evidence: Global warming skeptics often cite contradictory reports from a generation ago warning of global cooling. In 1975 Newsweek wrote of "ominous signs" that temperatures were dipping, and a year later National Geographic suggested the possibility of a worldwide chilling trend. Stephen Schneider, a climatologist at Stanford University, recalls those stories well. "I was one of the ones who talked about global cooling," he says. "I was also the one who said what was wrong with that idea within three years." Schneider coauthored a 1971 article in the journal Science about atmospheric aerosols—floating particles of soil dust, volcanic ash, and human-made pollutants. His research suggested that industrial aerosols could block sunlight and reduce global temperatures enough to overcome the effects of greenhouse gases, possibly triggering an ice age. But he soon realized that he had overestimated the amount of aerosols in the air and underestimated the role of greenhouse gases. "Back then this science was so new, so theoretical, it was really hard to sort it out," he says. He and other early climate researchers say they did not predict a global cooling trend but simply suggested the possibility. Evidence suggests that average worldwide temperatures did decrease between the 1940s and the 1970s. Some climatologists partially attribute the temporary cooling trend to industrial smog, which has since been overcome by the effects of growing greenhouse emissions and, ironically, by clean-air laws that have reduced atmospheric particulates. "Science is a self-correcting institution," Schneider says. "The data change, so of course you change your position. Otherwise, you would be dishonest."