How Well do We Understand the Planck Feedback?
Name
J Adv Model Earth Syst - 2023 - Cronin - How Well do We Understand the Planck Feedback.pdf
Description
Published version
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1.65 MB
Format
Adobe PDF
Checksum (MD5)
59a8501cb1bec67a6afd4ce86492bcd8
Author(s)
Cronin, Timothy W
Dutta, Ishir
Date Issued
July 17, 2023
Journal
Journal of Advances in Modeling Earth Systems
Publisher
American Geophysical Union
Citation
Cronin, T. W., & Dutta, I. (2023). How well do we understand the Planck feedback?Journal of Advances in Modeling Earth Systems, 15, e2023MS003729.
Version
Final published version
Abstract
A reference or “no-feedback” radiative response to warming is fundamental to understanding
how much global warming will occur for a given change in greenhouse gases or solar radiation incident
on the Earth. The simplest estimate of this radiative response is given by the Stefan-Boltzmann law as
𝐴𝐴 −4𝜎𝜎𝑇𝑇𝑒𝑒
3
≈ −3.8 W m−2 K−1 for Earth's present climate, where 𝐴𝐴 𝑇𝑇𝑒𝑒 is a global effective emission temperature.
The comparable radiative response in climate models, widely called the “Planck feedback,” averages
−3.3 W m−2 K−1. This difference of 0.5 W m−2 K−1 is large compared to the uncertainty in the net climate
feedback, yet it has not been studied carefully. We use radiative transfer models to analyze these two radiative
feedbacks to warming, and find that the difference arises primarily from the lack of stratospheric warming
assumed in calculations of the Planck feedback (traditionally justified by differing constraints on and time
scales of stratospheric adjustment relative to surface and tropospheric warming). The Planck feedback is
thus masked for wavelengths with non-negligible stratospheric opacity, and this effect implicitly acts to
amplify warming in current feedback analysis of climate change. Other differences between Planck and
Stefan-Boltzmann feedbacks arise from temperature-dependent gas opacities, and several artifacts of nonlinear
averaging across wavelengths, heights, and different locations; these effects partly cancel but as a whole slightly
destabilize the Planck feedback. Our results point to an important role played by stratospheric opacity in Earth's
climate sensitivity, and clarify a long-overlooked but notable gap in our understanding of Earth's reference
radiative response to warming.
MIT Department
Massachusetts Institute of Technology. Program in Atmospheres, Oceans, and Climate
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Creative Commons Attribution
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DOI of Published Version
10.1029/2023ms003729
