with major volcanic
eruptions is uncertain and differs between reconstructions. [9.2.2.3]

Detection and attribution results that are based on several models or several forcing
histories do provide information on the effects of model and forcing uncertainty that
leads towards a more reliable attribution of climate change to a specific cause. Such
results suggest that the attribution of a human influence on temperature change during
the latter half of the 20th century is robust. [9.1.2]

In addition to substantial uncertainty in the timing and amplitude of solar variations on
timescales of several decades to centuries, uncertainty also arises because the spatial
response of surface temperature to solar forcing resembles that due to anthropogenic
forcing. These uncertainties in interpretation of the role of different forcings reflects
substantial uncertainties in our knowledge about the size of past volcanic forcing and of
the timing and size of long-term variations in solar forcing, as well as differences in the
way these effects are taken into account in model simulations. [9.3.3.2]

There remains considerable uncertainty in the forcings that are used in climate models.
Estimates of the uncertainties in reconstructions of past solar forcing have increased
since the Third Assessment Report, and chemical and dynamical processes associated
with the atmosphere’s response to solar irradiance are omitted or not adequately
resolved in many climate models used in detection studies. Furthermore, some models
include the indirect effects of sulphate aerosols on clouds, whereas others consider only
the direct radiative effect. [9.4.1.8]

6.3 Assumptions
needed to attribute
climate change to
anthropogenic causes.

6.3a

Evidence for a human influence on climate relies on model-based detection studies.

The evidence that was available at the time of the Third Assessment Report consisted of
results from a range of detection studies of the instrumental record, relying on output
from several climate models for fingerprints and estimates of internal climate variability.
On this basis the Third Assessment Report stated that warming over the 20th century
was “very unlikely to be due to internal variability alone as estimated by current models”.
[9.1.3]

It is implicitly assumed in these studies that the surface observational record is not
affected by nonclimatic trends such as land use change. [3.2.2.2]

There are now a greater number of attribution studies than were available for the Third
Assessment Report, and these have used more recent climate data than previous studies
and a greater variety of model simulations. Increased confidence in detection of an anthro-
pogenic signal in the instrumental record refers to this proliferation of studies. [9.4.1.4]

Independent Summary for Policy Makers of the Text of the IPCC Fourth Assessment Report

6.3b

On average, models used for attributing recent climate change to human interference

assume that natural forcings alone would have yielded virtually no change over the 20th
century, and global cooling since 1979.

Climate models that include only natural forcings estimate that over the 20th century
there would have been no change or a slight cooling (up to 0.5C) everywhere on Earth.
[Figure 9.4.2]

When the same models are run over the post-1979 interval, they propose that natural
forcings alone would have yielded no change, or cooling, everywhere except for a small
portion of the Bering Strait and a few other locations. [Figure 9.4.2]

6.3c

Attribution studies to date do not take into account all known sources of possible

influence on the climate.

Studies have concentrated on what are believed to be the most important forcings:
greenhouse gases, direct solar effects, some aerosols and volcanism. Most analyses
exclude some forcings that could potentially have significant effects, particularly on
regional scales, but possibly on global scales as well. [9.4.18]

Observational campaigns have demonstrated the importance of black carbon in the
South Asia region and modeling studies have shown that the global forcing from black
carbon could be large. Yet few detection studies have explicitly included the temperature
response to black carbon aerosols because there are few transient coupled model
simulations including this forcing due to large modeling uncertainties. [9.4.18]

Land use changes are another forcing that could be potentially important, particularly on
regional scale. [9.4.18]

Attribution analyses that use recent model simulations which include carbonaceous
aerosols and land use changes continue to detect a significant anthropogenic influence
on 20th century temperature changes. [9.4.18]

6.3d

Due to the uncertainties involved, attribution of climate change to human cause is

ultimately a judgment call.

The approaches used in detection and attribution research described above can not fully
account for all uncertainties. [9.1.2]

Ultimately expert judgment is used to estimate the likelihood that a specific factor is
responsible for a given climate change. [9.1.2]

Overall conclusions

The following concluding statement is not in the Fourth Assessment Report, but was
agreed upon by the ISPM writers based on their review of the current evidence.

