clim_17_804.1671

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2004 American Meteorological Society

Contrails, Cirrus Trends, and Climate

ATRICK

INNIS

Atmospheric Sciences, NASA Langley Research Center, Hampton, Virginia

J. K

IRK

YERS

, R

ABINDRA

ALIKONDA

AND

UNG

HAN

Analytical Services and Materials, Inc., Hampton, Virginia

(Manuscript received 3 July 2003, in final form 28 October 2003)

ABSTRACT

Rising global air traffic and its associated contrails have the potential for affecting climate via radiative forcing.

Current estimates of contrail climate effects are based on coverage by linear contrails that do not account for
spreading and, therefore, represent the minimum impact. The maximum radiative impact is estimated by assuming
that long-term trends in cirrus coverage are due entirely to air traffic in areas where humidity is relatively
constant. Surface observations from 1971 to 1995 show that cirrus increased significantly over the northern
oceans and the United States while decreasing over other land areas except over western Europe where cirrus
coverage was relatively constant. The surface observations are consistent with satellite-derived trends over most
areas. Land cirrus trends are positively correlated with upper-tropospheric (300 hPa) humidity (UTH), derived
from the National Centers for Environmental Prediction (NCEP) analyses, except over the United States and
western Europe where air traffic is heaviest. Over oceans, the cirrus trends are negatively correlated with the
NCEP relative humidity suggesting some large uncertainties in the maritime UTH. The NCEP UTH decreased
dramatically over Europe while remaining relatively steady over the United States, thereby permitting an as-
sessment of the cirrus–contrail relationship over the United States. Seasonal cirrus changes over the United
States are generally consistent with the annual cycle of contrail coverage and frequency lending additional
evidence to the role of contrails in the observed trend. It is concluded that the U.S. cirrus trends are most likely
due to air traffic. The cirrus increase is a factor of 1.8 greater than that expected from current estimates of linear
contrail coverage suggesting that a spreading factor of the same magnitude can be used to estimate the maximum
effect of the contrails. From the U.S. results and using mean contrail optical depths of 0.15 and 0.25, the
maximum contrail–cirrus global radiative forcing is estimated to be 0.006–0.025 W m

depending on the

radiative forcing model. Using results from a general circulation model simulation of contrails, the cirrus trends
over the United States are estimated to cause a tropospheric warming of 0.2

–0.3

C decade

, a range that

includes the observed tropospheric temperature trend of 0.27

C decade

between 1975 and 1994. The magnitude

of the estimated surface temperature change and the seasonal variations of the estimated temperature trends are
also in good agreement with the corresponding observations.

1. Introduction

Condensation trails, or contrails, generated from

high-altitude aircraft exhaust may affect climate because
they can persist for many hours. Like their natural coun-
terparts, these anthropogenic cirrus clouds reflect solar
radiation and absorb and emit thermal infrared radiation
causing a radiative forcing that depends on many fac-
tors, especially contrail optical depth and coverage (Sas-
sen 1997; Meerko¨tter et al. 1999). Cloud radiative forc-
ings are balanced by changes in variables such as surface
and atmospheric temperatures, cloud cover, and precip-
itation (Hansen et al. 1997). Instantaneously, contrail
radiative forcing can warm the atmosphere and warm

Corresponding author address: Dr. Patrick Minnis, NASA Langley

Research Center, MS 420, Hampton, VA 23681.
E-mail: p.minnis@nasa.gov

or cool the earth’s surface, apparently reducing the di-
urnal range of surface temperature (Travis et al. 2002).
Although highly variable and uncertain, this forcing is
generally positive when averaged over time (Minnis et
al. 1999; Ponater et al. 2002; Meyer et al. 2002), re-
sulting in a net warming of the troposphere and surface
(Rind et al. 2000). The expected 2%–5% per annum
growth (Penner et al. 1999) of worldwide jet air traffic
through 2050 necessitates a more accurate assessment
of contrail climate effects.

Current estimates of contrail radiative forcing only

consider linear contrails young enough to be differen-
tiated from natural cirrus clouds in satellite images.
Those diagnoses of radiative forcing represent the min-
imum impact because they do not include the additional
cloud cover resulting from the spread of aging linear
contrails into natural-looking cirrus clouds (Minnis et
al. 1998; Duda et al. 2001) and from any other cirrus

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. 1. Hypothetical annual relative humidity probability distri-

bution at flight altitudes with T

5 2

C. Neither persistent contrails

nor cirrus clouds would occur for Rhi

100% (area A). Cirrus clouds

and persistent contrails can form when Rhi

100%, but the prob-

ability of forming cirrus through natural nucleation processes in-
creases as humidity rises (area C) leaving the potential for devel-
opment of additional cirrus coverage from contrails (area B) via nu-
cleation from mixing of moist jet engine exhaust with ice-saturated
ambient air. The portion of area B that is converted from clear air to
cloudy air should rise as the cumulative flight path length increases
over a given area with growing air traffic. Areas B and C correspond
to Rh between 60% and 100%.

clouds initiated by aerosols generated from aircraft ex-
haust (Jensen and Toon 1994). To reduce the uncertainty
in the climatic effects of aircraft-induced cirrus clouds,
it is necessary to determine the maximum change in
cirrus coverage due to the combination of linear con-
trails and the cirrus clouds derived from them and from
exhaust aerosols (Penner et al. 1999). The ratio of that
combination to the linear contrail coverage is denoted
as the spreading factor f

Increases in cirrus coverage or decreased sunshine

have been linked to jet air traffic over parts of the con-
tiguous United States (see Changnon 1981; Sassen
1997) and Alaska (Nakanishi et al. 2001) for many
years. Trends in regional cirrus frequency of occurrence
are strongly correlated with high-altitude airplane fuel
consumption between 1982 and 1991 (Boucher 1999).
Previous studies confirm the expected outcome of in-
creased air traffic, but are limited to small regions or to
time periods too short for minimizing the impact of the
11-yr cycle in cloud cover (Udelhofen and Cess 2001).
In this study, a 25-yr surface observation dataset is used
to examine the longer-term global trends in cirrus
amounts, relate them to the expected contrail coverage
to estimate f

, and then compute the maximum regional

contrail-induced radiative forcing and regional temper-
ature changes.

