20th Century Climate Change
There is now considerable evidence that indicates a relationship between the man-made enhancement of the Earth's natural greenhouse effect through greenhouse gas pollution of the atmosphere and the global warming that has been observed.
Measurements of surface temperature recorded around the world during the last 150 years indicate that global temperatures are now higher than in any decade over this period. During the 20th century a global average surface temperature increase of about 0.6°C years has taken place, although the warming trend has not been smooth and has taken place rather differently between the Northern and Southern Hemispheres. With this in mind, it is important to recognise that global average surface temperature, as a measure of the global climate, represents an over-simplification. Winter temperatures and night-time minimums for example, may have risen more than summer temperatures and daytime maximums.
With higher global temperatures one would expect an increase in rainfall and other forms of precipitation, because of the greater amount of moisture available within the atmosphere. Striking changes in precipitation have occurred on regional scales, most notably the drought in the African Sahel between the 1960s and 1980s. Nevertheless, the accuracy of many precipitation records should be treated with caution. Precipitation is more difficult to monitor than temperature due to its greater geographical variability.
Variations in land- and sea-ice coverage and the melting or growth of glaciers occur in response to changes in temperature, sunshine, precipitation, and for sea-ice changes in wind. Since 1966 Northern Hemisphere snow cover maps have been produced by the United States using satellite imagery. Consistent with the surface and tropospheric temperature measurements is the decrease (by about 10%) in snow cover and extent since the late 1960s. There has been a widespread retreat of mountain glaciers in non-polar regions during the 20th century. Variations in sea-ice extent have also been reported, with spring and summer sea-ice extent in the Northern Hemisphere decreasing by between 10 an 15% since the 1950s. Considerable interest is now focusing on Antarctica, where regional warming, as predicted by climate models, has been more rapid than global warming as a whole. In recent years the summertime disintegration of the Larsen Sea Ice Shelf adjacent the Antarctic continent has been occurring on an unprecedented scale. In view of the rapidity at which it is taking place, such an event has been viewed as a possible signal of global warming.
Increased global cloudiness, as for increased global evaporation and precipitation, would be an expected consequence of higher global temperatures. It is likely that there has been a 2% increase in cloud cover over mid - to high latitude land areas during the 20th century. In most areas the trends relate well to the observed decrease in daily temperature range (since cloudier nights tend to be warmer and cloudier days cooler).