Drought, Persistent and Severe, Strikes Again
By William K. Stevens, The New York Times, April 25, 2000
Tornadoes, hurricanes and floods are the stuff of television drama; they make people sit up and take notice.
Not so with drought. It is a far more subtle weather catastrophe that "sneaks up on you" and consequently commands little awe or respect, says Dr. Donald A. Wilhite, a Nebraska climatologist who is an expert on the subject. He calls it the Rodney Dangerfield of natural disasters.
Although it might not stir the emotions, in an average year drought is responsible for about as much economic damage as floods and hurricanes combined. Nor is it a rare phenomenon, as many people erroneously believe.
Instead, it is a persistent and permanent feature of weather and climate, severely affecting some part of the United States almost every year.
So it is that in 2000, much of the country's midsection and a broad swath of its southern tier from Arizona to Florida -- roughly a quarter of the territory of the contiguous 48 states in all -- is already experiencing a moderate to severe drought with the peak months for drought still ahead. If long-range forecasts of below-normal rainfall and above-normal temperatures are accurate, conditions may well get worse.
As a new growing season begins in the Iowa-Nebraska-Illinois breadbasket, severe deficiencies of soil moisture are creating a potential new threat to farmers' chronically sagging fortunes.
The water level of the Great Lakes is approaching all-time lows; not since 1965 have Lakes Michigan and Huron been so low. The prospect looms of huge financial losses in agriculture and recreation. And as always, drought raises the risk of wildfires.
So experts are plumbing the past and investigating today's changing climate as part of an effort to understand drought better and to gauge whether longer and more severe droughts may lie ahead.
For instance, could another Dust Bowl or an even worse drought be in the nation's future? Based on scientists' analyses of tree rings, ancient soils and other evidence, the answer is almost certainly yes.
Many experts believe that with the world's climate warming as it is, droughts will become more frequent and severe.
But apart from that, and regardless of whether humans are responsible for global warming, there is plenty of evidence that big, damaging droughts are inevitable. The only question is when they will come.
A 1998 study by federal scientists, for instance, found that droughts as widespread and severe as that of the Dust Bowl of the 1930's, when 65 percent of the country was affected at the drought's peak in 1934, have occurred once or twice a century over the last 300 to 400 years.
A decade-long drought, the study found, occurs about once every 500 years.
More recent analyses of tree-ring data have identified a 16th-century megadrought that affected much of the continent for years, far outstripping any drought of the 20th century in persistence and severity.
It was during this drought that the first English colony in America, at Roanoke Island in North Carolina, disappeared, and experts now believe the drought is what killed it off.
The report, by Dr. David W. Stahle, a tree-ring analyst at the University of Arkansas, Dr. Edward R. Cook, an analyst of ancient climates at the Lamont-Doherty Earth Observatory of Columbia University, and six colleagues, appeared in the March 21 issue of Eos, a scientific publication of the American Geophysical Union.
Megadroughts lasting a century or two are known to have occurred in what is now California over the last 3,500 years.
Droughts of similar severity have also been implicated in the downfall of the empire of the Maya in Central America a millennium ago; the Akkadian empire (the world's first) in Mesopotamia 4,200 years ago (that drought lasted 300 years) and several pre-Inca cultures in South America.
But it does not take a megadrought to bring disaster, and the emerging record of past dry spells contains several smaller ones that would surely devastate much of the United States if they materialized again.
In the 20th century, major droughts in the 1950's and in 1988 were second only to the Dust Bowl. The tree-ring studies showed that comparable droughts materialized in the 1750's, 1820's, 1850's and 1860's. The worst drought in recent years came in the summer of 1988, when a combination of intense heat and widespread moisture deficiency baked more than a third of the country for weeks.
It destroyed at least half the crops on the Great Plains, lowered water levels in the Mississippi River so much that barges ran aground and created the conditions for a summer-long run of forest fires in the West.
The most famous engulfed great swatches of Yellowstone National Park in that area's biggest conflagration in at least 200 years.
All of this makes it abundantly clear that drought is a standard feature of climate, not an exception to the rule.
But disaster planners often do not see it that way, and consequently pay more attention to other threats like floods, says Dr. Wilhite, who directs the National Drought Mitigation Center, a research and planning organization at the University of Nebraska.
If that is so, one reason may be that drought is rarely a cause of death or destruction of buildings, although drought-associated heat waves often send mortality temporarily skyrocketing.
(Summer heat makes drought worse by speeding evaporation of moisture; and when there is no more moisture to evaporate, the sun's energy goes entirely into heating the land, which in turn makes the air above it even hotter.)
Another possible reason for a relative lack of urgency about drought is that it is a creeping phenomenon. "It's hard to know when you're in a drought," Dr. Wilhite said.
"When do you ring the bell?" As things stand now, he said, people "essentially don't become aware of drought and the dimensions of its impact until they're in the middle of it; at that point, there's not a lot you can do about it."
According to figures compiled by Dr. Wilhite's organization, drought costs the nation $6 billion to $8 billion a year on average, compared with $2.4 billion for floods and $1.2 billion to $4.8 billion for hurricanes.
