Degrees of difference.

This article, the third of a series on the Copenhagen Consensus project, examines climate change. Global warming looms, in many people's minds, as one of the biggest threats facing the planet. Over the past 20 years researchers have gathered evidence that the burning of fossil fuels is causing temperatures to rise. However, the exact pace of global warming, as well as the size of mankind's contribution to the warming trend, remain uncertain. Aside from these issues is the question of precisely how greenhouse-gas emissions should be abated, assuming that they need to be reduced at all. In a new paper for the Copenhagen Consensus project, William Cline of the Centre for Global Development and the Institute for International Economics examines these topics. Rising temperatures are capable of causing great economic harm--though a lot depends not just on how big future rises prove to be but also on how quickly they happen.