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Arctic sea ice extent for September 2011 was 4.61 million km2 (1.78 million square miles). The magenta line shows the 1979 to 2000 median extent for that month. The black cross indicates the geographic North Pole. (Image courtesy of the National Snow and Ice Data Center, University of Colorado, Boulder, Colorado, US)

Source: http://nsidc.org/arcticseaicenews (website of the National Snow and Ice Data Center, Boulder, Colorado, US)

Moreover, rising levels of greenhouse gases and the loss of stratospheric ozone appear to be affecting wind patterns around Antarctica. Shifts in this circulation are referred to as the Antarctic Oscillation (AAO). As greenhouse gases have increased, and especially when ozone is lost in spring, there is a tendency for these winds to strengthen (a positive AAO index). The net effect is to push sea ice eastward and northward, increasing the ice extent. As the current sea ice anomaly has developed, the AAO index has been strongly positive. (See the National Oceanic and Atmospheric Administration (NOAA) AAO Index, www.noaa.gov.) For more information about the differences between sea ice dynamics in the Arctic and Antarctic, see the National Snow and Ice Date Center (NSIDC), www.nsidc.org/seaice.