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

We assess the effect of uncertainty in water vapor continuum absorption on CO2 forcing F, longwave feedback λ, and climate sensitivity S at surface temperatures Ts between 270K and 330K. 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. Emulating continuum uncertainty in observations, we hold total continuum absorption fixed at room temperature, but change its components: We assume a 10% decrease in self continuum absorption, which comprises interactions between water molecules, and a spectrally varying increase in foreign continuum absorption, which comprises interactions between water and non-water molecules. We find that continuum uncertainty mainly affects S through its effect on λ. Continuum uncertainty primarily impacts the surface feedback at Ts<290K and the atmospheric feedback at Ts>290 K. Under present-day conditions, those two effects have opposite signs and thus largely cancel each other, therefore the effect of continuum uncertainty on S is negligible (0.02K). At Ts>300K, however, the effect on S is much stronger (>0.2K). This is because at those Ts, the effects on λ 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.