Climate change affects the water cycle. Despite the improved accuracy of simulations of historical temperature, precipitation and runoff in the latest Coupled Model Intercomparison Project Phase 6 (CMIP6), the uncertainty of the future sensitivity of global runoff to temperature remains large. Here, we identify an emergent relationship between the historical sensitivity of precipitation to temperature change (1979–2014) and the future sensitivity of runoff to temperature change (2015–2100), which can be used to constrain future runoff sensitivity estimates. Using this constraint, we estimate that the uncertainties in future sensitivity of runoff have been reduced by 7.2 – 12.0%. The constrained sensitivity of runoff is much larger (36 – 104%) than that directly inferred from original CMIP6 projections. Our constrained sensitivities also indicate more extreme wet conditions and fewer dry conditions. These results suggest that the future global land water cycle is accelerating and comes with more hydroclimatic extremes than previously projected

The Budyko framework consists of a curvilinear relationship between the evaporative ratio (i.e., actual evaporation over precipitation) and the aridity index (potential evaporation over precipitation) and defines evaporation’s water and energy limits. A basin’s movement within the Budyko space illustrates its hydroclimatic change and can help identify the main drivers of change. Basins are expected to move along their Budyko curves when only long-term changes in the aridity index drive changes in the evaporative ratio. We hypothesize that the increasing effects of global warming on the hydrological cycle will cause basins to move along their Budyko curves. To test our hypothesis, we quantify the movement in Budyko space of 353 river basins from 1901 to 2100 based on the outputs of nine models from the Coupled Model Intercomparison Project - Phase 5 (CMIP5). We find that significant increases in potential evaporation due to global warming will lead to basins moving primarily horizontally in Budyko space accompanied by minor changes in the evaporative ratio. However, 37% of the basins will still deviate from their Budyko curve trajectories, with less evaporation than expected by the framework. We elaborate on how land-use change, vegetation changes, or shifts in precipitation or snow to rain ratios can explain these deviations.