2. Background

Contrails occupy a fairly special niche in atmospheric

thermodynamics because they form and produce ice
clouds at ambient relative humidities that are less than
those required for most natural cirrus cloud formation
(Gierens et al. 1999). The presence of cirrus or persistent
contrails depends on many factors such as temperature
T, humidity, vertical velocity, and cloud condensation
and freezing nuclei. The primary variable, however, is
relative humidity. The complex dynamical processes ini-
tiating cirrus formation ultimately must produce the nec-
essary water vapor. The most familiar moisture variable,
relative humidity with respect to liquid water (Rh), is
measured with radiosonde-borne hygrometers and used
in weather analyses. For cirrus processes, the relative
humidity with respect to ice (Rhi) is the relevant quan-
tity. It exceeds Rh for a given specific humidity at T

C. For T

, 2

C, Rh

100% typically corresponds

to Rhi

150%. Although cirrus clouds form naturally

through heterogeneous or homogeneous nucleation at
low temperatures when the ambient Rhi is around
140%–160% (Sassen and Dodd 1989), or less when the
air contains many nucleation aerosols (Stro¨m et al.
2003), they will persist as long as Rhi exceeds 100%.
Contrails can form and develop into cirrus clouds when
T

, 2

C and Rhi

100% because, in many instanc-

es, aircraft exhaust temporarily raises the local Rh above
100% as it mixes with the ambient air causing nucleation
of liquid droplets that freeze instantly. Therefore, con-
trails can add to the natural cirrus coverage when

, 2

C and Rhi

100% and no natural cirrus cloud

is already present (Fig. 1). If Rhi

100%, contrails can

form but will not persist.

At flight altitudes, conditions that can support con-

trail-generated cirrus clouds exist 10%–20% of the time
in clear air and within existing cirrus (Gierens et al.
1999; Jensen et al. 2001). Additionally, cirrus may form
at lower supersaturations on aerosols derived from jet
aircraft than on aerosols from natural sources (Jensen
and Toon 1994). Therefore, high-altitude air traffic has
the potential for increasing cirrus coverage and thick-
ening existing cirrus clouds by generating additional ice
crystals. If other relevant variables are steady over time,
then cirrus coverage should increase where air traffic is
significant.

3. Data

The surface-based cloud data consist of quality-con-

trolled surface synoptic weather reports from land sta-
tions and ships filtered by Hahn and Warren (1999).
Reports from automated weather stations and those with
obvious errors and inconsistencies were eliminated from
the dataset. The filtered observations include amounts
for low, middle, and high clouds as well as associated
weather conditions, solar zenith angle, and relative lunar
illuminance. Values of frequency of occurrence,
amount-when-present, and total cloud amount were cal-
culated using the methods outlined in Hahn and Warren
(1999). The cirrus amount is calculated by multiplying

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ABLE

1. Boundaries of the air traffic regions (ATR). Land–other

regions (LOR) and Ocean–other regions (OOR) consist of any land
or ocean area that is not contained in any of the other land or ocean
ATRs, respectively.

ATR

Latitude

Longitude

Western Asia (WASIA)
Europe (EUR)
Western Europe (WEUR)
United States of America (USA)
Land–other regions (LOR)
North Atlantic (NA)
North Pacific (NP)
Ocean–other regions (OOR)

–70

–60

–50

S–70

–70

S–70

E–180

W–40

W–15

–130

180

W–20

120

E–110

180

amount-when-present by frequency of occurrence, a
computation that implicitly assumes that the clouds are
randomly overlapped. Annual and seasonal mean cirrus
and total cloud amounts were calculated over land and
ocean regions between 1971 and 1995. The original
surface-based cloud dataset includes 1996. However, the
number of surface stations in the United States gradually
declined from 225 in 1971 to 110 in 1995. It dropped
to 20 in 1996, drastically affecting the sampling pat-
terns. Thus, only data prior to 1995 were used here to
ensure that the data were a fair representation of the
country as a whole.

Data from multiple stations within a single grid-box

for a given month were averaged first for each site and
then with the means from the other stations to yield a
monthly grid box average. Annual means for land were
calculated for 3

grid boxes having a minimum of 60

valid observations and 10 valid upper-level cloud ob-
servations for a minimum of 15 yr. Annual means for
ocean were calculated for 5

grid boxes having a min-

imum of 30 valid observations and 5 valid upper-level
cloud observations per month with a minimum of 7
months of data per year. Only data taken between 70

and 70

S were used to calculate averages for the air

traffic regions that are defined in Table 1.

Monthly mean cirrus and cirrostratus fractions from

the International Satellite Cloud Climatology Project
(ISCCP; Rossow and Schiffer 1999) D2 dataset were
summed to provide total cirrus coverage (CCI) on a 250

250 km

grid for 1984–96 as a consistency check on

the surface observations. Only daytime data taken be-
tween July 1983 and June 1991 and between July 1993
and June 1996 were used here to minimize the impact
of the Mount Pinatubo eruption on the satellite retriev-
als. The CCI averages were computed using only the
same grid boxes that were included in the surface air
traffic region analyses.

Annual mean relative humidities with respect to liq-

uid water at 300 hPa were computed on a 2.5

grid from

the National Centers for Environmental Prediction
(NCEP) reanalyses. RH(p) is used to indicate the annual
mean computed from the instantaneous values, Rh(p)
at pressure p. For brevity, the variable RH3 is used to
represent RH(300 hPa). RH3 values are used to evaluate

the changes in upper-tropospheric humidity (UTH) that
can affect cirrus variability (Kistler et al. 2001). The
mean annual distribution of expected linear contrail cov-
erage (ECON) on a 2.8

grid is based on a parameter-

ization of contrail formation tuned to satellite obser-
vations of linear contrails using air traffic data from
1992 and applied to 10 yr of global numerical weather
analyses of relative humidity and temperatures at se-
lected pressure levels (Sausen et al. 1998). The air traffic
data are based on the assumption of great circle routes
between two airports.

