LAUSR

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 H2O‐dominated and CO2‐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) H2O 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 CO2 with surface temperature.