Executive Summary, Final Report, Abrupt Climate Change

SAP 3.4: Abrupt Climate Change

Executive Summary

Lead Authors:

Peter U. Clark,* Department of Geosciences, Oregon State University,

Corvallis, OR

Andrew J. Weaver,* School of Earth and Ocean Sciences, University of Victoria, BC,

Canada.

Contributing Authors:

Edward Brook,* Department of Geosciences, Oregon State

University, Corvallis, OR

Edward R. Cook,* Lamont-Doherty Earth Observatory, Columbia University, New York,

Thomas L. Delworth,* NOAA Geophysical Fluid Dynamics Laboratory, Princeton, NJ

Konrad Steffen,* Cooperative Institute for Research in Environmental Sciences,

University of Colorado, Boulder, CO

*SAP 3.4 Federal Advisory Committee Member

Main Results and Findings

For this Synthesis and Assessment Report, abrupt climate change is defined as:

A large-scale change in the climate system that takes place over a few
decades or less, persists (or is anticipated to persist) for at least a few
decades, and causes substantial disruptions in human and natural systems.

This report considers progress in understanding four types of abrupt change in the

paleoclimatic record that stand out as being so rapid and large in their impact that if they

were to recur, they would pose clear risks to society in terms of our ability to adapt: (1)

rapid change in glaciers, ice sheets, and hence sea level; (2) widespread and sustained

changes to the hydrologic cycle; (3) abrupt change in the northward flow of warm, salty

water in the upper layers of the Atlantic Ocean associated with the Atlantic Meridional

Overturning Circulation (AMOC); and (4) rapid release to the atmosphere of methane

trapped in permafrost and on continental margins. While these four types of change pose

clear risks to human and natural systems, this report does not focus on specific effects on

these systems as a result of abrupt change.

SAP 3.4: Abrupt Climate Change

This report reflects the significant progress in understanding abrupt climate change that

has been made since the report by the National Research Council in 2002 on this topic,

and this report provides considerably greater detail and insight on these issues than did

the 2007 Intergovernmental Panel on Climate Change (IPCC) Fourth Assessment Report

(AR4). New paleoclimatic reconstructions have been developed that provide greater

understanding of patterns and mechanisms of past abrupt climate change in the ocean and

on land, and new observations are further revealing unanticipated rapid dynamic changes

of moderns glaciers, ice sheets, and ice shelves as well as processes that are contributing

to these changes. This report reviews this progress. A summary and explanation of the

main results is presented first, followed by an overview of the types of abrupt climate

change considered in this report. The subsequent chapters then address each of these

types of abrupt climate change, including a synthesis of the current state of knowledge

and an assessment of the likelihood that one of these abrupt changes may occur in

response to human influences on the climate system. Throughout this report we have

adopted the IPCC terminology in our expert assessment of the likelihood of a particular

outcome or result. The term

virtually certain

implies a >99% probability;

extremely

likely

: >95% probability;

very likely

: > 90% probability;

likely

: >65% probability;

more

likely than not

: >50% probability;

about as likely as not

: 33%–66% probability;

unlikely

<33% probability;

very unlikely

: < 10% probability;

extremely unlikely

: <5% probability;

exceptionally unlikely

: <1%.

Based on an assessment of the published scientific literature, the primary conclusions

presented in this report are:

•

Recent rapid changes at the edges of the Greenland and West Antarctic ice sheets

show acceleration of flow and thinning, with the velocity of some glaciers

increasing more than twofold. Glacier accelerations causing this imbalance have

been related to enhanced surface meltwater production penetrating to the bed to

lubricate glacier motion, and to ice-shelf removal, ice-front retreat, and glacier

ungrounding that reduce resistance to flow. The present generation of models

does not capture these processes. It is unclear whether this imbalance is a short-

term natural adjustment or a response to recent climate change, but processes

SAP 3.4: Abrupt Climate Change

causing accelerations are enabled by warming, so these adjustments will very

likely become more frequent in a warmer climate. The regions likely to

experience future rapid changes in ice volume are those where ice is grounded

well below sea level such as the West Antarctic Ice Sheet or large glaciers in

Greenland like the Jakobshavn Isbrae that flow into the sea through a deep

channel reaching far inland. Inclusion of these processes in models will likely

lead to sea-level projections for the end of the 21

century that substantially

exceed the projections presented in the IPCC AR4 report (0.28 ± 0.10 m to 0.42 ±

0.16 m rise).

