Climate and the Collapse of the Maya

February 9, 2014

The study of climate change, and realization that the Holocene, previously considered a period of great climatic stability experienced significant and dramatic disturbances, has occasioned new interdisciplinary study of the Maya, one of North and Central America’s great pre-Colombian civilizations. Scientists, anthropologists, archeologists, and historians have reinvigorated efforts to explain their sudden collapse, which took place from approximately 800-1000 CE in a period known as the Terminal Classic.

Peter Douglas is a postdoctoral scholar at the California Institute of Technology who recently earned his Ph.D. from Yale’s geology and geophysics department. His dissertation, “Plant-wax isotopes in Central American lakes and what they tell us about the collapse of the Classic Maya,” provides new evidence that the Maya experienced significant climate change during the Terminal Classic.

The collapse saw the fall of strong royal governments, the abandonment of cities, environmental degradation, a demographic plunge, and increased warfare (Webster, 2002). Actually “collapse” may be a misnomer, as there was no uniform decline throughout the civilization; some northern areas continued to flourish into the second millennium, but the southern area collapsed more quickly (Andrews, 1973).

Explanations for the collapse range from soil erosion to epidemics to foreign invasion, but drought is currently the leading theory (Aimers, 2007). However, most of the research for the drought has been conducted in the northern part of the former Maya civilization. “That’s been the key problem with this hypothesis,” said Douglas. “We have the strongest evidence for drought in the area where the collapse wasn’t really that bad.”

This gap has made some scholars skeptical of the drought hypothesis (Webster, 2002), but Douglas’ work may help silence the skeptics. He took samples from a lake in the south, in Guatemala, as well as a lake farther north, in the central Yucatán Peninsula. His findings suggest that early in the Mayan civilization the southern region was wetter than the north. That changed during the Terminal Classic period, when there was severe drying in both regions but especially so in the south. “The relative degree of drying was greater in the south, so that’s consistent with the idea that you have a stronger societal collapse there,” said Douglas.

In his research Douglas employed a technique that, to his knowledge, no one else has used to gather paleoclimate proxy data for a societal collapse. One method other researchers have used to find evidence of drought is to determine the ratio of oxygen isotopes in samples of fossilized shells and sedimentary rock (e.g., Hodell, 1995). Lighter isotopes of oxygen evaporate more readily, so times of greater evaporation and lower precipitation lead to a greater amount of heavy isotopes in the water, which get incorporated into the samples.

Douglas looked at what he termed “molecular fossils” – n-alkanoic acids that come from the waxy coatings on land plants, known as plant waxes or leaf waxes. These molecules contain hydrogen, so he used the same kind of reasoning as in the oxygen-isotope analysis, taking samples from lakebeds and measuring the ratio of heavy isotopes to light isotopes of hydrogen.

The cause of the drought was not the focus of Douglas’ research, but he thinks it is possible that it could have been caused by a shift in the Intertropical Convergence Zone (Atlantic wind currents) or El Niño (cf. Kennett, et al., 2012). Deforestation has also been proposed as the cause (Shaw, 2003). While Douglas agrees with recent findings that deforestation could have amplified a drought (Cook, et al., 2002), he does not think deforestation actually caused the Terminal Classic drought because the Maya had already cut down a sizable portion of the jungle a thousand years earlier.

Yale archeologist Harvey Weiss, an expert in societal collapses induced by climate change who was not involved in Douglas’ research, described his work as “brilliant, path-breaking research.” Weiss noted that there is increasing evidence that “megadroughts” led to collapses in several ancient societies such as Teotihuacán and the Akkadian Empire, and “Peter Douglas has made a very substantial contribution to this revolutionary view of past abrupt climate change magnitudes, temporalities, and societal collapse processes.”

Although Douglas is wary of direct comparisons, he still considers his work relevant to modern-day climate change. “My research has the potential to provide an example of how climate change in the past had a big impact on a major complex society, so it illustrates the potential for climate change to be very disruptive to our society,” he said.

An additional area of his research focused on carbon isotopes in the plant waxes, which allowed him to draw conclusions about how the Maya altered their agricultural system to adapt to drought, which could be instructive for policymakers. Douglas also hopes that future research into the distribution of water resources before and during the Terminal Classic can offer lessons about water management.

References:

Aimers, J. J., 2007, What Maya collapse? Terminal Classic variation in the Maya lowlands, Journal of Archaeological Research, v. 15, p. 329-377.

Andrews, E. W., IV, 1973, The Development of Maya Civilization After Abandonment of the Southern Cities, in Culbert, T. P., ed., The Classic Maya Collapse, University of New Mexico Press, Albuquerque, p. 243–265.

Cook, B. I., Anchukaitis, K. J., Kaplan, J. O., Puma, M. J., Kelley, M., and Gueyffier, D., 2012, Pre-Columbian deforestation as an amplifier of drought in Mesoamerica, Geophysical Research Letters, v. 39, L16706, doi: 10.1029/2012GL052565.

Hodell, D. A., Curtis, J. H., and Brenner, M., 1995, Possible role of climate in the collapse of Classic Maya civilization, Nature, v. 375, p. 391-394.

Kennett, D. J., Breitenbach, S. F. M., Aquino, V. V., Asmerom, Y., Awe, J. Baldini, J. U. L., Bartlein, P., Culleton, B. J., Ebert, I. C., Jazawa, C., Macri, I. M. J., Marwan, N., Polyak, V., Prufer, K. M., Ridley, H. E., Sodermann, H., Winterhalder, B., and Haug, G. H., 2012, Development and disintegration of Maya political systems in response to climate change, Science, v. 338, p. 788-791.

Shaw, J. M., 2003, Climate change and deforestation: Implications for the Maya collapse, Ancient Mesoamerica, v. 14, p. 157-167.

Webster, D., 2002, The Fall of the Ancient Maya: Solving the Mystery of the Maya Collapse, Thames & Hudson, New York, 368 p.