Prospects for a prolonged slowdown in global warming in the early 21st century.

Thomas R. Knutson,Larry W. Horowitz,Rong Zhang

doi:10.1038/ncomms13676

Thomas R. Knutson, Larry W. Horowitz + Show 1 more

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https://doi.org/10.1038/ncomms13676

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Abstract

Global mean temperature over 1998 to 2015 increased at a slower rate (0.1 K decade−1) compared with the ensemble mean (forced) warming rate projected by Coupled Model Intercomparison Project 5 (CMIP5) models (0.2 K decade−1). Here we investigate the prospects for this slower rate to persist for a decade or more. The slower rate could persist if the transient climate response is overestimated by CMIP5 models by a factor of two, as suggested by recent low-end estimates. Alternatively, using CMIP5 models' warming rate, the slower rate could still persist due to strong multidecadal internal variability cooling. Combining the CMIP5 ensemble warming rate with internal variability episodes from a single climate model—having the strongest multidecadal variability among CMIP5 models—we estimate that the warming slowdown (<0.1 K decade−1 trend beginning in 1998) could persist, due to internal variability cooling, through 2020, 2025 or 2030 with probabilities 16%, 11% and 6%, respectively.

Highlights

Global mean temperature over 1998 to 2015 increased at a slower rate (0.1 K decade À 1) compared with the ensemble mean warming rate projected by Coupled Model Intercomparison Project 5 (CMIP5) models (0.2 K decade À 1)
We focus on internal variability simulated by the Geophysical Fluid Dynamics Laboratory (GFDL) CM3 model control run, which simulates the strongest multidecadal internal variability among the CMIP5 models we examined
We find that there are some conditions under which a continued global warming slowdown could occur over the decade or more, due to either an overestimate of the transient climate response (TCR) by CMIP5 models, or due to a partial offsetting of a relatively strong forced warming signal by an internal variability cooling episode lasting up to a few decades

Summary

Introduction

Global mean temperature over 1998 to 2015 increased at a slower rate (0.1 K decade À 1) compared with the ensemble mean (forced) warming rate projected by Coupled Model Intercomparison Project 5 (CMIP5) models (0.2 K decade À 1). Several generations of climate models, as cited in the last three Intergovernmental Panel on Climate Change Assessment Reports[1,5,6] have produced a hiatus around 1940–1970 in response to a combination of anthropogenic (greenhouse gases, aerosols, etc.) and natural (volcanic eruptions, solar variability) forcing agents Those experiments suggest that the observed 1941–1973 hiatus and late 20th century warming result predominantly from external forcing. We find that there are some conditions under which a continued global warming slowdown could occur over the decade or more, due to either an overestimate of the TCR by CMIP5 models, or due to a partial offsetting of a relatively strong forced warming signal (as simulated by CMIP5 models) by an internal variability cooling episode lasting up to a few decades

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