LETTERS Australian climate–carbon cycle feedback reduced by soil black carbon

Annual emissions of carbon dioxide from soil organic carbon are an order of magnitude greater than all anthropogenic carbon dioxide emissions taken together 1. Global warming is likely to increase the decomposition of soil organic carbon, and thus the release of carbon dioxide from soils 2–5, creating a positive feedback 6–9. Current models of global climate change that recognize this soil carbon feedback are inaccurate if a larger fraction of soil organic carbon than postulated has a very slow decomposition rate. Here we show that by including realistic stocks of black carbon in prediction models, carbon dioxide emissions are reduced by 18.3 and 24.4 % in two Australian savannah regions in response to a warming of 3 ◦ C over 100 years 1. This reduction in temperature sensitivity, and thus the magnitude of the positive feedback, results from the long mean residence time of black carbon, which we estimate to b