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Also interacting with the climate change boundary is the option of desalination, which allows farms and cities in arid areas to use water from the sea by extracting the salt to yield fresh water. If Israel could use more desalinated water, then perhaps some of the flows of the River Jordan could be restored and the gradual drying up of the Dead Sea avoided. In desert countries like Saudi Arabia and small island states that have no rivers at all, desalination is the only option to support current human populations. But there is a price: Desalination is a very energy-intensive process, and where fossil fuels are used contributes substantially to greenhouse gas emissions. In Saudi Arabia, currently the world’s largest producer of desalinated water—on which it depends for 70 percent of its drinking water—the largest plant at Shoiba sits next to an oil-fired power plant, which supplies the required heat and energy.32 But desalination could be very compatible with intermittent renewable power sources like solar and wind, because fresh water—unlike electricity—can be stored easily, so needs to be produced only when the wind is blowing or the sun shining. Care however needs to be taken to ensure that the concentrated brine that is left over from desalination does not damage the marine environment when discharged into the ocean. Overall, desalination powered by renewables can likely make a major contribution to helping arid countries take more water from the sea, and thereby extract less from ecologically sensitive and overused river basins.

Once again, global warming will change everything. Although precipitation levels globally are expected to rise, the changing climate will alter the distribution of the planet’s water. Unfortunately most of the additional rainfall is expected in higher latitudes, while dry subtropical areas like the Mediterranean,33 southern Africa, Central America, and Australia can expect to get dryer still.34 Here existing dense populations may make additional water storage or desalination unavoidable. A changing climate will also change the seasonality of runoff: In the western United States, for example, earlier snowmelt and more winter rainfall (rather than snow) in a warmer world could advance the date of peak runoff by as much as two months.35 In high mountain areas like the Andes and Himalayas, melting glaciers will change the hydrology of downstream rivers, generally meaning less river flow in the dry season. For these regions new reservoirs are about the only conceivable adaptation option: Natural frozen stores of water must be replaced with human-engineered liquid ones.

MAKING WATER PAY

Two of the most effective remedies for water scarcity are policies more usually associated with the political right than with the left: increasing global trade and, most controversially of all, privatization. For years now it has become an accepted mantra that we should all eat locally produced food in order to reduce the volume of global trade and help the climate. There are many reasons why this is a good idea: I am a devotee of my local farmers’ market because I like to support growers in my area and get to know the people who are producing my family’s food. But environmentally the issues are less clear-cut. Although some greenhouse gases are involved in the shipping of bulk commodities like wheat and beef, in water-use terms it makes sense for most food to be produced in well-watered areas with high rainfall rather than in arid regions where irrigation can devastate the local ecology—as it has in Central Asia’s Aral Sea. The water used in producing different crops and foods is rated as “virtual water,” and its trade provides a little-known but enormous global environmental boon.

In Egypt, for example, each kilogram of corn imported from Europe saves this desert country half a cubic meter of water.36 Wheat imports from abroad in total therefore save Egypt 5.8 billion cubic meters of water each year, equivalent to 10 percent of its