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Water Vapor Feedback in Climate Models

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Abstract

General circulation models (GCMs) are highly sophisticated computer tools for modeling climate change, and they incorporate a large number of physical processes and variables. One of the most important challenges is to properly account for water vapor (clouds and humidity) in climate warming. In his Perspective, Cess discusses results reported in the same issue by Soden et al. in which water vapor feedback effects are tested by studying moistening trends in the upper troposphere. Satellite observations of atmospheric water vapor are found to agree well with moisture predictions generated by one of the key GCMs, showing that these feedback effects are being properly handled in the model, which eliminates a major potential source of uncertainty.
DOI: 10.1126/science.1119258
, 795 (2005);310 Science
Robert D. Cess
Water Vapor Feedback in Climate Models
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795
lished previously is 5.5 Å. In electron density
maps in that resolution range, nucleic acid
helices look like curved ribbons whose con-
stituent nucleotides are often difficult to
delineate, and protein density is hard to inter-
pret at all. Nevertheless, a great deal was
learned from those electron density maps
because relevant structures that had been
solved at higher resolution before could be
fitted into them. The problem with the 70S
model that emerged is that wherever its
structure deviated from that of the structures
being fit into its electron density maps, it was
difficult to be sure what was going on. In 3.5
Å resolution electron density maps, such as
those that led to the 70S E. coli structure
reported by Schuwirth et al., these ambigui-
ties disappear because individual nucleotides
are clearly visualized, and protein electron
density is independently interpretable.
What has been learned? The structures
presented by Schuwirth et al. are not the last
word about the information contained in the
particular crystals examined. Ribosomal
proteins are not fully modeled at this point,
and the structures are not fully refined.
In addition, the crystals analyzed by
Schuwirth et al. lack transfer RNAs or any
of the other proteins, nucleic acids, or small
molecules that interact with the ribosome
during protein synthesis. Nevertheless, sev-
eral themes clearly emerge. The structures
of the bridges that hold the two subunits
together are clear, which is important
because the bridges are critical function-
ally: The two subunits of the ribosome not
only communicate during protein synthesis,
they also engage in coordinated, relative
motions (8). In addition, the two 70S struc-
tures reported by Schuwirth et al. differ in
the orientation of the head domains of their
small subunits, and in neither is the head
domain position the same as it is in the T.
thermophilus 70S ribosome structure now
available (7). Movements of the small sub-
unit’s head domain like the ones reported by
Schuwirth et al. occur during protein syn-
thesis [e.g., (8)]. It is now possible to under-
stand how these motions occur at the
molecular level, and to propose models for
how they might be coupled to the events of
protein synthesis. It remains to be seen what
the small differences in conformation
between the large ribosomal subunit of
these E. coli ribosomes and the large ribo-
somal subunit structures of other organisms
actually mean. Thus, the ribosome struc-
tures obtained by Schuwirth et al. really do
advance our understanding of protein syn-
thesis. Now that high-quality crystals are
available for the E. coli 70S ribosome, the
rate at which new information is obtained
should increase.
References
1. B. S. Schuwirth et al., Science 310, 827 (2005).
2. N. Ban, P. Nissen, J. Hansen, P. B. Moore, T. A. Steitz,
Science 289, 905 (2000).
3. J. Harms et al., Cell 107, 679 (2001).
4. B.T. Wimberly et al., Nature 407, 327 (2000).
5. F. Schluenzen et al., Cell 102, 615 (2000).
6. M. Pioletti et al., EMBO J. 20, 1829 (2001).
7. M. M.Yusupov et al., Science 292, 883 (2001).
8. J. Frank, R. K. Agrawal, Nature 406, 318 (2000).
9. J. Cate, personal communication.
10.1126/science.1120539
G
eneral circulation models (GCMs)
are the most detailed computer sim-
ulations available for projecting cli-
mate change caused by increasing green-
house gases, as well as other anthropogenic
changes. These numerical models contain
numerous parameterizations of physical
processes occurring within the climate sys-
tem (that is, small-scale processes have to
be described within the models). As a
result, there is a need to devise ways of test-
ing these parameterizations and processes
within GCMs. On page 841 of this issue,
Soden et al. (1) report an important reality
check on one such process: the role of
atmospheric water vapor in climate change.
