The Role of Interactive Soil Moisture in Land Drying Under Anthropogenic Warming

Relative humidity (RH) is projected to increase over ocean but decrease over land under anthropogenic warming. The land RH decrease was previously attributed to ocean influences on land changes. Here, we show that interactive soil moisture (SM) is necessary and may be sufficient for anthropogenic warming to reduce the land RH. The land RH decrease is absent in simulations with realistic land-ocean geometry but fixed SM, and present in simulations with interactive SM but no oceans or changes in precipitation minus evaporation. The land RH decrease is coupled to the SM decline and the latter could be understood as a natural response of interactive SM to anthropogenic warming. Specifically, prior to adjustments in SM and RH, evaporation would increase by similar to 6% K-1 following the Clausius-Clapeyron relation, outpacing the radiatively constrained similar to 2% K-1 precipitation increase. This prior imbalance depletes SM and consequently reduces RH through the coupling between SM and RH. As a measure of air wetness, relative humidity (RH) is projected to decrease over land under anthropogenic warming, contributing to increasing evaporative demand and intensifying droughts, heatwaves and wildfires. The land RH decrease was previously attributed to influences from ocean, but here we propose an alternative explanation. We first show that the land RH decrease is coupled with the soil moisture (SM) decline. Then, by excluding interactive SM and ocean influences respectively in warming simulations, we illustrate that interactive soil moisture (i.e., soil moisture is fed by precipitation minus evaporation and can in turn affect evaporation through moisture availability) is necessary and may be sufficient to produce a drier land under anthropogenic warming. A conceptual understanding of this declined SM and RH is provided based on theoretical evaporation equations. We propose a new explanation for the projected decrease in land relative humidity under anthropogenic warmingComplex and idealized simulations show that interactive soil moisture (SM) is necessary and sufficient for anthropogenic warming to dry the landInteractive SM allows prior imbalance in evaporation and precipitation increases to dry the land independent of ocean influence

Automated summary

Relative humidity decreases over land under anthropogenic warming and this decrease is coupled with the decline in soil moisture. Interactive soil moisture is necessary and may be sufficient to explain the decrease in land relative humidity. The prior imbalance in evaporation and precipitation increases, facilitated by interactive soil moisture, leads to a drying effect on land.