The Earth’s climate is an extremely complex system and we must not understate the
difficulties involved in analyzing it. Despite the many data limitations and uncertainties,
knowledge of the climate system continues to advance based on improved and expanding
data sets and improved understanding of meteorological and oceanographic mechanisms.

The climate in most places has undergone minor changes over the past 200 years, and

the land-based surface temperature record of the past 100 years exhibits warming trends in
many places. Measurement problems, including uneven sampling, missing data and local
land-use changes, make interpretation of these trends difficult. Other, more stable data sets,
such as satellite, radiosonde and ocean temperatures yield smaller warming trends. The
actual climate change in many locations has been relatively small and within the range of
known natural variability. There is no compelling evidence that dangerous or unprecedented
changes are underway.

The available data over the past century can be interpreted within the framework of a

variety of hypotheses as to cause and mechanisms for the measured changes. The hypo-
thesis that greenhouse gas emissions have produced or are capable of producing a signifi-
cant warming of the Earth’s climate since the start of the industrial era is credible, and
merits continued attention. However, the hypothesis cannot be proven by formal theoretical
arguments, and the available data allow the hypothesis to be credibly disputed.

Arguments for the hypothesis rely on computer simulations, which can never be decisive

as supporting evidence. The computer models in use are not, by necessity, direct calculations
of all basic physics but rely upon empirical approximations for many of the smaller scale
processes of the oceans and atmosphere. They are tuned to produce a credible simulation
of current global climate statistics, but this does not guarantee reliability in future climate
regimes. And there are enough degrees of freedom in tunable models that simulations
cannot serve as supporting evidence for any one tuning scheme, such as that associated
with a strong effect from greenhouse gases.

There is no evidence provided by the IPCC in its Fourth Assessment Report that the

uncertainty can be formally resolved from first principles, statistical hypothesis testing or
modeling exercises. Consequently, there will remain an unavoidable element of uncertainty
as to the extent that humans are contributing to future climate change, and indeed whether
or not such change is a good or bad thing.

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Appendix 1– Expert Review

The ISPM was sent out to reviewers around the world. We hereby acknowledge with gratitude the
extremely helpful feedback given, at short notice, by dozens of colleagues, whose suggestions substan-
tially improved the final edition. The following individuals provided responses as of January 22, 2007.

Alberto Montanari

Hydrology

University of Bologna

Italy

Anastasios Tsonis

Mathematics

University of Wisconsin

USA

Anthony Lupo

Climatology

University of Missouri

USA

Arthur S. deVany

Mathematics

University of California-Irvine

USA

Barrie Jackson

Chemical Engineering

Queen’s University

Canada

Bjarne Andersson

Thermodynamics

Niels Bohr Institute

Denmark

Boris Winterhalter

Oceanography

Geological Survey of Finland

Finland

Christopher deFreitas

Climatology

University of Auckland

New Zealand

David Deming

Paleoclimatology

University of Oklahoma

USA

David Legates

Climatology

University of Delaware

USA

Demetris Koutsoyiannis Hydrology

University of Athens

Greece

Douglas Hoyt

Solar Physics

Raytheon Corp. (Retired)

USA

Eduardo Zorita

Paleoclimatology

GKSS Institute of Coastal Research

Germany

Einar Sletten

Chemistry

University of Bergen

Norway

Garth Paltridge

Atmospheric science

University of Tasmania

Australia

Gösta Walin

Oceanography

Goteborg University

Sweden

Harry Lins

Hydrology

United States Geological Survey

USA

John Maunder

Climatology

WMO Commission for Climatology

New Zealand

(ret’d)

