Water and Food in Developing Countries in the Next Century

Abstract

The world’s supply of water is fixed. It is estimated that 97% of the world’s water exists in the oceans, 2.2% exists as ice and snow, mostly in the polar regions, and only about 0.7% of the total supply is the freshwater that sustains mankind, including the global agricultural system. This quantity of freshwater — around 40,500 km3 — which is the difference between precipitation and evapotranspiration, is continuously replenished by nature’s hydrological cycle. Most climatologists and hydrologists agree that there is no natural process short of climate change, especially global warming, that can increase the world’s rainfall and so the supply of freshwater. The greater the warming, the larger the expected increase in precipitation. One “simple level of analysis” suggests that global warming of 30° C could well lead to a 10% increase in evaporation and an average increase in precipitation of 10%. The biggest increases would be at high latitudes, smaller increases would occur close to the equator (Gleick, 1992). The weight of evidence suggests that this is unlikely to happen within the next several decades (Rosenzweig, 1994). It is an open question, though, as to what might happen in the second half of the next century. There are some manmade processes that can increase the supply of fresh water. One of the most important of these is the conversion of saline water from the ocean into fresh water by removing salt through desalinization or by filtration. Thus far, however, the processes that have been developed are highly energy intensive and costly; the plants presently in operation are mostly in the oil-rich, water-poor nations of the Persian Gulf. It is estimated that there are more than 11,000 desalting plants operating worldwide, but together they produce less than 0.2% of the world’s total fresh water (Postel, 1991). The costs of desalting sea water range currently from about $0.80 to $1.60 m-3, and costs of treating brackish water are about $0.30 m -3, well above the costs of fresh water used for irrigation (Wolf, 1996).