4. Results and discussion

a. Cirrus trends

Linear trends,

CC/

t, in annual mean cirrus cov-

erage (CC) with time, t, were computed for each grid
box having averages for more than 15 yr. Values of CC
increased over the United States, the North Atlantic and
Pacific, and Japan (Fig. 2a), but dropped over most of
Asia, Europe, Africa, and South America. Increases are
also evident around coastal Australia, sub-Saharan west-
ern Africa, and over the South Atlantic west of Africa.
Areas with no trends were insufficiently sampled. Many
of the trends are statistically significant at the 90% con-
fidence level (Fig. 2b). The largest concentrated in-
creases occurred over the northern Pacific and Atlantic
and roughly correspond to the major air traffic routes
reflected in the ECON distribution (Fig. 2c). Increased
CC over the United States and around Australia and
Japan coincides with relatively well-traveled routes.
Over Europe, the CC trends are mixed, while

CC/

is negative over South American routes. The coincident
NCEP RH3 dropped dramatically over Europe and
northeastern Asia and less so over the northeastern Unit-
ed States, much of Asia, and the temperate ocean regions
(Fig. 2d). RH3 rose over much of the Tropics, northern
China, and southwestern United States and adjacent wa-
ters.

Although there appears to be close correspondence

between many air traffic routes and increased CC, some
differences exist between the local maxima in

CC/

and ECON. Assuming that contrails caused the changes
in CC, then such differences could arise from slight
discrepancies in gridding, idealization of air routes, and
contrail movement. Contrails often advect hundreds of
kilometers as they develop. Thus, their region of origin
may differ from the area with maximum change in CC.
To reduce this regional noise, the data were grouped
into larger areas of contrail influence. The boxes in Fig.
2c show the air traffic regions while the remaining areas
constitute the other-region categories (see Table 1). The
western European region (WEUR) is a subset of the
European region (EUR). If contrails induce more cirrus
coverage, then the largest effects should occur over the
United States and WEUR where ECON is greatest (Ta-
ble 2).

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. 2. Trends in cirrus coverage and 300-hPa relative humidity (1971–95) and estimated 1992 linear contrail coverage. (a) Trends in

cirrus coverage for all regions with more than 15 yr of data. (b) Subset of (a) for all regions having trends significant at the 90% confidence
level according to Student’s t test. (c) Estimated linear contrail coverage. Black and white boxes determine the boundaries for the land and
ocean air traffic regions, respectively. Only observations taken from land stations and from ships are used for the land and ocean air traffic
regions, respectively. (d) Trends in annual mean NCEP relative humidity at 300 hPa.

ABLE

2. Contrails, mean cirrus cover, and cloudiness trends (% decade

) over air traffic regions from surface (CC) and ISCCP (CCI)

data. The numbers in parentheses indicate the interannual variability in CC. The 1971–75 trends in CC are all significant at the 99% confidence
level, except over WEUR where no trend is apparent.

Region

1992 ECON

(%)

Mean CC 1971–95

(%)

CC trend

1971–95

Total cloud trend

1971–95

CC trend

1983–95

CCI trend

1983–95

WASIA
EUR
WEUR
USA
LOR
NA
NP
OOR

0.08
0.60
1.52
1.75
0.09
0.32
0.16
0.13

36.2 (1.0)
18.5 (1.3)
19.8 (1.4)
29.2 (1.1)
24.5 (0.5)
15.3 (0.7)
15.7 (0.8)
14.4 (0.6)

0.9

1.2
0.0
1.0

1.6
0.7
0.9
0.7

0.7

0.4

0.7
0.5

1.4
0.0
0.8
1.2

2.0

0.4
1.8
0.3

1.5
0.3
1.6
0.8

2.1
0.0
0.9
2.3

0.6
0.2

0.4
0.1

The yearly air traffic region CC averages (Fig. 3) rose

over the United States, remained steady over WEUR,
and decreased over EUR, western Asia (WASIA), and
the land–other regions (LOR), which include all land
areas not included in the land air traffic regions. Values
of CC also increased over the North Pacific (NP) and
North Atlantic (NA) as well as over the ocean–other
regions (OOR). Interannual variability in CC (Table 2)

is greatest over Europe and least over the LOR. Trends
in CC computed from the data in Fig. 3 are mostly of
the same sign as the total cloud cover trends (Table 2)
and can account for most of the total cloud cover change
over WASIA, the LOR, and the NP. The increase in CC
over the United States is compensated by slight de-
creases in other cloud types, while the opposite is true
for EUR. Because CC is based on the assumption of

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. 3. Annual variation of CC over (a) five land regions [WASIA, WEUR, LOR, ERA, and United States (USA)]

and (b) ocean regions (NA, NP, and OOR).

random overlap, that is, CC

Ci(1

OC), the total

cloud coverage is

OC,

(1)

where OC is other cloud coverage and Ci is the actual
observed cirrus coverage. Using the original annual
means of CC and TC for the United States, it can be
shown that OC decreased by only 0.65% between 1971
and 1995, while Ci rose from 20.1% to 21.9%. Thus,
the trend in OC is only

0.3% decade

. Over WEUR,

the 0.7% decade

drop in total cloudiness is likely due

to a decrease in low and midlevel cloud cover because
CC remained constant. Total cloudiness increased more
than CC over the OOR while

CC/

t over the NA

appears to have been compensated by decreases in other
cloud types.

