Abstract
Climate risks increase with mean global temperature1, so knowledge about the amount of future global warming should better inform risk assessments for policymakers. Expected near-term warming is encapsulated by the transient climate response (TCR), formally defined as the warming following 70 years of 1% per year increases in atmospheric CO2 concentration, by which point atmospheric CO2 has doubled. Studies based on Earth’s historical energy budget have typically estimated lower values of TCR than climate models, suggesting that some models could overestimate future warming2. However, energy-budget estimates rely on historical temperature records that are geographically incomplete and blend air temperatures over land and sea ice with water temperatures over open oceans. We show that there is no evidence that climate models overestimate TCR when their output is processed in the same way as the HadCRUT4 observation-based temperature record3,4. Models suggest that air-temperature warming is 24% greater than observed by HadCRUT4 over 1861–2009 because slower-warming regions are preferentially sampled and water warms less than air5. Correcting for these biases and accounting for wider uncertainties in radiative forcing based on recent evidence, we infer an observation-based best estimate for TCR of 1.66 °C, with a 5–95% range of 1.0–3.3 °C, consistent with the climate models considered in the IPCC 5th Assessment Report.
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Acknowledgements
M.R. is funded by the Cloudsat and OCO-2 projects. The research described in this paper was performed at the Jet Propulsion Laboratory, California Institute of Technology, sponsored by NASA. E.H. is funded by the UK Natural Environment Research Council and the National Centre for Atmospheric Science. We thank Piers Forster for providing support regarding CMIP5 radiative forcing time series and R. Knutti, P. Jacobs and P. Kalmus for substantial helpful comments. M.R. thanks G. Stephens for advisory support and helpful scientific discussions.
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M.R. performed the main analysis, produced the figures and drafted the article. K.C. provided code for temperature reconstruction methods and performed sensitivity tests. E.H. provided input on experimental design and helped write the article, M.B.S. provided input on experimental design, helped write the article and performed sensitivity tests.
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Richardson, M., Cowtan, K., Hawkins, E. et al. Reconciled climate response estimates from climate models and the energy budget of Earth. Nature Clim Change 6, 931–935 (2016). https://doi.org/10.1038/nclimate3066
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DOI: https://doi.org/10.1038/nclimate3066
