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Tropospheric Adjustment Induces a Cloud Component in CO2 Forcing

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  • 1 Walker Institute for Climate System Research, Department of Meteorology, University of Reading, Reading, and Hadley Centre, Met Office, Exeter, United Kingdom
  • | 2 Hadley Centre, Met Office, Exeter, United Kingdom
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Abstract

The radiative forcing of CO2 and the climate feedback parameter are evaluated in several climate models with slab oceans by regressing the annual-mean global-mean top-of-atmosphere radiative flux against the annual-mean global-mean surface air temperature change ΔT following a doubling of atmospheric CO2 concentration. The method indicates that in many models there is a significant rapid tropospheric adjustment to CO2 leading to changes in cloud, and reducing the effective radiative forcing, in a way analogous to the indirect and semidirect effects of aerosol. By contrast, in most models the cloud feedback is small, defined as the part of the change that evolves with ΔT. Comparison with forcing evaluated by fixing sea surface conditions gives qualitatively similar results for the cloud components of forcing, both globally and locally. Tropospheric adjustment to CO2 may be responsible for some of the model spread in equilibrium climate sensitivity and could affect time-dependent climate projections.

Corresponding author address: Jonathan Gregory, Meteorology Building, University of Reading, P.O. Box 243, Reading RG6 6BB, United Kingdom. Email: j.m.gregory@reading.ac.uk

Abstract

The radiative forcing of CO2 and the climate feedback parameter are evaluated in several climate models with slab oceans by regressing the annual-mean global-mean top-of-atmosphere radiative flux against the annual-mean global-mean surface air temperature change ΔT following a doubling of atmospheric CO2 concentration. The method indicates that in many models there is a significant rapid tropospheric adjustment to CO2 leading to changes in cloud, and reducing the effective radiative forcing, in a way analogous to the indirect and semidirect effects of aerosol. By contrast, in most models the cloud feedback is small, defined as the part of the change that evolves with ΔT. Comparison with forcing evaluated by fixing sea surface conditions gives qualitatively similar results for the cloud components of forcing, both globally and locally. Tropospheric adjustment to CO2 may be responsible for some of the model spread in equilibrium climate sensitivity and could affect time-dependent climate projections.

Corresponding author address: Jonathan Gregory, Meteorology Building, University of Reading, P.O. Box 243, Reading RG6 6BB, United Kingdom. Email: j.m.gregory@reading.ac.uk

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