New evidence shows that droughts were a significant factor in the fall of the Mayan civilization. Paleoclimatologist Douglas Kennett of Pennsylvania State University led an international team of researchers that analyzed a 2,000 year–old stalagmite from a cave in South Belize (1). Kennett obtained semiannual climate records through his analysis, and compared this record to historical events. The team proposes that high rainfall favored Mayan population growth between 440 and 660 C.E. Then, between 660 and 1000 C.E., a drying trend led to increased warfare and the division of political states (2). A drought lasting between 1020 and 1100 C.E. occurred in the midst of the population collapse, which marked the end of the Classic Mayan civilization (2,3).
The Mayan civilization experienced its peak during the Classic period (250–950 C.E.). The Mayans lived in the lowlands of the Yucatán Peninsula, which includes parts of southern Mexico, Guatemala, and Belize. Between 750-950 C.E., the Mayan civilization deteriorated as the Mayans abandoned many of their major cities. Archaeologists call this period the Terminal Classic collapse, and the cause of this demise remains unclear (3).
Figure 1: A map of the Classical Mayan Civilization. Image Courtesy of Wikimedia.
Early Theories for Mayan Decline
Prior to the development of the drought theory on Mayan civilization’s demise, researchers had suggested soil erosion as the cause of the civilization’s downfall. The Mayans chopped down forests to create greater farmland, resulting in soil erosion that would have made farming more difficult. However, recent studies of Guatemalan Lake Salpetén’s sediments show little erosion during the Terminal Classic period. In fact, Flavio Anselmetti of the Swiss Federal Institute of Aquatic Science and Technology found that clay soil deposits mostly accumulated during the early Mayan era (4). Though a variety of other reasons have been suggested, drought currently remains the most accepted.
The Yucatán Peninsula, where the Mayans resided, is a seasonal desert. The region depends on heavy summer rains that provide as much as 90 percent of the annual precipitation. Annual precipitation also varies drastically across the peninsula. Surface water often dissolves the limestone bedrock of the Yucatán, and also creates caves and underground rivers. Because of these underground formations, surface water is scarce. Therefore, the Mayans used artificial reservoirs as their source of water during their four- to five-month-long winter dry spells. Tikal, a Maya city, had enough reservoirs to supply 10,000 people for 18 months. However, the reservoirs still depended on seasonal rain to replenish their supply (3).
While Douglas Kennet’s data provides greater support for the drought theory, it is not a new concept. David A. Hodell had proposed the idea in 1995 after analyzing sediment records in Lake Chichancanab. The lake is located in Yucatán, Mexico and possesses gastropod and ostracod shells with varying levels of the isotope 18O. A small percent of H218O naturally resides in the lake water, but when temperatures rise, the proportion of H218O becomes greater. This is due to unequal levels of evaporation between H218O and H216O. Since H216O is lighter than H218O, H216O water evaporates more readily. Higher levels of H218O occur with greater evaporative loss, or in dry climates. David Hodell studied gastropod and ostracod shells because the carbonate in the shells reflects the higher 18O levels. The shells also incorporated gypsum and calcite. Lake Chichancanab is saturated in both calcite and gypsum, but in drier climates there is a higher gypsum/calcite ratio. The shells also demonstrate this pattern.
Hodell’s analysis of the shells suggested the existence of a drought during the latter portion of the Mayan Classical period, but it was too broad. The dating for the shells has an error margin of 35 to 65 years. Hodell’s analysis was too broad a time frame to link with historical events (5).
Other scientists have studied lake and marine sediments to infer the climate as well. Studies conducted in Lake Punta Laguna in Mexico and the Macal Chasm in Belize also provided time frames that were too broad to compare with the complex political changes. A study performed in the Cariaco Basin in Venezuela had greater precision, but the location was too far from the Mayans. The different regions may have experienced varying climate.
Current Climate Research
In comparison to previous climate studies, Douglas Kennett’s dating of the samples has a smaller margin of error. Kennett’s team uses high resolution uranium-thorium dating. The steady radioactive decay of uranium-234 to thorium-230 has a half-life of 245,500 years and a small error range of 17 years. With a higher precision provided by the uranium-thorium dating, the research team can project dates within a couple of decades rather than centuries (6,7). Additionally, York Balum Cave, the original location of the Chaac stalagmite, is located within 200 km of major Mayan centers such as Tikal and Copan. The close proximity supports the validity of Kennett’s data, as the two sites should experience the same climate system. The precise nature of the stalagmite’s dating allows the research team to compare their climate data with complex historical records.
To obtain climate data, Kennett measured the concentrations of 18O2 and 16O2 in 0.1 mm increments of the stalagmite. Each 0.1 mm increment represented half of a year. Higher relative levels of 18O suggest a drier climate while lower relative levels a suggest wetter climate (2). The validity of the stalagmite’s climate information was confirmed using a Hendy test. The Hendy test assesses isotopic equilibrium based on two conditions: a lack of correlation between the presence of isotopes 18O2 and 13C, and a consistent level of 18O2 for each growth layer of the stalagmite (8).
