Spectral longwave radiative forcing and feedbacks in the AIRS satellite record

We find that satellite observations of Earths outgoing longwave radiation (OLR) spectrum from the Atmospheric Infrared Sounder (AIRS) instrument during 2002-2019 display significant decreases in OLR (heating) in the CO2, O3, CH4, and N2O bands as well as increases in OLR (cooling) in the H2O rotational, vibrational, and window bands. The observed trends at each wavenumber are a convolution of radiative forcing and feedback. We use a benchmark line-by-line radiative transfer model with meteorological inputs from a climate model (GFDL CM3/AM3) to disentangle the total OLR into its forcing and feedback components at each wavenumber. We find that the instantaneous radiative forcing from increases in well-mixed greenhouse gases is the dominant contributor to the observed decreases in OLR from their corresponding bands. Surprisingly, the 10 m CO2 band, which is much weaker than the main 15 m band of CO2, contributes to the instantaneous radiative forcing as much as CH4 and N2O over this time period. Next, adjustments dominated by stratospheric cooling supplement the reduction in OLR in the CO2 and O3 bands. Finally, the warming in response to these greenhouse gas increases and its accompanying feedback comprising temperature and humidity changes, caused increased emission from all bands in the far-infrared. Together, the feedback slightly outcompetes the greenhouse gas radiative forcing and yields a small positive trend in broadband OLR. We conclude that the spectral satellite radiation record provides clear evidence of a human-influenced climate system.