Direct observation of Earth’s spectral longwave feedback parameter

The spectral longwave feedback parameter λ_ν represents how Earth’s outgoing longwave radiation adjusts to temperature changes and is thus the detailed fingerprint of all longwave feedbacks, directly impacting Earth’s climate sensitivity. Most research so far has focused on the spectral integral of λ_ν. Spectrally resolving λ_ν permits inferring information about the vertical distribution of longwave feedbacks, thus gaining a better understanding of the underlying processes. However, investigations of λ_ν have so far been largely limited to model studies, and no observational study we are aware of has inferred the global all-sky λ_ν.

Here we show that it is possible to directly observe the global all-sky λ_ν using satellite observations of seasonal and interannual variability taken by the Infrared Atmospheric Sounding Interferometer (IASI). We find that spectral bands subject to strong water vapour absorption exhibit a substantial stabilising net feedback. We demonstrate that this stabilising feedback is partly caused by changes in relative humidity with warming, the radiative fingerprints of which can be directly observed. Therefore, our findings emphasise the importance of better understanding processes affecting future trends in relative humidity. This first observational constraint on the global all-sky λ_ν can be used as a powerful tool to evaluate the representation of longwave feedbacks in global climate models and to better constrain Earth’s climate sensitivity.