The growth in ISCCP cirrus coverage over the United

States greatly exceeds that from the surface observations
(Table 2). Conversely, the CCI changes over the LOR
and WEUR are less than half of those in CC. Except
over the NA, the ocean CCI trends are small compared
to those from the surface data. Discrepancies between
the two trends are expected given the differences in
sampling and observation techniques. Over the NP,

CC/

t is positive but the ISCCP trend is negative,

perhaps reflecting the difficulty in detecting cirrus from
the visible and infrared satellite data when low clouds
are prevalent underneath the cirrus. From atlases of
cloud co-occurrence (Warren et al. 1986, 1988), it was
found that the chance of cirrus occurring over stratus,
midlevel clouds, and nimbostratus is 20%–40% more
likely over the NP than over the NA. Additionally, cirrus
occurs without any other clouds present nearly twice as
often over the NA than over the NP. Conversely, cirrus
occurs 20% more often over cumulus clouds, which
have large areas of clear between them, in the NA than
in the NP. Thus, any trends in cirrus coverage detected

over the NP with the ISCCP data are likely to be di-
minished in magnitude relative to those over the NA.
The surface observations account for the overlap effects
by applying a random overlap correction, but do not
misclassify cirrus as some other cloud type. Except for
the NP case, the sign of

CC/

t is consistent with the

ISCCP trends for the period common to both suggesting
that the trends are not artifacts of the observation meth-
ods.

The frequency of clear-sky observations was also

computed for the land air traffic regions. Over the Unit-
ed States and WASIA, the frequency of clear skies
changed by

1.3% and 1.8% decade

, respectively.

Over EUR and WEUR, clear-sky frequency decreased
by 0.6% and 0.7% decade

, respectively. Thus, over

the United States, the increase in cirrus coverage was
accompanied by fewer totally clear skies indicating that
both the amount and frequency of cirrus coverage in-
creased during the period.

The seasonal variations in CC and CCI trends over

the United States (Fig. 4a) are generally consistent with
the seasonal variations of contrail occurrence frequency
(Minnis et al. 2003), satellite contrail coverage (Pali-
konda et al. 1999, 2004), and ECON. The greater spring
and winter trends are accompanied by maxima in
contrail frequency and coverage while all five quantities
dip during summer. The greatest discrepancies in the
relative seasonal values occur during fall when the CC
trend drops slightly, a change more in line with the
surface-based contrail frequency variation than with
changes in the other quantities. Over WEUR,

CC/

averaged 1.0% decade

during summer and fall and

1.0% decade

during winter and spring (Fig. 4b), a

seasonal variation differing from the satellite-based con-
trail coverage (Meyer et al. 2002) and ECON. Over
WASIA, EUR, and the LOR,

CC/

t was negative dur-

ing all seasons except for summer over WASIA. Max-

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. 4. Seasonal trends in cirrus and contrail amounts and frequencies over (a) the United States and (b)

WEUR. The ISCCP and CC lines are cirrus trends. The ECON lines correspond to expected contrail coverage
for 1992. Satellite denotes mean linear contrail coverage derived from satellite data during the mid-1990s.
Surface data denote persistent contrail frequency observed from the surface during the 1990s. The U.S. winter
and spring trends are significant at the 99% confidence level compared to 84% for summer and fall. Only the
summer CC trend over WEUR is significant at the 99% confidence level, while the winter and fall trends are
significant at the 84% confidence level. The WEUR spring trend is not statistically significant. If cirrus coverage
is increasing because of contrail generation, then it should increase proportionately with contrail coverage. This
idea is borne out by data over the United States, but not over WEUR.

imum

CC/

t occurred during spring, summer, and fall

over the NA, NP, and OOR, respectively.

Because the number of observing sites over the Unit-

ed States decreased during the period, the sensitivity of
the CC trends to the number of years sampled was ex-
amined to ensure that the 15-yr requirement for using
a grid box in the regional analysis did not introduce any
artificial trend in the results. The U.S. trends were com-
puted using 20-, 24-, and 25-yr requirements reducing
the number of U.S. grid boxes from 104 to 99, 67, and
43, respectively. The corresponding U.S. trends were
0.95%, 1.18%, and 1.11% decade

indicating that the

sampling uncertainty is on the order of 10%–12%. The
number of observing sites was generally more stable
over other land areas.

b. Humidity impact

Most contrails form between 200 and 300 hPa where

often T

, 2

C. The lowest pressure with Rh reported

in the NCEP reanalyses is 300 hPa. When Rhi

100%,

the minimum threshold for cirrus and contrail formation,
Rh should be about 68% at

C and 60% at

(Fig. 1). Given the known biases and uncertainties in
radiosonde Rh measurements at low temperatures (Sas-
sen 1997; Miloshevich et al. 2001), a more conservative
estimate of the formation threshold would be 40% at

C, and less at lower temperatures. Thus, persistent

contrails are likely to form if Rh

40% and T

, 2

as measured from radiosondes, while cirrus clouds are
more likely to form at a higher Rh threshold (Ponater

et al. 2002). The annual frequency of Rh

40% at high

altitudes should provide a measure of how often cirrus
or contrails form or how much area they cover during
a given year. Although only the annual mean value of
Rh at 300 hPa (RH3) is available from the NCEP data,
Minnis et al. (2003) showed that it was highly correlated
with the frequency of Rh(300 hPa)

40% over the

United States. Figure 5 shows the regression of RH3
with the annual frequency of Rh(300 hPa)

40% from

radiosonde data at 78 locations in the Comprehensive
Aerological Research Data Set (CARDS; see Eskridge
et al. 1995). It yields a squared linear correlation co-
efficient R

of 0.94 and shows that RH3 is an excellent

predictor of the frequency of humidity levels conducive
to contrail and cirrus formation at 300 hPa. Comparisons
with radiosonde data (Minnis et al. 2003) show that RH3
is also highly correlated with the frequencies of
Rh(250 hPa) and Rh(200 hPa) greater than 40%, yield-
ing R

values of 0.74 and 0.52, respectively. Thus, RH3

can be used as a measure of the long-term variations in
the frequency of persistent contrail conditions within
the typical contrail altitude range.

