Geophysical Research Letters | 2019
Climatology Explains Intermodel Spread in Tropical Upper Tropospheric Cloud and Relative Humidity Response to Greenhouse Warming
Abstract
The response of upper tropospheric clouds and relative humidity (RH) to warming is important to the overall sensitivity of the Earth to increasing greenhouse gas concentrations. Previous research has shown that changes in hydrologic fields should closely track rising isotherms in a warming climate. Here we show that the distribution of tropical clouds and RH in general circulation models is approximately constant under greenhouse warming when using temperature as a vertical coordinate. By assuming that these fields are an invariant function of atmospheric temperature and that temperature change follows a dilute moist adiabat, we are able to accurately predict cloud fraction and RH changes in the tropical upper troposphere (150–400 hPa) in 27 general circulation models. Our results indicate that intermodel spread in changes of tropical upper tropospheric clouds and RH is closely related to differences in model climatology and could be substantially reduced if model ensembles reliably reproduced observed climatologies. Plain Language Summary As the Earth warms due to increasing greenhouse gas concentrations, many aspects of the climate adjust. Clouds, for example, tend to shift upward in response to surface warming. Such processes, known as climate feedbacks, alter the radiative energy that reaches the Earth s surface and, as a result, can amplify or damp global warming. One method for investigating climate feedbacks is to analyze their behavior in global climate models. Because clouds and relative humidity exhibit widely varying responses to warming in climate models, uncertainty in the Earth s sensitivity to changes in atmospheric carbon dioxide is exacerbated. Building on previous theoretical research, we demonstrate that the simulated response of clouds and relative humidity to global warming is closely associated with model representation of the present-day climate. Theory, observations, and climate models indicate that moistening of the troposphere and rising of the highest cloud tops cause large positive feedbacks. Our research suggests that the substantial model spread in representing these feedbacks may be reduced by improving simulations of the observed present-day climate.