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