The British government recently issued the most comprehensive study to date of the economic costs and risks of global warming, and of measures that might reduce greenhouse gas emissions, in the hope of averting some of the direst consequences. Written under the leadership of Sir Nicholas Stern of the London School of Economics, who succeeded me as Chief Economist of the World Bank, the report makes clear that the question is no longer whether we can afford to do anything about global warming, but whether we can afford not to.