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Did changes in climate cause the decline of Mayan Civilization?

Chaac, the ancient Mayan god of rain and lightening.

The classic period of the Mayan civilization stretched for 13 centuries, during which huge stepped pyramids were built as monuments to kings who ruled as gods. The Mayans had a written language and an advanced understanding of astronomy. This hugely productive period came to an end abruptly at about 1000 A.D. A new interdisciplinary study by Douglas J. Kennett and his colleagues links the rise and fall of the Mayan civilization to precipitation levels. 

Kennett et al used an interesting technique to create a climate record for the classic period. They harvested a large stalagmite from a cave in Belize, near the heart of Mayan territory, and dated changes in oxygen isotope ratios using the decay of uranium to thorium down the column. A low ratio of 18oxygen to the more common lighter 16oxygen indicates a period of relative drought. Kennett's data show that a large increase in Mayan populations occurred during a long period of relatively high precipitation levels.  This was followed by a period of intense competition and warfare when dryer periods became more common. The final collapse was coincident with a century long drought.

MemberCentral discussed the new findings with AAAS member Jason Yaeger, a professor of anthropology at the University of Texas and an expert on the Mayan civilization.

AAAS Members Central:  Mayans were still around to greet the Spanish conquistadors, so what constitutes a "collapse" of Mayan civilization?
Jason Yaeger, professor of anthropology at the University of Texas:   Well, an extinction event, it was not, although there was a period when the population may have declined by as much as 90%. And there was a political collapse; the kings were no longer seen as divine. The Spanish found the Mayans living in huts surrounding these magnificent monuments that lay in ruins. But it is true that the Mayans continue even today. There are about 24 different languages still spoken that are related to the original Mayan tongue.

AAASMC: Does the new study by Kennett convince you that climate was a driver of the Mayan collapse?
Yaeger:  If you substitute the word 'factor' for the word 'driver' than I agree. Scholars have grappled with the role of climate change in the Mayan civilization for 150 years. Kennett's new climate record puts the influence of climate on much firmer ground. Of course, the Mayan civilization was never monolithic and there were significant differences in various microclimates among different Mayan groups.

AAASMC:  It's hard to imagine a multi-decadal drought in the Caribbean when today it is so lush.  Can we say from the isotope ratios how dry it really was?
Yaeger:  That is one of the "known unknowns" to paraphrase Donald Rumsfeld. We know that there were relatively wet and relatively dry periods, but we don't have a perfect correlation between isotope ratios and annual rainfall.  It's possible, for instance, that instead of having two crops of corn in a year, as normal, maybe there was only one in the dry periods. When you're at subsistence levels already, that becomes significant.

AAASMC:  The period of peak monument building, as shown in Kennett's paper, coincided with periods of war-related activities.  Is there a reason for this link?
   The monument-building represented the culmination of Mayan artistry and religion. Of course, these were also peak periods for population density, warfare, human sacrifices, decapitations etc. The monuments were political tools for the kings, as symbols of authority and competition. Building a pyramid was also a significant drain on resources, since the artisans and workers were not participating in farming, while still consuming food. So strife in these times could be related to food shortages. The royal household always did well, but studies of the bones of workers reveal signs of malnutrition and disease.

Politics probably played a role in the Mayan collapse. Jared Diamond is right in that players make decisions that are short term and self-interested, even sometimes when they understand the long term ramifications.

AAASMC:  In the paper that you sent me by yourself and David Hodell, you mention that the use of the famous long calendar was lost.  Were there other technologies, like writing, that disappeared along with the collapse?
Yaeger:  The long calendar stopped being used to record political events, but it was not lost entirely. It was still used to date astronomical events. The Mayan writing system survived the collapse of the Classic era but was eclipsed by the Spanish invasion. The Spanish specifically targeted the intelligentsia and nobility.  Most books were burned. We only have a few texts left that are limited in scope. 

AAASMC:  Paleoclimatology was a new word for me.  It occurs to me that archaeology is a lot harder than it used to be...
Yaeger:  Yes, archaeology has become much more interdisciplinary and collaborative than it used to be.  No one person can do it anymore. You need an epigrapher, an iconographer, a paleoclimatologist, a geologist, maybe a paleobotanist as well. Of course, you still need trowels, picks and shovels like you did 100 years ago, but now we also use x-ray fluorescence, neutron activation, strontium-90 for dating teeth.  An important new tool is LIDAR—light detection and ranging—a remote sensing technology which allows you to detect ruins through thick jungle canopies.

AAASMC:  I have to ask this question.  Do you have any plans for the end of the long count calendar?
Yaeger:  Yes, my wife, who is also a Mayan scholar, and I are going to a Belize for a Mayan celebration. Some of the shamans think that this event will usher in a new era of peace and cooperation and respect for indigenous people.

AAASMC:  So you don't think this is the end of the world?
Yaeger:  No, we have plans for next year.

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Chaac, the ancient Mayan god of rain and lightening.
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