Keith Hage

Meteorology

University of Alberta

Canada

Larry Hulden

Biology

Finnish Museum of Natural History

Finland

Lena Hulden

Historical Biology

University of Helsinki

Finland

Marcel Leroux

Climatology

University of Lyon

France

Nicola Scaffeta

Solar Physics

Duke University

USA

Oddbjörn Engvold

Physics

University of Oslo

Norway

Olav Kvalheim

Physical Chemistry

University of Bergen

Norway

Ole Humlum

Physical Geography

University of Oslo

Norway

Olev Trass

Chemical Engineering

University of Toronto

Canada

Oliver Frauenfeld

Meteorology

University of Colorado

USA

Patrick Michaels

Climatology

Virginia Tech

USA

Peter Robinson

Meteorology

University of North Carolina-Chapel

USA

Hill

Peter Stilbs

Physical Chemistry

Royal Institute of Technology, Sweden

Sweden

Piia Post

Meteorology

University of Tartu

Estonia

Richard Lindzen

Climatology

Massachusetts Institute of Technology

USA

Ramesh Kriplani

Meteorology

Indian Institute of Tropical

India

Meteorology

Richard McNider

Meteorology

University of Alabama

USA

Robert Balling

Climatology

Arizona State University

USA

Robert Carter

Paleoclimatology

James Cook University

Australia

Robert S. Knox

Physics

University of Rochester

USA

Terence Mills

Statistics

Loughborough University

Thomas N. Chase

Meteorology

University of Colorado

USA

Tim Patterson

Paleoclimatology

Carleton University

Canada

William Alexander

Biosystems

University of Pretoria

South Africa

Engineering

William Gray

Meteorology

Colorado State University

USA

Independent Summary for Policy Makers of the Text of the IPCC Fourth Assessment Report

In addition, 11 reviewers asked to remain anonymous.

Reviewers were asked to respond to the following questions on the indicated scale from 1—5.
The scores given are based on 54 reviews received.

1. To what extent does the ISPM cover the range of topics you consider important for policy makers

and other general readers who want to understand climate change?

1 (Quite Inadequately)
2 (Somewhat Inadequately)
3 (Neutral)

4 (Adequately)

Mean response = 4.2

5 (Quite Adequately)

2. To what extent do you consider the ISPM to convey the current uncertainties associated with the

science of climate change?

1 (Generally overstates the uncertainties)
2 (In some cases overstates the uncertainties)

3 (Is about right)

Mean response = 3.3

4 (In some cases understates the uncertainties)
5 (Generally understates the uncertainties)

3. To what extent to you agree with the Overall Conclusions?

1 (Strongly disagree)
2 (Disagree)
3 (Neutral)

4 (Agree)

Mean response = 4.4

5 (Strongly Agree)

4. Do you support the publication of the ISPM as a means of communicating the current state of

climate science to policy makers and other general readers?

1 (No, strongly opposed)
2 (No, somewhat opposed)
3 (Neutral)
4 (Yes, somewhat in support)

5 (Yes, strongly in support)

Mean response = 4.7

About the Authors

Coordinator

Dr. Ross McKitrick

holds a BA in economics from Queen’s University, and an MA and Ph.D. in

economics from the University of British Columbia. He was appointed Assistant Professor in the
Department of Economics at the University of Guelph in 1996 and Associate Professor in 2001. In
the fall of 2002 he was appointed as a Senior Fellow of the Fraser Institute in Vancouver B.C. His
research focuses on the relationship between economic growth and pollution; regulatory mechanism
design; and various aspects of the climate change policy debate. He has published numerous scholarly
articles in both economics and science journals. His coauthored book

Taken By Storm: The Troubled

Science, Policy and Politics of Global Warming

was awarded a prestigious Donner Prize for Best

Book on Canadian Public Policy. Professor McKitrick’s research has been discussed in such places as

Nature, Science, The Economist, Natuurwetenschap&Techniek, The National Post, The Globe and
Mail

and in a front page article in the

The Wall Street Journal

(Feb 14 2005). He has made invited

academic presentations in Canada, the US and Europe, as well as professional briefings to the
Canadian Parliamentary Finance and Environment Committees, to government staff at the US
Congress and Senate and to the US National Research Council.

