Changes in the satellite-observed radiation budget: Manifestations of radiative forcing, feedbacks, and internal variability

Abstract

Earth’s radiation budget is a fundamental indicator of the state of the climate. Satellite observations over 2000-2020 display significant monthly-to-decadal changes in the clear-sky greenhouse effect and outgoing longwave (LW) radiation (OLR). Radiative transfer experiments show that these OLR changes are driven by surface and atmospheric temperature increases (decreases) that are only partially offset by the accompanying water vapor increases (decreases). Water vapor in the middle troposphere, in addition to the upper troposphere, substantially contributes to the OLR changes. In addition to OLR changes, the observed net radiative imbalance displays a significant positive trend. Using climate model simulations, we show that it is exceptionally unlikely (< 1% probability) that this trend can be explained by internal variability alone. The trend is driven by a large decrease in reflected shortwave (SW) radiation and a small OLR increase. Model experiments show that this is because recent changes in effective radiative forcing and feedbacks are additive in the SW, while being nearly offset by each other in the LW. The SW forcing, dominated by cloud changes induced by rapid adjustment to greenhouse gas increases and aerosol decreases, helps explain the positive trend (heating) in the SW cloud radiative effect (CRE). LW greenhouse gas forcing decreases OLR more in clear skies than all-sky conditions because clouds mask the impact of greenhouse gas increases. This cloud masking helps explain the negative trend (cooling) in LWCRE. In contrast to the well-identifiable forcing contributions to CRE, cloud feedback contributions have a wide range. In fact, their signs are uncertain. To further understand the LW changes over this period, we investigate the changes across the LW spectrum. The observed spectrum displays significant OLR decreases in the CO2, O3, CH4, and N2O absorption bands as well as OLR increases in the H2O rotational, vibrational, and window bands. Line-by-line radiative transfer calculations and model simulations show that increases in well-mixed greenhouse gases caused an instantaneous radiative forcing and stratospheric cooling that decreased OLR, while the climate response, comprising temperature and humidity changes, increased OLR. We conclude that the satellite radiation record provides clear evidence of a human-influenced climate system.