The Detection and Attribution of Human Influence on Climate*

This paper describes the field of the detection and attribution of climate change and highlights recent progress, major issues, and future directions. The attribution of global temperature variations over the past century to a combination of anthropogenic and natural influences is now well established, with the anthropogenic factors dominating. Other aspects of the climate system, including regional quantities, are increasingly being found to also show a detectable signal of human influence. Of particular interest, though, is the attribution of changes in nonmeteorological quantities, such as hydrological and ecological measures, and of changes in the risk of extreme weather events to anthropogenic emissions. Methods are being developed for tackling these two problems, but are still in the early stages. As the field gradually includes a service focus, the biggest challenges will become the integration of various approaches into an overall framework and the communication of the capabilities and limitations of that framework to the outside community.