Wednesday, February 29, 2012

More on the Collapse

Ironically, just as I was writing my last post, on the Classic Maya Collapse, the Early Edition of the Proceedings of the National Academy of Sciences was releasing online the articles in a special section devoted to historical collapses. It appears to have been organized by Karl Butzer, the doyen of environmental archaeology. The special feature includes two articles on the Maya:

Dunning, Nicholas P., Timothy P. Beach, and Sheryl Luzzadder-Beach (2012). Kax and Kol: Collapse and Resilience in Lowland Maya Civilization.

Luzzadder-Beach, Sheryl, Timothy P. Beach, and Nicholas P. Dunning (2012). Wetland Fields as Mirrors of Drought and the Maya Abandonment.

I haven't had the chance to read them yet, but the authors are at the top of the field and have already made major contributions to the research on the question, so I expect the articles to be both interesting and important.

Monday, February 27, 2012

Ancient Maya Droughts Caused Collapse of Journalism

Last Friday an article [1] came out in Science on the perennial issue of the role played by drought in the collapse of Classic Maya civilization. Medina-Elizade and Rohling present a quantitative model of precipitation changes using four paleoclimate proxy records from Yucatán: 1) the high resolution (1-yr) δ18O record from the Chaac stalagmite from the cave of Tzabnah in Tecoh; 2) the lacustrine δ18O record from Punta Laguna; 3) the lacustrine δ18O record from Lake Chichancanab; and 4) the sediment density record from Lake Chichancanab. They authors conclude that the Terminal Classic period droughts were somewhat less severe, in part because they were discontinuous, than previously thought. The discontinuities are apparent in the high-resolution data from the Chaac stalagmite but were masked by the coarser temporal resolution of the lacustrine data sets. It’s a nice article. It builds on Medina-Elizade’s impressive work on the Chaac stalagmite [2].

The article generated considerable buzz in the popular press, but the journalists seem to have gotten the message wrong. Most of the newspaper articles have headlines like “Maya Collapse Caused by Drought” when the central message of the article is that the droughts were less severe than previously thought. Moreover, Medina-Elizade and Rohling interpret the isotopic signal to imply that most of the decrease in precipitation resulted from a decrease in tropical storms and hurricanes. (Water from hurricanes has a slightly lighter isotopic content than regular precipitation. Fascinating, no?) While cyclones provide a significant percentage of the total rainfall in the Yucatán Peninsula, they are also deleterious to human settlements and the agricultural economy because they destroy houses and crops and are usually followed (the next year) by huge forest fires (from the dead trees knocked down by the storm) and also sometimes by plagues of locusts. So, not only were the droughts more intermittent than previously believed, but the reduction in cyclonic activity may have actually been beneficial to some degree. Finally, we need to remember than the collapse started before the Terminal Classic droughts, in the west and southwest Maya lowlands (Petexbatun region, Palenque, etc.), where there is essentially no evidence of drought.

An additional interesting observation: an article just came out in the journal Weather in which the authors analyze climate data drawn from Arabic historical documentary sources from Iraq for the period A.D. 816 to 1009 [3]. This period corresponds precisely to the Maya Terminal Classic period. The authors list seven droughts in this period, of which two (A.D. 897 and 913) are approximately contemporaneous with two of the major Maya droughts (A.D. 895 and 909), each of which lasted for several years [see Ref. 2].

So, I’m not saying there weren’t any droughts, but their relationship to the collapse is murky and uncertain. They probably weren’t the essential underlying cause of the collapse, which was almost certainly severe overpopulation that in turn created a critically unstable political, economic, and ecological regime, which was vulnerable to almost any kind of shock—war, drought, or environmental degradation.

References Cited

1. Medina-Elizalde, Martín and Eelco J. Rohling (2012). Collapse of Classic Maya Civilization Related to Modest Reduction in Precipitation. Science 335:956-959.

2. Medina-Elizade, Martín, Stephen J. Burns, David W. Lea, Yemane Asmerom, Lucien von Gunten, Victor Polyak, Mathias Vuille, and Ambarish Karmalkar (2010). High Resolution Stalagmite Climate Record from the Yucatán Peninsula Spanning the Maya Terminal Classic Period. Earth and Planetary Science Letters 298: 255-262.

3. Domínguez-Castro, Fernando, José Manuel Vaquero, Manuela Marín, María Cruz Gallego, and Ricardo García-Herrera (2012). How Useful Could Arab Documentary Sources Be for Reconstructing Past Climate? Weather 67(3): 76-82.