James E. Hansen wiped sweat from his brow. Outside it was a record-high 98 degrees on June 23, 1988, as the NASA scientist testified before the Senate Committee on Energy and Natural Resources during a prolonged heat wave, which he decided to cast as a climate event of cosmic significance. He expressed to the senators his “high degree of confidence” in “a cause-and-effect relationship between the greenhouse effect and observed warming.”

With that testimony and an accompanying paper in the Journal of Geophysical Research, Mr....