CO₂‐Dependence of Longwave Clear‐Sky Feedback Is Sensitive to Temperature

CO 2 absorbs and emits radiation, which allows it to act both as radiative forcing and feedback. Recent work has shown CO 2 ’s feedback effect becomes dominant in hothouse climates, giving rise to a non‐monotonic climate sensitivity around 310 K. However, CO 2 ’s feedback effect in colder climates is less clear. We use a line‐by‐line model to explore the CO 2 ‐dependence of the longwave clear‐sky feedback and identify a dividing temperature. Above 290 K, feedback increases with CO 2 concentration; below 290 K, feedback decreases with CO 2 concentration. We explain this dependence in terms of spectral competition under CO 2 increases. In hot climates, CO 2 ’s moderate feedback replaces near‐zero feedback from the H 2 O bands; in cold climates, CO 2 ’s moderate feedback replaces the large feedback from the surface. Given that global mean temperature is currently close to 290 K, our results suggest that feedback CO 2 ‐dependence is weak at present but can be important in past and future climates.