Humidity-dependence of Climate Sensitivity

We study how the vertical distribution of relative humidity (RH) affects climate sensitivity (CS), even if it remains unchanged with warming. Using a one-dimensional radiative-convective equilibrium model, konrad, we show that the climate sensitivity depends on the shape of the vertical distribution of humidity, an effect we call humidity-dependence: Moister atmospheres were shown to have a larger CS, increasingly so with warmer temperature, consistent with our understanding of how water vapor influences the transmissivity of the atmospheric window (Nakajima et al., 1992; Koll & Cronin, 2018). CS is further shown to increase with increasing humidity in the upper troposphere but decreases with increases in humidity in the lower mid-troposphere. We interpret these effects in terms of the effective emission height of water vapor. Differences in the vertical distribution of RH are shown to explain a large part of the 10 to 30% differences in clear-sky sensitivity seen in climate and storm-resolving models. The results imply that convective aggregation reduces climate sensitivity, even when the degree of aggregation does not change with warming. Combining our findings with relative humidity trends in reanalysis data shows a tendency toward Earth becoming more sensitive to forcing over time. These trends and their height variation merit further study.