Observed Shortterm climate changes and Their Effects

Instrumental and ice core records show that several components of the atmosphere and surface have significant changes in recorded climate history. First, the concentration of greenhouse gases such as carbon dioxide has increased dramatically since 1850, causing an increase in the atmospheric absorption of outgoing radiation, warming the atmosphere. Aerosols, which are microscopic droplets or airborne particles, have also increased, and these have the effect of reflecting and absorbing incoming solar radiation.

The most obvious change to the short-term climate is the increase in temperature of the atmosphere and sea surface. Eleven of the 12 years between 1995 and 2006 rank among the hottest on record since instrumental records were in widespread use since 1850, with the rate of temperature rise increasing each interval since 1850. The total temperature increase since 1850 is estimated by the Intergovernmental Panel on Climate Change to be 1.4°F (0.76°C). Atmospheric water vapor has been measured to be increasing with increasing temperature of the atmosphere, although measurements of water vapor only extend back to about the middle 1980s.

Sea level has been rising at about .07 inches per year (.18 cm/yr.) since 1961, and at .12 inches per year (.31 cm/yr.) since 1993. The temperature of the oceans to a depth of 1.9 miles (3 km) has been increasing

Cornfield blighted by El Nino drought in Denton, Texas, July 22, 1998 (CORBIS)

since at least 1961, and most (~80 percent) of the heat energy associated with global warming is being absorbed by seawater. This increase in temperature of the seawater is causing the water to expand, contributing to sea level rise. Also contributing to sea level rise is a dramatic melting of mountain glaciers in both the Northern and Southern Hemispheres. Changes in the ice caps on Greenland and Antarctica show an increase in 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 globally 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 estimated to be about 7 percent. Permafrost locks a huge amount of peat and carbon into a closed system, so there are fears that 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 discernable. Much of the Antarctica region is isolated from other parts of the global climate belt, so overall, 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 2005 show long-term drying and potential desertification over parts of the sub-Saharan Sahel, the Mediterranean region, parts of southern Asia, and 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 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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