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 1°F in the 20^th century
  
• Decrease of snow cover and sea ice extent and the retreat of mountain glaciers in the latter half of the 20^th 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 shift of plant and animal ranges
  
• Decline of some plant and animal species
  
• Earlier flowering of trees
  
• Earlier emergence of insects
  
• Earlier egg-laying in birds