Did Pulses of Climate Change Drive the Rise and Fall of the Maya?

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For nearly 13 centuries, from 300 B.C.E. to 1000 C.E., the Classic Maya civilization flourished. Maya kings reigned as gods over dozens of city-states, Maya artists ornamented the walls of temples and palaces, and Maya scribes recorded the triumphs and defeats of their rulers in hieroglyphic inscriptions. But despite the vast ruins the Maya left behind, archaeologists have long puzzled over what fueled the ascent and downfall of this civilization—and whether climate played a starring role, a bit part, or was offstage altogether at key turning points in Maya history.

Now, an international research team led by geoarchaeologist Douglas Kennett of Pennsylvania State University, University Park, has produced the best climate record yet of Maya times: a subannual reconstruction of rainfall in the Maya heartland that extends back 2000 years. Comparing this reconstruction to records of major Maya building episodes and warfare, they conclude that an extended period of generous rainfall helped spur the growth and proliferation of Maya city-states, and a prolonged dry period played a key part in their collapse. “Climate does matter, and it matters in a very complex way,” Kennett says.

The new climate record “is a really nice first cut at a high-resolution record,” says paleoclimatologist Lawrence Edwards of the University of Minnesota, Twin Cities, who was not involved in the work. But many archaeologists remain reluctant to adopt climate as the chief driver of Maya society. Even so, the study's abundant new data will help clarify the role of climate in Maya history, says archaeologist Jason Yaeger of the University of Texas, San Antonio. “We can start to think of climate change as a more concrete variable, and obviously an important one, in Maya history,” Yaeger says.

Paleoclimatologists and archaeologists agree that the record itself is outstanding. Previous studies have wrested paleoclimate data from sources such as marine muds off the Venezuelan coast and lake sediments from the Yucatán Peninsula (Science, 18 May 2001, p. 1293), building a burgeoning literature. But many archaeologists have argued that such records are too far from the Maya heartland or too broad-brush to show causal links between climate and culture.

The new record, from a 56-centimeter-long stalagmite (an upward-pointing cave-floor pillar of limestone) came from the right neighborhood: It grew from incessantly dripping water in Yok Balum Cave in southern Belize, less than 200 kilometers from many major Classic Maya city-states, including Tikal, Caracol, and Calakmul. And “the real advancement is their uranium-thorium dating,” says paleoclimatologist David Hodell of the University of Cambridge in the United Kingdom. The clocklike radioactive decay of uranium-234 to thorium-230 provided geochemist Yemane Asmerom of the University of New Mexico, Albuquerque, with precise ages for mineral deposition down through the stalagmite, with strikingly small error bars of plus or minus 1 to 17 years. “Within a couple of decades, they've got [the dating] nailed,” Edwards says.

The only aspect of the Yok Balum record that falls well short of ideal is the drought record preserved in the changing oxygen-isotope composition down the stalagmite. The isotopic composition of rainwater seeping down into the cave—the ratio of the rare, heavier oxygen-18 isotope to oxygen-16—reflects the amount of rainfall on the surface. But unfortunately, evaporation within the cave alters the isotopic signal of declining rainfall before it is preserved in the stalagmite, making it hard to speak quantitatively about cycles of drought on the surface. Instead, the authors write of “wet periods,” “drying trends,” and “dry periods.”

To match the paleoclimatic data to cultural history, the team charted the rise and fall of complex Maya polities and the prevalence of warfare. Graduate student Claire Ebert of Pennsylvania State University searched an existing Maya hieroglyphic database for records of dated monuments in the heartland. She also searched for dated inscriptions containing key words or phrases related to war, such as “vassalage after defeat.” Then she and Kennett charted the frequencies of these monuments and inscriptions over time, and compared them with the stalagmite data.

The team found that the Maya heartland boomed during a pronounced moist period between 400 and 640 C.E., when the number of Maya polities rose and many new royal dynasties were founded. But in the 340-year-long drying trend that followed, warfare spiked repeatedly, and many Maya rulers embarked on massive building projects, displaying their power in an increasingly fragmented and fractious political landscape. This period culminated in a major political collapse in the 9th century C.E. Then came the final blow: the driest interval in the record, a century of drought beginning in 1000 C.E., when Maya populations sharply declined. “So it's really the idea of a two-stage collapse,” Kennett says.

The high-resolution paleoclimate record is “an amazing accomplishment,” says archaeologist Arlen Chase of the University of Central Florida, Orlando, who was not involved in the project. But he thinks that more climate records are still needed, because the study probably underestimates the amount of microclimatic variation in the Maya heartland. Climate at the Yok Balum site could well have been different from that at, say, Caracol, on the other side of the Maya Mountains. And at the time of collapse, some Maya migrated to a much drier region, the northern Yucatán—an odd move if drought was a driver, notes archaeologist Andrew Scherer of Brown University. He adds that although the authors suggest that less rainfall meant meager harvests, that link has yet to be established. Corn, the chief crop, requires only 400 to 600 millimeters of precipitation, and a broad swath of the Maya heartland now receives between 2000 and 3000 millimeters of rainfall annually. So if precipitation dropped 40% in the 9th century—as another study has estimated—the change might not have seriously impacted food supplies in some areas, he says. “To suggest that a drop in rainfall will lead to agricultural failure is something we don't know,” Scherer says.

But while archaeologists continue to puzzle over the links between climate change and Maya culture, many consider this interdisciplinary study, which puts climate variation in a societal context, to be a major step forward. “The more we engage in interdisciplinary study,” Yaeger says, “the more nuanced and robust our models will be.”