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Climate Models Overheat Antarctica

08.05.2008
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.

Peter Weiss | American Geophysical Union
Further information:
http://www.agu.org
http://dx.doi.org/10.1029/2007GL032630
http://www.ucar.edu/news/releases/2008/antarctica.jsp

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