A Strong Constraint on Radiative Forcing of Well-mixed Greenhouse Gases

Radiative forcing from well-mixed greenhouse gases (WMGHGs) is a primary driver of Earth’s energy imbalance but remains difficult to constrain due to its dependence on atmospheric state and model discrepancies. We present the first global benchmark of longwave (LW) instantaneous radiative forcing (IRF) for major WMGHGs under realistic, all-sky conditions. This benchmark is supported by (1) global line-by-line radiative transfer simulations using 24 years of reanalysis-based atmospheric and cloud conditions, and (2) a robust linear relationship between LW IRF and outgoing longwave radiation, enabling direct observational constraints from satellites. Using this framework, we estimate that the rising WMGHG concentrations increased LW IRF by 3.71±0.07W/m² from 1850 to 2024. Applying this benchmark to Earth System Models, we reduce the uncertainty in CO2 effective radiative forcing by 60%. This observation-informed, physically grounded method offers a more accurate and consistent constraint on greenhouse gas forcing for future climate assessment reports.