Quantifying the Sources of Intermodel Spread in Equilibrium Climate Sensitivity

Peter M. Caldwell Lawrence Livermore National Lab, Livermore, California

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Mark D. Zelinka Lawrence Livermore National Lab, Livermore, California

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Karl E. Taylor Lawrence Livermore National Lab, Livermore, California

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Kate Marvel Lawrence Livermore National Lab, Livermore, California

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Abstract

This study clarifies the causes of intermodel differences in the global-average temperature response to doubled CO2, commonly known as equilibrium climate sensitivity (ECS). The authors begin by noting several issues with the standard approach for decomposing ECS into a sum of forcing and feedback terms. This leads to a derivation of an alternative method based on linearizing the effect of the net feedback. Consistent with previous studies, the new method identifies shortwave cloud feedback as the dominant source of intermodel spread in ECS. This new approach also reveals that covariances between cloud feedback and forcing, between lapse rate and longwave cloud feedbacks, and between albedo and shortwave cloud feedbacks play an important and previously underappreciated role in determining model differences in ECS. Defining feedbacks based on fixed relative rather than specific humidity (as suggested by Held and Shell) reduces the covariances between processes and leads to more straightforward interpretations of results.

Corresponding author address: Peter M. Caldwell, L-103, Lawrence Livermore National Laboratory, P.O. Box 808, Livermore, CA 94566. E-mail: caldwell19@llnl.gov

Current affiliation: NASA Goddard Institute for Space Studies, New York, New York.

Abstract

This study clarifies the causes of intermodel differences in the global-average temperature response to doubled CO2, commonly known as equilibrium climate sensitivity (ECS). The authors begin by noting several issues with the standard approach for decomposing ECS into a sum of forcing and feedback terms. This leads to a derivation of an alternative method based on linearizing the effect of the net feedback. Consistent with previous studies, the new method identifies shortwave cloud feedback as the dominant source of intermodel spread in ECS. This new approach also reveals that covariances between cloud feedback and forcing, between lapse rate and longwave cloud feedbacks, and between albedo and shortwave cloud feedbacks play an important and previously underappreciated role in determining model differences in ECS. Defining feedbacks based on fixed relative rather than specific humidity (as suggested by Held and Shell) reduces the covariances between processes and leads to more straightforward interpretations of results.

Corresponding author address: Peter M. Caldwell, L-103, Lawrence Livermore National Laboratory, P.O. Box 808, Livermore, CA 94566. E-mail: caldwell19@llnl.gov

Current affiliation: NASA Goddard Institute for Space Studies, New York, New York.

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