This study finds that the physiological response of plants to increased atmospheric CO2 affects the global hydrological cycle even more than does the greenhouse effect and changes in precipitation. The authors conclude:
This highlights the key role of vegetation in controlling future terrestrial hydrologic response and emphasizes that the carbon and water cycles are intimately coupled over land [LeMordant 2018: 1].
With increasing [CO2] at the leaf surface, the density of stomata at the leaf surface is decreased and their individual opening is reduced and therefore less water is transpired per unit leaf area. In other words, leaf-level water use efficiency increases, potentially increasing surface soil moisture and runoff. On the other hand, leaf biomass tends to also increase with increasing [CO2] … generating a larger evaporative surface that can partly offset the reduction in stomatal conductance and negate the soil water savings. Our objective is therefore to quantify how such plant [CO2] effects influence future hydrological variable responses compared with radiative effects ––the atmospheric impact of the “greenhouse effect.” Radiative effects impact precipitation, i.e., water supply, and evaporative demand, through increase in radiation, temperature, and atmospheric dryness as estimated by the vapor pressure deficit (VPD), i.e., saturation minus actual vapor pressure [LeMordant 2018: 1].
Our study illustrates how deeply the physiological effects [on vegetation] due to increasing atmospheric [CO2] impact the continental water cycle. Contrary to previous wisdom, changes in precipitation and radiation [greenhouse effect] do not play the primary role in future drying and moistening in most regions. Rather, biosphere physiological effects and related biosphere–atmosphere interactions are key for predicting future continental water stress as represented by ET [evapotranspiration], long-term runoff, EF, or leaf area index. In turn, vegetation water stress largely regulates land carbon uptake, further emphasizing how tightly the future carbon and water cycles are coupled so that they cannot be evaluated in isolation [LeMordant 2018: 5].
Lemordant, Leo, et al., 2018, Critical impact of vegetation physiology on the continental hydrologic cycle in response to increasing CO2, PNAS: http://www.pnas.org/content/early/2018/03/29/1720712115