Kennett’s climate data is skewed due to evaporation within the caves. Because there was another small level of evaporation, the 18O2 levels are higher and suggest slightly drier climates (2). The conversion of the 18O2 levels to quantitative rainfall measurements is currently unobtainable, so Kennett uses the data to describe periods as either wet or dry (6).
Figure 2: A Mayan calendar created by a modern crafsman. Image courtesy of Truthanado.
Correlations Between Data and History
Douglas Kennett’s research confirms the long “drying trend” predicted by earlier studies. He maps the drying trend to have started in 640 C.E. and peaked in 1020 C.E. Kennett’s data also show particularly long droughts between 200-300 C.E., 820-870 C.E., 1020-1100 C.E., and 1530-580 C.E. Short but very severe droughts also occurred in 420, 930, and 1800 C.E.
The drying trend shown in the data from 200-300 C.E. matches the demise of El Mirador, a pre-Mayan settlement located in northern Guatemala. However, this correlation is contrasted with the persistence and expansion of other societies such as the
Evidence points to high levels of rainfall between 400 and 500 C.E., the same time period as the Early Classic expansion. The heavy rainfall continually recharged the urban water storage systems and would explain the growing influences of Tikal and other Mayan centers. The period of 440-500 C.E. also had the best recorded ruling Maya lineages. The 820-870 C.E. drought predicted by the research matches the Terminal Classic period. Kennett’s research also supports a previous prediction of a 40 percent decrease in summer rainfall during the Terminal Classic period (2). Kennet’s research team also found that war related events increased during the early stages of a drying trend that spans from 640-1020 C.E. From 750-775, C.E. rulers commissioned monuments at unprecedented rates. These monuments possessed text that pointed to status rivalry, war, and strategic alliances. A sudden drop in the number of texts at key Maya centers that followed provides evidence for the failure of Mayan political systems.
The Classical Mayan civilization ended with gradual depopulation and a shift of political power to northern parts of the Yucatán Peninsula.
Kennett’s research team suggests a two stage collapse of the Classical Maya civilization. The first stage starts with the 660 C.E. drying trend that increased warfare and political destabilization. This primary stage worsens with the dry interval between 800-900 C.E., which causes a reduction of agricultural activity and more political disintegration. The second stage is a more gradual population decline punctuated by spurts of more drastic population reductions during the driest interval between 1020-1100 C.E (2).
While the drought that occurred from 1530-1580 C.E. was beyond the Mayan Classical period, Kennett’s team compared their climate data with Yucatán records. Records matched the data to a drought that occurred between 1535-1575 C.E. Historical accounts also linked this drought to famine, disease, death, and population relocation. This example shows the impact of dry conditions to populations.
Criticisms and Conclusions
Kennett’s research accomplishes a more accurate estimate of the region’s paleoclimate. However, Arlen Chase of the University of Central Florida believes that more climatic records are needed, because of microclimatic variations within the Mayan civilization. The Yucatán Peninsula does experience drastically different annual precipitation among its regions. The Maya migration to northern Yucatán, which is much drier, also conflicts with the drought theory. Andrew Scherer, an archaeologist of Brown University, also questions the link between less rainfall and meager harvests. Corn, a major crop for the Maya, requires only 400 to 600 mm of rain, and the Yucatán peninsula gets an average of 2000 to 3000 mm of annual rainfall. If precipitation dropped only 40 percent during the Terminal Classic period, then the decrease in rainfall might not have affected food supplies as drastically as the drought theory assumes.
Kennett’s interdisciplinary study shows climate change in a social context (6).
Contact Na Eun Oh at
Na.Eun.Oh.16@dartmouth.edu
References
1. H. Shen, Drought hastened Maya decline (2012). Available at http://www.nature.com/news/drought-hastened-maya-decline-1.11780 (05 January 2013).
2. D. J. Kennett et al., Science 338, 788-791 (2012).
3. L. C. Peterson, G.H. Haug, Am. Sci. 93, 322-329 (2005).
4. S. Williams, Not So Clear-Cut: Soil erosion may not have led to Mayan downfall (2007). Available at http://www.sciencenews.org/view/generic/id/9062/description/Not_So_Clear-Cut_Soil_erosion_may_not_have_led_to_Mayan_downfall (05 January 2013).
5. D. A. Hodell, J. H. Curtis, M. Brenner, Nature 375, 391-393 (1995).
6. R. F. Service, Science 338, 730-731 (2012).
7. M. Medina-Elizalde and E. J. Rohling, Science 335, 956-959 (2012).
8. Proxies of Hydrology, Pt.2 (2010). Available at http://courses.washington.edu/proxies/Hydrol_Lec10_stalag_js5_hndt.pdf (05 January 2013).