Subtle variations in the sun's brightness helped trigger a drastic climate change in the Yucatán Peninsula, researchers suggest, and that, in turn, may have helped doom the Maya civilization. The new study, based on a mucky lake-bottom core, confirms that the area's worst drought in many millennia struck just as Maya civilization began its accelerating decline.
Solar activity varies in a "bicentennial oscillation" with a period variously reported to be between 206 and 208 years. The pattern is recorded in the abundance of radioactive carbon-14 produced by cosmic rays and preserved in tree rings, which is thought to correspond to a varying number of sunspots and the brightness of the sun.
The new record is a well-dated, high-resolution core from the bottom of Lake Chichancanab, Mexico. Varying amounts of gypsum deposited on the lake floor record the lake's climate: Whenever rainfall decreased, evaporation concentrated salts and the lake water precipitated gypsum. When paleoclimatologist David Hodell of the University of Florida, Gainesville, and colleagues compared the core's gypsum record to records of solar activity, they found that the bicentennial oscillations in each were in step throughout.
The drought record shows just how hard times were for the Maya at the end of their heyday. Dry spells pepper the 2600-year record, but its most intense, most prolonged drought runs from about A.D. 750 to 850, the researchers report in the 18 May issue of Science. In fact, this was the region's worst drought in 7000 years, according to a longer, less detailed record of Hodell's from the same lake, and was one of the bicentennial droughts. The megadrought came just as Maya civilization entered its decline, which ran from about A.D. 750 to 900, as measured by the number of sites where people were building the massive temples and stone monuments that typify the Maya Classic Period.
Mayanists are guardedly receptive of the climate-culture connection. "It's quite possible it was a major factor, but I don't think climate itself is the sole factor of Maya collapse," says archaeologist Takeshi Inomata of the University of Arizona in Tucson. Climatologists are perhaps more enthusiastic about the sun-climate connection. "The Hodell result adds to a string of recent papers that document the importance of solar variability for climate change," notes paleoceanographer Peter deMenocal of the Lamont-Doherty Earth Observatory in Palisades, New York.