Is global warming already happening?
Yes. The IPCC concluded in its Third Assessment Report, “An increasing body of observations gives a collective picture of a warming world and other changes in the climate system”. The kinds of changes already observed that create this consistent picture include the following: Examples of observed climatic changes * Increase in global average surface temperature of about 0.6 degrees Celsius in the 20th century * Decrease of snow cover and sea ice extent and the retreat of mountain glaciers in the latter half of the 20th century * Rise in global average sea level and the increase in ocean water temperatures * Likely increase in average precipitation over the middle and high latitudes of the Northern Hemisphere, and over tropical land areas * Increase in the frequency of extreme precipitation events in some regions of the world Examples of observed physical and ecological changes * Thawing of permafrost * Lengthening of the growing season in middle and high latitudes * Poleward and upward
