The greenhouse effect will bring large climate changes, possibly a hostile environment, and social consequences that will range from business as usual to disaster. At present uncertainty dominates forecasts of both the climate change and derivative ecosystem and social consequences. Policy analysis must proceed in the midst of vast uncertainty, seeking programs that are unlikely to be harmful or costly if the greenhouse consequences are more benign than predicted and likely to help if the worst happens. The worst consequences are likely to occur to unmanaged ecosystems and to developing countries; in contrast, the industrial nations may experience only minor irritations, apart from the eventual rise in sea level.