Water: The Arid Side of Climate Changes

America is blessed with ample water--on an average day 4,200 billion gallons of rain fall on the lower 48 states. Most of that evaporates, leaving only about 1,435 billion gallons a day, of which in 1985, only about 340 billion gallons a day are withdrawn for human use.

It seems like more than enough.

However, as anyone who has ever flown across the nation (and looked out the window) can attest, the water is not spread evenly. Vast sections of the West are arid, though not necessarily unpopulated.

Total water use exceeds average stream flow in 24 of 53 Western water-resource regions, a difference made up by “mining” dwindling ground water stocks and importing water. Much of the Colorado’s flow, for example, is dammed, diverted and consumed upstream by irrigation and by the millions upon millions of people living (and insisting on green lawns) where it would otherwise be too dry.

And matters may get much worse. After studying the temperature and stream-flow records, scientists have concluded that if a “conservative” 2-degree Celsius increase in temperature occurs, the virgin flow of the Colorado could fall by nearly a third.

If, as some of the computer models suggest, this is accompanied by a 10% fall in precipitation in the Southwest as a result of new weather patterns, water supply in the upper Colorado could fall by 40%, but even if precipitation went up 10%, runoff would still drop nearly a fifth.

Across the West, the picture is similar--in the Missouri, Arkansas, Texas Gulf Coast and California irrigation regions, runoff could fall by 40% or more. In the Missouri, Rio Grande and Colorado basins, even current water needs could not be met by stream flows after the expected climatic changes.

“One model we’re looking at,” says Texas Agriculture Commissioner Jim Hightower, “predicts a 25% increase in the demand for irrigation water” from the Ogallala Aquifer, the great subterranean lake that irrigates the plains and is already badly depleted. “You can’t pump more water if the well has already gone dry.”

Even areas that we’re used to thinking of as ruined may be ruined in new and interesting ways. Lake Erie and the Great Lakes in general became symbols of the environmental decline in the 1970s. They have recovered somewhat, but a change in climate may subject them to unprecedented stresses (which is like subjecting the South Bronx to unprecedented decay). Under EPA models of doubled carbon dioxide levels, the average level of Lake Superior could fall by a foot and a half--which doesn’t sound like so much, except that the fleet of ships working the Great Lakes are designed to pass within a foot of the bottom of the locks and channels. Ships will have to carry smaller loads and sail more frequently (burning more fuel, which will . . . you know).

Shippers may be aided by a longer shipping season. The central basin of Lake Erie currently sees 83 days of ice, but if the temperatures rise, only areas near shore will freeze, and then for three weeks or less.

But in that case, the erosion along the shoreline, which has been protected by the ice, will increase, since the winter is the season of big storms. And if the ships do keep sailing, it may also increase the supply of sad folk songs--it was a winter gale in 1975 that sank the Edmund Fitzgerald with all 29 hands.

Declining water levels can cause a variety of miscellaneous mischief. When droughts lowered Lake Michigan in the 1960s, dry rot set in along the piers and pilings of Chicago’s shoreline. Hydropower production may drop as flows along the Niagara River fall, pollutants in the lakes may become less diluted, and the warmer lake waters will almost certainly lead to algal blooms and a return of the oxygen deprivation that “killed “ Lake Erie once before.

That “increased eutrophication could make the Lake Erie Central Basin uninhabitable for fin fish and shellfish during the summer,” concluded the EPA.

Across the country and across the world the usual endless list of multiplying dangers can be compiled.

“Water quality appears to be vulnerable to deterioration because of increased use of agricultural pesticides as a response to climate change,” reported the EPA. There is an increased risk of forest fires like the 1988 Yellowstone blaze (“The biggest difference between this year and other years is no rain,” the park ecologist Donald Despain said after the fire). As usual, no one knows exactly what will happen.

But the computer model used by Syukuroa Manabe of the National Oceanic and Atmospheric Administration insists there’s a greater than 90% chance that soil moisture across North America, Western Europe and Siberia will decrease.

An obvious question is what all this means for agriculture (or, since “agriculture” has become abstracted from everyday life in the same fashion, as, say, “the military,” it might be better to ask what this will mean as regards dinner). The answer comes on several levels, the first that of the individual plant.

Quite apart from heat and drought, the simple increase in the amount of carbon dioxide in the atmosphere affects plants. Ninety percent of the dry weight of a plant comes from the conversion of carbon dioxide to carbohydrates by photosynthesis.

If nothing else limits a plant’s growth--if it has plenty of sunshine, water and nutrients--then increased carbon dioxide should increase the yield. And in ideal laboratory conditions this is what happens: As a result, some journalists have rhapsodized about “super-cucumbers,” and found other green linings to the cloud of greenhouse gas.

But there are drawbacks. If some crops grow more quickly, farmers may need to buy more fertilizer. Leaves may become richer in carbon but poorer in nitrogen, reducing food quality not only for humans but for nitrogen-craving insects who may eat more leaf to get their fix.

In the best case, direct effects of increased carbon dioxide on yield are expected to be small: Annual harvests of well-tended corn crops might rise about 5% when carbon dioxide levels reach 400 parts per million, all other things being equal.

But all other things, of course, won’t be equal. All other things--moisture, temperature, growing season--will be different.

It is an obvious point, but one worth repeating: Everything we eat, except fish and a few hothouse vegetables, spends its growing life in the open air, “exposed,” in the words of biologist Paul Waggoner, “to the annual lottery of the weather.” Today’s Lean Cuisine frozen entree stood rooted in some Kansas field last year, where it survived attacks from insects and disease, and grew as fast as its supply of water and sunshine and nutrients allowed. About 50 million acres of America’s cropland and rangeland is irrigated, but even that depends on the weather over any long stretch. And we can’t just stick the wheat crop under glass.

The uncertainty ahead, in other words, extends to the pastures and the fields--to our food supply. Too many unknowns and too many variables make even the broadest predictions difficult.

As usual, there is a strong temptation to clutch at every reassurance: If the models say there will be enough food to go around, whew!

But when computers are modeling something as complex as all of agriculture, the potential for error is enormous (or the potential for accuracy is small).

The effect of the heat and drought of 1988 made liars of most of the computer programs in just a few weeks. They had concluded that a doubling of carbon dioxide, which will not happen for several decades, might make the weather hot and dry enough to cut American corn and soybean yields as much as 27%. But in the summer of 1988, when the rains held off, the American corn crop fell more than 35%, down 2.6 billion bushels.

Even if, as seems likely, the heat wave had little to do with the greenhouse effect, we now have some idea of what it will feel like once it does kick in. By early this year, grain storage around the world amounted to only about 250 million metric tons, enough to last 54 days--the lowest level since 1973. Worldwide consumption of grain outpaced worldwide production by 152 million metric tons last year.

One can run a budget deficit for quite a while, but when the food runs out there’s no central bank to mint some more. A second year of drought would be a “catastrophe,” the assistant secretary of agriculture said. “If we return to a normal situation in the weather, we’ll be OK,” said Nelson Denlinger, executive vice president of the United States Wheat Assn.

But there is no normal situation in the weather to return to--that’s the point. The weather of the future cannot be predicted from the weather of the past, nor can its effects.

(From “The End of Nature,” by Bill McKibben. Copyright, 1989, by William McKibben. Reprinted with the permission of Random House Inc.)

COMING UP

SUNDAY: Can the consequences of the “greenhouse effect” be ameliorated? Unfortunately, there are no quick fixes.