02 Jan 2023
02 Jan 2023
Status: this preprint is open for discussion and under review for Atmospheric Chemistry and Physics (ACP).

A Colorful look at Climate Sensitivity

Bjorn Stevens and Lukas Kluft Bjorn Stevens and Lukas Kluft
  • Max Planck Institute for Meteorology, Hamburg

Abstract. The radiative response to warming, and to changing concentrations of CO2, is studied in spectral space. If relative humidity does not change with temperature, clear-sky emissions over spectral intervals in which water vapor is optically thick become independent of surface temperature, giving rise to the idea of spectral masking. It is demonstrated that this idea allows one to derive simple, physically informative, and surprisingly accurate, expressions for the clear sky radiative forcing, radiative response to warming and hence climate sensitivity. Extending these concepts to include the effects of clouds, leads to the expectation that (i) clouds damp the clear-sky response to forcing, (ii) that diminutive clouds near the surface, which are often thought to be unimportant, may be particularly effective at enhancing the clear-sky sensitivity over deep moist tropical boundary layers; and (iii) even small changes in high-clouds over deep moist regions in the tropics makes these regions radiatively more responsive to warming that previously believed. The analysis demonstrates that the net effect of clouds on warming is ambiguous, justifying the assertion that the clear-sky (fixed RH) climate sensitivity – which after accounting for clear-sky surface albedo feedbacks, is about 3 K – provides a reasonable prior for Bayesian updates accounting for how clouds are distributed, how they they might change, and for deviations associated with changes in relative humidity with temperature. These effects are best assessed by quantifying the distribution of clouds and water vapor, and how they change, in temperature, rather than geographic space.

Bjorn Stevens and Lukas Kluft

Status: open (until 13 Feb 2023)

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • RC1: 'Comment on egusphere-2022-1460', Anonymous Referee #1, 10 Jan 2023 reply

Bjorn Stevens and Lukas Kluft

Bjorn Stevens and Lukas Kluft


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Short summary
Analytic expressions are derived for the clear-sky climate sensitivity in an atmosphere within which the relative humidity depends only on temperature. The expressions have quantitative fidelity and are physically insightful. The ideas leading to this derivation also help better understand how clouds modify the clear sky sensitivity, demonstrating a more ambiguous role of clouds, and in so doing providing a better theoretical underpinning for the climate sensitivity itself.