High Earth-system climate sensitivity determined from Pliocene carbon dioxide concentrations

Climate sensitivity-the mean global temperature response to a doubling of atmospheric CO2 concentrations through radiative forcing and associated feedbacks-is estimated at 1.5-4.5 degrees C (ref. 1). However, this value incorporates only relatively rapid feedbacks such as changes in atmospheric water vapour concentrations, and the distributions of sea ice, clouds and aerosols(2). Earth-system climate sensitivity, by contrast, additionally includes the effects of long-term feedbacks such as changes in continental ice-sheet extent, terrestrial ecosystems and the production of greenhouse gases other than CO2. Here we reconstruct atmospheric carbon dioxide concentrations for the early and middle Pliocene, when temperatures were about 3-4 degrees C warmer than preindustrial values(3-5), to estimate Earth-system climate sensitivity from a fully equilibrated state of the planet. We demonstrate that only a relatively small rise in atmospheric CO2 levels was associated with substantial global warming about 4.5 million years ago, and that CO2 levels at peak temperatures were between about 365 and 415 ppm. We conclude that the Earth-system climate sensitivity has been significantly higher over the past five million years than estimated from fast feedbacks alone.