Increasing water‐use efficiency (WUE), the ratio of carbon gain to water loss, is a key mechanism that enhances carbon uptake by terrestrial vegetation under rising atmospheric CO 2 ( c a ). Existing theory and empirical evidence suggest a proportional WUE increase in response to rising c a as plants maintain a relatively constant ratio between the leaf intercellular ( c i ) and ambient ( c a ) partial CO 2 pressure ( c i / c...