The Paris Agreement1 commits ratifying parties to pursue efforts to limit the global temperature increase to 1.5 °C relative to pre-industrial levels. Carbon budgets2,3,4,5 consistent with remaining below 1.5 °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 forcings8,9. 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 rate11,12. 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 °C for 2006–2015 relative to 1861–188013,14,15,16,17,18). 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 °C is undoubtedly challenging19,20,21, our results indicate it is not impossible, as might be inferred from the IPCC AR5 carbon budgets2,8.
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The authors thank M. Berkley for assistance with data acquisition, V. K. Arora and V. Kharin for providing comments on the initial version of the manuscript, and M. Eby, A. P. Schurer and A. R. Friedman for helpful discussions. The authors acknowledge support from the Natural Sciences and Engineering Research Council of Canada (NSERC) Discovery Grant Program and the UK Natural Environment Research Council SMURPHS project (grant no. NE/N006143/1). The authors acknowledge the World Climate Research Programme’s Working Group on Coupled Modelling, which is responsible for CMIP, and thank the climate modelling groups for producing and making available their model output. For CMIP the US Department of Energy’s Program for Climate Model Diagnosis and Intercomparison provides coordinating support and led development of software infrastructure in partnership with the Global Organization for Earth System Science Portals. The authors acknowledge Met Office Hadley Centre for providing observational HadCRUT4 data sets, K. Cowtan and R. Way for the filled-in HadCRUT4 data set, the Berkeley Earth Surface Temperature data set, NOAA/OAR/ESRL PSD for providing the NASA(GISS/GISTEMP and NOAAGlobalTemp) global surface temperature data.
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Nature Geoscience (2018)