IN THE long debate over population and resources, food has '' drawn the most attention -- and the worst forecasts. From Thomas Malthus in 1798 to Paul Ehrlich in 1968, predictions of imminent famine have been off the mark. Human ingenuity so far has managed to come up with new ways to out-reap the Grim Reaper in the global harvest.
Dodging a bullet, however, is no guarantee of immortality. Perhaps a better example of a finite natural resource pressed to its limits by population growth is the fresh water all human beings need for health and survival. Lack of access to safe water is a daily problem for many people in the developing world, but in the past year concerns about the safety of drinking water have led Americans to shun their taps in Milwaukee, Des Moines and -- last month -- the nation's capital.
Difficult to purify, expensive to transport and impossible to substitute, renewable fresh water may be the most unforgiving of the planet's natural resources. About 10,000 cubic miles of renewable fresh water move through the hydrological cycle each year, a small fraction of all the Earth's water and a volume relatively unchanged throughout human history.
World population, however, has expanded from a few million in prehistoric times to 5.5 billion today, with increases in per capita water consumption that further strain the water cycle. It's a renewable resource; we can't run out of it. But we can spread it so thin that health and development are stymied. And that's what increasingly is happening as many countries drop below widely recognized benchmarks of per capita water availability needed for health and development.
Today, from the deserts of Saudi Arabia to the Texas panhandle, the expansion of irrigated agriculture is drawing on non-renewable groundwater to feed the world. In many nations in Africa, reservoir and lake levels are falling and water tables are dropping, leaving wells dry as greater numbers of users press on limited supplies. For some countries with rapidly expanding populations, the danger is that now-abundant non-renewable water resources eventually will be sucked dry. Much more modest renewable resources would then be pressed to support populations that could not have grown so numerous in the absence of non-renewable water.
There are many terms for what happens when population growth races past critical natural thresholds of resource availability, including "overshoot" and "population crash." A less alarmist and more accurate term describes water use in much of today's world: unsustainable.
