Broad range of 2050 warming from an observationally constrained large climate model ensemble

Incomplete understanding of three aspects of the climate system—equilibrium climate sensitivity, rate of ocean heat uptake and historical aerosol forcing—and the physical processes underlying them lead to uncertainties in our assessment of the global-mean temperature evolution in the twenty-first century 1,2 . Explorations of these uncertainties have so far relied on scaling approaches 3,4 , large ensembles of simplified climate models 1,2 , or small ensembles of complex coupled atmosphere‐ocean general circulation models 5,6 which under-represent uncertainties in key climate system properties derived from independent sources 7‐9 . Here we present results from a multi-thousand-member perturbed-physics ensemble of transient coupled atmosphere‐ocean general circulation model simulations. We find that model versions that reproduce observed surface temperature changes over the past 50 years show global-mean temperature increases of 1.4‐3 K by 2050, relative to 1961‐1990, under a mid-range forcing scenario. This range of warming is broadly consistent with the expert assessment provided by the Intergovernmental Panel on Climate Change Fourth Assessment Report 10 , but extends towards larger warming than observed in ensemblesof-opportunity 5 typically used for climate impact assessments. From our simulations, we conclude that warming by the middle of the twenty-first century that is stronger than earlier estimates is consistent with recent observed temperature changes and a mid-range ‘no mitigation’ scenario for greenhouse-gas emissions. In the latest generation of coupled atmosphereocean general circulation models (AOGCMs) contributing to the Coupled Model Intercomparison Project phase 3 (CMIP-3), uncertainties in key propertiescontrollingthetwenty-firstcenturyresponsetosustained anthropogenic greenhouse-gas forcing were not fully sampled, partially owing to a correlation between climate sensitivity and aerosol forcing 7,8 , a tendency to overestimate ocean heat uptake 11,12 and compensation between short-wave and long-wave feedbacks 9 . This complicates the interpretation of the ensemble spread as a direct uncertainty estimate, a point reflected in the fact that the ‘likely’ (>66% probability) uncertainty range on the transient response was explicitly subjectively assessed as40% to C60% of