Reduced Evapotranspiration Decreases Land Precipitation Despite Enhanced Moisture Convergence: Insights from Perturbed Parameter Ensembles of Earth System Models

Both canonical theory and climate model simulations predict an increase in globally averaged precipitation with radiatively-forced warming. However, terrestrial processes control surface water and energy fluxes, and therefore can impact surface temperatures independently of radiative forcing. Here, we investigate the sensitivity of the global hydrological cycle to warming induced by changes in land surface conditions using perturbed parameter ensembles from two climate models, which sample a broad range of evapotranspiration (ET) states through modifications to land model parameters. The parameters are perturbed within the range of observationally constrained uncertainty and alter ET through different land surface pathways. We find that reduced ET drives warming yet decreases land-mean precipitation, which we link to strong decreases in near-surface relative humidity from warming. These decreases in relative humidity overcome increases in specific humidity and moisture convergence over land, which are also caused by shifts in large-scale circulation. We explain the tropical precipitation response to ET forcing with a simple scaling that relates changes in the land-ocean contrast in relative humidity to land-mean precipitation change. Further, the suppressed ET induces a shortwave cloud response that amplifies warming. Our results highlight that changes to land surface properties at the magnitude of parametric uncertainty influence warming and precipitation. ET-driven warming reduces land precipitation, opposite in sign to the precipitation increase from radiatively-forced warming. This response is not directly captured by scalings of the global hydrological cycle with radiative forcing. We therefore establish the potential significance of terrestrial ecosystem functioning, and its response to climate change, for hydroclimate.