When I served in 1995 on the Intergovernmental Panel on Climate Change, the scientific group that periodically assesses the science of global warming, there was overwhelming evidence that the concentration of greenhouse gases in the atmosphere had increased markedly since the beginning of the industrial revolution, that human activity had contributed significantly to those increases, and that they would have profound effects on climate and sea levels. But few saw, for instance, the Artic ice cap melting as rapidly as now seems to be the case.