Observational Evidence of Increasing Global Radiative Forcing

Change in atmospheric composition, such as increasing greenhouse gases, causes an initial radiative imbalance to the climate system, quantified as the instantaneous radiative forcing. All climate change is a response to this imbalance. Although modeled with high accuracy, this fundamental metric has not been directly observed globally. In part, this is because current space-based instruments cannot distinguish the instantaneous radiative forcing from the climate's radiative response. We apply radiative kernels to satellite observations to separate these components and find all-sky instantaneous radiative forcing has increased 0.53 +/- 0.11 W/m² from 2003 through 2018 in CERES radiative flux observations, almost entirely accounting for positive trends in the total radiative imbalance. We demonstrate that a combination of rising well-mixed greenhouse gases and recent reductions in aerosol emissions account for the trend and that the observed changes closely agree with reanalysis, offline radiative transfer calculations and well-established empirical estimates. This lends credence to the ability of our current satellite observing system to measure long-term climate change, despite instrumental uncertainties. We find these radiative forcing trends are detectable within 4 years. Our results highlight the distinct fingerprints of anthropogenic activity in observations of Earth's changing energy budget.