Writing Team

Joseph S. D’Aleo

has over three decades of experience as a meteorologist and climatologist. He holds

BS and MS degrees in Meteorology from The University of Wisconsin and was in the doctoral program
at New York University. Mr. D’Aleo was a Professor of Meteorology at the college level for over 8 years
(6 years at Lyndon State College in Vermont) and was a co-founder and the first Director of Meteorol-
ogy at the cable TV Weather Channel. From 1989 to 2004, D’Aleo was Chief Meteorologist at WSI
and Senior Editor for WSI’s popular Intellicast.com web site. Mr. D’Aleo is a Certified Consultant
Meteorologist and was elected a Fellow and Councilor of the American Meteorological Society. He has
served as member and chairman of the American Meteorological Society’s Committee on Weather
Analysis and Forecasting. He has authored and/or presented numerous papers focused on advanced
applications enabled by new technologies, and the role of natural solar and ocean cycles on weather
and climate. His published works include a resource guide for Greenwood Publishing on El Niño and
La Nina. He is currently Executive Director for ICECAP, an organization and international web site
that will bring together the world’s best climate scientists to shed light on the true complex nature of
climate change.

Dr. Madhav L. Khandekar

holds a B.Sc. in Mathematics and Physics, an M.Sc. in Statistics from Pune

University, India, and both M.S. and Ph.D. degrees in Meteorology from Florida State University.
Khandekar has worked in the fields of climatology, meteorology and oceanography for almost 49
years and has published well over 100 papers, reports, book reviews and scientific commentaries as
well as a book on Ocean Surface Wave Analysis and Modeling, published by Springer-Verlag in 1989.
Khandekar spent 25 years as a Research Scientist with Environment Canada (now retired) and has
previously taught meteorology and related subjects at the University of Alberta in Edmonton (1971-74)
and for two United Nations training programs: Barbados, West Indies (1975-77, World Meteorological
Organization lecturer in meteorology) and Qatar, Arabian Gulf (1980-82, ICAO expert in aeronautical
meteorology). He has published research on surface waves, arctic sea ice, ENSO/monsoon and global
weather, numerical weather prediction, boundary-layer meteorology, and tropical cyclones. He presently
serves on the editorial board of the international Journal,

Natural Hazards

(Kluwer, Netherlands) and

was an editor of

Climate Research

(Germany) from 2003-2005. Khandekar acted as a guest editor for

a special issue of the journal

Natural Hazards

on global warming and extreme weather, published in

June 2003. Khandekar has been a member of the American Meteorological Society since 1966, the
Canadian Meteorological and Oceanographic Society since 1970, and the American Geophysical
Union since 1986.

Independent Summary for Policy Makers of the Text of the IPCC Fourth Assessment Report

Mr. William Kininmonth

has a B.Sc. from the University of Western Australia, an M.Sc from Colorado

State University, and an M.Admin from Monash University. He is a consulting climatologist, and worked
with the Australian Bureau of Meteorology for 38 years in weather forecasting, research and applied
studies. For 12 years until 1998 he was head of its National Climate Centre. William Kininmonth was
Project Manager of an Australian Government project of assistance to the Meteorology and Environ-
mental Protection Administration of Saudi Arabia, based in Jeddah (1982-85). Mr. Kininmonth was
Australian delegate to the World Meteorological Organization’s Commission for Climatology (1982-
1998) and served two periods on its Advisory Working Group (1985-89 and 1993-97). He participated
in Expert Working Groups of the Commission and carried out regional training activities in relation to
climate data management and climate monitoring. Between 1998 and 2002 he consulted to the
Commission, including coordinating an international review of the 1997-98 El Niño event and
preparation of a WMO publication,

Climate into the 21st Century

(Cambridge). He was a member of

Australia’s delegations to the preparatory meetings for the Ministerial Declaration of the Second World
Climate Conference (1990) and to the United Nations Intergovernmental Negotiating Committee for a
Framework Convention on Climate Change (1991-92). William Kininmonth is author of the book,

Climate Change: A Natural Hazard

(Multi-Science Publishing Co, UK – 2004)

