Background
The nine UK authors write that "freshwater is an essential societal requirement and is consumed through domestic, industrial, agricultural and environmental withdrawals," but they state that "regionally, freshwater resources are under pressure due to demographic, economic and climatic drivers of change," and they report that these forces result in a large proportion of the global population currently having to live in water scarce conditions, according to the World Water Assessment Programme (2009), which situation could conceivably get worse, on the one hand (Fung et al., 2011), or better, on the other hand (Arnell et al., 2011).

What was done
In a study designed to help determine the most likely of these two alternative consequences, Wiltshire et al. used an ensemble of HadCM3 climate models forced with a range of future emission scenarios that they combined with a simple water scarcity index to assess the contribution of each of three important factors to the projected population living in water stress over the 21st century, these factors being (1) population, (2) climate-change-driven water availability, and (3) a carbon dioxide physiological-forcing effect on evaporation and run-off that was described some 30 years ago by Idso and Brazel (1984).

What was learned
The four main conclusions of this endeavor were that (1) "projected population growth alone is expected to lead to an increased number of people living in water stress in the future," that (2) "the global increase in runoff due to climate change is insufficient to offset the increase in water stress caused by population growth," but that (3) "the effect of CO2 on plant water use efficiency is likely to have a direct effect on run-off causing increased water availability," and that (4) "the reduction in the number of people living in water stress caused by this physiological forcing is of the same order of magnitude as the effect of climate change."

What it means
In the concluding words of Wiltshire et al., "this study highlights the importance of considering more than just the radiative forcing of different greenhouse gases when evaluating mitigation scenarios and the need to better constrain the CO2 water-use interaction in ecosystem models," for they say "the effect of rising CO2 is to increase available water and to reduce the number of people living in high water stress by around 200 million compared to climate only projections."

Therefore, chalk this study up as suggesting a better freshwater future for humanity in a CO2-enriched world of the future.

References
Arnell, N.W., van Vuuren, D.P. and Isaac, M. 2011. The implications of climate policy for the impacts of climate change on global water resources. Global Environmental Change - Human and Policy Dimensions 21: 592-603.

Fung, F., Lopez, A. and New, M. 2011. Water availability in +2 degrees C and +4 degrees C worlds. Philosophical Transactions of the Royal Society A - Mathematical, Physical and Engineering Sciences 369: 99-116.

Idso, S.B. and Brazel, A.J. 1984. Rising atmospheric carbon dioxide concentrations may increase streamflow. Nature 312: 51-53.

World Water Assessment Programme. 2009. The United Nations World Water Development Report 3: Water in a Changing World. UNESCO, London, United Kingdom.