Determining whether global warming is happening or not may seem to be a simple problem: simply measure temperature changes over time. What has tended to be more problematic is determining whether warming has been caused by humans or whether it was occurring naturally, or some combination of the two. It turns out that much of the climate data we have may be wildly inaccurate with a bias towards warming. Why? Simply put, weather monitoring stations have been located in areas that violate the most basic needs of accurate measurement.
According to Anthony Watts, a veteran meteorologist from Chico, California:
Far more stations fail to meet prescribed standards than are properly maintained. Using a scale from 1 to 5, by which stations that are properly maintained receive a rating of 1 and stations that are severely compromised by artificial temperature signatures (being located adjacent to an artificial heating source, such as a building, rooftop, parking lot, or concrete surface, for example) receive a 5 while fully 70 percent of official temperature stations receive a 4 or 5 rating, and only 4 percent receive a 1.
For more information on Watts and his work to ensure that weather monitoring stations are as accurate as possible, go to http://www.surfacestations.org/.