Radiosondes at a given location may give accurate

profiles of Rh instantaneously, but are subject to con-
siderable error in the long term because of instrument
changes and variations in reporting practices (Elliot and
Gaffen 1991; Gaffen 1993). Over land, NCEP incor-
porates radiosonde data, adjusting the profiles when nec-
essary to maintain consistency with the model physics
(Kistler et al. 2001). Over oceans, only radiosonde data
from islands and coastlines are assimilated by NCEP;

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. 5. Variation of RH3 with the frequency of occurrence of

Rh(300 hPa)

40% from CARDS rawinsonde data from 77 stations,

1971–2000. Contrail formation and cirrus persistence are highly prob-
able if Rh(300 hPa)

40%. Thus, RH3 can serve as a predictor for

cirrus persistence.

ABLE

3. Trends in mean 300-hPa relative humidity from NCEP:

1971–95.

Air traffic region

Trend in RH3

(% decade

)

Mean temperature

(K)

WASIA
EUR
WEUR
USA
LOR
NA
NP
OOR

3.2

5.0

6.2

0.2

1.2

1.8

1.7

1.3

223.1
225.7
226.7
230.5
230.4
227.6
230.0
235.4

no other maritime humidity data are used. Comparisons
of NCEP RH3 values with those from other sources
yield mixed results, but CARDS and NCEP are posi-
tively correlated at statistical significance levels of 88%
over WASIA and 99% over WEUR, EUR, and LOR
(see appendix A). There is no correlation between the
two parameters over the United States (see appendix A).
NCEP and CARDS are most consistent over Europe
where the negative RH3 trend (Fig. 2d) is strongest. The
NCEP data used here are currently the best available
estimate of UTH over land for the study period (see
appendix A).

Trends in RH3,

RH3/

t, are negative everywhere

except over the United States (Table 3), where the trend
is insignificant. Mean temperatures at 300 hPa are below

C everywhere except over the OOR (Table 3). Fig-

ure 6 shows scatterplots of and linear regression fits to
RH3 and CC data over the land regions. The lack of a
RH3–CC correlation over the United States (Fig. 6c)
and WEUR (Fig. 6b) indicates that the cirrus variation
cannot be explained by the NCEP UTH trends. How-
ever, RH3 explains 39%, 46%, and 33% of the variance
in CC over WASIA (Fig. 6a), EUR (Fig. 6b), and the
LOR (Fig. 6d), respectively, where ECON is relatively
small. The slopes of the respective linear fits (Fig. 6)
are 0.3, 0.2, and 0.6%CC (%RH3)

, suggesting that

CC is less sensitive to changes in RH3 over EUR, where
ECON is relatively large, than over the LOR. Over
ocean, CC and RH3 are negatively correlated with R

ranging from 0.30 to 0.39 highlighting the large un-

certainty in UTH over the oceans. Hereafter, only hu-
midity impacts over land are considered.

The trend in total cirrus coverage CC can be estimated

as the sum of the trend in contrail cirrus and the de-
pendence of natural cirrus coverage c

nat

on RH3,

RH3

ECON

nat

f ,

(2)

RH3

where

ECON/

t is estimated from ECON (Table 2)

and the growth in air traffic (ECON is assumed to be
zero in 1970 for WASIA and the LOR). Values of CC
increased over the United States despite no change in
RH3, while it remained relatively steady over WEUR
as RH3 plummeted. Between 1970 and 1995, air traffic
increased by factors of 3.7 and 5.2 over the United States
and Europe (Penner et al. 1999), respectively, aug-
menting the respective contrail-forming potentials by
approximately 15% and 21% each year relative to 1970
air traffic. If the frequency distribution of Rh remained
steady throughout the period, then CC should have in-
creased over those regions. Additional air traffic each
year would produce more contrail cirrus for 100%

Rhi

140% (area B in Fig. 1) while natural cirrus

coverage would remain constant, resulting in a net in-
crease in CC as seen for the United States. If the fre-
quency of Rh

40% decreased each year as indicated

by RH3, then natural cirrus coverage would decrease
proportionately as seen for the LOR, EUR, and WASIA.
A simultaneous rise in air traffic would produce pro-
portionately more contrail cirrus for 100%

Rhi

140%. In that instance, CC should decrease at a smaller
rate than expected for aircraft-free conditions. The
amount of counterbalancing would depend on the ab-
solute annual increase in flights and the reduction rate
in RH3. Over WEUR, the greater number of flights is
apparently sufficient to offset the decrease in RH3, while
over WASIA and the LOR, the smaller amount of air
traffic would have less impact and is probably insuffi-
cient to offset the drop in RH3. Large interannual chang-
es in RH3 can affect the occurrence of both cirrus and
contrails (Minnis et al. 2003), but over the long term,
increases in air traffic should offset some of the reduc-
tion in CC caused by decreases in RH3.

Assuming that ECON increased linearly with air traf-

fic, the correlation between ECON and CC is negative

1678

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J O U R N A L O F C L I M A T E

over the LOR, WASIA, and EUR and near zero over
WEUR. A fit to the U.S. data for ECON increasing by
15% yr

relative to 1971 yields R

0.42. This degree

of correlation is comparable to that between RH3 and
CC over LOR and EUR. Thus, over the one air traffic
region where RH3 appears to be essentially invariant,
air traffic can account for changes in CC as well as RH3
can in the air traffic regions where RH3 changes sig-
nificantly. Over WEUR, a two-parameter fit using RH3
and ECON to predict CC yields a negligible increase
(0.0006 to 0.014) in R

relative to the correlation in Fig.

6b suggesting that the interannual variability is too large
or the dramatic decrease in RH3 during the period had
an inordinate impact on the frequency distributions of
Rhi.

Any conclusion about the role of aircraft in the U.S.

