H 2 O Windows and CO 2 Radiator Fins: A Clear‐Sky Explanation for the Peak in Equilibrium Climate Sensitivity

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.