Assessment of polar climate change using satellite technology

Results from general circulation models (GCMs) have indicated that a predicted climate warming resulting from an increase in atmospheric carbon dioxide (CO 2 ) will be amplified in the polar regions and that temperature increases in the polar regions will be several times greater than the global average. Some GCMs predict a 4°–5°C average air temperature increase in the Arctic by the middle of the next century. Evidence from the polar regions indicates that a warming in the cryosphere may already be in progress. A 2°–4°C rise in permafrost temperature, measured in northern Alaska, is believed to have occurred during the last 100 years. In addition, many small valley glaciers and ice caps have experienced retreat and appear to have contributed up to 50% to the observed rise (10–15 cm) in sea level during the last century. Other work shows that increased snowfall which can be associated with warmer temperatures has caused thickening of some Alaskan glaciers. Though a decrease in snow and sea ice cover would be a likely consequence of global warming, a sustained decrease in global snow and sea ice extent has not been found from analysis of more than 20 years of image data (1.1‐km pixel resolution) from National Oceanic and Atmospheric Administration meteorological satellites and more than 7 years of scanning multichannel microwave radiometer snow data (30‐km pixel resolution) on the Nimbus 7 satellite. Snow and sea ice are sensitive to atmospheric temperature changes because of their large surface to volume ratio. While measurements of snow and sea ice extent, snow depth, and sea ice concentration are possible using visible, near‐infrared, or microwave sensors on satellites, it is not feasible to measure the mass balance of the ice sheets with these sensors directly. Estimates by glaciologists show that the Greenland Ice Sheet is in approximate equilibrium and that the Antarctic Ice Sheet has a positive mass balance. Satellite radar altimetry (and in the future, laser altimetry) is a promising technique for measuring the surface elevation of ice sheets. Satellite‐borne laser altimetry in conjunction with imagery on ice sheet extent will permit direct measurements of changes in mass balance of the ice sheets through time. The terminus positions and ablation area boundaries of valley glaciers are indicative of glacier mass balance; these can be studied using visible and near‐infrared data from the Landsat satellite series and data from the French Systeme Probatoire d'Observation de la Terre (SPOT) satellite and synthetic aperture radar data. Lake ice freeze‐up and breakup dates are sensitive to regional air temperature and may also be good indicators of climate trends. Monitoring the onset of lake freeze‐up and breakup dates is feasible with radar and visible image data. The very important role of snow and ice in global processes is being highlighted as large‐scale, satellite‐derived geophysical data sets have become available and are beginning to be used as realistic input to GCMs.