Caltech Michelin Lecture: "But it is impossible to ignore how closely the history of global warming fits on the previous template for nuclear winter. Just as the earliest studies of nuclear winter stated that the uncertainties were so great that probabilites could never be known, so, too the first pronouncements on global warming argued strong limits on what could be determined with certainty about climate change. The 1995 IPCC draft report said, 'Any claims of positive detection of significant climate change are likely to remain controversial until uncertainties in the total natural variability of the climate system are reduced.' It also said, 'No study to date has positively attributed all or part of observed climate changes to anthropogenic causes.' Those statements were removed, and in their place appeared: 'The balance of evidence suggests a discernable human influence on climate.' "