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: For example, Ross McKitrick and Patrick J. Michaels conducted a statistical study of surface-temperature data regressed against socioeconomic indicators, and concluded that about half of the observed warming trend ( for 1979–2002 ) could be accounted for by the residual UHI effects in the corrected temperature data set they studied—which had already been processed to remove the ( modeled ) UHI contribution.