An examination of anthropogenic climate forcing in the 21st century: greenhouse gases and aerosols - direct and indirect

The projection of future climate change - and its consequent impact on societies and individuals - depends not only on society´s direct emissions of greenhouse gases and aerosols, but also on the Earth system´s response to these changes insofar as they additionally alter the radiative forcing of climate. These feedbacks involve coupling across the physical climate and biogeochemical systems, and they are often also designated indirect effects. This report examines some of the more well identified and quantified indirect effects related to atmospheric composition, as well as those that, although identified, remain highly uncertain. Although the unpredictability of anthropogenic emissions of greenhouse agents over the 21st century dominates the uncertainty of future climate, these feedbacks are a major contribution; and the big question is how such global feedbacks could be detected and quantified. The detection of feedbacks, even from an integrated Earth observing system, is a major challenge. This report is based in large part on results from the IPCC/TAR and immediate follow-on studies, and this rapporteur thanks the many lead and contributing authors to the TAR.