It has long been known (2) that cloud-
climate interactions constitute a major
uncertainty in attempting to project future
climate change with a GCM. As an illustra-
tive example, if global cloud cover were
to decrease because of climate warming,
then this decrease reduces the infrared
greenhouse effect due to clouds. Thus, the
climate system is able to emit infrared
radiation more efficiently, moderating the
warming and so acting as a negative feed-
back mechanism. But there is a related
positive feedback in this example that
would increase the warming: The solar
radiation absorbed by the climate system
increases because the diminished cloud
cover causes a reduction of reflected solar
radiation by the atmosphere.
The situation is actually far more com-
plicated than in this simple example,
because changes in cloud cover will
undoubtedly depend on cloud type and geo-
graphical location. Moreover, there would
likely be associated changes in cloud alti-
tude and cloud optical depth. One test of
cloud-climate interactions within a GCM is
to determine, relative to satellite observa-
tions, how well a GCM represents the radia-
tive impact of clouds on the model’s climate
during the 5 years encompassing 1985 to
1989, and the top panel of the figure
demonstrates that many models do rather
poorly in this respect. And with regard to
those models that do agree well with Earth
Radiation Budget Satellite observations, it
must be emphasized that this test is a neces-
sary, but not sufficient, test of a model.
Another feedback mechanism is water
vapor feedback. Water vapor is the atmo-
sphere’s dominant greenhouse gas, and a
change in its concentration associated with a
change in climate would alter the greenhouse
effect of the atmosphere, thus producing a
feedback mechanism. In 1967 it was pro-
posed (3) that the atmosphere might conserve
its relative humidity, and if so, this would lead
to a positive feedback because a warmer
atmosphere would contain more water vapor,
thus amplifying the warming. And indeed,
GCMs do tend to conserve global mean
atmospheric relative humidity, as is shown
for one such model in the bottom panel of the
figure. But for more than a decade there has
been considerable debate on this issue, with
suggestions that water vapor feedback might
actually be a negative feedback mechanism.
Soden et al. (1) present a very clever
way of testing one aspect of water vapor
feedback. As they point out, observed mois-
tening trends in the lower troposphere have
been linked to corresponding changes in
surface temperature. But attempts to
observe a moistening trend in the upper tro-
posphere have proven to be unsuccessful,
and this is the issue that Soden et al.
address. They accomplish this by using
clear-sky satellite radiance measurements
from the High Resolution Infrared
Radiometer Sounder channel centered at
6.7 µm (channel 12), which measures a por-
tion of the 6.3-µm water vapor absorption
band and therefore is sensitive to water
vapor in the upper troposphere. They then
compare the channel 12 observations of
global mean blackbody temperature, for the
ATMOSPHERIC SCIENCE
Water Vapor Feedback
in Climate Models
Robert D. Cess
The author is at the Institute for Terrestrial and
Planetary Atmospheres, Marine Sciences Research
Center, State University of New York, Stony Brook, NY
11794, USA. E-mail: rcess@notes.cc.sunysb.edu
www.sciencemag.org SCIENCE VOL 310 4 NOVEMBER 2005
P ERSPECTIVES
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796
period January 1982 to December 2004, to
those computed from the temperature and
moisture profiles of the Geophysical Fluid
Dynamics Laboratory atmospheric GCM,
which uses prescribed sea surface tempera-
tures. The temporal trends of the observed
and modeled channel 12 observations are in
very good agreement, and this agreement
persists when the GCM
results are repeated
with the assumption of
constant atmospheric
relative humidity. On
the other hand, there is
considerable disagree-
ment with the channel
12 observations when
the GCM results are
repeated by assuming
no change in the water
vapor content of the
upper troposphere. Soden et al. then use
additional satellite observations to empha-
size that global mean relative humidity is
being conserved by the upper troposphere
in response to atmospheric warming.
This work by Soden et al. provides the
clearest evidence yet that GCMs are prop-
erly representing water vapor feedback.
This is an important contribution because it
eliminates one potential uncertainty within
these climate models. There remains, how-
ever, an uncertainty in other climate feed-
back mechanisms, the most notable of
which is cloud feedback as described above.
The reduction of these uncertainties will
require a suite of cleverly designed neces-
sary, but not sufficient, tests of the models.
References
1. B. J. Soden, D. L. Jackson, V. Ramaswamy, M. D.
Schwarzkopf, X. Huang, Science 310, 841 (2005);
October 2005 (10.1126/science.1115602).
2. R. D. Cess et al., Science 245, 513 (1989).
3. S. Manabe, R. T. Wetherald, J. Atmos. Sci. 24, 241
(1967).
