claim: A paper by Ross McKitrick, an economics professor at the  University of Guelph, and Patrick Michaels, an environmental studies  professor at the University of Virginia, concludes that half of the  global warming trend from 1980 to 2002 is caused by Urban Heat Island.
evidence: Consequently, summers are 2.3 °C (4 °F) warmer in the Northern Hemisphere than in the Southern Hemisphere under similar conditions.