  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.
