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Downward longwave radiation (DLR) is often assumed to be an independent forcing on the surface energy budget in analyses of Arctic warming and land-atmosphere interaction. We use radiative kernels to show that the DLR response to forcing is largely determined by surface temperature perturbations. We develop a method by which vertically integrated versions of the radiative kernels are combined with surface temperature and specific humidity to estimate the surface DLR response to greenhouse forcing. Through a decomposition of the DLR response, we estimate that changes in surface temperature produce at least 63% of the clear-sky DLR response in greenhouse forcing, while the changes associated with clouds account for only 11% of the full-sky DLR response. Our results suggest that surface DLR is tightly coupled to surface temperature; therefore, it cannot be considered an independent component of the surface energy budget.

Plain Language Summary Longwave radiation, often referred to as "thermal" or "infrared" radiation, emitted downward by Earth's atmosphere is a primary contributor to the surface energy budget. Numerous studies have invoked longwave radiation as a driver of surface warming. This paper shows that this line of reasoning fails to account for the strong control surface temperature exerts on longwave radiation. Using radiative kernels, matrices that quantify the longwave radiation response to a climate perturbation (like global warming), we argue that any surface temperature anomaly will generate a downward longwave radiation response. This constitutes a feedback between the Earth's surface and its atmosphere. The kernels show large longwave responses to perturbations in the lowest part of the atmosphere and almost no response to perturbations at high levels; by vertically integrating the kernels, we can ignore the vertical structure of climate perturbations. Using this modification, we predict the longwave radiation response to a warming world using only the surface changes. Our prediction agrees with climate model output, suggesting that the longwave radiation response is determined primarily by surface temperature. Further, the cloud contribution to changes in longwave radiation is small. These results provide clarity on how changes in the surface energy budget should be analyzed.