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 by showing that interactive soil moisture (SM) itself can dry the land under anthropogenic warming. We first show that the land RH decrease is coupled with the SM decline. Then, by excluding interactive SM and ocean influences respectively in warming simulations, we illustrate that interactive SM (i.e., SM is fed by precipitation minus evaporation but in turn affects evaporation) is necessary and sufficient to produce a drier land under anthropogenic warming. Specifically, 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. Theoretically, without adjustments in SM and RH, evaporation would increase by ~6-7% K^-1 following the Clausius–Clapeyron relation, outpacing the radiatively constrained ~2-3% K^-1 precipitation increase. This prior imbalance depletes SM and reduces RH until the balance is restored. In essence, interactive SM allows a tug of war between the Clausius–Clapeyron relation and the radiative constraint to dry the land.

https://eos.org/editor-highlights/warming-reduces-relative-humidity-through-soil-moisture