The Earth's global mean surface temperature has warmed by about half a degree Centigrade over the last 100 years. The six warmest years this century have occurred since 1990, and eight of the next ten warmest have occurred since 1980. The amount and spatial and temporal patterns of precipitation are changing. Average sea level has increased by I 5-25 centimeters during the last 100 years. Glaciers are retreating worldwide.
Greenhouse gases warm the atmosphere, while aerosols tend to cool the atmosphere.
The balance of scientific evidence suggests a discernible human influence on the Earth's climate. Projected impact of human activities on the climate system Without globally coordinated policies to specifically address human-Induced climate change, the concentrations of greenhouse gases in the atmosphere are projected to increase significantly. Global mean surface temperatures are projected to increase by between I.0 and 3.5 degrees Centigrade by 2 I 00, a rate faster than anything observed during the last I 0,000 years. Sea levels are projected to rise by l5-95 centimeters by 2100. Global warming caused by increases in the atmospheric concentrations of greenhouse gases can only be reversed very slowly because of the century-scale atmospheric residence times of the gases and the large thermal inertia of the oceans.