Great Plains or Great Desert? The Sea of Dunes Lies in Wait
Published: May 28, 1996
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Evidence is also mounting that severe droughts and floods devastated a number of early civilizations. The Peruvian coast of South America is a case in point. An analysis of Andean ice cores dating back to the fifth century A.D. has revealed that the region endured alternating droughts and wet periods lasting up to 400 years each. Dr. Lonnie G. Thompson, a paleoclimate expert at Ohio State University, said that archeological evidence indicated that the rise and fall of a number of pre-Inca cultures "are in sync with these precipitation changes."
The wet-dry variations might have been caused by long-term oscillations in the ocean-atmosphere climate system, analogous to the short-term changes typical of El Nino -- the vast pool of warm water in the tropical Pacific that appears from time to time, altering regional climates around the world. That theory is bolstered, Dr. Thompson said, by core samples from Tibet that show wet-dry cycles matching those in Peru.
Other evidence of global or at least hemisphere-wide climatic changes affecting earlier civilizations has previously emerged. Two years ago, Dr. Scott Stine of California State University at Hayward reported that what is now California had experienced two droughts beginning about 1,100 years ago, the first lasting 220 years and the second 140 years. Both droughts were much more intense than the six-year dry spells that afflict modern California from time to time. They also coincided roughly with similar droughts in South America that caused the collapse of that continent's most advanced pre-Inca empire, a state called Tiwanaku -- and they coincided with a warm period in Europe that allowed the Vikings to colonize Greenland and vineyards to flourish in England.
Last year, a University of Florida team led by Dr. David A. Hodell reported that an unusually severe drought about 1,200 years ago might have contributed to the abrupt decline of the classic Maya civilization in Yucatan.
And in 1993, an American-French archeological team headed by Dr. Harvey Weiss of Yale University reported that the sudden onset of a 300-year drought had brought about the collapse, 4,200 years ago, of the Akkadians, the Middle East's earliest known empire.
Until recently, paleoclimatologists have focused more on the climatic record of the ice ages because those temperature swings were much larger, and therefore more interesting to scientists, than those of the Holocene. By comparison, "people figured that the Holocene is truly boring," Dr. Alley said. Then, in 1993, attention shifted to the last warm interglacial period before this one, which lasted from about 130,000 to 120,000 years ago. Reports in that year on studies of Greenland ice cores suggested that the last interglacial period had experienced abrupt and previously unexpected swings of global temperature. But that finding turned out to be ambiguous.
Now, as the results of studies under way for some time are beginning to come in from around the world, the spotlight is shifting to the current era, and a broad pattern of abrupt and severe climatic change in the Holocene is being seen for the first time. The studies show clearly that the Holocene has abounded in climatic swings "large enough to dwarf changes seen in the instrumentally based climate record of the last 150 years," Dr. Jonathan T. Overpeck wrote recently in the journal Science. Dr. Overpeck directs the paleoclimatology program of the National Geophysical Data Center in Boulder, Colo.
Some of most arresting findings are those having to do with the Great Plains. Vast stretches of the plains, from Texas and New Mexico to Nebraska and South Dakota, consist of classic sand dunes covered thinly by vegetation. One area, the Nebraska Sand Hills, is the biggest dune field in the Western Hemisphere. When atmospheric circulation settles into a characteristic pattern of no rain and high heat over the region, the vegetation dies. The dunes, naked to the wind, go into their Sahara mode.
Through radiocarbon dating of strata in the dunes and historical accounts from the 1700's and 1800's, Dr. Muhs and colleagues have found that the stripping of vegetation caused by droughts has several times activated the dunes over large areas in the last 3,000 years.
The historical accounts make it clear that when 19th-century textbooks referred to the Great American Desert, the allusion was not just born of some effete Eastern bias. The Nebraska Sand Hills, for instance, were described by an explorer in 1796 as a "great desert of drifting sand, without trees, soil, rock, water or animals of any kind, excepting some little varicolored turtles, of which there were vast numbers." In 1855, a sketch by another explorer depicted bare, peaked dunes in the Sand Hills stretching to the horizon.
In the 1860's, a drought much more severe than the one that caused the Dust Bowl apparently brought the desertification of the West to its most recent peak; after that, the dunes stabilized again. Dr. Muhs said that it might take less than a decade of drought to kill enough vegetation to create desert conditions again.
Aerial photographs show that in much of the plains in the mid-1930's, including the Dust Bowl region, some dunes were already "going active." Had the drought not ended when it did, Dr. Muhs said, a repeat of the 1860's could have resulted.
The mega-droughts have come so often, he said, that it is impossible to judge whether the "normal" condition of the plains is to have controlled or active dunes.
The present drought in the southern Great Plains, which threatens cattle ranchers and wheat growers with bankruptcy, is a little more than six months old. It would probably take at least two or three more years of drought, Dr. Muhs said, for the desert to begin to be unleashed again. He and his colleagues have concluded that the predicted greenhouse effect would probably mobilize the dunes once again. "If you superimpose that on one of the warm, dry periods we know has recurred repeatedly in the last 1,000 years or more," Dr. Muhs said, "you're looking at an enormous impact on the landscape."
Either way, Dr. Overpeck wrote in Science, future climatic changes of the kind produced earlier in the Holocene "may be our biggest worry in the years to come."
Photo: Nebraska Sand Hills, biggest dune field in Western Hemisphere,are stabilized by a thin cover of vegetation, above; when high heat and drought settle into the area, the vegetation dies and the dunes shift like the Sahara. (Dr. Daniel Muhs/U.S.G.S.) (pg. C9) Diagram/Map: "How Midwestern Dunes Can Migrate" show the direction of winds strong enough to move sand-sized particles across areas of the Midwest.; Before a Drought Today in the Great Plains, dunes are covered by a thin layer of short grass, bunch grass or sagebrush, giving them the appearance of hills.; During a Drought Heat and drought kill most vegetation, leaving bare sand, which is blown into parabolic dunes by strong northwesterly winds. This type of dune is common in the Great Plains.; Creating a Desert The wind continues to blow enough sand to bury roads, railroads and farmhouses in its path. The dunes creep forward, with reinforcement sand coming from the river. (Source: Dr. Daniel Muhs, United States Geological Survey, Denver)