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UC Berkeley physicist Richard Muller and others were looking at the so-called urban heat island effect - the notion that because more urban temperature stations are included in global temperature data sets than are rural ones, the global average temperature was being skewed upward because these sites tend to retain more heat. Hence, global warming trends are exaggerated. Using data from such urban heat islands as Tokyo, they hypothesized, could introduce "a severe warming bias in global averages using urban stations." In fact, the data trend was "opposite in sign to that expected if the urban heat island effect was adding anomalous warming to the record. The small size, and its negative sign, supports the key conclusion of prior groups that urban warming does not unduly bias estimates of recent global temperature change."