During periods of hydrologic stress, vegetation productivity is limited by soil moisture supply and atmospheric water demand. This study shows that atmospheric demand has a greater effect in many biomes, with implications for climate change impacts.

Via air temperature increases and relative humidity changes, climate change will modify vapor pressure deficit (VPD), which is an important determinant of water vapor and CO₂ exchange between the land surface and atmosphere. VPD is the difference between the water vapor the air can hold at saturation (e_s) and the actual amount of water vapor (e_a). Here we assess changes in VPD, e_s, and e_a in the United States (U.S.) for the recent past (1979–2013) and the future (2065–2099) using gridded, observed climate data and output from general circulation models. Historically, VPD has increased for all seasons, driven by increases in e_s and declines in e_a. The spring, summer, and fall seasons exhibited the largest areal extent of significant increases in VPD, which was largely concentrated in the western and southern portions of the U.S. The changes in VPD stemmed from recent air temperature increases and relative humidity decreases. Projections indicate similar, amplified patterns into the future. For the summer, the general circulation model ensemble median showed a 51% projected increase (quartile range of 39 and 64%) in summer VPD for the U.S., reflecting temperature‐driven increases in e_s but decreases or minimal changes in relative humidity that promotes negligible changes in e_a. Using a simple model for plant hydraulic functioning, we also show that in the absence of stomatal acclimation, future changes in VPD can reduce stomatal conductance by 9–51%, which is a magnitude comparable to the expected decline in stomatal conductance from rising CO₂.

Elevated vapor pressure deficit (VPD) due to drought and warming is well-known to limit canopy stomatal and surface conductance, but the impacts of elevated VPD on ecosystem gross primary productivity (GPP) are less clear. The intrinsic water use efficiency (iWUE), defined as the ratio of carbon (C) assimilation to stomatal conductance, links vegetation C gain and water loss and is a key determinant of how GPP will respond to climate change. While it is well-established that rising atmospheric CO₂ increases ecosystem iWUE, historic and future increases in VPD caused by climate change and drought are often neglected when considering trends in ecosystem iWUE. Here, we synthesize long-term observations of C and water fluxes from 28 North American FLUXNET sites, spanning eight vegetation types, to demonstrate that ecosystem iWUE increases consistently with rising VPD regardless of changes in soil moisture. Another way to interpret this result is that GPP decreases less than surface conductance with increasing VPD. We also project how rising VPD will impact iWUE into the future. Results vary substantially from one site to the next; in a majority of sites, future increases in VPD (RCP 8.5, highest emission scenario) are projected to increase iWUE by 5%–15% by 2050, and by 10%–35% by the end of the century. The increases in VPD owing to elevated global temperatures could be responsible for a 0.13% year⁻¹ increase in ecosystem iWUE in the future. Our results highlight the importance of considering VPD impacts on iWUE independently of CO₂ impacts.

Key Points: Stream temperature is expected to increase by 1 to 5.5 oC Dissolved oxygen concentrations are expected to decrease Sediment concentrations are projected to decrease by about 50%