Human health may not remain sustainable if damage to the global environment continues. The argument is simple: Earth is essentially a closed system; humans are proliferating and commandeering more surface area, food and energy; the resultant accumulation of waste gases, depletion of soil and water, and loss of biodiversity is starting to overload Earth's carrying capacity. There are limits in any closed system and our species is now pressing against some of them. These are new problems and we cannot be certain of the consequences for human health. A warmer world will probably have more frequent heatwaves, unstable weather, increased spread of mosquito-borne infectious diseases, and disruptions to agriculture. Ozone depletion, if sustained, may cause moderate increases in skin cancer and cataracts, and may damage crop growth and marine stocks. Depletion of agricultural resources, overfishing, and loss of genetic resources from species extinction all entail potentially serious consequences for human health. The manifest uncertainties of these global change processes and the need for prediction, rather than empirical observation, create new challenges to health scientists. Likewise, policy-makers will have to deal with best estimates and long time-frames, informed by understanding of ecological realities.