Graphs of global average temperature, sea level, and snow cover for the past 160 years (Data from IPCC 2007)
Northern and the Southern Hemispheres. Changes in the ice caps on Greenland and Antarctica show an increased outflow of glacial ice and meltwater, so melting of the polar ice caps is very likely contributing to the measured sea-level rise. Both of these ice caps show significant thinning, much due to increased melting, but some (especially on Greenland) due also to decreased snowfall.
Many specific regions of the planet are showing dramatic changes in response to the global average warming surface conditions. For instance, the surface temperatures measured in the Arctic have been increasing at about twice the global rate for the past 100 years, although some fluctuations on a decadal scale have been observed as well. The sea ice that covers the Arctic Ocean may be on the verge of collapse, as the sea ice thins and covers a smaller area each year. Since 1978 the Arctic sea ice has diminished in aerial extent by 2.7 percent each decade. On land in Arctic regions the thick permafrost layer is also warming, by 4-5°F (~3°C), and a total decrease in the area covered by permafrost since 1900 is estimated to be about 7 percent. Permafrost locks a huge amount of peat and carbon into a closed system, so there are fears outlined in the IPCC report that the melting of the permafrost layer may release large amounts of carbon into the atmospheric system. The sea ice around Antarctica shows greater variations on interannual scales and no longer-term trends are yet discernible. Much of the Antarctic region is isolated from other parts of the global climate belt, so overall it shows less change than northern polar regions.
Precipitation patterns across much of the planet are changing as a result of global warming. Observations from 1900 to 2007 show long-term drying and potential desertification over parts of the sub-Saharan Sahel, the Mediterranean region, parts of southern Asia, and much of southern Africa. Deeper and longer droughts have been occurring over larger areas since the 1970s, and some of these conditions can be related to changes in ocean temperature, wind patterns, and loss of snow cover. Westerly winds in the mid latitudes have become stronger in both the Northern and the Southern Hemispheres since the 1960s.
Weather extremes show an increase in frequency, including heavy precipitation events over land, as well as heat waves and extreme temperatures over land. Many studies suggest that oceanic cyclones or hurricanes may also be becoming stronger and more frequent, but some decadal variations in oceanic cyclones may also complicate determination of these trends. Most studies support an increase in tropical cyclone activity since the 1970s over the North Atlantic, and relate this to the increase in sea surface temperatures.
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