Water, Water, Nowhere?
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"Human waste has an enormous impact on water quality," says Vörösmarty. Even where there are sewers, they often just move the problem to the nearest river. "There's a delicate balance between human health and the health of ecosystems, and if you begin to err on the side of human health, that means that the ecosystems may be placed in jeopardy."

The wild card

The wild card in the world's water supply is global warming, which most scientists now accept is occurring, according to Vörösmarty. "I think there is a fairly strong signature of climate change. The big question is, what impact will it have on water supplies?"

In January, the United Nations' Intergovernmental Panel on Climate Change (IPCC) issued its latest report on global warming, stating that the Earth's average temperature is likely to rise by between 2.5 and 10.4 degrees Fahrenheit in this century. (By comparison, the Earth's temperature has risen about 9 degrees since the last ice age.) Sea levels could rise by as much as 35 inches.

George Laney, Newmarket, N.H., water and sewer superintendent, at the town's water treatment plant. A shortage of water has forced the town to start using its water treatment facility.

Higher average temperatures have a multitude of effects on the hydrologic cycle: increased evaporation rates, higher rainfall amounts and decreased snowfall. However, because more of the precipitation may come in the form of intense storms, floods will be more likely, and less of the rainwater will be available for use. Increased evaporation could create more droughts. Hydrologic records--on which modern irrigation, dam construction and water releases are based--will become more difficult to interpret. "At a time when water scarcity calls for intensified planning," wrote Jacques Leslie in last July's Harper's Magazine, "planning itself may be stymied."

Vörösmarty and his group wanted to assess the importance of climate change on the world's water supply. They incorporated currently-accepted climate-change scenarios into their computer model to predict the water supply in 2025. What they found disputes much of the current thinking on global warming and water scarcity.

Climate change will account for only about 20 percent of the impending global crisis in water scarcity, the study found, while population changes and economic development account for the remaining 80 percent. "What global warming does is make the world only about 5 percent drier on average, and on a regional scale you have big winners and losers," says Vörösmarty.

With population growth, economic development and climate change all in its model, the UNH group was able to obtain a wide-ranging, geographically detailed view of the world's water future. Most of the globe will have a more difficult time meeting its water needs, especially already dry Africa and the Asian subcontinent. Some areas will actually gain freshwater resources. Parts of central and northern Asia, especially, will become relatively wetter, as will regions of the western U.S. and Canada.

Vörösmarty's research underlines the difficulty of predicting regional effects. Take China, for example. The scenarios show that climate change is likely to mitigate many of the effects of an increased population and water demands on the Yellow River. By contrast, only 500 miles to the south, the conjunction of climate change and economic development means that demands on the Chang Jiang (Yangtze) River will more than quadruple. (It is in the middle course of the Chang Jiang that the controversial Three Gorges Dam project is being constructed.)

As that example demonstrates, the water situation is extremely complicated, and relatively small regions differ significantly in their response to a host of factors. "It's the interaction between climate, population growth and economic development together that gives you the relative water demand signature," Vörösmarty says. The current focus on climate change masks the strain an increasing population will place on the Earth's freshwater resources.

"It could be a real problem for us, as a society, to cope with this, because we're going to have all these hot spots popping up. I think in the United States the issue will be more a matter of economic concern than a question of survival, because we can pay our way out of problems. People who live in developing countries are already quietly suffering." ~

David Appell is a physicist and free-lance writer who lives in Gilford, N.H. He specializes in stories about scientific topics.

For more information, see the Water Systems Analysis Group Web site at http://www.watsys.sr.unh.edu.

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