How well do we understand the Planck feedback?
Abstract
The Planck feedback, or rate of increase of infrared energy loss per unit vertically-uniform warming of the surface and troposphere, is the dominant stabilizing feedback in planetary climate. An estimate of its value is given by the derivative of the Stefan-Boltzmann law with respect to temperature, -4σTe3 - evaluated at an effective emission temperature Te, which is defined so that σTe4 equals the planetary-average outgoing longwave radiation (OLR). Earth's average OLR of 240 W m-2 corresponds to an effective emission temperature of 255 K, and a consequent Planck feedback estimate of -3.76 W m-2 K-1. Feedback analysis of comprehensive global climate models, however, gives a Planck feedback of -3.2 W m-2 K-1 -- roughly 0.5 W m-2 K-1 less stabilizing than the simple estimate (e.g., Soden and Held, 2006). This missing 0.5 W m-2 K-1 has apparently not been a major source of concern because it varies little across climate models, but it represents a notable gap in our understanding of Earth's dominant stabilizing climate feedback: its magnitude is roughly double that of the surface albedo feedback. In this work, we define and describe the contributions of four factors -- stratospheric masking, temperature-dependent opacity, nonlinear averaging, and meridional covariance -- which cause the Planck feedback to deviate from 4σTe3. We estimate that stratospheric masking, temperature-dependent opacity, and meridional covariance each likely increase the Planck feedback (making it less stabilizing) by +0.1-0.2 W m-2 K-1 relative to the simple estimate 4σTe3, and that nonlinear averaging slightly decreases the Planck feedback (making it more stabilizing) by about -0.1 W m-2 K-1. We also discuss whether these four correction factors could differ greatly in magnitude for exotic climates on Earth or for climates of other worlds.
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2020
- Bibcode:
- 2020AGUFMA113.0018C
- Keywords:
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- 3310 Clouds and cloud feedbacks;
- ATMOSPHERIC PROCESSES;
- 3311 Clouds and aerosols;
- ATMOSPHERIC PROCESSES;
- 3337 Global climate models;
- ATMOSPHERIC PROCESSES;
- 3360 Remote sensing;
- ATMOSPHERIC PROCESSES