Global warming changed from a scientific topic to a public concern in 1988—and it has remained a public issue ever since. That summer was unusually hot. A major drought had shriveled farmers' crops. On a scorching June day in Washington, D. C. , James E. Hansen, a scientist with the National Aeronautics and Space Administration (NASA), announced at a congressional hearing, " The greenhouse effect has been detected and it is changing our climate now." Hansen claimed that he had "99 percent confidence" that the greenhouse signal had arrived. 4 Hansen's claim was picked up by the news media, and it resonated throughout the hot summer. The ensuing discussion launched a major government program to study global warming and marked the beginning of a new level of environmental concern.

The intensity of discussion remained high because, as the 1990s progressed, it got hotter. According...