The climate response to five trillion tonnes of carbon

Concrete actions to curtail greenhouse gas emissions have so far been limited on a global scale(1), and therefore the ultimate magnitude of climate change in the absence of further mitigation is an important consideration for climate policy(2). Estimates of fossil fuel reserves and resources are highly uncertain, and the amount used under a business-as-usual scenario would depend on prevailing economic and technological conditions. In the absence of global mitigation actions, five trillion tonnes of carbon (5 EgC), corresponding to the lower end of the range of estimates of the total fossil fuel resource(3), is often cited as an estimate of total cumulative emissions(4-6). An approximately linear relationship between global warming and cumulative CO2 emissions is known to hold up to 2 EgC emissions on decadal to centennial timescales(7-11); however, in some simple climate models the predicted warming at higher cumulative emissions is less than that predicted by such a linear relationship(8). Here, using simulations(12) from four comprehensive Earth system models(13), we demonstrate that CO2-attributable warming continues to increase approximately linearly up to 5 EgC emissions. These models simulate, in response to 5 EgC of CO2 emissions, global mean warming of 6.4-9.5 degrees C, mean Arctic warming of 14.7-19.5 degrees C, and mean regional precipitation increases by more than a factor of four. These results indicate that the unregulated exploitation of the fossil fuel resource could ultimately result in considerably more profound climate changes than previously suggested.