Leaving Mesa Verde: Peril and Change in the Thirteenth-Century Southwest

4. The Climate of the Depopulation of the Northern Southwest

The Climate of the Depopulation of the Northern Southwest Aaron M. Wright 4 In this chapter, I examine the climate of the northern Southwest between AD 1000 and 1400, a period with major cultural and demographic transitions , including the complete depopulation of the northern San Juan region and the “Northern Periphery” (see Allison, this vol.) during the thirteenth century. My goal is to present an up-to-date synthesis of southwestern climatic variability, highlighting its potential relevance for understanding the demographic, social, economic, and ritual transformations discussed elsewhere in this volume. Ultimately, the interplay of environmental and social factors underlies the multifaceted, complex history of Ancestral Pueblo society and movement during the late prehispanic period. The paleoclimate of the northern Southwest, the heartland of Ancestral Pueblo peoples, is one of the best known in North America. This is partly because a deteriorating climate has long been posited as a prime motivator for regional depopulation (e.g., Douglass 1929; Hewett 1908; Kidder 1924). Moreover, the number of trees from local archaeological sites, on which most southwestern climatic reconstructions are based, is by far the greatest of any region in western North America. As a result, the regional paleoclimate is well known and the impacts of climatic variability on local farmers thoroughly studied, although new dendrochronological research and analyses of other proxies continue to provide new insights on climate at various temporal and spatial scales. This chapter couples some of these previous climatic data with the results of new studies to provide an up-to-date picture of temperature and precipitation variability to better understand the conditions facing farmers in the late prehispanic central Mesa Verde region. Since the impact of climatic downturns on subsistence is often invoked as a causal force in the depopulation of the northern Southwest, 76 Aaron M. Wright this chapter also considers the climatic conditions of regions that might have come to host migrants from the north. Cordell and colleagues (2007) have considered the precipitation differences between the northern Rio Grande and central Mesa Verde regions. This chapter expands their approach by considering temperature as well as precipitation, and it broadens the geographic scope to include regions often neglected as potential migrant destinations. I begin by reviewing known climate processes and patterns across the northernSouthwest.Recentresearchandmodelinghavegreatlyenhanced current understandings of both high- and low-frequency processes, some of which account for the well-known droughts that occurred during the late prehispanic period. The climatic requirements for maize farming in the northern Southwest are then addressed to show that fluctuations in precipitation and temperature patterns—some of great magnitude and extreme duration—were relevant to Ancestral Pueblo farming practices. Precipitation and temperature reconstructions for various regions across the northern Southwest and Mogollon Rim are then presented. Finally, I can then compare regional climatic conditions to gauge the influence climate may have had on attracting Mesa Verdean migrants into other regions. Understanding Southwestern Climate The following discussion distinguishes between environment and climate , the latter being more specific and my chief concern here. Although climatic variability is continuously scalar through both space and time, for analytical purposes archaeologists have come to address southwestern climate in terms of high- and low-frequency processes and variability (e.g., Dean 1988; Dean et al. 1985; Dean, Doelle, and Orcutt 1994; Euler et al. 1979). Short-term climatic variability, such as fluctuations in seasonal and annual climate, is referred to as high frequency and tends to be more localized than low-frequency variability. High-frequency variability—for example, early and late frosts, droughts, and floods—is archaeologically important because such occurrences can, among other things, be detrimental to crops and significantly influence the distributions of annually derived wild resources. Low-frequency climatic variability refers to those patterns that are realized in periods of twenty-five The Climate of the Depopulation 77 years or more. Such fluctuations are dictated largely by extraregional, and sometimes global, processes. Low-frequency variability can have longterm effects on regional ecosystems and can induce significant environmental changes. However, low-frequency environmental change can also be human-induced (e.g., Redman 1999), for example through deforestation (e.g., Kohler 1992; Kohler and Matthews 1988) and overhunting (e.g., Driver 2002). It is traditionally hypothesized that, unless a population has overexploited a local environment or has reached some threshold of resource availability, demographic shifts should respond more closely to low-frequency than to high-frequency climatic and environmental variability (e.g., Dean et al. 1985). Precipitation Variability Southwestern precipitation is driven by dynamic...