4. E. F. Harrison et al., J.Geophys. Res.95, 18687 (1990).
5. G. L. Potter, R. D. Cess, J. Geophys. Res. 109, D02106
(2004).
6. A. Dai et al., J.Clim. 14, 485 (2001).
10.1126/science.1119258
B
ond order and the division of chemical
bonding into single or multiple bonds
are among the most fundamental con-
cepts in molecular chemistry. Elements in
the main group of the periodic table may
have up to three bonds to the same bonding
partner (that is, the maximum bond order
can only be three). It was long believed that
this is the highest bond order that can be
achieved in a stable molecule. Because of
this conventional wisdom, the 1964 report
by Cotton et al. (1) on the synthesis of a
molecule with bond order four caused a
sensation. The analysis of transition metal
salt compounds containing the anion
[Re
2
Cl
8
]
2
revealed a quadruple bond
between the rhenium atoms. This finding
opened the door to a new field of chemistry
and led to the synthesis of a large number of
hitherto unknown molecules with multiple
bonds having bond orders up to four
between transition metal atoms (2). It has
been speculated that a further extension to
bond order five should in principle be pos-
sible, but attempts to make a compound
with a quintuple bond have been unsuccess-
ful until now. On page 844 of this issue,
Nguyen et al. (3) report the synthesis of a
stable compound with fivefold bonding
between two chromium atoms (see first fig-
ure on the following page).
Chemical bonding between two atoms
is usually discussed in terms of bonding
and antibonding combinations of the
valence atomic orbitals (AOs) that yield
molecular orbitals (MOs). The pivotal AOs
of the transition metal atoms are the five d
orbitals. The figure shows schematically
the combination of the d-AOs that give five
components for the bonding MOs (σ, π, δ)
and five components for the antibonding
MOs (σ*, π*, δ*). The diagram also quali-
tatively indicates the expected ordering for
the energy levels of the orbitals. A quintu-
ple bond between two transition metals
requires that 10 electrons occupy the
lowest lying MOs. This yields one σ bond,
one degenerate π bond, and one degen-
erate δ bond (that is, the π and σ bonds
each have two levels with the same
energy). Transition metal compounds like
[Re
2
Cl
8
]
2
with a quadruple bond have
only one (not degenerate) δ bond.
Theoretical analysis (1) showed that the
d
x
2
y
2
AOs that form the second component
of the δ bond (see the figure) interact pri-
marily with ligand orbitals such as the
chlorine AOs in [Re
2
Cl
8
]
2
. All previous
attempts to synthesize a molecule with the
general formula L
n
TM-TML
n
(where L is
ligand, TM is transition metal) in which the
d
x
2
y
2
AOs of TM engage in the “missing”
fifth metal-metal bonding rather than in
TM-L bonding have failed.
CHEMISTRY
Building a Quintuple Bond
Gernot Frenking
0
–10
–20
–30
–40
–50
–60
1 3 5 7 9 11 13 15 17 19
Model
60°N to 60°S means
Net CRF (W/m
2
)
Measured CRF
0
200
400
600
800
1000
0 10 20 30 40 50 60 70 80
Relative humidity (%)
Pressure (mbar)
Cloudy predictions. (Top) Actual effect of clouds on climate (measured CRF)
compared to the effect predicted by 19 global climate models. Some of the
models significantly overestimate cloud-induced cooling. Clouds can poten-
tially cool climate (by reflecting solar radiation) and simultaneously heat the
system (by increasing the atmospheric greenhouse effect).The net effect illus-
trated in the figure is cooling, as indicated by the negative values of CRF. Actual
net CRF (cloud-radiative forcing), measured by the Earth Radiation Budget
Satellite (4) and averaged from 60°N to 60°S, is –22 W/m
2
.(Bottom) Average
relation between atmospheric pressure and humidity for a 120-year (1870 to
1989) simulation of global warming. The profile is an average of 120 annual
mean profiles; the bars represent two standard deviations, indicating that
global mean atmospheric relative humidity is conserved over the entire 120-
year period. The simulation is from the National Center for Atmospheric
Research Community Climate System Model Version 1 (6).
The author is at the Fachbereich Chemie, Philipps-
Universität Marburg, Hans-Meerwein-Strasse, D-
35039 Marburg, Germany. E-mail: frenking
@chemie.uni-marburg.de
4 NOVEMBER 2005 VOL 310 SCIENCE www.sciencemag.org
P ERSPECTIVES
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... Although the proportion of water vapor content in the atmosphere is small (0.1-3%), it is the most active element in the atmospheric circulation and Earth climate system [2,3]. The most direct impact of global warming is the change of water vapor content, with warmer temperature increasing water vapor in the atmosphere [4]. ...
