By Rachel Hauser   


Eric
Steig (University of Washington) aboard the National Science Foundation
research vessel Lawrence M. Gould, en route to the Antarctic Peninsula.
Livingston Island can be seen in background. Until recently,
the Antarctic ice sheet looked to be bucking the global warming trend.
This assessment relied on temperature data collected from a sparse
network of mostly coastal weather stations. To provide a more complete
picture of Antarctica’s historic surface temperature regime, a team of
U.S. scientists employed an innovative technique to construct 50-year
estimates of the near-surface temperature anomalies for the entire
continent. The resulting climate field reconstructions for 1957-2006
show an overall warming trend across Antarctica, with this trend being
strongest over the West Antarctic ice sheet.


Mark
Fahnestock, a research associate professor at University of New
Hampshire, installs an Automatic Weather Station (AWS) at the Megadunes
site, located on the Antarctic Plateau near Vostok Base in late 2002.
This site was placed at one of the coldest, windiest areas in
Antarctica to study the formation of cold snow dunes that span large
areas of the high plateau part of the ice sheet. Image courtesy Ted
Scambos, National Snow and Ice Data Center. Photo credit: Ted Scambos,
National Snow and Ice Data Center. Funded by the National Science Foundation’s Office of Polar Programs and led by Eric Steig,
a glaciologist and isotope geochemist at the University of Washington,
the team based their temperature reconstructions on observations
collected at manned weather stations; temperature data from these
stations are the continent’s most reliable and continuous records,
extending back to 1957. However, these observations are available at
only a handful of locations, with all but two located on the coast,
making it difficult to assess temperatures across the vast continent.
To obtain more information about the continental interior, the
scientists used two independent sources of data, thermal infrared (IR)
measurements from satellites and observations from automatic weather
stations (AWS) located in remote parts of the continent and near
research bases. The thermal IR and AWS data, which extend from
approximately 1980 to present, provide information about the spatial
relationships of temperature anomalies between regions. The researchers
then applied these spatial relationships to the long-term manned
temperature records to fill in the areas between stations, resulting in
an Antarctic-wide temperature time-line that stretches back to 1957.

Using
the two independent data sources provided an important validation of
the results. Satellite thermal IR observations are deemed reliable only
under clear-sky conditions – under cloudy skies, the thermal IR channel
captures cloud-top temperature, rather than surface temperature. And,
in the case of the AWS, lacking human intervention these measurements
can have inherent biases. For instance, a temperature sensor might be
snow covered and so measure under-snow temperature rather than surface
air temperature for a given period of time. Cross-checking the two data
sources provided an important confirmation of the results. 


Red
represents areas where temperatures have increased the most during the
last 50 years, particularly in West Antarctica, while dark blue
represents areas with a lesser degree of warming. Temperature changes
are measured in degrees Celsius. Credit: NASA/GSFC Scientific
Visualisation Studio.“The results from the different reconstructions agreed well,” says David Schneider,
a visiting climate scientist at the National Center for Atmospheric
Research. “The strongest near-surface warming has occurred in the
winter and spring, while the summer and autumn trends are comparatively
weak, making the overall annual trend positive.” 

Steig adds
that their results also agree well with similar work by previous
researchers. “The main thing we added is information about what is
happening in West Antarctica, a large region of the continent that is
particularly lacking in weather stations,” he explains.

The
study sheds light on an outstanding question in Antarctic climatology:
has the strong warming observed on the Antarctic Peninsula also
extended inland over continental West Antarctica? The reconstructions
indicate that the answer to this question is yes, with warming trends
extending well into West Antarctica in excess of 0.1°C per decade
during the past 50 years. This finding is supported by measurements
from nearby ice cores and boreholes that also indicate warming during
the last half century, notes Schneider.  

“This isn’t a
surprising result,” says Steig. “The same weather systems that bring
warm, moist air from the Pacific onto the Antarctic Peninsula also
affect West Antarctica.” 

The scientists’ historic temperature
timeline indicates that for the 50-year time period considered, warming
across Antarctica is comparable to that of the Southern Hemisphere.
However, this doesn’t mean that the same pattern of Antarctic warming
of the last 50 years is necessarily going to continue.  While the
overall warming trend is difficult to explain without considering the
influence of higher greenhouse gas concentrations, the pattern of
change – with greater warming in West Antarctica – is the result of the
combined effects of changing stratospheric ozone concentrations, and
changes in atmospheric circulation and sea ice extent. As the ozone
hole is gradually repaired in the future (see sidebar story below),
this will probably lead to greater warming in East Antarctica, but not
necessarily in West Antarctica.