Computer analyses of global climate have consistently
overstated warming in Antarctica, concludes new research by scientists
at the National Center for Atmospheric Research (NCAR) and Ohio State
University. The study can help scientists improve computer models and
determine if Earth's 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 NCAR scientist Andrew Monaghan, the
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 records of the past 50 to 100 years of Antarctic climate with
simulations run on computer models. Researchers have used atmospheric
observations to confirm that computer models are accurately simulating
climate for the other six continents. The models, which are mathematical
representations of Earth's climate system, are a primary method for
scientists to project future climate.
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.
The study was published on April 5 in Geophysical Research Letters.
It was funded by the National Science Foundation, NCAR's primary
sponsor, and the Department of Energy.
The authors compared recently constructed temperature data sets from
Antarctica, based on data from ice cores and ground weather stations, to
20th century simulations from computer models used by scientists to
simulate global climate. While the observed Antarctic temperatures rose
by about 0.4 degrees Fahrenheit (0.2 degrees Celsius) over the past
century, the climate models simulated increases in Antarctic
temperatures during the same period of 1.4 degrees F (0.75 degrees C).
The error appeared to be caused by models overestimating the amount
of water vapor in the Antarctic atmosphere, the new study concludes. The
reason may have to do with the cold Antarctic atmosphere handling
moisture differently than the atmosphere over warmer regions.
A chilling ozone hole
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 models generally capture these wind changes, although sometimes
incompletely.
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 7 to 23 inches (18-59 centimeters) this
century, in part because of melting glaciers worldwide. The Geophysical
Research Letters paper suggests that warming in Antarctica over the next
century could offset that by about 2 inches if the continent warms by
5.4 degrees F (3 degrees C), 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 coming 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 21st 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."
The University Corporation for Atmospheric Research manages the
National Center for Atmospheric Research under primary sponsorship by
the National Science Foundation (NSF). Opinions, findings, conclusions,
or recommendations expressed in this document are those of the author(s)
and do not necessarily reflect the views of the National Science
Foundation, NASA, or other funding agencies.
