In a study published this month, UC Berkeley researchers examined the mysterious abandonment of an ancient Mexican city, analyzing the effect dry weather might have had on the city’s rise and fall.
By using lake sediment to establish a timeline for the region’s climatic events and comparing it to an archaeological timeline for the same area, researchers showed how drought may have contributed to the city of Cantona’s population flux about 1,000 years ago. They found that environmental stresses often interacted with the region’s political forces in nuanced ways.
The study, which involved researchers from universities in Mexico and Germany, was recently published in the Proceedings of the National Academy of Sciences. The paper’s lead author was Tripti Bhattacharya, a UC Berkeley graduate student in the department of geography.
In 2007, a team recovered 12-meter-long cores of sediment from Aljojuca, a lake near Cantona. At its peak, the city was estimated to be home to 90,000 people and was an important center of production for obsidian in a region controlled by powerful city-states. Cantona was abandoned around A.D. 1000, however.
The sediment samples provided a record of 6,200 years of climate history and enabled researchers at UC Berkeley and Lawrence Livermore National Laboratory to extract information about how climate patterns varied on a multidecadal scale.
In one technique, researchers examined the ratio of two isotopes of oxygen — one heavier and one lighter — in the sample. A rise in the prevalence of the heavier isotope meant more evaporation occurred in the lake, indicating drier weather.
They found that from about A.D. 500 to 1150, the region experienced a period of aridity marked by below-average rainfall that was “unprecedented in 4,000 years,” according to Bhattacharya.
The researchers then compared the climatic timeline to the region’s cultural history previously established through archaeological records. They found that around A.D. 700 — a time of intensifying aridity — the city swelled in population. Their paper posits that perhaps Cantona grew in importance even during this dry spell because of migration caused by turmoil, both climatic and political, in nearby areas.
“Human responses to climate are often nonlinear,” Bhattacharya said. “Climatic stress played a role in reorganizing the power dynamics of the time.”
According to Bhattacharya, examining the vulnerability or resilience of past cultures to climatic changes — a task that requires a nuanced understanding of both physical and political geography — can help us grapple with modern questions of how to use our natural resources.
Roger Byrne, a UC Berkeley professor of geology and another author of the study, cautioned against using prehistoric examples as perfect analogs to today’s problems but noted that California’s current drought might induce population shifts.
Though climate change is a pressing challenge that needs to be addressed, looking to the past can help develop a response, according to Sheryl Luzzadder-Beach, a professor of geography at the University of Texas at Austin who did not work on this particular study.
“The past is important because it provides what are essentially case studies for climate extremes,” Bhattacharya said. “Maybe this is a cautionary tale for the present.”