End of rain, end of reign

For over four centuries, the rains came regularly and plentifully, nourishing the growth of one of history’s great civilizations. Rising from the jungle floor of parts of present-day Mexico and Central America, ancient Mayan cities bustled with activity, sustained by abundant cropland and governed by a complex social and political order. And then, the skies began to dry, leading to political discord, economic upheaval and ultimate collapse.

At least, that’s the story the stones tell.

By comparing millennia-long rainfall records from a natural mineral formation in Belize, with glyph records carved into stone monuments by the area’s former inhabitants, a multidisciplinary team of researchers has provided new evidence that climate change played a central role in the disintegration of Classic Maya civilization, which lasted from approximately 300 to 1000 CE.

The results of the study, published in the Nov. 9 issue of Science, address what has been a longstanding mystery surrounding the disappearance of a scientifically advanced society whose peak population may have reached up to 10 million — far more people than live in the region today.

“Ultimately we hope this will help us understand the question of what causes civilizations to persist and what causes them to come apart,” said Bruce Winterhalder, a UC Davis anthropologist who participated in the study.

According to Winterhalder and his colleagues, the demise of the Maya appears to have come in two stages. The first was a prolonged period of political instability likely triggered by a decline in agricultural productivity. They believe that the shift in climate tipped the balance on a society whose dense urban populations were already putting high demands on the political system as well as the surrounding environment.

“Imagine slowing down the flow of the water in the California aqueduct to L.A.,” said lead author Douglas Kennett, a professor of environmental anthropology at Penn State University. “You have a population built up on that specific flow of water. With the L.A. example, you’d also have to reduce the flow of semi trucks into the city carrying food. What we’re interested in is how that destabilizes the social and political fabric.”

With the collapse of the political infrastructure of Mayan society came the end of monument building, inter-city commerce, dynastic alliances and rivalries — but not all of Mayan culture. It was only later, when a severe drought struck the region between 1020 and 1100 CE, that the remaining population was largely wiped out.

To obtain a high-resolution record of rainfall in the region inhabited by the Maya, researchers examined a stalagmite formed from dripping rainwater inside a cave near the ancient Maya city of Uxbenka. Radioisotope dating of uranium and thorium gave the age of the stalagmite, while analysis of oxygen isotope ratios — rainwater is relatively enriched in the heavier form — provided the rainfall record, with variations on timescales of less than a year.

The second data set came from the Maya themselves, in the form of hieroglyphic inscriptions left on raised stone monuments called stelae. Containing what amount to political histories of successive Mayan periods, marked by changes in leadership, wars and other major events, the stelae are also exquisitely dated. And because the Maya adhered to a precise, astronomically based calendar system, the dates recorded for each of these events, as well as the dates of the completion of the stelae themselves, could be correlated precisely to the climatological chronology deposited in the stalagmite.

“The Maya area is unique in having so many written texts that are absolutely dateable and locatable in space,” said Martha Macri, a UC Davis linguist and professor of Native American studies who provided the hieroglyphics database used for the correlation.

The glyph record, comprised from widely distributed sites, allowed investigators to see the synchrony of both the rise and fall of the Classic Maya.
The period from about 300 to 700 CE, during which the Classic Maya were flourishing, was anomalously wet. With few major rivers that could be tapped for irrigation, the Maya depended on the seasonal rains to keep the good times going. But beginning about 700 CE, the climate became noticeably, but not catastrophically, drier.

Nonetheless, symbols for warfare begin to increase in the glyph record, perhaps a sign of increasing competition for scarce resources. This was attended by rapid turnovers in ruling families, signaling political instability, and perhaps a loss of faith in the divine mandate of the rulers who were now powerless to bring back the rains.

The authors of the study say there is still much more to be done to untangle the social and ecological dynamics that drove Mayan society. Still, what they have learned so far may already provide a cautionary tale.

“People are always asking the question, what does this tell us about ourselves? It tells us we ought to be fairly cautious about our assumptions of the stability of our economic system and culture in the face of natural change,” Winterhalder said. “Just prior to the collapse I can imagine that the leadership of these city states were quite confident, looking out over their realm, and seeing productive agriculture, and scientists, and armies, and monuments going up, and just thinking, ‘Man, this is going to last forever.’ And three generations later it was gone.”

OYANG TENG can be reached at science@theaggie.org.