CO2-dependence of Longwave Clear-sky Feedback is sensitive to Temperature

CO2 is widely appreciated as a radiative forcing agent, but recent work showed that CO2 also acts as a climate feedback (Seeley & Jeevanjee 2020). CO2’s ability to emit longwave radiation allows the atmosphere to shed more energy in response to surface warming, and gives rise to a “radiator fin” effect which dominates Earth’s climate sensitivity in hot-and-high-CO2 climates. However, the general CO2-dependence of the longwave feedback is still poorly understood.

Here we explore the CO2-dependence of Earth’s longwave clear-sky feedback using a line-by-line model. We report a dividing surface temperature (Ts) of ~290 K for typical relative humidities. Above 290K, CO2 increases the feedback; below 290K, CO2 decreases the feedback; around 290K, the feedback is CO2-independent. We explain our results via a spectral competition between CO2 radiator fins, which enhance the feedback, and CO2 blocking the surface’s emission, which decreases the feedback. Only at high Ts, once H2O shuts down all window regions, does CO2 enhance the feedback.

Given that Earth’s global-mean temperature is close to ~290K, our results explain why feedback CO2-dependence is weak in our current climate but could have been important for paleoclimates. Finally, because feedback CO2-dependence is identical to forcing Ts-dependence, our results also explain the temperature-dependence of the CO2 forcing. Analogous to the clear-sky feedback, CO2 forcing also changes its behavior above versus below ~290K.