The worst droughts easily outstrip the worst floods and hurricanes; for instance, the 1988 drought exacted a toll of about $40 billion, while the Mississippi Valley floods of 1993, the worst of the century, cost $28 billion at most, and Hurricane Andrew in 1992, the most expensive on record, caused $25 billion to $33 billion in damages.
Damages might be reduced by better planning, Dr. Wilhite said. For example, farmers might switch to drought-resistant crops, and water conservation measures could be planned before a drought occurred instead of in a crisis.
Soil management and tillage practices have improved since the Dust Bowl days, when vast expanses of topsoil simply dried up and blew away, and to some extent the improvements might ease the impact of a comparable drought today, he said. But "some things you can't do anything about," he said, including depletion of aquifers and the dropping of water levels in reservoirs, lakes, rivers and streams.
What does the future hold?
Over the last 25 years, the earth's average surface temperature has risen at a rate equal to about 3.5 degrees per century.
Mainstream scientists believe it will continue to rise at that rate in the 21st century if emissions of heat-trapping waste industrial gases like carbon dioxide are not reduced.
A warming atmosphere has two effects related to precipitation, experts say: it causes more moisture to evaporate from the oceans and fresh waters, thus producing heavier rains.
Scientists at the National Climatic Data Center in Asheville, N.C., have detected a clear trend in that direction.
But when it does not rain for a long time, a warmer atmosphere also intensifies drought by making more moisture evaporate from the land, faster. No trend toward increased drought has been detected over the last century, said Dr. David R. Easterling, a researcher at the Climatic Data Center.
But he said that eventually, in the interior of the continent, drought could win out over increasing precipitation if the atmosphere warmed enough.
Global warming could also affect the size and frequency of forest fires. A recent study of charcoal layers in Yellowstone, reflecting the last 17,000 years of fire history there, found that the number of fires peaked before 7,000 years ago, a time when summers were warmer than now.
The implication is that while fires would become more frequent in a warmer climate, they would also become smaller because less fuel, in the form of dead trees and forest litter, would have a chance to build up, said Dr. Cathy Whitlock, a geographer at the University of Oregon who was a co-author of the study. The other authors, also at Oregon, were Dr. Sarah H. Millspaugh and Dr. Patrick J. Bartlein. The paper appeared in March in the journal Geology.
A longtime expert on wildfire in the West, Dr. William Romme of Fort Lewis College in Durango, Colo., said the study suggested that "if the warming trend over the last century continues, as it appears it will, we can expect some really dramatic changes in fire behavior." It is likely, he said, that more frequent fires will allow less fuel to build up, in time making big conflagrations like that of 1988 less probable.
As for drought itself, some scientists believe, the frequency and severity of future dry spells could increase as a result of natural, periodic oscillations in Pacific Ocean surface temperatures on a scale of decades. The Stahle-Cook paper in Eos suggests that the 16th-century megadrought could have been caused by a prolonged cooling phase in some areas of the Pacific.
Back-and-forth oscillations in the Pacific temperatures alter atmospheric circulation patterns that deliver moisture to North America. One phase, which favors development of the phenomenon known as La Niña, is associated with drought in much of the United States.
That phase has prevailed for the last two years -- La Niña years -- and some scientists predict that it will predominate in the decades just ahead.
For that matter, Dr. Easterling said, the big drought of 1988 was "pretty well tied" to La Niña. So was last summer's severe drought in the Middle Atlantic region and Northeast, many scientists believe. And so is this year's developing drought across the southern tier and midsection.
So far, the drought of 2000 has been most clearly manifest in the Great Lakes region.
A combination of lower precipitation and high temperatures over the last three years, coupled with relatively little snow, has reduced stream flows in the region to well below normal.
In the lakes themselves, there has been "an unprecedented drop" in water levels over the last two to three years, said Dr. Frank H. Quinn, senior research hydrologist for the National Oceanic and Atmospheric Administration's Great Lakes Environmental Research Laboratory in Ann Arbor, Mich.
Lakes Huron and Michigan, which are really one body of water, have dropped about three feet, the biggest such drop in 140 years of record keeping, ending a 30-year run of above-average levels.
This has some benefits: "We will have the best beaches in 35 years and considerably less erosion," Dr. Quinn said.
But in a region whose economy is in so many ways tied to the lakes, he said, there will also be large negative effects: the region's billion-dollar water recreation industry is likely to be hit hard, as many marinas no longer have enough water to hold boats.
Lake freighters will not be able to load to full capacity. Less forceful water flows will reduce hydropower production downstream of the lakes.
The drought has not yet really pinched the midcontinent agricultural regions, since the growing season is only beginning, Dr. Wilhite said. Nor have the high evaporation levels of late spring and summer set in, not to mention the normal summertime increase in demand for water.
The squeeze is likely to come, Dr. Wilhite said, if and when long-range forecasts of abnormal warmth and dryness materialize in the next three months.
The most recent observations show that much of the southern tier from Arizona to Florida is also in a severe drought, but the latest forecasts call for normal rainfall in most of that swath over the next three months.
But the forecasts can and do change from month to month.
So for those in the dry regions, it is a time to prepare, watch and wait.