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NSF Press Release


Embargoed until 2 P.M., EDT

NSF PR 01-43 - May 17, 2001

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 Cheryl Dybas

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This material is available primarily for archival purposes. Telephone numbers or other contact information may be out of date; please see current contact information at media contacts.

Changes in Sun's Intensity Tied to Recurrent Droughts in Maya Region

The Maya were talented astronomers, religiously intense in their observations of the sun, moon and planets. Now, new research shows that something in the heavens may have influenced their culture and ultimately helped bring about their demise.

In an article set to appear in this Friday's issue of the journal Science, a team of researchers funded by the National Science Foundation (NSF) and led by a University of Florida at Gainesville geologist reports finding that the Yucatan Peninsula, seat of the ancient Maya civilization, was buffeted by recurrent droughts. More importantly, the research shows, the droughts - one of which is thought to have contributed to the collapse of the Maya civilization -- appear to have been caused by a cyclical brightening of the sun.

"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 in turn influenced the Maya's evolution," said David Hodell, a geologist at the University of Florida, and the paper's lead author.

Comments David Verardo, director of NSF's paleoclimate program, which funded the research, "Hodell and his colleagues have unearthed a classic tale of climate's influence on society's resources -- in this case, water. The lesson here is that natural variability is the 500-pound gorilla of climate science. If we don't understand how nature manifests itself in climate, we can't hope to understand how humans impact climate over the long-term."

In 1995, Hodell and colleagues published results suggesting that the ninth-century collapse of the Maya civilization may have been influenced by a severe drought that lasted for more than 150 years. The paper was based on analysis of a sediment cores from Lake Chichancanab on the north central Yucatan Peninsula in Mexico.

For the latest research, Hodell and his colleagues returned to the lake and collected a new series of cores. The scientists discovered layers of calcium sulfate, or gypsum, concentrated at certain levels in the cores. Lake Chichancanab's water is nearly saturated with gypsum. During dry periods, lake water evaporates and the gypsum falls to the lake bottom. The layers therefore represent drought episodes. The geologists found the recurrence of the deposits is remarkably cyclical, occurring every 208 years, although they varied in intensity.

The 208-year cycle caught the researchers' attention because it is nearly identical to a known 206-year cycle in solar intensity, Hodell said. As part of that cycle, the sun is most intense every 206 years, something that can be tracked through measuring the production of certain radioactive substances such as carbon-14. The drought episodes occurred during the most intense part of the sun's cycle.

The droughts also occurred at times when archeological evidence reflects downturns in the Maya culture, including the 900 A.D. collapse. Such evidence includes abandonment of cities or slowing of building and carving activity.

The energy received by the earth at the peak of the solar cycle increases less than one-tenth of one percent, so it's likely that some mechanism in the climate is amplifying the impact in the Yucatan, Hodell explained.

Archaeologists know the Maya were capable of precisely measuring the movements of the sun, moon and planets, including Venus. Hodell said he is unaware, however, of any evidence the Maya knew about the bicentenary cycle that ultimately may have played a role in their downfall. "It's ironic that a culture so obsessed with keeping track of celestial movements may have met their demise because of a 206-year cycle," he said.




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