Emergent constraints on the hydrological impacts of land use and land cover change.

Land use and land cover changes have substantial effects on the terrestrial water cycle, but their sign and magnitude remain elusive at large scales. State-of-the-art Earth system models disagree on how these changes affect terrestrial evapotranspiration. Here we use the observation-based transpiration-specific Bowen ratio to correct modelled evapotranspiration changes induced by land use and land cover changes globally and regionally within a hierarchical emergent constraint framework. We show that the constraint reverses the sign of the original model estimates at the global scale and over Central and South America, and narrows the inter-model spread. The misrepresentation of transpiration-specific Bowen ratio and its variations across plant functional types in models is the main source of this bias. Applying an analogous constraint framework to a future afforestation scenario, the constrained simulations project stronger evapotranspiration enhancements and weaker decreases in terrestrial water availability compared to the original simulations, particularly in tropics and subtropics.