CC trends must consider all of the evidence rather than
the humidity trends alone because of the uncertainties
in RH3. Prior to 1994, few measurements were taken
over the United States when T

, 2

C. However, CC

and RH3 are also uncorrelated over WEUR, where RH3
is consistent with both satellite and radiosonde obser-
vations (see appendix A) and ECON is comparable to
that over the United States. This similarity supports the
absence of an RH3 trend over the United States. The
seasonal variations in the U.S. CC trends from both
surface and satellite data have the same general char-
acteristics as the magnitudes of the trends in contrail
coverage. No statistically significant seasonal trends
were found in RH3 over the United States, further sug-
gesting that relative humidity is not responsible for the
CC trends. Given the positive CC trends in the surface
and satellite observations, the seasonal consistency be-
tween contrails and

CC/

t, the lack of an RH3 trend,

an increase in cirrus frequency, and the known impacts
of contrails on cirrus coverage, it is concluded that air
traffic is most likely responsible for the 1% decade

growth in CC over the United States. It appears that
cirrus coverage would have diminished over WEUR
without the continually increasing high-altitude air traf-
fic. In a similar vein, CC probably would have decreased
more over WASIA, the remainder of Europe, and the
LOR without contrails. Air traffic probably contributed
to the increase of CC over the studied ocean areas. How-
ever, without improved estimates of UTH over the
ocean, it will be difficult to determine the magnitude of
that impact.

Based on the air traffic increase, the estimated linear

contrail coverage rose by 0.55% decade

over the Unit-

ed States compared to the 1% decade

growth rate in

CC. Assuming that the remaining CC increase is due to
spreading contrails not detected by satellite analysis, f

1.8. If the U.S. case is representative, the maximum

impact of linear contrails plus the extra contrail-induced
cirrus clouds should be roughly twice that determined
for ECON. If the ECON values differ from the actual
linear contrail coverage as derived from satellites, then

would change accordingly. Assuming that f

1.8

for all land areas, the change of natural cirrus with the
change in RH3 over EUR, WASIA, and the LOR is
estimated to be 0.3, 0.3, and 1.4% (%RH3)

, respec-

tively. The value for the United States cannot be de-
termined because RH3 was invariant.

c. Climate effects

As of 1999, the ‘‘best’’ estimate of maximum net

global radiative forcing, F

NET

0.017 W m

, at the

top of the atmosphere computed for ECON was based
on a contrail optical depth

of 0.3 and the occurrence

of contrails only at a pressure level of 200 hPa (Minnis
et al. 1999). Recent studies indicate that, on average,

is less than 0.30 and the mean contrail pressure is closer
to 225 hPa. From satellite measurements, Meyer et al.
(2002) found a mean linear contrail optical depth of
0.11 over Europe. Ponater et al. (2002) determined the-
oretically that the mean global linear contrail optical
depth is 0.15 at 250 hPa and is greater over the United
States than over Europe. Using satellite data taken over
the Midwest, Duda et al. (2004) derived average contrail
optical depths of

;

0.20 over the life cycles of contrails

that developed into cirrus clouds. In a detailed manual
analysis of satellite data over the northeastern United
States, Minnis et al. (2002) derived a mean optical depth
of 0.26 for a large area of contrails from initiation to
dissipation. A similar result (mean

of 0.26) was found

by Palikonda et al. (2004) using an automated analysis
of linear contrails over the entire United States using
data from two satellites during all of 2001. Given the
variability in theory and observations, the value of the
global mean contrail optical depth remains uncertain but
is probably between 0.15 and 0.25 and is likely to be
larger over the United States than over Europe.

A new estimate of the potential global radiative forc-

ing by contrails and the resulting cirrus clouds can be
made using this new range of optical depths and the
spreading factor. Retaining the contrail pressure of 200
hPa assumed by Minnis et al. (1999), the maximum
value of F

NET

(

t 5

0.25) including both linear contrails

and the resulting contrail-generated cirrus clouds would
increase to 0.0255 W m

because f

would more than

compensate for the reduction in

from 0.30 to 0.25.

Increasing the pressure to 225 hPa would reduce the
estimate slightly because the temperature difference be-
tween the two levels is approximately 3.5 K. Using

0.15 would reduce the forcing to 0.0153 W m

. The

minimum global estimate of F

NET

for linear contrails

(Marquart and Mayer 2002) is 0.0032 W m

, a value

that would almost double if f

is considered. Thus, for

the combination of contrails and aged-contrail cirrus
coverage, the global F

NET

for 1992 air traffic is most

likely between 0.006 and 0.0255 W m

assuming that

is globally representative.

The immediate response to F

NET

is warming of the

atmosphere below the contrail and cooling or warming
of the surface depending on the time of day (Meerko¨tter

15 A

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2004

1679

M I N N I S E T A L .

. 6. Correlation of RH3 and mean annual CC, 1971–95, for (a) WASIA, (b) Europe, (c) USA, and (d)

LOR.

ABLE

4. Tropospheric and surface temperature trends, computed using the observed cirrus trends (cirrus) over the United States and from

radiosonde date (labeled Angell) over North America between 20

and 50

N from Angell (1999). Computed results are given for two contrail-

cirrus optical depths.

t (

C decade

)

t (

C decade

)

Cirrus

t 5

0.15

t 5

0.25

Angell

Cirrus

t 5

0.15

t 5

0.25

Angell

Winter
Spring
Summer
Fall
Annual

0.25
0.26
0.13
0.12
0.19

0.41
0.44
0.22
0.21
0.32

0.46
0.32
0.17
0.19
0.29

0.21
0.23
0.11
0.10
0.16

0.35
0.38
0.18
0.17
0.27

0.43
0.38
0.15
0.13
0.27

et al. 1998). Long-term responses to aircraft-induced
cirrus have been estimated by inserting small percent-
ages of cirrus into a general circulation model (GCM)
at various time steps along the air traffic routes and then
running the model to equilibrium (Rind et al. 2000).
The GCM results account for many of the feedbacks
and the redistribution of the radiative energy in the sys-
tem. For a 1% change in absolute cirrus coverage with

t 5

0.33, the GCM yielded surface temperature changes

(

) of 0.43

and 0.58

C over the globe and Northern

Hemisphere, respectively. The GCM mean tropospheric

temperature between 3 and 10 km,

, rose by

;

0.78

C. The regional variation in

CC/

t (Table 3)

and in the RH3 trends would prevent a direct application
of the GCM results to estimate the global temperature
impact of contrail-induced CC changes. Estimation of
the temperature impact of

CC/

t is more straightfor-

ward over the United States because the UTH is rela-
tively steady and the CC trend accounts for most of the
total cloudiness trend.

