Droughts and the decline and rise of urban civilizations

The role of climate change in the collapse of ancient civilizations has become a popular topic, among both scholars and the public. Jared Diamond’s book Collapse (Diamond 2004) remains a best-seller, and it has generated considerable discussion and debate. This week a new technical study on ancient climate in central Mexico was reported (Stahle et al. 2011) (click here for more information) and it may transform our views of urban development in central Mexico. The authors have constructed a tree-ring chronology in the Mexican state of Queretaro that extends back to ca. AD 800. Although there has been a good amount of prior paleo-climatic research on central Mexico, the chronological detail given by tree rings – estimates of year-to-year rainfall patterns – dwarfs the resolution of prior work by limnologists.

Stahle et al. point to four periods of severe multi-decadal drought within their data:

1.      AD 897 – 922.  The authors try to extend the first drought range all the way to the Maya area to correlate with prior work on droughts at the end of the Classic period that may be related to the Classic Maya collapse. To be accepted, however, such an extension will require a lot more data from intermediate areas. But what they do not mention is that this coincides rather closely with the fall of several major cities, such as Xochicalco and Teotenango, at the end of the central Mexican Epiclassic period (ca. 900-950).

2.      AD 1149 – 1167.  The second drought, as pointed out by the authors, falls close to current estimates for the decline of Tula and the Toltec state (they use the incorrect term “empire”).

3.      AD 1378- 1404.  The third drought period does not match any clear demographic or political downfall. In fact, it came at the end of a massive demographic upsurge in central Mexico, and coincided with the expansion of the poorly understood Tepanec Empire (1380-1428). This was the most severe of the four droughts.

4.      AD 1514 – 1539.  The final drought covers the period of the conquest of the Aztec by Cortés (1519-1521) and the early colonial period.

To me, one of the most intriguing findings is NOT one of these periods of drought, but rather the relative absence of major drought between the second and third episodes. This interval, mostly contained within the “Early Aztec” period and the first part of the “Late Aztec” period, witnessed the single most dramatic population surge of the pre-Columbian New World. Populations in central Mexico exploded with a growth rate of over 1% (average annual increase), a very high rate for a preindustrial population. The population data are in Sanders et al. (1979), and I discuss the implications in Smith (2003). Prior paleoclimatological research had suggested a general increase in precipitation starting in the twelfth century AD (e.g., Metcalfe and Davies 2007), and the absence of major prolonged droughts fits this picture.

Not only did populations rise, but it was also a major period of urbanization in central Mexico. Most Aztec cities were founded and grew rapidly into capitals of city-states at this time (Smith 2008).

So, are we ready to read social changes directly off the rainfall record? Not so fast! Archaeologists and paleo-climate scientists have so far not done a very good job relating episodes of climate change (whether drought or temperature change) to episodes of social change. Scholars tend to be polarized into two camps: the environmental determinists and the cultural interpretivists.

Many climate specialists and some archaeologists fall into the first category, environmental detereminists. There was a drought, the civilization collapsed, so we now have it figured out. End of story. This is far too simplistic, however.

Quite a few archaeologists and anthropologists, on the other hand, are cultural interpretivists. In their view, environmental processes never directly cause changes in human society. People are smart, society is resilient, and environmental effects are always mediated and interpreted through cultural understandings. These people get very upset at environmental determinist accounts. Some of them held a whole conference whose purpose was to attack Jared Diamond’s book, Collapse (McAnany and Yoffee 2010).

To me, neither approach is satisfactory, and neither will lead to real understanding of past social change and the role of drought, climate change, and other environmental factors. What is missing is the perspective of historical social science. The environmental determinists apply a simplified natural-science understanding to human society, which is inadequate to model human social dynamics. Their cultural opponents, on the other hand, seem resistant to scientific approaches to the past that employ causal mechanisms. But modern comparative and historical social science research transcends these limitations.

One promising avenue for integrating past climate data with social developments is the field of historical ecology (Balée 2006), and another can be found in transdisciplinary approaches to human-environment interaction (Costanza et al. 2007). But I find these approaches too limited in the realm of social dynamics of complex urban societies. They have little to say about urbanization, social inequality, poverty, collective action, and innumerable other social phenomena that have dominated human society since the Urban Revolution. It seems to me that when the insights of comparative historical social science (e.g., Hedström 2005; Steckel 2007; Tilly 2010) are applied to archaeological and historical research on the ancient past, our understanding of urbanization and other processes will increase tremendously.

In the meantime, the new data by Stahle et al. are of the utmost importance for the history of urbanization and social change in pre-Spanish central Mexico, and I hope it does not take too long to develop appropriate explanatory models to use the new data effectively.

References:

Balée, William

2006    The Research Program of Historical Ecology. Annual Review of Anthropology 35:75-98.

Costanza, Robert, Lisa J. Graumlich, and Will Steffen (editors)

2007    Sustainability or Collapse? An Integrated History and Future of People on Earth. MIT Press, Cambridge, MA.

Diamond, Jared

2004    Collapse: How Societies Choose to Fail or Succeed. Viking, New York.

Hedström, Peter

2005    Dissecting the Social: On the Principles of Analytical Sociology. Cambridge University Press, New York.

McAnany, Patricia A. and Norman Yoffee (editors)

2010    Questioning Collapse: Human Resilience, Ecological Vulnerability, and the Aftermath of Collapse. Cambridge University Press, New York.

Metcalfe, Sarah E. and Sarah Davies

2007    Deciphering Recent Climate Change in Central Mexican Lake Records. Climatic Change 83:169-186.

Sanders, William T., Jeffrey R. Parsons, and Robert S. Santley

1979    The Basin of Mexico: Ecological Processes in the Evolution of a Civilization. Academic Press, New York.

Smith, Michael E.

2003    The Aztecs. 2nd ed. Blackwell Publishers, Oxford. (** 3rd edition coming soon; I shipped the files off to Blackwell today!)

2008    Aztec City-State Capitals. University Press of Florida, Gainesville.

Stahle, David W., José Villanueva-Díaz, Dorian J. Burnette, Julián Cerano Paredes, Richard Heim, Jr., Falko K. Fye, Rodolfo A. Soto, Matthew D. Therrell, Malcolm K. Cleaveland, and D. K. Stahle

2011    Major Mesoamerican Droughts of the Past Millennium. Geophysical Research Letters 38:(in press).

Steckel, Richard H.

2007    Big Social Science History. Social Science History 31:1-34.

Tilly, Charles

2010    Cities, States, and Trust Networks: Chapter 1 of Cities and States in World History. Theoretical Sociology 39:265-280.

Stahle, David W., José Villanueva-Díaz, Dorian J. Burnette, Julián Cerano Paredes, Richard Heim, Jr., Falko K. Fye, Rodolfo A. Soto, Matthew D. Therrell, Malcolm K. Cleaveland, and D. K. Stahle (2011). Major Mesoamerican Droughts of the Past Millennium Geophysical Research Letters