Dr. Christopher Essex

Christopher Essex is a full Professor of Applied Mathematics at the University

of Western Ontario, where he is Director of the Program in Theoretical Physics. He holds a B.Sc (hon.),
from the University of Western Ontario, an M.S. from Rice University and a Ph.D. from York University.
He was an NSERC Postdoctoral Fellow at the Canadian Climate Centre and held a Humboldt
Research Fellowship at the University of Frankfurt in Germany, and was recently a visiting scientist at
the Niels Bohr Institute in Denmark. Dr. Essex specializes in the underlying mathematics, physics and
computation of complex dynamical processes such as climate. His research, including discoveries in
statistical physics, has been published in leading scientific journals, and he is a frequently invited
speaker at professional international science symposia. He is a recipient of the $10,000 Donner Prize
(2002) for his book on global warming and is a life member of the Canadian Industrial and Applied
Mathematics Society. In 2006 he was appointed by the Governor General of Canada to the Natural
Sciences and Engineering Research Council.

Dr. Wibjörn Karlén

received a Masters degree at the University of Maine in 1972. The focus of his

thesis was the pattern and possible cause of Holocene climatic variations. A few years later he defended
a Ph.D. at the Department of Physical Geography at Stockholm University. He has during the following
years maintained his interest in the climate of the Holocene. He has collected field data in a number
of areas around the world, including Scandinavia, Svalbard, Alaska, Kenya and Antarctica. Between
1984 and 2004 he was appointed professor at Stockholm University, and between 1985 and 1995
was in charge of the Tarfala Research Station, where research focuses on glaciology. After retirement
he is now the editor of an international scientific journal,

Geografiska Annaler

. Since 1992 he has

been a member of the Royal Swedish Academy of Sciences.

Dr. Olavi Kärner

studied mathematics at the University of Tartu, Estonia before receiving his Ph.D. in

Atmospheric Physics from the Leningrad Hydrometeorological Institute in 1974

In 1966, Dr. Kärner

joined the Tartu Observatory in Tõravere, Estonia, and since 1977 has held the position of Senior
Research Associate, Atmospheric Sensing Group. His scientific interests include time series analysis
for climate studies, and the development of satellite cloud classification methods for radiation budget
calculations. In 1993, Dr. Kärner and co-author, Dr. Sirje Keevallik, published

Effective Cloud Cover

Variations

(A. Deepak Publishing)

He was born in 1942 in Tartu, Estonia, and is married with three

children.

Independent Summary for Policy Makers of the Text of the IPCC Fourth Assessment Report

Dr. Ian Clark

holds a B.Sc. and M.Sc. in Earth Sciences from the University of Waterloo, and a Ph.D.

Sciences de la Terre from the Université de Paris-Sud. Dr. Clark is a Professor in the Department of
Earth Sciences at the University of Ottawa. He conducts research on past climates and environmental
change in the Arctic since the last ice age. Current programs involve field work with students in the
Yukon Territory and on the Mars environment analogue site on Devon Island in Nunavut, which is
supported by the Canadian Space Agency. He teaches courses on Quaternary Geology and Climate
Change and on Groundwater Geochemistry. Dr. Clark is director of the G.G. Hatch Isotope Laboratory,
an internationally-renown facility supporting research in Earth and environmental science.

Dr. Tad Murty

completed his early education in India and later received an M.S. and Ph.D. in Meteorol-

ogy and Oceanography from the University of Chicago. Dr. Murty was a Senior Research Scientist with
the Canadian Department of Fisheries and Oceans for 27 years and a Professor of Earth Sciences at
Flinders University, Adelaide, Australia. Murty has also served as the Director of Australia’s National
Tidal Facility, and as a Senior Scientist with Baird & Associates Coastal Engineers in Ottawa, Canada.
Dr. Murty retired in 2004 and is now an Adjunct Professor in the Departments of Earth Sciences and
Civil Engineering at the University of Ottawa. Dr. Murty has authored, co-authored and edited 18
books and monographs and more than 250 papers in peer reviewed scientific journals. He has served
on various national and international committees, and received several awards for original and
outstanding research on mathematical modelling of marine hazards. At present, he is the leader of a
World Meteorological Organization group preparing a manual on storm surges from Hurricanes and
extra-tropical cyclones. Dr. Murty is also the Editor of

Natural Hazards

published by Springer Associate,

and the Editor of

Marine Geodesy

published by Taylor& Francis.

