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H 2 O Windows and CO 2 Radiator Fins: A Clear‐Sky Explanation for the Peak in Equilibrium Climate Sensitivity
Author(s) -
Seeley Jacob T.,
Jeevanjee Nadir
Publication year - 2021
Publication title -
geophysical research letters
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 2.007
H-Index - 273
eISSN - 1944-8007
pISSN - 0094-8276
DOI - 10.1029/2020gl089609
Subject(s) - longwave , adiabatic process , troposphere , atmospheric sciences , environmental science , climate sensitivity , thermodynamic equilibrium , materials science , absorption (acoustics) , climate model , computational physics , meteorology , thermodynamics , physics , climate change , radiation , geology , optics , oceanography , composite material
Recent explorations of the state‐dependence of Earth’s equilibrium climate sensitivity (ECS) have revealed a pronounced peak in ECS at a surface temperature of ∼310 K. This ECS peak has been observed in models spanning the model hierarchy, suggesting a robust physical source. Here, we propose an explanation for this ECS peak using a novel spectrally resolved decomposition of clear‐sky longwave feedbacks. We show that the interplay between spectral feedbacks in H 2 O‐dominated and CO 2 ‐dominated portions of the longwave spectrum, along with moist‐adiabatic amplification of upper‐tropospheric warming, conspire to produce a minimum in the feedback parameter, and a corresponding peak in ECS, at a surface temperature of 310 K. Mechanism‐denial tests highlight three key ingredients for the ECS peak: (1) H 2 O continuum absorption to quickly close spectral windows at high surface temperature; (2) moist‐adiabatic tropospheric temperatures to enhance upper‐tropospheric warming; and (3) energetically consistent increases of CO 2 with surface temperature.