The uncertainties are great but so are the certainties. Venus and Mars show what too much, or too little, greenhouse gas can do to the possibility of life. Carbon dioxides ability to absorb infra-red radiation can be measured. But the analysis of how much warming, what changes in climate, what impact on agriculture, on health and comfort, how well different countries can adapt, is still in progress; too little is known to predict what concentration will be too much, so no global rationing scheme is likely. Certainly the worst climate impacts will be on the rural poor; economic development is an important defense. An ambitious program of research, development, and exploration for new economical energy sources and locations for carbon sequestration is urgently needed. The already-developed nations will have to provide financial and technological assistance to the less developed.