Effect of Uncertainty in Water Vapor Continuum Absorption on CO2 Forcing, Longwave Feedback, and Climate Sensitivity

Abstract

We investigate the effect of uncertainty in water vapor continuum absorption at terrestrial wavenumbers on CO₂ forcing $F$, longwave feedback λ, and climate sensitivity $S$ at surface temperatures T_s between 270 and 330 K. We calculate this uncertainty using a line-by-line radiative-transfer model and a single-column atmospheric model, assuming a moist-adiabatic temperature lapse-rate and 80% relative humidity in the troposphere, an isothermal stratosphere, and clear skies. Due to the lack of a comprehensive model of continuum uncertainty, we represent continuum uncertainty in two different idealized approaches: In the first, we assume that the total continuum absorption is constrained at reference conditions; in the second, we assume that the total continuum absorption is constrained for all atmospheres in our model. In both approaches, we decrease the self continuum by 10% and adjust the foreign continuum accordingly. We find that continuum uncertainty mainly affects $S$ through its effect on λ. In the first approach, continuum uncertainty mainly affects λ through a decrease in the total continuum absorption with T_s; in the second approach, continuum uncertainty affects λ through a vertical redistribution of continuum absorption. In both experiments, the effect of continuum uncertainty on $S$ is modest at T_s = 288 K (≈0.02 K) but substantial at T_s ≥ 300 K (up to 0.2 K), because at high T_s, the effects of decreasing the self continuum and increasing the foreign continuum have the same sign. These results highlight the importance of a correct partitioning between self and foreign continuum to accurately determine the temperature dependence of Earth's climate sensitivity.