The contrail–cirrus effect on the surface and tropo-

spheric temperatures over the United States is estimated

1680

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J O U R N A L O F C L I M A T E

using the Rind et al. (2000) values of

for the North-

ern Hemisphere and

for the globe. The latter value

was used because it was the only tropospheric value
available. The temperature trends were computed for
the observed U.S. cirrus cover changes as follows:

(

/0.33)

T D

CC/

(3)

where

T is either the surface or tropospheric temper-

ature change for each percent cirrus coverage and the
ratio of

/0.33 accounts for the differences in the contrail

optical depths used here and by Rind et al. (2000). Here,
D is a correction factor to account for diurnal variability
in air traffic and for contrails overlapping clouds. The
GCM study did not include the diurnal variation of con-
trail coverage, which is considerably reduced between
0000 and 0600 local time over the United States (Garber
et al. 2004). The GCM also inserted the simulated con-
trails during clear-sky conditions. The value of D used
here is 0.55. Its derivation is described in appendix B
and assumes that temperature change is proportional to
radiative forcing. The unit radiative forcing for

t 5

0.25

is 16.1 W m

, a value equal to the global maximum

value mentioned earlier divided by the total contrail-
cirrus coverage. Thus, a 1% increase in contrail cov-
erage over the United States would correspond to F

NET

0.16 W m

. From (3), the respective surface and

air temperature trends over the United States due to

CC/

t are 0.16

and 0.19

C decade

t 5

0.15 and

0.27; and 0.32

C decade

t 5

0.25.

Angell (1999) determined the zonal trends in surface

and atmospheric temperatures over North America be-
tween 1973 and 1994. For comparison with the trends
estimated here from the cirrus data, the average tem-
perature trends for the surface and the troposphere be-
tween 850 and 300 hPa were taken from the layer means
in Figs. 3 and 4 of Angell (1999) for the zones: 40

–

N, 30

–40

N, and 20

–30

N. The trends for the

southernmost zone were given a weight of only 0.5
because the boundaries of the contiguous United States
are north of 25

N. The mean annual and seasonal trends

(Table 4) computed for

t 5

0.15 and 0.25 bound all of

the corresponding observations from Angell (1999), ex-
cept for winter when the observed temperature trends
slightly exceed those estimated for

t 5

0.25. From the

U.S. linear contrail observations noted earlier, it is likely
that the appropriate value of

is closer to 0.25 than to

0.15. The U.S. seasonal

trends are opposite those

for the entire Northern Hemisphere where the weakest
increases in temperature occur during winter and early
spring, and the mean annual hemispheric trends are less
than half those over the United States (Pielke et al.
1998). These results demonstrate that the increased cir-
rus coverage, attributable to air traffic, could account
for nearly all of the surface and tropospheric warming
observed over the United States between 1975 and 1994.
They should not be extrapolated to conclude that con-
trails are responsible for warming in other regions.

The value of F

NET

, 0.16 W m

(%CC)

, for the

United States estimated from the modified results of
Minnis et al. (1999) corresponds to a 0.32

C decade

trend in temperature (Table 4). These values suggest that
a 1

C temperature change results from a 0.5 W m

top-

of-the-atmosphere forcing, a response-forcing ratio of
2.0. Because the radiative forcing at the tropopause is
slightly larger than that at the top of the atmosphere
(Ponater et al. 2002), the response ratio would be some-
what smaller. The results from Rind et al. (2000) in-
dicate that the response ratio for F

NET

at the tropopause

is highly variable. For a cirrus increase of 0.4%, the
response ratios are 1.7 and 3.0 for the Northern Hemi-
sphere and the globe, respectively. The respective values
for an increase of

;

1.3% are 0.75 and 1.5. Thus, the

results in Table 4 are reasonable given the estimated
radiative forcing.

The actual response to increased cirrus coverage via

contrails, however, is not likely to be in the equilibrium
state used by the GCM. Contrail outbreaks are more
sporadic than the regular insertion technique used by
Rind et al. (2000) and the steadily increasing air traffic
does not allow the actual atmospheric system to reach
an equilibrium. The instantaneous response would also
not be appropriate because of the long time period con-
sidered and the tendency of significant contrails to last
many hours and occur at different times of day. The
results in Table 4 assume that the system response occurs
in the region of the radiative forcing. The Rind et al.
(2000) equilibrium study shows that the heating by con-
trails is often displaced from the location of the contrail
insertion. It remains uncertain, without more realistic
modeling of the contrail behavior, whether the long-term
temperature change will occur in the region where the
cirrus coverage increases or whether it is displaced and
smoothed by the global circulation.

5. Concluding remarks

The estimated temperature changes are based on a

simple application of limited GCM calculations and as-
sume that cirrus coverage is the only parameter changing
during the period. Other GCM formulations may yield
different results than the Rind et al. (2000) study used
here. Changes in aerosol concentrations, greenhouse
gases, and the geographical distribution of clouds, as
well as other air traffic effects, were not taken into ac-
count. For example, ozone formed from air traffic ex-
haust is expected to produce a positive radiative forcing
comparable to that from contrails and would result in
additional tropospheric warming below the flight levels
(Penner et al. 1999). Also, contrails that form in existing
cirrus clouds and do not contribute additional cloud cov-
er can increase the cirrus optical depth further affecting
F

NET

. Despite the good correlations between RH3 and

CC outside of heavy air-traffic land areas, UTH esti-
mates must be viewed with some skepticism because
filtering of observations may have eliminated significant
portions of the soundings available for assimilation by

15 A

PRIL

2004

1683

M I N N I S E T A L .

