Clouds reduce downwelling longwave radiation over land in a warming climate.

Clouds greatly influence the Earth's energy balance. Observationally constraining cloud radiative feedback, a notably uncertain climate feedback mechanism, is crucial for improving predictions of climate change but, so far, remains an elusive objective, and the feedback may be different over the ocean versus over land. Here we show a local negative surface longwave cloud feedback over land at the southern Great Plains site, constrained by direct long-term observation of spectrally resolved downwelling longwave radiance. This negative cloud feedback at the southern Great Plains site causes a -1.77 ± 1.15 W m per decade change in downwelling longwave radiation and suggests that cloud changes may partially modulate the warming effect of increased greenhouse gas concentrations and atmospheric temperatures over land. Specifically, our results are derived from an optimal spectral fingerprinting method designed to separate surface longwave cloud feedback from other surface forcings and feedbacks, by making use of their unique spectral signatures in the long-term record of spectrally resolved radiances. Furthermore, we show that the results are not site specific: negative surface longwave cloud feedbacks, primarily induced by decreasing low cloud cover in warming climates, are commonly observed over land in reanalysis and satellite datasets. Our findings establish a pivotal observational benchmark of radiative forcing and feedback needed for validating climate model performance over land.