The history of Earth has been a history of temperature change; people and animals have historically moved to better climes, richer pastures, and areas of more abundant food. The problem now is that "somewhere else" is already occupied; meanwhile, temperatures appear to be rising, with foreseeably disastrous consequences. The gases in the air which cause the greenhouse effect have made life possible; too many gases, however, and catastrophe looms. Observation, historical records, and computer models lead to the conclusion that temperatures are rising, though no one can be certain how high they will go, or when, or what will happen. Low-lying areas will be under water, of course, as ice melts and sea levels rise; diseases and predators will proliferate; rains will increase in some parts of the world, while drought strikes elsewhere and mass starvation results. (26 minutes)