Dr. James J. O’Brien

Dr. James J. O’Brien is the Robert 0. Lawton Distinguished Professor, Meteorology

& Oceanography, and the Director of the Center for Ocean-Atmospheric Prediction Studies at Florida
State University. After receiving his Ph.D. in meteorology from Texas A&M University in 1966, O’Brien
has published more than 115 scientific publications, and has significantly contributed to the advance-
ment of the science of atmospheric and ocean modeling. O’Brien is a Fellow of the American
Meteorological Society, the American Geophysical Union, the Royal Meteorological Society, and the
American Association for the Advancement of Science. He is also a Member of the Norwegian Academy
of Science and Letters, and a Foreign Fellow of the Russian Academy of Natural Science. He has been
the Editor of the

Journal of Geophysical Research:Oceans

, and the Associate Editor of

Monthly Weather

Review

, and

Continental Shelf Research.

He is currently an Associate Editor of the

International

Journal of Math and Computer Modeling

. A member of Florida State University’s Faculty for more

than 35 years, he is perhaps best known for his early, basic research into El Niño. Since 1999, O’Brien
has been the Florida State Climatologist, and in 2006 he received the prestigious Uda Prize from the
Japanese Oceanographic Society.

Glossary

Accumulated Cyclone

a measure used by the National Oceanic and Atmospheric

Energy (ACE)

Administration (NOAA) to express the activity of Atlantic hurricane
seasons

Acidity

the level of hydrogen ion concentration in a solution measured on the
pH scale such that the majority of readings range from
1 (very high acidity) to 14 (very high alkalinity)

Aerosols

tiny solid particles or liquid droplets that remain suspended in the
atmosphere for at least several hours. Aerosols include volcanic dust,
sea spray and its particulate products, wind generated dust, smoke from
natural forest fires, and particles emitted during combustion

Albedo

the extent to which an object reflects light; the ratio of scattered to
incident electromagnetic radiation power. For example, snow covered
surfaces have a high albedo, and dark bare ground has a low albedo.

Altimetry

the measurement of altitude

Altitude

the elevation of an object above a known level; commonly, the elevation
of an object above mean sea level

Anthropogenic

resulting from or produced by human beings

Areal

the adjective of area; relating to or involving an area. For example,
average rainfall over an area could be referred to as the areal average

Aspect

in geography, aspect refers to the direction a slope is facing

Atlantic Multidecadal

an ongoing series of long-duration changes in the sea surface

Oscillation (AMO)

temperature of the North Atlantic Ocean, with cool and warm phases
that may last for 20-40 years at a time; these changes are natural and
have been occurring for at least the last 1,000 years

Biosphere

the outer part of the Earth (including the land, air, and water) in which
life occurs

Biota

the flora (plant) and fauna (animal) of a region or time period

Black carbon

a term describing a group of compounds consists mainly of soot,
charcoal, and possible light-absorbing organic matter.

Carbonate concentration

the number of molecules of a carbonate (a compound containing
carbon and oxygen such as calcium carbonate, which is limestone) in a
given volume

Chlorofluorocarbons (CFC)

a family of chemical compounds composed of carbon, fluorine, chlorine
and hydrogen that were used extensively as propellants, refrigerants,
and solvents.

Conservation of mass

a law of physics that states that matter cannot be created or destroyed
only changed in form.

