Observations of the Clear-Sky Spectral Longwave Feedback at Surface Temperatures between 210 and 310 K

Abstract The longwave feedback λ characterizes how Earth’s outgoing longwave radiation changes with near-surface air temperature T as , directly impacting climate sensitivity. Compared to λ , its spectrally resolved counterpart λ ν offers deeper insights into the underlying physical processes. Both λ and λ ν vary with T as , but this T as dependence has so far only been investigated using models. Here, we derive the clear-sky λ ν for T as between 210 and 310 K based on observations of the AIRS instrument. We disentangle the radiative signatures of the atmospheric general circulation by simulating λ ν based on a single-column model with different degrees of idealization. We find that at low T as , the observed λ ν is dominated by the surface response and sensitive to biases in Earth’s skin temperature. At higher T as , changes in the vertical distributions of atmospheric temperature and relative humidity play an important role in shaping λ ν . These changes impact both the absorption of surface emission in the atmospheric window and the atmospheric emission in the water vapor and CO 2 absorption bands. Our results demonstrate that we can fully understand the observed λ ν at a wide range of T as using a simple model of Earth’s atmosphere, lending further support to estimates of the clear-sky longwave λ , Earth’s most fundamental climate feedback. They also highlight the effect of different assumptions about Earth’s atmosphere on λ . Similar approaches can be used to better constrain changes in relative humidity and temperature with warming using satellite observations, as well as for paleoclimate and exoplanet studies. Significance Statement The longwave feedback describes how much more thermal radiation Earth emits to space as it warms. It is the strongest feedback in Earth’s climate and determines the warming in response to an increase in CO 2 concentration. We use satellite observations to analyze the dependence of the longwave feedback on radiation frequency at a wide range of surface temperatures which allows insights into the underlying physical processes. We reproduce these observations using a simple model of Earth’s atmosphere, demonstrating the complete physical understanding of this fundamental climate feedback. Our results can be used to constrain changes in atmospheric temperature and humidity with warming from satellite observations and to better understand the climates of Earth’s past and of other planets.