September 5, 2007

Antarctica: Warming, Cooling, or Both?

Filed under: Antarctic, Polar

The ice caps are melting – right? If you visit thousands of websites on climate change, watch Gore’s film or many similar documentaries, you would be left with no doubt that the icecaps are warming and melting at an unprecedented rate. However, with respect to Antarctica, you might be surprised when you examine what the United Nations Intergovernmental Panel on Climate Change says in their 2007 Summary for Policymakers. Believe it or not, IPCC reports “Antarctic sea ice extent continues to show inter-annual variability and localized changes but no statistically significant average trends, consistent with the lack of warming reflected in atmospheric temperatures averaged across the region.” Furthermore, they note “Current global model studies project that the Antarctic ice sheet will remain too cold for widespread surface melting and is expected to gain in mass due to increased snowfall.”

A major article on this subject appears in a recent issue of the Journal of Climate by William Chapman and John Walsh of the University of Illinois. The two scientists extensively review the literature on temperature trends in Antarctica and conclude “These studies are essentially unanimous in their finding that the Antarctic Peninsula has warmed since the 1950s, when many of the surface stations were established.” They note “Recent summaries of station data show that, aside from the Antarctic Peninsula and the McMurdo area, one is hard-pressed to argue that warming has occurred, even at the Antarctic coastal stations away from the peninsula and McMurdo.” Furthermore, they write “Recent attempts to broaden the spatial coverage of temperature estimates have shown a similar lack of evidence of spatially widespread warming.” We completely agree having covered this subject at World Climate Report many times in the past – there is some warming in Antarctica but it is largely confined to the relatively small peninsula extending away from the bulk of the continent and is largely confined to the winter season (see below – the Antarctic Peninsula extends toward to southern tip of South America).


Figure 1. Locations of surface temperature data color-coded by data source: red - land surface stations, yellow – automated weather stations, and blue - sea surface temperatures (from Chapman and Walsh, 2007).

Chapman and Walsh collected temperature in and around Antarctica from 460 locations including 19 manned surface observation stations located on the continent, 73 automated weather stations, and the 2° latitude by 2° longitude gridded sea surface temperature time series. They made every attempt to have complete records from 1958 to 2002, and the scientists used a “correlation length scales” method to further extract information from the observations.

When areally-averaged over the 60°S to 90°S, Chapman and Walsh found “The 45-yr linear temperature change is largest in winter (+0.776°C) and spring (+0.405°C), and smallest in summer (+0.193°C) and autumn (+0.179°C). These temperature changes correspond to linear trends of +0.172°C/decade (winter), +0.090°C/decade (spring), +0.045°C/decade (summer), and +0.040°C/decade (autumn). The 45-yr (1958–2002) linear temperature change of annual mean Antarctic temperatures is +0.371°C with a corresponding trend of +0.082°C/decade” (Figure 2). Furthermore, the authors find “Statistically significant warming is confined to the Antarctic Peninsula and a small region along the eastern coast of the continent. Temperature trends over the remainder of the Antarctic continent do not exceed significance thresholds.” So at this point, we have learned that Antarctica is warming in its bitterly cold winter season and most of the warming is confined to a small area surrounding the Antarctic Peninsula (Figure 3).


Figure 2. Annual and seasonal mean time series and linear trends of area-averaged surface air temperature anomalies for 1958–2002. Means shown are for the Antarctic domain of 60° to 90°S (from Chapman and Walsh, 2007).


Figure 3. Linear trends of annual mean surface air temperature (°C/decade) for the period 1958–2002. Greens and blues denote cooling; yellows and reds denote warming. Significant trends are indicated by hatching: 95% - single hatching; 99% - crosshatching (from Chapman and Walsh, 2007).

Here is the interesting twist to the story. Notice in the graph of seasonal and annual temperature trends that the coldest years occurred at or near the beginning of the record. Chapman and Walsh find “Trends computed using these analyses show considerable sensitivity to start and end dates with starting dates before 1965 producing overall warming and starting dates from 1966 to 1982 producing net cooling rates over the region.”

Incredibly, if you are interested in Antarctica temperature trends from the present back to 1982, the region has cooled. If you go from present back to 1966, the region has cooled. Like it or not, over the past four decades, and during the time of the greatest build-up of greenhouse gases, Antarctica has been cooling!

Reference:

Chapman, W.L. and J.E. Walsh. 2007. A Synthesis of Antarctic Temperatures. Journal of Climate, 20, 4096-4117.




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