Climate

The IPCC defines climate in a narrow sense as the “average weather”,
or more rigorously as the statistical description in terms of the mean
and variability of relevant quantities over a period of time ranging from
months to thousands or millions of years

Carbon Dioxide (CO

)

a molecule consisting of one carbon atom bonded to two oxygen
atoms. At room temperature it is a colourless and odourless gas

Coral series

coral growth is influenced by temperature (but not temperature alone),
and can be used much like tree-ring widths to make inferences about
the climate in historical times. A coral series generally refers to a coral
sample(s) that is used to estimate past climate changes

Independent Summary for Policy Makers of the Text of the IPCC Fourth Assessment Report

Cryosphere

refers to the portions of the Earth’s surface where water is in solid
(frozen) form, and includes snow, ice, and frozen ground (including
permafrost)

Diurnal temperature range

the difference between the maximum and minimum temperature in a

(DTR)

day

Downscaling

a method for obtaining high-resolution climate or climate change
information from relatively coarse-resolution global climate models
(GCMs). Typically, GCMs have a resolution of 150-300 km by 150-300
km, but many models require information at scales of 50 km or less, so
some method is needed to estimate the smaller-scale information

Dynamical downscaling

a method for obtaining high-resolution climate data from relatively
coarse-resolution global climate models which uses a limited-area, high-
resolution model (a regional climate model, RCM) driven by boundary
conditions from a GCM to derive smaller-scale information; used
whenever models require small-scale data

El Niño

otherwise known as the El Niño-Southern Oscillation (ENSO) is a
coupled air-sea phenomenon that has its origins in the Pacific Ocean
but affects climate globally

Emissions

in the climate change context, emissions refers to the release of a
greenhouse gas or its precursors into the atmosphere

ENSO

see El Niño

Equilibrium climate

the change in surface air temperature following a unit change in

sensitivity

radiative forcing

Extratropical

The extratropics refer to an area outside of the tropics.

Extratropical

often used to describe storms or cyclones that originate outside of the
tropics

Firn

A type of snow that has survived at least one season and has become
granular and dense, almost an ice. It is often found under snow that
accumulates at the head of glaciers

Fossil fuel

refers generally to fuels such as coal, oil, and natural gas that were
formed from decayed plants and animals by exposure to heat and
pressure over hundreds of millions of years in the Earth’s crust

General Circulation Model

a time-dependent, numerical, three-dimensional computer model of the

(GCM)

climate system, representing the effects of such factors as reflective and
absorptive properties of atmospheric water vapor, greenhouse gas
concentrations, clouds, annual and daily solar heating, ocean tempera-
tures and ice boundaries

Glacial Isostatic Adjustment the process whereby the earth’s shape and gravitational field are

(GIA)

modified in response to large scale changes in surface mass load
related to glaciation and deglaciation

Greenhouse gas

any gas that absorbs infra-red radiation in the atmosphere. Greenhouse
gases include water vapor, carbon dioxide (CO

), methane (CH

nitrous oxide (N

O), halogenated fluorocarbons (HCFCs) , ozone (O

perfluorinated carbons (PFCs), and hydrofluorocarbons (HFCs)

Gulf Stream

a warm, swift, relatively narrow ocean current that flows along the east
coast of the United States

Heat island

an urban area that is significantly warmer than the surrounding
countryside; otherwise called an Urban Heat Island (UHI) or the ‘urban
heat island effect’.

Independent Summary for Policy Makers of the Text of the IPCC Fourth Assessment Report

Heinrich events

sudden intense cold and dry phases which are the most extreme of a
spectrum of abrupt, brief cold events which seem to have occurred very
frequently over the last 115,000 years

Holocene

the post-glacial period, between the present and 10,000 years before
present

Ice cores

a cylinder of ice removed from an ice sheet containing layers of
compacted ice useful for the reconstruction of past environments

Irradiance

a measure of the amount of light energy incident on a unit area per
unit wavelength interval (Watts per meter square per nanometer, for
example) from all directions

kelvin

the base unit of temperature in the International System of Units. Zero
kelvin (0 K) is defined as absolute zero – the lowest possible tempera-
ture where no heat energy remains in a substance. A temperature
change of one kelvin is equal to a temperature change of one degree
Celsius.

