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Relationship of tropospheric stability to climate sensitivity and Earth’s observed radiation budget

  1. Jonathan M. Gregoryb,c
  1. aDepartment of Meteorology, University of Reading, Reading RG6 6BB, United Kingdom;
  2. bNational Centre for Atmospheric Science–Climate, University of Reading, Reading RG6 6BB, United Kingdom;
  3. cMet Office Hadley Centre, Exeter EX1 3PB, United Kingdom
  1. Edited by Dennis L. Hartmann, University of Washington, Seattle, WA, and approved November 3, 2017 (received for review August 14, 2017)

Significance

In current climate models, the anticipated amount of warming under greenhouse gas forcing, quantified by the “effective climate sensitivity,” increases as time passes. Consequently, effective climate sensitivity values inferred from the historical record may underestimate the future warming. However, the mechanisms of this increase in effective climate sensitivity are not understood, limiting our confidence in climate model projections of future climate change. Here, we present observational and modeling evidence that the magnitude of effective climate sensitivity partly depends on the evolution of the vertical profile of atmospheric warming. In climate models, as the Earth warms overall, the warming becomes increasingly muted aloft, and this alters the strength of feedbacks controlling the radiative response to greenhouse gas forcing.

Abstract

Climate feedbacks generally become smaller in magnitude over time under CO2 forcing in coupled climate models, leading to an increase in the effective climate sensitivity, the estimated global-mean surface warming in steady state for doubled CO2. Here, we show that the evolution of climate feedbacks in models is consistent with the effect of a change in tropospheric stability, as has recently been hypothesized, and the latter is itself driven by the evolution of the pattern of sea-surface temperature response. The change in climate feedback is mainly associated with a decrease in marine tropical low cloud (a more positive shortwave cloud feedback) and with a less negative lapse-rate feedback, as expected from a decrease in stability. Smaller changes in surface albedo and humidity feedbacks also contribute to the overall change in feedback, but are unexplained by stability. The spatial pattern of feedback changes closely matches the pattern of stability changes, with the largest increase in feedback occurring in the tropical East Pacific. Relationships qualitatively similar to those in the models among sea-surface temperature pattern, stability, and radiative budget are also found in observations on interannual time scales. Our results suggest that constraining the future evolution of sea-surface temperature patterns and tropospheric stability will be necessary for constraining climate sensitivity.

Footnotes

  • ?1To whom correspondence should be addressed. Email: p.ceppi{at}reading.ac.uk.

Published under the PNAS license.

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