A cyclical brightening of the sun appears to have triggered a severe 150-year drought that brought down the civilization of some of the ancient world's most accomplished astronomers, the Mayans, according to a study published today.
Writing in the journal Science, researchers at the University of Florida and the Lawrence Livermore Laboratory in California report that evidence from lake sediments indicates the Yucatan Peninsula, center of the Mayan empire, undergoes a drought every 208 years.
This interval is almost identical to a known 206-year cycle in the sun's intensity, said David Hodell, professor of geology at the university and lead author of the study.
"Looking at this series of sediment cores, it looks like changes in the sun's energy output are having a direct effect on the climate of the Yucatan and causing the recurrence of drought, which is in turn influencing the Maya evolution," Hodell said.
Hodell and colleagues had suggested in a 1995 study that the ninth-century collapse of the classic Mayan civilization came in the midst of a period that was the driest in more than 1,000 years.
They based the conclusion on analysis of a sediment core from Lake Chichancanab, on the north-central Yucatan Peninsula in Mexico. Sediments deposited layer by layer give scientists a time line of changes in climate, vegetation and land use.
In a return trip to the lake last year, the researchers collected a new series of core samples and noted layers of gypsum concentrated at certain levels in the cores. The lake's water is nearly saturated with the mineral, but during dry periods more water evaporates and the gypsum builds up at the bottom of the lake, giving a marker for ancient droughts.
Although they vary in depth, and thus the intensity of the drought they represent, the deposits occur almost exactly every 208 years. The researchers found that this cycle closely matched a previously documented solar cycle that's tracked by measuring certain radioactive substances in the soil. These tend to peak during the most intense part of a 206-year cycle of solar activity.
During those periods, the energy received by Earth from the sun increases by less than one-tenth of 1 percent, Hodell noted. But the additional energy could have been enough to change any of several circulation patterns that affect tropical weather generally and rainfall over the Yucatan specifically, the researchers said.
Climate scientists remain uncertain how much effect the long-term solar cycles have on global temperatures. Most doubt the sun has more than a minor influence on the global warming trend that's been going on for several decades, but some contend the influence is still poorly understood and may turn out to be significant.
"The Maya were highly dependent on rainfall and surface reservoirs as their principal water supply," the researchers said, so dry spells lasting decades and even centuries would have had a particularly "detrimental impact on Maya food production and culture."
The scientists found evidence not only for arid events in the ninth century, but also for several other long droughts before and after that period. Archaeological evidence suggests social upheaval resulting from each of the droughts.