Lacustrine

pertaining to or living in lakes or ponds

Last Interglacial (LIG)

the most recent time (115 000 to 125 000 years ago) during which
global temperatures were as high as or higher than in the postglacial,
when continental glaciers were limited to the Arctic and Antarctic, and
sea levels were near current positions

Meltback

a periodic melting of a glacier

Meridional Overturning

sinking and spreading of cold water; for instance, the Atlantic meridional

Circulation (MOC)

overturning circulation carries warm upper waters into far-northern
latitudes and returns cold deep waters southward across the Equator

Monsoon

a thermally driven wind arising from differential heating between a land
mass and the adjacent ocean that reverses its direction seasonally

Nonstationarity

see Stationarity

Northern Annular Mode

large-scale modes of climate variability in the Northern Hemisphere
also known as the Arctic Oscillation or the North Atlantic Oscillation.

Ozone

a molecule made up of three atoms of oxygen that occurs naturally in
the stratosphere and filters out much of the sun’s ultraviolet radiation;
ozone builds up in the lower atmosphere as smog pollution

Pacific Decadal Oscillation

a pattern of climate and ocean conditions occurring in the north Pacific
Ocean that results in shifts in sea surface temperatures and plankton
abundance on a decades-long time scale

Paleohydrology

the study of hydrologic processes and events using proxy measures that
occurred before the beginning of the systematic collection of hydrologic
data

Paleolithic

the Old Stone Age; the archaeological period before c.10,000 BC,
characterized by the earliest known stone tool manufacture

Parameterization

the representation of physical effects as simplified parameters in a
dynamic model; for instance, cloud formation is calculated from
quantities like water vapor, depending on the exact parameterization
scheme employed

a measure of the acidity or alkalinity of a solution; pH scale typically
ranges from 0 to 14 such that 7 indicates neutral solutions, small
numbers indicating greater acidity and large numbers indicating greater
alkalinity

Independent Summary for Policy Makers of the Text of the IPCC Fourth Assessment Report

Power Dissipation Index

for a tropical cyclone PDI is defined as the sum of the maximum one-

(PDI)

minute sustained wind speed cubed, at six-hourly intervals, for all
periods when the cyclone is at least tropical storm strength; The PDI
takes into account the frequency, strength, and duration of tropical
cyclones

Proxy record

a substitute measure when direct measurement is not possible

Radiation

energy that comes from a source and travels through some material or
through space; light, heat and sound are types of radiation.

Radiation budget

the balance between incoming energy from the Sun and outgoing
thermal (longwave) and reflected (shortwave) energy from the earth

Radiosonde

a measuring device attached to weather balloons that directly records
various atmospheric parameters

Radiative Forcing (RF)

the net flux of radiation into or out of a system such that there must be
some change to the non-radiative energy states of the system such as a
change in its temperature

Sea level

the position of the boundary between the air and the sea; serves as the
reference point from which all land elevations and water depths are
measured. The sea level at any location changes constantly with tide,
atmospheric pressure, and wind conditions and is therefore commonly
defined as mean sea level (msl)

Sink strength

the degree to which a process capable of removing energy or a sub-
stance from the atmosphere; a sink provides storage for a substance;
for example, plants act as sinks through photosynthesis, as they
transform carbon dioxide in the air into organic matter which either
stays in the plants or is stored in the soils

Stationarity

a condition of time series data in which both the mean and variance
are finite and constant with respect to time, and the covariance across
fixed intervals is constant across time.

Statistical downscaling

a method for obtaining high-resolution climate data from relatively
coarse-resolution global climate models which derives statistical
relationships between observed small-scale (often station level) vari-
ables and larger (GCM) scale variables, using regression analysis or
neural network methods

Stratosphere

the atmosphere is categorized into layers; the stratosphere is the layer
above the troposphere and below the mesosphere; it is generally
defined as beginning at 10km above the earth’s surface and ending at
50km above the earth’s surface and is characterized by an increase in
temperature with height

Sublimation

a phase change of a substance from solid directly to gas

Subsidence

the sinking or downward settling of the Earth’s surface

Surface thermometer

an interconnected system of temperature-measuring devices

network

Thermohaline Circulation

the flow of ocean water caused by changes in density, which depends
on both temperature (thermo) and salinity (haline)

Top of the Atmosphere

the upper limit of the atmosphere defined differently depending on the

(TOA)

application; in climatology, TOA is the altitude at which air becomes so
thin that atmospheric pressure or mass becomes negligible