Yue Xu, Daniel Koll, Nicholas Lutsko

If temperature is held constant, increasing atmospheric CO2 reduces atmospheric radiative cooling and suppresses precipitation. Global Climate Models suggest this “direct” precipitation response ranges from -2% to -3% per CO2 doubling and hence contributes significantly to the net precipitation response of +3% to +9% per CO2 doubling. Our study aims to explain the magnitude and state-dependence of the direct precipitation response by developing an analytical model for CO2 surface forcing. The model is grounded in idealized CO2 spectroscopy that considers CO2 absorption bands both at 667 cm−1 and 1000 cm−1 and is validated against line-by-line radiative transfer calculations. Surface forcing increases with higher surface temperatures and less atmospheric water vapor. By combining our model with a previously established analytical model for top-of-atmosphere CO2 forcing, we derive a simplified model to estimate atmospheric forcing and quantify the global-mean precipitation sensitivity to CO2 changes. Atmospheric forcing increases with surface temperature and decreases with tropopause temperature. Despite ignoring shortwave changes and clouds, our analytical results compare favorably with the surface forcing in CMIP6 models and capture the bulk of these models’ direct precipitation response. Our surface forcing model thus provides a theoretical understanding for how CO2 increases suppress precipitation; it also has implications for understanding the precipitation response under solar geoengineering and how CO2 changes affect land climates.