Researchers Find Evidence Of 16th Century Epic Drought Over North America

Researchers from the University of Arkansas, the Lamont-Doherty Earth Observatory, the University of Arizona, Valdosta State University and the University of Western Ontario will report their findings in an upcoming issue of the journal EOS, Transactions of the American Geophysical Union.

The researchers used drought-sensitive tree ring chronologies that extend back before A.D. 1500 from trees in Western North America, the Southeast and the Great Lakes. They found that dry conditions extended from the Sierra Madre Occidental in Mexico and the Southwest to the Rocky Mountains and the Mississippi Valley throughout the last half of the 1500s. Severe conditions occurred at times in Mexico, the Southwest, Wyoming and Montana, and the Southeast.

Looking back as far as A.D. 1200, no other drought appears to have been as intense, prolonged and widespread as the 16th century megadrought, the researchers found.

Climate varies within a certain envelope, with a drier spell one year and a damp one the next, but in the 1500s "the basement collapsed and went down to another level," said David Stahle, professor of geosciences at the University of Arkansas.

The tree ring records tell of the worst drought in 1,000 years, with an extended period of dryness lasting 40 years in places. Early records from Spanish and English settlements in the Carolinas and Virginia corroborate these findings. You can actually see the correlation between the annual weather variation written Ain archival records and the annual "reports" of the tree rings, Stahle said.

Archival records from the Spanish colony of Santa Elena on Parris Island, S.C., indicate a severe drought from 1566-69. In 1587 -- the year Sir Walter Raleigh's colony on Roanoke Island disappeared -- the Parris Island settlers abandoned their colony. Tree ring records show the year was the region's worst drought in 800 years.

"Drought is the most severe natural disaster," Stahle said. "Year-in and year-out, over the long haul, drought extracts the most from humanity."

An historic drought of this magnitude should serve as a warning to nations to learn to use their water resources wisely, Stahle said.

"If there's any lesson to be taken home from the paleo-record, it's that we need to conserve our water resources," Stahle said. "It would help prepare us for the inevitable return of drought."

Tree growth depends upon the amount of water and nutrients the plant receives in a given year. Tree cells grown during spring and summer differ from one another, and researchers peering through microscopes can tell much about a region's climatic history by looking at the recorded tree ring growth from year to year, using pencil-thin core samples from living trees.

The scientists compare the tree ring characteristics to the climate data gathered over the past 100 years. Then they use statistical models to reconstruct past climate changes based on the tree ring structures, going back hundreds of years.

Individual trees have their own personal histories, but a group of 30-40 tiny core samples from trees in the same region form a library with a shared recording of the climatic past. The scientists used some chronologies that date back more than 1,000 years to reconstruct the past climate of North America and Mexico and unearth the epic drought of the 16th century.

The severely dry weather over the Southwest and northern Mexico may explain why some American Indians in these areas abandoned their pueblos between 1540 and 1598, the researchers contend. And one of the fiercest and longest battles between American Indians and European settlers, the Chichimeca War in Mexico, raged for 40 years beginning in 1550, during the most severe part of the drought.

Ironically, the lack of water may have been linked to ocean currents. Because the drought-affected area looks like a pattern formed on a smaller scale in today's climate-ocean current phenomenon La Nina, Stahle speculates that cold ocean currents in the equatorial Pacific may have caused the prolonged drought since the weather blows across America from the Pacific Ocean.

"This drought was not a consequence of global warming. We don't know what caused it. The factors that did cause it could return," Stahle said. Further studies of ocean sediments or coral reefs may reveal the ocean's role, if any, in this past, prolonged, severe drought.

Discovering why the drought occurred may help researchers predict future droughts, Stahle said.

"If such a drought were to occur today, it would wipe out certain agricultural activities. It would change economic activities on the land. And it would put enormous stress on water resources. This would have a dramatic effect on society," Stahle said.