 The study investigates the effects of climate change and rising atmospheric CO2 concentration on global terrestrial ecosystem water use efficiency, WUE, using a process-based ecosystem model, finding that future changes would increase WUE in high-latitude regions but decrease it in low-latitude regions. Rising atmospheric CO2 concentration could ameliorate climate change-induced WUE decline, and more accurate prediction of WUE requires deeper understanding on the responses of ET to rising atmospheric CO2 concentrations and its interactions with climate.