Climate-Scale Variability in Soil Moisture Explained by a Simple Theory

Abstract

There is no basic explanation for soil moisture variability in the current climate, and models diverge on the sign of expected changes in a warming world. Here, we present a diagnostic physical theory for soil moisture at large scales. The theory is radically simpler than published alternatives, dependent only on precipitation and surface net radiation with no free parameters. Minor variations improve its performance. The theory answers two basic questions: (a) Why does soil moisture exhibit a W-shaped latitudinal profile, even though precipitation over land does not? Poleward declines in net radiation resolve this discrepancy. (b) Why does soil moisture decrease with warming in some regions where precipitation increases? The theory predicts this phenomenon where fractional increases in net radiation exceed those in precipitation. Common alternative mechanisms, which invoke changes in vapor pressure deficit or plant responses to (Formula presented.), are inessential to explaining first-order changes in soil moisture with warming.