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Abstract

Earth's spectral long-wave feedback parameter can be directly observed
using satellite measurements, revealing the influence of relative
humidity on climate feedbacks.
The spectral long-wave feedback parameter represents how Earth's
outgoing long-wave radiation adjusts to temperature changes and directly
impacts Earth's climate sensitivity. Most research so far has focused on
the spectral integral of the feedback parameter. Spectrally resolving
the feedback parameter permits inferring information about the vertical
distribution of long-wave feedbacks, thus gaining a better understanding
of the underlying processes. However, investigations of the spectral
long-wave feedback parameter have so far been limited mostly to model
studies. Here we show that it is possible to directly observe the global
mean all-sky spectral long-wave feedback parameter using satellite
observations of seasonal and interannual variability. We find that
spectral bands subject to strong water-vapour absorption exhibit a
substantial stabilizing net feedback. We demonstrate that part of this
stabilizing feedback is caused by the change of relative humidity with
warming, the radiative fingerprints of which can be directly observed.
Therefore, our findings emphasize the importance of better understanding
processes affecting the present distribution and future trends in
relative humidity. This observational constraint on the spectral
long-wave feedback parameter can be used to evaluate the representation
of long-wave feedbacks in global climate models and to better constrain
Earth's climate sensitivity.