How much observed climate change is man-made?

That human activities influence global climate is beyond doubt, but the level of this anthropogenic contribution to climate change remains uncertain. In a paper published today in the journal Nature Climate Change, Potsdam-based climate scientist Gerrit Hansen and her Berkeley Lab colleague Dáithí Stone present a systematic assessment of anthropogenic climate change for the range of impacts of regional trends reported in the Fifth Assessment Report of the United Nations Intergovernmental Panel on Climate Change (IPCC).

Applying a statistical algorithm that relates observational data with climate model outputs, Hansen and Stone investigate the degree to which atmospheric carbon dioxide emissions are a necessary condition for simulations to reproduce observed climate trends. With this procedure, the researchers find that almost two-thirds of the impacts related to atmosphere and ocean temperature can be attributed to human effects.

Hansen and Stone go on to report that anthropogenic climate change has been a major influence for approximately three-quarters of the impacts observed on continental scales. The effects of carbon emissions can thus be seen not only worldwide, but also at regional and local scales.

Hansen and Stone’s analysis is significant, as it provides a missing link between the effects of observed climate change as reported by the IPCC, and the role of anthropogenic climate forcing for the regional trends related to these observations.