Background
The authors write that "due to the rapidly increasing population and water use per capita in many areas of the world, around one third of the world's population currently lives under physical water scarcity (e.g. Vorosmarty et al., 2000; Alcamo et al., 2003; Oki and Kanae, 2006)." But they state that despite the large number of water scarcity studies that have been conducted over the years, "no global assessment is available of how this trend has evolved over the past several centuries to millennia."

What was done
To fill this gaping void, Kummu et al. conducted a study that covered the period of time from AD 0 to 2005. This analysis was carried out for ten different time slices, defined as those times at which the human population of the globe was approximately double the population of the previous time slice. Global population data for these analyses were derived from the 5' latitude x 5' longitude-resolution global HYDE dataset of Klein Goldewijk (2005) and Klein Goldewijk et al. (2010), while evaluation of water resources availability over the same period was based on monthly temperature and precipitation output from the climate model ECBilt-CLIO-VECODE, as calculated by Renssen et al. (2005).

What was learned
The four researchers report that "moderate water shortage first appeared around 1800, but it commenced in earnest from about 1900, when 9% of the world population experienced water shortage, of which 2% was under chronic water shortage (<1000 m³/capita/year)." Thereafter, from 1960 onwards, they say that "water shortage increased extremely rapidly, with the proportion of global population living under chronic water shortage increasing from 9% (280 million people) in 1960 to 35% (2300 million) in 2005." Currently, as they continue, "the most widespread water shortage is in South Asia, where 91% of the population experiences some form of water shortage," while "the most severe shortage is in North Africa and the Middle East, where 77% and 52% of the total population lives under extreme water shortage (<500 m³/capita/year), respectively."

What it means
To alleviate these freshwater shortages, Kummu et al. say that measures have generally been taken to increase water availability, such as building dams and extracting groundwater. But they state that "there are already several regions in which such measures are no longer sufficient, as there is simply not enough water available in some regions." And they say that "this problem is expected to increase in the future due to increasing population pressure (e.g. United Nations, 2009), higher welfare (e.g. Grubler et al., 2007) [and] production of water intensive biofuels (e.g. Varis, 2007, Berndes, 2008)." Hence, they conclude there will be an increasing need for many non-structural measures to be implemented, the first and foremost of which they indicate to be "increasing the efficiency of water use," which characteristic of nearly all of earth's plants is almost universally promoted by atmospheric CO2 enrichment, and which is therefore something that we simply cannot do without. We must continue to let the air's CO2 content rise, as a beneficent and essential byproduct of the burning of fossil fuels.

References
Alcamo, J., Doll, P., Henrichs, T., Kaspar, F., Lehner, B., Rosch, T. and Siebert, S. 2003. Global estimates of water withdrawals and availability under current and future 'business-as-usual' conditions. Hydrological Sciences Journal 48: 339-348.

Berndes, G. 2008. Future biomass energy supply: the consumptive water use perspective. International Journal of Water Resources Development 24: 235-245.

Grubler, A., O'Neill, B., Riahi, K., Chirkov, V., Goujon, A., Kolp, P., Prommer, I., Scherbov, S. and Slentoe, E. 2007. Regional., national, and spatially explicit scenarios of demographic and economic change based on SRES. Technological Forecasting and Social Change 74: 980-1021.

Klein Goldewijk, K. 2005. Three centuries of global population growth: a spatial referenced population (density) database for 1700-2000. Population and Environment 26: 343-367.

Klein Goldewijk, K., Beusen, A. and Janssen, P. 2010. Long-term dynamic modeling of global population and built-up area in a spatially explicit way: HYDE 3.1. The Holocene 20: 565-573.

Oki, T. and Kanae, S. 2006. Global hydrological cycles and world water resources. Science 313: 1068-1072.

Renssen, H., Goosse, H., Fichefet, T., Brovkin, V., Driesschaert, E. and Wolk, F. 2005. Simulating the Holocene climate evolution at northern high latitudes using a coupled atmosphere-sea-ice-ocean-vegetation model. Climate Dynamics 24: 23-43.

United Nations. 2009. World Population Prospects: The 2008 Revision. Population Division of the Department of Economic and Social Affairs of the United Nations Secretariat. http://esa.un.org/unpp.

Varis, O. 2007. Water demands for bioenergy production. International Journal of Water Resources Development 23: 519-535.

Vorosmarty, C.J., Green, P., Salisbury, J. and Lammers, R.B. 2000. Global water resources: vulnerability from climate change and population growth. Science 289: 284-288.