Inhomogeneous forcing and transient climate sensitivity

Abstract

Understanding climate sensitivity is critical to projecting climate change in response to a given forcing scenario. Recent analyses1,2,3 have suggested that transient climate sensitivity is at the low end of the present model range taking into account the reduced warming rates during the past 10–15 years during which forcing has increased markedly4. In contrast, comparisons of modelled feedback processes with observations indicate that the most realistic models have higher sensitivities5,6. Here I analyse results from recent climate modelling intercomparison projects to demonstrate that transient climate sensitivity to historical aerosols and ozone is substantially greater than the transient climate sensitivity to CO2. This enhanced sensitivity is primarily caused by more of the forcing being located at Northern Hemisphere middle to high latitudes where it triggers more rapid land responses and stronger feedbacks. I find that accounting for this enhancement largely reconciles the two sets of results, and I conclude that the lowest end of the range of transient climate response to CO2 in present models and assessments7 (<1.3 °C) is very unlikely.

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Figure 1: Comparison of transient climate response for well-mixed greenhouse gas forcing and for aerosol + ozone + land-use forcing.
Figure 2: Ratios of regional temperature responses to well-mixed greenhouse gas and inhomogeneous forcings in CMIP5 simulations.
Figure 3: Global mean temperature change estimates based on anthropogenic forcings obtained from a multi-model analysis11.

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Acknowledgements

I acknowledge the World Climate Research Programme’s Working Group on Coupled Modelling and the US Department of Energy’s Program for Climate Model Diagnosis and Intercomparison, and I thank the climate modelling groups from CMIP and the Atmospheric Chemistry and Climate Model Intercomparison Project (listed in Supplementary Table 1) for making available their model output. I thank G. Faluvegi and G. Milly for assistance with data analysis and US taxpayers and D. Considine for their support through NASA’s Modeling, Analysis and Prediction Program.

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Correspondence to Drew T. Shindell.

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Shindell, D. Inhomogeneous forcing and transient climate sensitivity. Nature Clim Change 4, 274–277 (2014). https://doi.org/10.1038/nclimate2136

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