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Water vapor content plays an important role in climate change and the ecosystem in the Tibetan Plateau (TP) through its complicated interaction with the meteorological elements. However, due to the complex topography of the Tibetan Plateau, it is unreliable to attempt to understand the variation pattern of water vapor content using only observational data. Satellite and reanalysis data can be a good substitute for observational data, but their accuracy still needs to be evaluated. Therefore, based on radiosonde stations data, comprehensive assessment of water vapor content on the TP and surrounding areas derived from ERA-5, Second Modern-Era Retrospective analysis for Research and Applications (MERRA2), Atmospheric Infrared Sounder (AIRS)-only, and weighted ensemble data was performed in the context of spatial and temporal distribution at the annual and seasonal scale. Based on precipitation from Gauge V3.0 and Tropical Rainfall Measuring Mission satellite (TRMM) and temperature from ERA-5, the relationship between water vapor content and temperature and precipitation was analyzed. The results show that water vapor content decreases from southeast to northwest, and ERA-5, MERRA2, and AIRS-only can reasonably reproduce the spatial distribution of annual and seasonal water vapor content, with ERA-5 being more reliable in reproducing the spatial distribution. Over the past 50 years, the water vapor content has shown a gradual increasing trend. The variation trends of AIRS-only, MERRA2, ERA-5, and weighted ensemble data are almost consistent with the radiosonde stations data, with MERRA2 being more reliable in capturing water vapor content over time. Weighted ensemble data is more capable of capturing water vapor content characteristics than simple unweighted products. The empirical orthogonal function (EOF) analysis shows that the first spatial mode values of water vapor content and temperature are positive over the TP, while the values of precipitation present a “negative-positive-negative” distribution from south to north over the TP. In the second spatial mode of EOF analysis, the values of water vapor content, air temperature, and precipitation are all negative. The first temporal modes of EOF analysis, water vapor content, air temperature, and precipitation all show an increasing trend. In conclusion, there is a clear relationship of water vapor content with temperature and precipitation.
... There was generally more precipitation in the southern part of the TP and less precipitation in the northern part. Cess 16 and Xie 17 concluded that there is a positive feedback effect between air temperature and water vapor content on the TP. Li et al. 18 studied the relationship between atmospheric water vapor content and temperature and precipitation in Changchun, and found that water vapor content had a signi cant correlation with temperature, while the trend and amplitude of precipitation and water vapor content variation were not consistent. ...
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Based on radiosonde stations and V3.0 data, Atmospheric Infrared Sounder (AIRS)-only, Tropical Rainfall Measuring Mission satellite (TRMM) and MERRA2, and ERA-5 data, we evaluated the ability of each dataset to reproduce water vapor content and explored its relationship with precipitation and temperature over the Tibetan Plateau and its surroundings. The results showed that the southern part of the surrounding area had high water vapor content and a low water vapor content zone appeared in the inner part of the Tibetan Plateau. The largest water vapor content appeared in summer and the smallest in winter. Most of the products could capture the spatial distribution of water vapor content, ERA-5 had the smallest bias and the highest correlation coefficient with the radiosonde data. The water vapor content has shown a gradually increasing trend over the last 50 years, with the most obvious increase in summer. Several sets of products had the same fluctuation trend and value is greater than the radiosonde data. There was a significant positive correlation between air temperature and water vapor content in the Tibetan Plateau, especially in the south. As the latitude increased, the correlation between precipitation and water vapor content gradually decreased and a negative correlation appeared.
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Climate feedbacks have been usually estimated using changes in radiative effects associated with increased global-mean surface temperature. Feedback uncertainties, however, are not only functions of global-mean surface temperature increase. In projections by global climate models, it has been demonstrated that the geographical variation of sea surface temperature change brings significant uncertainties into atmospheric circulation and precipitation responses at regional scales. Here we show that the spatial pattern of surface warming is a major contributor to uncertainty in the combined water vapour-lapse rate feedback. This is demonstrated by computing the global-mean radiative effects of changes in air temperature and relative humidity simulated by 31 climate models using a methodology based on radiative kernels. Our results highlight the important contribution of regional climate change to the uncertainty in climate feedbacks, and identify the regions of the world where constraining surface warming patterns would be most effective for higher skill of climate projections.
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