. A3. The 1985–96 trends in RH3 from (a) ECMWF and (b)

NCEP reanalyses.

223 K, the measured Rh would be nearly 21% less than
the true value at ice saturation compared to that at 230
K, which would be only 14% less than the correspond-
ing true value at ice saturation. If such biases hold for
the NCEP data, then approximately 7% should be added
to the RH3 values over Asia resulting in the Asian val-
ues exceeding their U.S. counterparts. Given the poor
state of UTH measurements and their use in assimilation
models, it is not expected that the absolute value of
RH3 will be highly accurate for a given region.

Trends in RH3 and in the average RH between 300

and 500 hPa were computed for 1979–98 for compar-
ison with satellite-derived values of UTH (Bates and
Jackson 2001). A comparison with Fig. 1 from Bates
and Jackson (2001) indicates that the NCEP trends are
generally opposite in sign to those from the satellite
data over most land areas. Better agreement was found
over the oceans. The trends in RH3 from NCEP were
also compared to those from the European Centre for
Medium Range Forecasts (ECMWF) monthly Tropical
Ocean Global Atmosphere (TOGA) Global Tropospher-
ic Analyses for 1985–96 (Fig. A3) when ECMWF data
are available (ECMWF 1995). In general, the ECMWF
trends are more extreme than their NCEP counterparts.
They agree in sign over Europe, western Asia south of
50

N, and over much of the NP and NA. The trends are

opposite in sign over many other areas including the

United States and Australia. The NCEP trends are only
significant at the 90% confidence level over the western
United States, Europe, Siberia, China, and Australia.
The ECMWF trends are significant over the eastern
United States, Europe, China, southern Asia, Siberia,
and Australia. Thus, the two datasets show some sig-
nificant disagreement. Both datasets were correlated
with CC for the various air traffic regions. The values
of R

for the ECMWF RH3 were all less than 0.12 and

none of the correlations were statistically significant.

The only UTH data available for 1971–96 are from

CARDS or NCEP. ECMWF reanalyses are not yet avail-
able for the entire time period. Of all of the data avail-
able for any significant time period, only the NCEP RH3
values show significant correlations with CC for areas
where contrails should play a minor role in cirrus cov-
erage. The correlations over ocean are significant but of
the wrong sign. Further study is needed to understand
this behavior over oceans. The CARDS data are avail-
able only over a limited number of sites and overall are
not correlated with CC any better than the NCEP data.
Thus, because of data quality, consistency, and cover-
age, the NCEP dataset is the best available for com-
parison with the CC trends. From all of the available
evidence, however, it appears that the NCEP data over
land make the most sense and, therefore, constitute the
most appropriate UTH dataset for the study period or
any significant portion of it.

APPENDIX B

Determination of Diurnal and Cloud Correction

Factor for Estimating Contrail-Induced

Temperature Changes

If it is assumed that radiative forcing is directly pro-

portional to a temperature change, then radiative forcing
values can be used to establish the desired correction
factor D to account for the lack of an air traffic diurnal
cycle and the absence of contrail–cloud overlap in the
Rind et al. (2000) estimates of temperature change due
to contrails. Here, D is estimated using the global ra-
diative forcing results for

t 5

0.3 in Table 1 from Minnis

et al. (1999), which included an air traffic diurnal cycle
and overlapping clouds and contrails. The shortwave
(SW), longwave (LW), and net radiative forcings from
Minnis et al. (1999) are

5 2

0.008,

0.025,

and

0.017 W m

, respectively.

NET

The absence of a diurnal cycle in the GCM results

only affects the longwave forcing. Air traffic over the
United States during the night is roughly 42% of that
during the day (Garber et al. 2004). Assuming that day
and night are 12 h each and that the unit nighttime LW
radiative forcing over land (

) is 90% of that during

nite

the day due to heating and cooling of the surface, the
total LW forcing accounting for a diurnal cycle in air
traffic is

0.025

0.5F

0.5

0.42

0.9F

(B1)

day

1684

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J O U R N A L O F C L I M A T E

where

is the daytime LW forcing. Solving (B1)

day

yields

0.0363 W m

and

0.0326 W m

day

nite

If the air traffic was constant as in Rind et al. (2000),
then the total radiative forcing would be

0.5 F

day

nite

(B2)

Thus,

0.0345, a value that is 38% greater than

that for a day–night ratio of 2.4. Assuming random cloud
overlap, the SW and LW radiative forcings from Minnis
et al. (1999) can be approximated as

A )

A F

and

(B3)

clr

cld

A )

A F

(B4)

clr

cld

respectively. The subscripts, cld and clr refer to cloudy
and clear areas, respectively, and A

is the mean cloud

fraction. The relative clear and cloudy forcings were
estimated using the results from Fig. 5 of Meerko¨tter et
al. (1999) that show the unit radiative forcings for a
clear scene and for the same scene with a low-level
cloud. The unit SW forcings for the clear and cloudy
scenes are

14 and

2 W m

, respectively. Thus,

0.14 F

cld

(B5)

Assuming an average cloud height of 5 km over the
United States and a linear variation of

with cloud

cld

height, the unit LW radiative forcing at 5 km in the
Meerko¨tter et al. (1999) would be 31 W m

compared

to 57 W m

for the clear case and

0.54 F

cld

clr

(B6)

Substituting (B5) and (B6) into (B3) and (B4), respec-
tively, and assuming A

is 50%,

and

are

0.0140

clr

and 0.0325 W m

, respectively. Thus, in the absence of

cloud cover and the air traffic diurnal cycle,

1.38F

and

(B7)

clr

(B8)

clr

The equivalent net forcing for the GCM case,

, is

NET

the sum of (B7) and (B8), which produces a value of
0.0309 W m

. The correction factor D is simply the

ratio of

, which yields a value 0.55.

NET

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