Humanity's Future: Will There Be Enough Water ... to Grow Enough Food?
Fereres, E., Orgaz, F. and Gonzalez-Dugo, V. 2011. Reflections on food security under water scarcity. Journal of Experimental Botany 62: 4079-4086.
But what about developing new sources of water, or transferring water from one place to another, in order to increase food production where water shortages exist? The three Spanish researchers say that such measures have "limited potential in some areas," but they indicate that they are "no longer possible in other world regions." They do note, however, that agriculturalists have significantly increased crop water use efficiency "by reducing water losses (and some of the water consumed in evaporation from soil) through improved agronomy and engineering of irrigation systems." But they lament the fact, as they put it, that "science has been much less successful so far in reducing the water consumed in transpiration." Fortunately for us, however, mankind en masse has had a measurable amount of success in this area, albeit unintentionally.
What we are talking about here is the extraction of fossil fuels from the crust of the Earth, which has provided so much coal, gas and oil to fuel the engines of industry that the carbon dioxide given off to the air in the combustion process has raised the atmosphere's CO2 concentration by some 40% since the inception of the Industrial Revolution. And that phenomenon has had two major effects on man's production of food. It has significantly increased the leaf photosynthetic rates of our crops, while it has significantly reduced their transpiration rates, which has led to significant increases in leaf water use efficiency, or the amount of biomass produced per unit of water transpired in the process.
In spite of these well-documented facts, to quote Morgan et al. (2011), "many believe that CO2-induced reductions in transpiration at the leaf level will be largely offset at the canopy level by increases in leaf area," and that "global warming is predicted to induce desiccation in many world regions through increases in evaporative demand." But in a real-world test of these two potentially negative phenomena in a Prairie Heating and CO2 Enrichment (PHACE) experiment conducted in a native mixed-grass prairie in Wyoming (USA), they found that the positive effects of elevated CO2 prevailed, indicating, in their words, that "in a warmer, CO2-enriched world, both soil water content and productivity in semi-arid grasslands may be higher than previously expected," providing what Baldocchi (2011) describes as "one of the first and best views of how a mixed-grass ecosystem growing in a semi-arid climate will respond to future CO2 and climatic conditions," while a full decade earlier, in fact, Robock et al. (2000) had already developed a massive collection of soil moisture data from more than 600 stations spread across a variety of climatic regimes, including the former Soviet Union, China, Mongolia, India and the United States; and in analyzing those observations, they had determined that "in contrast to predictions of summer desiccation with increasing temperatures, for the stations with the longest records, summer soil moisture in the top one meter has increased while temperatures have risen."
Nevertheless, prudence suggests that we still pursue all avenues available to us to further increase both individual plant water use efficiency and whole-field crop water use efficiency. Mankind's CO2 emissions may ultimately prove a godsend to humanity, as they just might make the difference between our being able to adequately feeding our expanding population in the very near future or our failing to do so in a catastrophe of unimaginable proportions.
Baldocchi, D. 2011. The grass response. Nature 476: 160-161.
Morgan, J.A., LeCain, D.R., Pendall, E., Blumenthal, D.M., Kimball, B.A., Carrillo, Y., Williams, D.G., Heisler-White, J., Dijkstra, F.A. and West, M. 2011. C4 grasses prosper as carbon dioxide eliminates desiccation in warmed semi-arid grassland. Nature 476: 202-205.
Nature. 2010. Can science fed the world? http://www.nature.com/news/specials/food/index.html
Robock, A., Vinnikov, K.Y., Srinivasan, G., Entin, J.K., Hollinger, S.E., Speranskaya, N.A., Liu, S. and Namkhai, A. 2000. The global soil moisture data bank. Bulletin of the American Meteorological Society 81: 1281-1299.
Science. 2010. Food security. http://www.sciencemag.org/site/special/foodsecurity/