ONE effect of global warming will be to accelerate the decomposition of soil organic matter, thereby releasing CO2 to the atmosphere, which will further enhance the warming trend1–7. Such a feedback mechanism could be quantitatively important, because CO2 is thought to be responsible for ∼55% of the increase in radiative forcing arising from anthropogenic emissions of gases to the atmosphere8, and there is about twice as much carbon in the top metre of soil as in the atmosphere9. Here we use the Rothamsted model for the turnover of organic matter in soil3 to calculate the amount of CO2 that would be released from the world stock of soil organic matter if temperatures increase as predicted, the annual return of plant debris to the soil being held constant. If world temperatures rise by 0.03 °C yr−1 (the increase considered as most likely by the Intergovernmental Panel on Climate Change8), we estimate that the additional release of CO2 from soil organic matter over the next 60 years will be 61 × 1015 gC. This is ∼19% of the CO2 that will be released by combustion of fossil fuel during the next 60 years if present use of fuel continues unabated.