While terrestrial precipitation is a societally highly relevant climate variable, there is little consensus
among climate models about its projected 21st century changes. An important source of precipitable
water over land is plant transpiration. Plants control transpiration by opening and closing their
stomata. The sensitivity of this process to increasing CO2 concentrations is uncertain. To assess the
impact of this uncertainty on future climate, we perform experiments with an intermediate
complexity Earth System Climate Model (UVic ESCM) for a range of model-imposed transpirationsensitivities
to CO2. Changing the sensitivity of transpiration to CO2 causes simulated terrestrial
precipitation to change by −10% to +27% by 2100 under a high emission scenario. This study
emphasises the importance of an improved assessment of the dynamics of environmental impact on
vegetation to better predict future changes of the terrestrial hydrological and carbon cycles.