Different times, different worries. Four hundred years ago, many Europeans could see a major energy crisis on the horizon. Fuel wood was being used up faster than it could be replenished, the forests of Europe were shrinking, and it was only a matter of time before they would all be gone. What would civilized nations then use to provide heat for cooking and for industries such as smelting and forging iron?

We know the answer to that. People switched to fossil fuels, first coal, then oil and natural gas. Now we worry about these resources running out. A few months ago, I reviewed the situation in this column and concluded that we are centuries away from genuine worldwide shortages, although distribution is and will be a major issue. Of course, burning fossil fuels may also lead to other difficulties, from rising sea levels to increased air pollution, but I'm not going to focus on those today. Instead I want to discuss another shortage, one which will have major effects long before we deplete our supplies of coal and gas - and is in fact affecting us already.

I want to talk about water. If you look at this planet from space, on the face of it a water shortage sounds ludicrous. We call our world "Earth," but a better name for it might be "Water." Seventy percent of the whole globe is covered by oceans, with an average depth of a mile or more.

Unfortunately, these oceans are too salty to drink. What we need is fresh water.

At first sight, this seems like a bogus problem. We have apparently inexhaustible amounts of fresh water. The arithmetic runs as follows: There are about 1.4 billion cubic kilometers of water in the outer layers of the planet, including seas, lakes, oceans, and underground storage. All but three percent of that is salt brine, which means we have 42 million cubic kilometers of fresh water. Of this, roughly three-quarters is locked away in the "cold storage" of glaciers and polar ice caps. Unless we find a way to use these, we are down to ten million cubic kilometers. However, all but two percent of this resides in the hard-to-access forms of deep underground aquifers, soil moisture, and atmospheric water vapor. We are reduced to 200,000 cubic kilometers of fresh water, held in rivers and lakes and swamps, or within plants.

We do not usually think of water in terms of cubic kilometers. We are more familiar with gallons, barrels, or bathtubs. Fortunately, the conversion is not hard to do. We find that 200,000 cubic kilometers equates to 50,000 trillion gallons of fresh water. That's enough to provide 250 gallons a day to every person on the planet, if we all live to be 90. And this does not take into account the fact that the sun's heat is continuously evaporating about 1,250 cubic kilometers a day from the salty oceans, and returning it as fresh water in the form of rain or snow. That is more than enough to replenish daily use.

Simple arithmetic like this can be highly misleading. It might lead you to say, what water shortage? How can there possibly be a shortage, when not only water but fresh water is clearly so abundant?

Yet acute water shortages are predicted for the coming century. To see why, note first that even in this country, where clean and abundant water is often taken for granted, what runs out of our household faucets is only a small fraction of our actual use. About one-tenth of fresh water goes to home supply and sewage systems, two-fifths to agriculture, and the rest to industrial needs.

Second, water is one of Nature's best solvents. That makes a water supply terribly easy to pollute. With a little carelessness, the output from an industrial plant or a sewage system can spoil many billions of gallons. To take one example, kepone deposited into Virginia's James River made the waters of the whole estuary toxic and unusable for many years.

The biggest problem, though, is not in locating fresh water. It derives from the hugely unbalanced distribution of what we have. Fresh water is even more unevenly available around the world than fossil fuels. A Canadian can rely on a supply of 20 million gallons a day; a Libyan has little more than a hundred. Los Angeles and Phoenix are parched; visitors to Seattle sometimes feel that it rains all the time. On a continental scale, Africa has only one-fifth as much fresh water runoff as South America, and is close to twice the size.

Water wars are already common. One-fifth of the planet's land area is too arid to permit agriculture, and squabbles over water in such regions are inevitable. In this country, the fights usually take place in courtrooms, where ownership questions combine with shipment and access rights to create a lawyer's paradise. Supplies of fresh water in California, discovered years ago, remain untapped, locked up in endless legal battles. In other parts of the world, such as the sub-Saharan region of Africa, battles over water are more direct and more bloody.

The safeguards against future water shortages for a water-rich country (and the United States is one, even though Canada is far more so) are rather simple. If we chose to adopt them, elementary conservation measures such as are already practiced in many countries and on board any ship, could immediately reduce fresh water use by a factor of five. Tightening of pollution laws would double the potable water supply. Recycling could be increased at little cost or inconvenience.

Maybe it is past time for us to do all these things. Where I live, the water often tastes lousy and smells worse. A gallon of bottled water costs close to as much as a gallon of gas. And to see what real water shortages can do to a society we need not peer into an uncertain future. We can look backward in time 5,000 years, to the developing civilizations in the valley of the Nile or between the Tigris and Euphrates. There, the control of water supplies produced "watershed empires" and "hydraulic states," despotic, tightly controlled, and thoroughly unpleasant to someone used to modern freedoms.

It will probably do no good, but I think I'll mention all this the next time that my daughter spends more than 20 minutes in the shower.

"Borderlands of Science" is syndicated by: