Global environmental change threatens the habitability of the planet and the health of its inhabitants. Toxic pollution of air and water, acid rain, destruction of stratospheric ozone, waste, species extinction and, potentially, global warming are produced by the growing numbers and activities of human beings. Progression of these environmental changes could lead to unprecedented human suffering. Physicians can treat persons experiencing the consequences of environmental change but cannot individually prevent the cause of their suffering. Physicians have information and expertise about environmental change that can contribute to its slowing or prevention. Work to prevent global environmental change is consistent with the social responsibility of physicians and other health professionals.