2 and climate on intrinsic water-use efficiency in South Asian tropical moist forest trees. Tree Physiology 40: 904-916. Numerous scientific studies have been conducted in both laboratory and field environments to examine the impact of rising atmospheric CO 2 on plant water use efficiency (WUE). In nearly all instances those studies reveal rising CO 2 improves plant WUE, typically by reducing plant stomatal apertures, which reduces water lost to the air via transpiration. Consequently, at higher levels of CO 2, plants tend to need less water to produce the same (or more) amount of tissue, which portends great benefits for the future of agricultural production, especially in arid and irrigated regions.