Two reasons to be concerned about climate change are its unjust distributional impact and its negative aggregate effect on economic growth and welfare. Although our knowledge of the impact of climate change is incomplete and uncertain, economic valuation is difficult and controversial, and the effect of other developments on the impacts of climate change is largely speculative, the authors find that poorer countries and people are more vulnerable than are richer countries and people. A modest global warming is likely to have a net negative effect on poor countries in hot climates, but may have a net positive effect on rich countries in temperate climates. If one counts dollars, the world aggregate may be positive. If one counts people, the world aggregate is probably negative. Negative impacts would become more negative, and positive impacts would turn negative for more substantial warming. The marginal costs of carbon dioxide emissions are uncertain and sensitive to assumptions that partially reflect ethical positions, but are unlikely to be larger than $50 per tonne carbon.