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Beyond equilibrium climate sensitivity

Nature Geoscience volume 10, pages 727–736 (2017)Cite this article

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Abstract

Equilibrium climate sensitivity characterizes the Earth's long-term global temperature response to increased atmospheric CO2 concentration. It has reached almost iconic status as the single number that describes how severe climate change will be. The consensus on the 'likely' range for climate sensitivity of 1.5 °C to 4.5 °C today is the same as given by Jule Charney in 1979, but now it is based on quantitative evidence from across the climate system and throughout climate history. The quest to constrain climate sensitivity has revealed important insights into the timescales of the climate system response, natural variability and limitations in observations and climate models, but also concerns about the simple concepts underlying climate sensitivity and radiative forcing, which opens avenues to better understand and constrain the climate response to forcing. Estimates of the transient climate response are better constrained by observed warming and are more relevant for predicting warming over the next decades. Newer metrics relating global warming directly to the total emitted CO2 show that in order to keep warming to within 2 °C, future CO2 emissions have to remain strongly limited, irrespective of climate sensitivity being at the high or low end.

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Figure 1: Overview of published best estimates and ranges for the transient climate response constrained by different lines of evidence.
Figure 2: Overview of published best estimates and ranges for equilibrium climate sensitivity constrained by different lines of evidence.
Figure 3: Overview of published best estimates and ranges for equilibrium climate sensitivity constrained by different lines of evidence.
Figure 4: Illustration of feedbacks changing in result to various boundary conditions.
Figure 5: Illustrative example of combining multiple constraints for climate sensitivity.

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Acknowledgements

R.K. acknowledges support by the European Union's Horizon 2020 research and innovation program under grant agreement 641816 (CRESCENDO), and by NCAR and the Regional and Global Climate Modeling Program (RGCM) of the US Department of Energy, Office of Science (BER), Cooperative Agreement DE-FC02-97ER62402. The National Center for Atmospheric Research is sponsored by the National Science Foundation. G.C.H. was supported by the ERC funded project TITAN (EC-320691), by the Wolfson Foundation and the Royal Society as a Royal Society Wolfson Research Merit Award (WM130060) holder, and by the NERC-funded SMURPHS project. We acknowledge the World Climate Research Programme's Working Group on Coupled Modelling, which is responsible for CMIP, and we 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.

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Authors and Affiliations

  1. Institute for Atmospheric and Climate Science, ETH Zurich, Zurich, CH-8092, Switzerland

    Reto Knutti & Maria A. A. Rugenstein

  2. National Center for Atmospheric Research, Boulder, 80307, Colorado, USA

    Reto Knutti

  3. School of Geosciences, University of Edinburgh, Edinburgh, EH9 3FE, UK

    Gabriele C. Hegerl

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All authors wrote the Review. M.A.A.R. produced Figs 1, 2, 3, 4. R.K. produced Fig. 5.

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Correspondence to Reto Knutti.

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Knutti, R., Rugenstein, M. & Hegerl, G. Beyond equilibrium climate sensitivity. Nature Geosci 10, 727–736 (2017). https://doi.org/10.1038/ngeo3017

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