Cumulative carbon emissions budgets consistent with 1.5 degrees C global warming

The Paris Agreement(1) commits ratifying parties to pursue efforts to limit the global temperature increase to 1.5 degrees C relative to pre-industrial levels. Carbon budgets(2-5) consistent with remaining below 1.5 degrees C warming, reported in the IPCC Fifth Assessment Report (AR5)(2,6,8), are directly based on Earth system model (Coupled Model Intercomparison Project Phase 5)(7) responses, which, on average, warm more than observations in response to historical CO2 emissions and other forcings(8,9). These models indicate a median remaining budget of 55 PgC (ref. 10, base period: year 1870) left to emit from January 2016, the equivalent to approximately five years of emissions at the 2015 rate(11,12). Here we calculate warming and carbon budgets relative to the decade 2006-2015, which eliminates model-observation differences in the climate-carbon response over the historical period9, and increases the median remaining carbon budget to 208 PgC (33-66% range of 130-255 PgC) from January 2016 (with mean warming of 0.89 degrees C for 2006-2015 relative to 1861-188013-18). There is little sensitivity to the observational data set used to infer warming that has occurred, and no significant dependence on the choice of emissions scenario. Thus, although limiting median projected global warming to below 1.5 degrees C is undoubtedly challenging(19-21), our results indicate it is not impossible, as might be inferred from the IPCC AR5 carbon budgets(2,8).