Natural variability, radiative forcing and climate response in the recent hiatus reconciled

Global mean surface warming over the past 15 years or so has been less than in earlier decades and than simulated by most climate models(1). Natural variability(2-4), a reduced radiative forcing(5-7), a smaller warming response to atmospheric carbon dioxide concentrations(8,9) and coverage bias in the observations(10) have been identified as potential causes. However, the explanations of the so-called 'warming hiatus' remain fragmented and the implications for long-term temperature projections are unclear. Here we estimate the contribution of internal variability associated with the El Nino/Southern Oscillation (ENSO) using segments of unforced climate model control simulations that match the observed climate variability. We find that ENSO variability analogous to that between 1997 or 1998 and 2012 leads to a cooling trend of about -0.06 degrees C. In addition, updated solar and stratospheric aerosol forcings from observations explain a cooling trend of similar magnitude (-0.07 degrees C). Accounting for these adjusted trends we show that a climate model of reduced complexity with a transient climate response of about 1.8 degrees C is consistent with the temperature record of the past 15 years, as is the ensemble mean of the models in the Coupled Model Intercomparison Project Phase 5 (CMIP5). We conclude that there is little evidence for a systematic overestimation of the temperature response to increasing atmospheric CO2 concentrations in the CMIP5 ensemble.