•

There is no clear evidence to date of human-induced global climate change on

North American precipitation amounts. However, since the IPCC AR4 report,

further analysis of climate model scenarios of future hydroclimatic change over

North America and the global subtropics indicate that subtropical aridity is likely

to intensify and persist due to future greenhouse warming. This projected drying

extends poleward into the United States Southwest, potentially increasing the

likelihood of severe and persistent drought there in the future. If the model results

are correct then this drying may have already begun, but currently cannot be

definitively identified amidst the considerable natural variability of hydroclimate

in Southwestern North America.

•

The AMOC is the northward flow of warm, salty water in the upper layers of the

Atlantic, and the southward flow of colder water in the deep Atlantic. It plays an

important role in the oceanic transport of heat from low to high latitudes. It is very

likely that the strength of the AMOC will decrease over the course of the 21

century in response to increasing greenhouse gases, with a best estimate decrease

of 25-30%. However, it is very unlikely that the AMOC will undergo an abrupt

transition to a weakened state or collapse during the course of the 21

century,

and it is unlikely that the AMOC will collapse beyond the end of the 21

century

because of global warming, although the possibility cannot be entirely excluded.

•

A dramatic abrupt release of methane (CH

) to the atmosphere appears

very unlikely, but it is very likely that climate change will accelerate the pace of

SAP 3.4: Abrupt Climate Change

persistent emissions from both hydrate sources and wetlands. Current models

suggest that a doubling of northern high latitudes CH

emissions could be realized

fairly easily. However, since these models do not realistically represent all the

processes thought to be relevant to future northern high-latitude CH

emissions,

much larger (or smaller) increases cannot be discounted. Acceleration of release

from hydrate reservoirs is likely, but its magnitude is difficult to estimate.

Major Questions and Related Findings

1. Will There Be an Abrupt Change in Sea Level?

This question is addressed in Chapter 2 of this report, with emphasis on documenting (1)

the recent rates and trends in the net glacier and ice-sheet annual gain or loss of ice/snow

(known as mass balance) and their contribution to sea level rise (SLR) and (2) the

processes responsible for the observed acceleration in ice loss from marginal regions of

existing ice sheets. In response to this question, Chapter 2 notes:

The record of past changes in ice volume provides important insight to the

response of large ice sheets to climate change.

•

Paleorecords demonstrate that there is a strong inverse relation between

atmospheric carbon dioxide (CO

) and global ice volume. Sea level rise

associated with the melting of the ice sheets at the end of the last Ice Age

~20,000 years ago averaged 10-20 millimeters per year ( mm a

-1

) with large

“meltwater fluxes” exceeding SLR of 50 mm a

-1

and lasting several centuries,

clearly demonstrating the potential for ice sheets to cause rapid and large sea

level changes.

Sea level rise from glaciers and ice sheets has accelerated.

•

Observations demonstrate that it is extremely likely that the Greenland Ice

Sheet is losing mass and that this has very likely been accelerating since the

mid-1990s. Greenland has been thickening at high elevations because of the

increase in snowfall that is consistent with high-latitude warming, but this

gain is more than offset by an accelerating mass loss, with a large component

from rapidly thinning and accelerating outlet glaciers. The balance between

SAP 3.4: Abrupt Climate Change

gains and losses of mass decreased from

near-zero in the early 1990s to net

losses of 100 gigatonnes per year (Gt a

-1

) to more than 200 Gt a

-1

for the most

recent observations in 2006.

•

The mass balance for Antarctica is a net loss of about 80 Gt a

-1

in the mid

1990s, increasing to almost 130 Gt a

-1

in the mid 2000s. Observations show

that while some higher elevation regions are thickening, substantial ice losses

from West Antarctica and the Antarctic Peninsula are very likely caused by

changing ice dynamics.

