Not so long ago, people talked about global warming in apocalyptic terms—imagining the Statue of Liberty up to its chin in water or an onslaught of tropical diseases in Oslo. Recently, however, advances in our understanding of climate have moved global warming from a subject for a summer disaster movie to a serious but manageable scientific and policy issue. The greenhouse effect is nothing new; it has been operating ever since the earth formed. Without it, the surface of the globe would be a frigid –20 ℃ (–4 °F), the oceans would have frozen, and no life would have developed.