Radiative forcing by CO$_2$ observed at top of atmosphere from 2002-2019

Abstract: Spectroscopic measurements at top of atmosphere are uniquely capable of attributing changes in Earth's outgoing infrared radiation field to specific greenhouse gasses. The Atmospheric Infrared Sounder (AIRS) placed in orbit in 2002 has spectroscopically resolved a portion of Earth's outgoing longwave radiation for over 17 years. Concurrently, atmospheric CO$_2$ rose from 373 to 410 ppm, or 28% of the total increase over pre-industrial levels. The Coupled Model Intercomparison Project Phase 6 (CMIP6) multi-model ensemble average predicts 0.477 Wm$^{-2}$ clear-sky longwave effective radiative forcing from this increase. Global measurements under nighttime, clear-sky conditions reveal 0.360$\pm$0.026 Wm$^{-2}$ of CO$_2$-induced longwave radiative forcing, or 75$\pm$5% of model predictions.