Soil moisture-evaporation coupling shifts into new gears under increasing CO2

Abstract When soil moisture content (SM) falls within a transitional regime between dry and wet, it controls evaporation rates, affecting atmospheric heat and humidity. Accordingly, different SM regimes correspond to different gears of land-atmosphere coupling, affecting climate. Determining patterns of SM regimes and their future changes is imperative. Global distributions of shifts between SM regimes from eight climate models are examined. Under increasing CO2, the range of SM extends into unprecedented coupling regimes in many locations. SM in many arid regions migrates from the transitional toward the dry regime, while an opposite migration appears in semiarid regions. Transitional regimes also emerge in currently humid areas of the tropics and high latitudes. These changes imply that a larger fraction of the world will evolve to experience more than one gear of land-atmosphere coupling, with the strongly coupled transitional regime growing the most. Furthermore, the locally predominant gear may change under global warming.