Major droughts may have spurred the demise of multiple cultures and cities in pre-Hispanic Mexico over the last millennium.
A new study, which used tree rings to add many hundreds of years to the region's climate record, pinpointed four severe droughts in the region over the last 1,200 years. Some were far more intense and prolonged than anything ever seen in modern meteorological records, and many coincided with major historical events.
One, for example, lasted for 25 years around the year 900 and accompanied the end of a flourishing era of Mayan city-states.
By dating droughts to precise periods of time, the work puts the region's current drought problems into perspective, said David Stahle, a geoscientist at the University of Arkansas, Fayetteville. The findings should also help researchers figure out the factors that have driven moisture out of the area again and again throughout history.
"Clearly the drought in 1400 was not caused by the accumulation of heat-trapping gasses in the atmosphere because it precedes the industrial revolution," Stahle said. "But something caused it that stimulated very unusual climate conditions elsewhere in the world. This may allow us to determine the dynamics of drought-forcing over Mexico."
There are at least two points of view about the links between climate and culture, Stahle added. And controversy surrounds the conversation.
"Books have been written asserting the point that droughts ended civilizations," he said. "Others say that's too simplistic. The fact remains that there were these physical events in the environment. Surely, that had some kind of consequences."
Whatever the implications, simply piecing together the ancient climate history of Mesoamerica fulfills a long-standing goal in the field of tree-ring analysis, Stahle said. Until now, studies have been able to look back only about 500 years.
As a result, researchers could only speculate about how climate change may have affected a series of highly developed cultures, whose accomplishments included huge pyramids and elaborate temples.
To extend the record, Stahle and colleagues plunged through rugged terrain into a steep gorge at Barranca de Amealco. The gorge sits about 90 kilometers (56 miles) north of Mexico City. It is 60 km (37 miles) from Tula, the capital city of the ancient Toltec state, and 90 km (56 miles) from the Aztec capital of Tenochitlan.
Although surrounding areas have been deforested and developed, the gorge has held onto a grove of gargantuan Montezuma baldcypress trees that span as wide as four meters (14 feet) in diameter. Without harming any trees, the researchers drilled and removed hundreds of meter-long cores. They analyzed 74 of the cores, which were about the thickness of a pencil, from 30 trees.
By comparing annual growth rings between samples, the researchers were able to look back at the region's climate, year by year, going back 1,238 years. The thickness of a given ring reveals how dry or wet a single growing season was.
According to their analyses, one of the most severe periods of drought occurred between 897 and 922, the researchers reported in the journal Geophysical Research Letters, just as the Terminal Classic period came to an end. Another 19-year drought around 1150 coincided with the fall of the Toltec state, which was the dominant civilization of central Mexico at the time.
The most severe drought of the last 1,000 years, which lasted from 1378 to 1404, actually occurred during a period of rapid Aztec expansion. But a drought from 1514 to 1539 led up to and continued through the arrival of Cortez and the Spanish conquest.
It's possible, the researchers speculate, that the climate, along with epidemic disease, led to the rapid decline of Aztec Mexico as the Colonial period began.
Together, the findings demonstrate how helpful tree-ring analysis can be at pinning down the history of droughts on Earth, said Raymond Bradley, director of the Climate System Research Center at the University of Massachusetts, Amherst. Ancient groves of trees in other parts of the world might hold similar treasure troves of climatic information.
With the new evidence, Bradley said, the study also adds to a long-standing debate about what caused a series of cultural declines in ancient Mexico. Some experts argue that the changes were driven purely by socioeconomic and political factors. Others think that, because the region has long been heavily dependent on agriculture, climate was what pulled the trigger.
"Dave's study adds one more line of evidence that climate really did play an important role in reconditioning the situation for societal disintegration," Bradley said. "It was not a level environmental playing field, and the environment did change over time. That may have tipped the balance in many cases."