EIGHTH CENTURY MEGADROUGHT IMPACTED MUCH OF NORTH AMERICA

FAYETTEVILLE, Ark. - A drought that lasted three times as long as the Dust Bowl of the 1930s wreaked ecological havoc over much of the western United States and Mexico, and occurred at about the same time as the fall of Teotihuacan and classic Mayan civilization 13 centuries ago, say University of Arkansas researchers.

David Stahle, professor of geosciences, presented some of his findings today at the annual Council for the Advancement of Science Writing meeting in St. Louis, Mo.

"The megadrought of the eighth century was three times longer than the Dust Bowl," Stahle said. "The consequences, ecologically and socially, must have been enormous."

Stahle and colleagues at the University of Arkansas (Matthew Therrell, Malcolm Cleaveland, and Falko Fye), the National University in Mexico City (Rodolfo Acuna-Soto), the University of Tennessee (Henri Grissino-Mayer), and Columbia University's Lamont-Doherty Earth Observatory (Edward Cook) have compiled tree ring and lake sediment records to study prolonged multi-decadal drought over the past 2,000 years.

The tree-ring data indicate a severe drought in the eighth century that lasted from AD 736 to 765, a period of 30 years. Rainfall over New Mexico averaged only 90 percent of normal for this entire 30-year period and only 83 percent of normal during the worst decade of the eighth century "megadrought." In Nebraska, during the heart of the Dust Bowl drought of the 1930s, annual rainfall averaged just 80 percent of normal. A single year with only 80 percent of normal rainfall constitutes a serious deficit, but decadal or multi-decadal dry spells of this magnitude can have catastrophic environmental and social consequences, as witnessed during the Dust Bowl.

Widespread, intense droughts like the eighth century event can impact the ecology of a region. Long term drought conditions affect a region’s plant life, animal life and hydrology and have been linked historically to outbreaks of disease.

"They leave behind a big footprint," Stahle said. Indeed, Stahle and his colleagues have shown evidence in the tree-ring record of another continental-scale megadrought that occurred in the 16th century. The 16th century drought conditions lasted 40 to 50 years, probably the most serious drought in the last 2000 years over North America, a megadrought that certainly affected the first European settlements in North America. The 16th century drought seems to have contributed to the virulence of hemorrhagic fever outbreaks that swept through the native populations in Mexico in 1545 and 1576 and caused the deaths of millions of people.

The tree-ring chronologies document eighth century megadrought over the southwestern United States, but lake sediment records indicate that this severe drought may have extended into the northern Great Plains and across central Mexico, possibly including the Yucatan peninsula. Severe drought has been well documented over central Mexico and the Yucatan during the late first millennium AD, and the new tree-ring data suggest that this period of prolonged drought may have begun in the middle of the eighth century over the southwestern United States and northern Mexico.

The lake sediment records of epic drought in the Yucatan have been linked to the collapse of classic Mayan culture. And the new research indicates that the eighth century drought may have also contributed to the decline of Teotihuacan in central Mexico, one of the six largest cities in the world at the time.

"Any society would be stressed under the conditions of one of these megadroughts," said Stahle. "And we now believe that prolonged drought may have been a factor in the collapse of Teotihuacan."