There is broad consensus that, via changes in stomatal conductance, plants moderate the exchanges of water and carbon between the biosphere and atmosphere, playing a major role in global hydroclimate. Tree rings record atmospheric CO₂ concentration (c_a) and its isotopic composition (¹³C/¹²C)—mediated by stomatal and photosynthetic influences—that can be expressed in terms of intrinsic water-use efficiency (W). Here, we compile a global W dataset based on 422 tree-ring isotope series and report that W increased with c_a over the twentieth century, but the rates of increase (dW/dc_a) declined by half. Angiosperms contributed more than gymnosperms to the slowdown, and in recent decades, dW/dc_a for angiosperms was close to zero. dW/dc_a varies widely across climatic regions and reflects pauses in emissions during the Great Depression and after World War II. There is strong spatial variability in climate forcing via an increasing W, which is weakening globally with time.

广泛的共识是，通过气孔导度的变化，植物可调节生物圈与大气之间的水和碳交换，在全球水文气候中发挥重要作用。年轮记录了气孔和光合影响介导的大气中CO ₂浓度（c _a）及其同位素组成（¹³ C / ¹² C），可以用内在用水效率（W）表示。在这里，我们基于422个树环同位素系列编制了一个全球W数据集，并报告说在20世纪W随着c _a的增加而增加，但是增加的速率（d W / dc _a）下降了一半。被子植物比裸子植物对减缓的贡献更大，近几十年来，被子植物的d W / d c _a接近于零。d W / d c _a在不同气候区域之间差异很大，反映了大萧条时期和第二次世界大战后排放的停顿。随着W的增加，气候强迫具有很大的空间变异性，而W随着时间的推移逐渐减弱。