•

The best estimate of the current (2007) mass balance of small glaciers and

ice caps is a loss that is at least three times greater (380 to 400 Gt a

-1

) than

the net loss that has been characteristic since the mid-19

century.

Recent observations of the ice sheets have shown that changes in ice dynamics

can occur far more rapidly than previously suspected.

•

Recent observations show a high correlation between periods of heavy surface

melting and increase in glacier velocity. A possible cause is rapid meltwater

drainage to the base of the glacier, where it enhances basal sliding. An

increase in meltwater production in a warmer climate will likely have major

consequences on ice-flow rate and mass loss.

•

Recent rapid changes in marginal regions of the Greenland and West

Antarctic ice sheets show mainly acceleration and thinning, with some glacier

velocities increasing more than twofold. Many of these glacier accelerations

closely followed reduction or loss of their floating extensions known as ice

shelves. Significant changes in ice-shelf thickness are most readily caused by

changes in basal melting induced by oceanic warming. The interaction of

warm waters with the periphery of the large ice sheets represents one of the

most significant possibilities for abrupt change in the climate system. The

likely sensitive regions for future rapid changes in ice volume by this process

are those where ice is grounded well below sea level, such as the West

Antarctic Ice Sheet or large outlet glaciers in Greenland like the Jakobshavn

Isbrae that flow through a deep channel that extends far inland.

SAP 3.4: Abrupt Climate Change

•

Although no ice-sheet model is currently capable of capturing the glacier

speedups in Antarctica or Greenland that have been observed over the last

decade, including these processes in models will very likely show that IPCC

AR4 projected sea level rises for the end of the 21

century are too low.

2. Will There Be an Abrupt Change in Land Hydrology?

This question is addressed in Chapter 3 of this report. In general, variations in water

supply and in particular protracted droughts are among the greatest natural hazards facing

the United States and the globe today and in the foreseeable future. In contrast to floods,

which reflect both previous conditions and current meteorological events, and which are

consequently more localized in time and space, droughts occur on subcontinental to

continental scales and can persist for decades and even centuries.

On interannual to decadal time scales, droughts can develop faster than human societies

can adapt to the change. Thus, a severe drought lasting several years can be regarded as

an abrupt change, although it may not reflect a permanent change in the state of the

climate system.

Empirical studies and climate model experiments conclusively show that droughts over

North America and around the world are significantly influenced by the state of tropical

sea-surface temperatures (SSTs), with cool La Niña-like SSTs in the eastern equatorial

Pacific being especially responsible for the development of droughts over the

southwestern United States and northern Mexico. Warm subtropical North Atlantic SSTs

played a role in forcing the 1930s Dust Bowl and 1950s droughts as well. Unusually

warm Indo-Pacific SSTs have also been strongly implicated in the development of global

patterns of drought observed in recent years.

Historic droughts over North America have been severe, but not nearly as prolonged as a

series of “megadroughts” reconstructed from tree rings from about A.D. 900 up to about

A.D. 1600. These megadroughts are significant because they occurred in a climate

system that was not being perturbed in a major way by human activity (i.e., the ongoing

anthropogenic changes in greenhouse gas concentrations, atmospheric dust loadings, and

land-cover changes). Modeling experiments indicate that these megadroughts may have

SAP 3.4: Abrupt Climate Change

occurred in response to cold tropical Pacific SSTs and warm subtropical North Atlantic

SSTs externally forced by high irradiance and weak volcanic activity. However, this

result is tentative, and the exceptional duration of the droughts has not been adequately

explained, nor whether they also involved forcing from SST changes in other ocean

basins.

Even larger and more persistent changes in hydroclimatic variability worldwide are

indicated over the last 10,000 years by a diverse set of paleoclimatic indicators. The

climate conditions associated with those changes were quite different from those of the

past millennium and today, but they show the additional range of natural variability and

truly abrupt hydroclimatic change that can be expressed by the climate system.

With respect to this question, Chapter 3 concludes: