Dependence of Climate Sensitivity on the Given Distribution of Relative Humidity

We study how the vertical distribution of relative humidity (RH) affects climate sensitivity, even if it remains unchanged with warming. Using a radiative-convective equilibrium model, we show that the climate sensitivity depends on the shape of a fixed vertical distribution of humidity, tending to be higher for atmospheres with higher humidity. 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%-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 RH 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. Plain Language Summary Equilibrium Climate Sensitivity is the change in surface temperature in response to a doubling of atmospheric CO2. We study how the assumed vertical distribution of relative humidity affects this sensitivity. Theoretical considerations show that the more moist an atmosphere is, the more it warms as a response to an increase in CO2. Adding water vapor to the lower troposphere has the counter effect, lowering the sensitivity. We emphasize the importance of climate simulations taking humidity into account, as it is largely responsible for the difference in projections among models without clouds. We note surprising trends in humidity-with substantial drying of the lower troposphere over the ocean-in the last four decades as reported by two reanalyses of meteorological observations. Subject to the accuracy of these reconstructions, there appears to be a change with less moistening than expected, but with moistening/drying profiles which will condition Earth to become more sensitive to forcing over time. We stress the need for a study of observations to more critically evaluate these trends, and know better what models should aim for.

Automated summary

The study shows that climate sensitivity depends on the vertical distribution of humidity, with higher humidity leading to a higher temperature increase. Differences in the vertical distribution of relative humidity explain differences in climate sensitivity among models. Recent trends in humidity suggest Earth is becoming more sensitive to forcing over time.