The great famines predicted for the 1970s were averted by new varieties of rice, wheat and maize, whose development was known as the "green revolution". They produce tremendous yields, but require plenty of water. This has been provided by irrigation, much of which uses underground reserves. Unfortunately, many of them are being exploited much faster than they are being replenished. In India, for example, some 250 cubic kilometres (a cubic kilometre is a billion cubic metres or a trillion litres) are extracted for irrigation every year, of which about 150 are replaced by the rain. "Two hundred million people [are] facing a waterless future. The groundwater boom is turning to bust and, for some, the green revolution is over."Yikes. Meanwhile, salty oceans will start encroaching on freshwater supplies lying around the coast, and many of the proposed solutions to the shift would only accelerate global warming -- such as moving agriculture inland (which would require cutting down trees in the Amazon) or building desalination plants (which are energy-intensive). The obvious step would be to cut carbon emissions and forestall the whole disaster, although Monbiot reports that forestalling global drying would require a whopping 90 percent cut in carbon emissions by wealthy nations before 2030. Who'd take those odds?
In China, 100 million people live on crops grown with underground water that is not being refilled: water tables are falling fast all over the north China plain. Many more rely on the Huang He (the Yellow river), which already appears to be drying up as a result of abstraction and, possibly, climate change. Around 90% of the crops in Pakistan are watered by irrigation from the Indus. Almost all the river's water is already diverted into the fields -- it often fails now to reach the sea. The Ogallala aquifer that lies under the western and south-western United States, and which has fed much of the world, has fallen by 30 metres in many places. It now produces half as much water as it did in the 1970s.