The effects of Antarctic water vapor have been overestimated.
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Climate Models Overheat Antarctica
WASHINGTON--Computer analyses of global climate have consistently
overstated warming in Antarctica, concludes a new study. The findings
can help scientists improve computer models and determine if the
southernmost continent will warm significantly this century, a major
research question because of Antarctica's potential impact on global
sea-level rise.
"We can now compare computer simulations with observations of actual
climate trends in Antarctica," says Andrew Monaghan of the National
Center for Atmospheric Research (NCAR) in Boulder, Colo., lead author of
the study. "This is showing us that, over the past century, most of
Antarctica has not undergone the fairly dramatic warming that has
affected the rest of the globe. The challenges of studying climate in
this remote environment make it difficult to say what the future holds
for Antarctica's climate."
The study marks the first time that scientists have been able to compare
the past 50 to 100 years of Antarctic climate with simulations run on
computer models. The models are a primary method for researchers to
project future climate. Scientists have used atmospheric observations to
confirm that computer models are accurately simulating climate for the
other six continents.
Antarctica's climate is of worldwide interest, in part because of the
enormous water locked up in its ice sheets. If those vast ice sheets
were to begin to melt, sea level could rise across the globe and
inundate low-lying coastal areas. Yet, whereas climate models accurately
simulate the last century of warming for the rest of the world, they
have unique challenges simulating Antarctic climate because of limited
information about the continent's harsh weather patterns.
Monaghan and his colleagues at NCAR and Ohio State University, in Columbus,
published their findings last month in Geophysical Research Letters, a
journal of the American Geophysical Union (AGU).
The authors compared recently constructed temperature data sets from
Antarctica, based on data from ice cores and ground weather stations, to
twentieth century simulations from computer models used by scientists to
simulate global climate. While the observed Antarctic temperatures rose
by about 0.2 degrees Celsius (0.4 degrees Fahrenheit) over the past
century, the climate models simulated increases in Antarctic
temperatures during the same period of 0.75 degrees C (1.4 degrees F).
The error appeared to be caused by models overestimating the amount of
water vapor in the Antarctic atmosphere, the new study concludes.
The models, however, have correctly captured trends in Antarctic snowfall,
including increases in snowfall in the late twentieth century, prior to
a decrease over the last decade.
Part of the reason that Antarctica has barely warmed has to do with the
ozone hole over the continent. The lack of ozone is chilling the middle
and upper atmosphere, altering wind patterns in a way that keeps
comparatively warm air from reaching the surface. Unlike the rest of the
continent, the Antarctic Peninsula has warmed by several degrees, in
part because the winds there are drawing in warmer air from the north.
The study delivered a mixed verdict on Antarctica's potential impact on
sea-level rise. The Intergovernmental Panel on Climate Change, which
operates under the auspices of the United Nations, has estimated that
sea-level rise could amount to 18 to 59 centimeters (7 to 23 inches)
this century, in part because of melting glaciers worldwide. The new
findings suggest that other effects of warming in Antarctica over the
next century could reduce that by about 5 centimeter (2 inches) if the
continent warms by 3 degrees C (5.4 degrees F) as computer models have
indicated. The reason is that the warmer air over Antarctica would hold
more moisture and generate more snowfall, thereby locking up additional
water in the continent's ice sheets.
But the authors caution that model projections of future Antarctic
climate may be unreliable.
"The research clearly shows that you can actually slow down sea-level
rise when you increase temperatures over Antarctica because snowfall
increases, but warmer temperatures also have the potential to speed up
sea-level rise due to enhanced melting along the edges of Antarctica,"
says Monaghan, who did some of his research at Ohio State University
before going to NCAR. "Over the next century, whether the ice sheet
grows from increased snowfall or shrinks due to more melt will depend on
how much temperatures increase in Antarctica, and potentially on erosion
at the ice sheet edge by the warmer ocean and rising sea level."
"The current generation of climate models has improved over previous
generations, but still leaves Antarctic surface temperature projections
for the twenty-first century with a high degree of uncertainty," adds
co-author and NCAR scientist David Schneider. "On a positive note, this
study points out that water vapor appears to be the key cause of the
problematic Antarctic temperature trends in the models, which will guide
scientists as they work to improve the climate simulations."
This study was funded by National Science Foundation and by the
Department of Energy.
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Images:
Available for download at
http://www.ucar.edu/news/releases/2008/antarctica.jsp
are a color map of Antarctica showing warming and cooling trends and a
photo of researcher Andrew Monaghan.
Title:
"Twentieth century Antarctic air temperature and snowfall simulations by
IPCC climate models"
Authors:
Andrew J. Monaghan and David H. Bromwich: Polar Meteorology Group, Byrd
Polar Research Center, Ohio State University, Columbus, Ohio, USA.; Bromwich
also with Atmospheric Sciences Program, Department of Geography, Ohio State
University, Columbus, Ohio, USA;
David P. Schneider: Climate and Global Dynamics Division, National
Center for Atmospheric Research, Boulder, Colorado, USA.
Citation:
Monaghan, A. J., D. H. Bromwich, and D. P. Schneider (2008), Twentieth
century Antarctic air temperature and snowfall simulations by IPCC climate
models, Geophys. Res. Lett., 35, L07502, doi:10.1029/2007GL032630.
