"Super droughts" may have helped bring down the Toltec and Aztec civilizations.
Using dendrochronology -- the study of tree rings -- a team from the University of Arkansas created a model using thousand-year-old Montezuma baldcypress (Taxodium mucronatum) from Barranca de Amealco in Querétaro state.
The drought observed through tree rings was "more severe and prolonged than anything we've seen in the modern era," said David Stahle, lead researcher of the study, to be published in the American Geophysical Union's Geophysical Research Letters.
The study's home base in Mexico is critical, as the climate models used by the U.N. Intergovernmental Panel on Climate Change (IPCC) to measure climatic change have predicted a "drying out" of the country leading up to 2050, said Richard Seager, a research professor at the Lamont-Doherty Earth Observatory at Columbia University, another lab that works with tree rings to decode past climate patterns.
"The fact that they occurred in the past means they could occur again," said Seager, but the events would be "much greater and more extreme."
Connie Woodhouse, a professor at the University of Arizona specializing in the climatology of western North America, said the records indicate that a similar event could happen in the future, with the added exacerbation of human-induced global warming.
"It's not a forecast or a predictive tool," she said. "It's sort of a heads-up."
Through studying bald cypress rings, the team was able to reconstruct the soil moisture balance during the rise and fall of the Toltecs and Aztecs, two of Mexico's great indigenous civilizations. The Toltecs flourished from 800 to 1000. Prime time for the Aztecs ran from 1500 to 1700.
The data identified droughts as long as 19 years, and are linked with destabilizing events that eventually brought down the Toltec state. Droughts during Aztec rule coincided with devastating famines.
The shift to a severe climate had secondary effects, as well. "Prolonged drought over Mesoamerica during the early Colonial era may have interacted with epidemic disease to contribute to the catastrophic depopulation of Aztec Mexico in the aftermath of the [Spanish] conquest," states the study.
So will modern civilizations finish off like the Toltecs? Stahle is careful not to draw a direct parallel.
"We don't know for sure if it caused a decline," he said. "We don't know for sure if it caused the collapse of the ancient city of Tula [the Toltec capital]." But the drought's role is apparent.
Dendrochronology is an effective method for constructing climate models back to 2,000 years because it directly calibrates with ancient weather patterns, said Stahle. The bald cypress is a rare variety in the region to survive more than 1,000 years. One tree in southern Mexico is ranked one of the oldest in the world.