Claim: Climate Change Likely Led to Violence in early Andean Populations

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[Round and round the p-hacking wheel goes. Where it stops, nobody knows. It is PEER-REVIEWED. It says so right down there. I wonder if they were even looking for this before they found this “correlation”.~cr]

UC Davis archaeological study points to potential competition for limited resources

Peer-Reviewed Publication


Climate change in current times has created problems for humans such as wildfires and reduced growing seasons for staple crops, spilling over into economic effects. Many researchers predict, and have observed in published literature, an increase in interpersonal violence and homicides when temperatures increase.

Violence during climatic change has evidence in history. University of California, Davis, researchers said they have have found a pattern of increased violence during climatic change in the south central Andes between A.D. 470 and 1500. During that time, which includes the Medieval Climatic Anomaly (ca. A.D. 900-1250), temperatures rose, drought occurred, and the first states of the Andes collapsed.

Climate change and potential competition for limited resources in the south central Andes likely led to violence among people living in the highlands at that time, researchers suggest in a new paper. Their study looked at head injuries of the populations living there at that time, a commonly used proxy among archaeologists for interpersonal violence.

“We found that decreased precipitation predicts increased rates of cranial trauma,” said Thomas J. Snyder, a doctoral candidate in the Department of Anthropology’s Evolutionary Wing and the primary author of the study.

“This observation suggests that climate change in the form of decreased precipitation exerted a significant effect on rates of interpersonal violence in the region.”  

The study was published June 5 in Quaternary Research, Cambridge University Press. Co-author of the paper is Randall Haas, formerly of the same lab at UC Davis and currently a professor at Wayne State University.

Violence not found in coastal, mid-elevation regions

The same results were not found in coastal and mid-elevation regions, indicating they chose nonviolent solutions to climate change or were not affected by it, researchers said. There was also more agricultural and economic diversity there, potentially buffering against the onset of climate change. Drought-induced resource scarcity in the highlands, however, seems like a likely explanation for the violence there, researchers said.

Snyder said looking at the history of people’s interaction with nature is important when considering possible effects of current climate change challenges and people’s interaction with their climate.  

“Our findings reinforce the idea that people living in already marginal environments are the most likely to be hit hardest by climate change,” he said. “Archaeological research can help us predict how best to handle the challenges faced by people in precarious positions in a rapidly changing climate.”

UC Davis researchers recorded violence during early years in the Andes by analyzing existing data of nearly 3,000 skeletal fractures of humans found at 58 archaeological sites — comparing them to ice accumulation at the time at the Quelccaya glacier — in what is now Peru, Chile and Bolivia. At the same time, there was widespread abandonment of Wari and Tiwanaku sites in the region, indicating a sociopolitical unraveling after the onset of the centuries-long global climate changes.

The archaeology of the Andes provides an excellent opportunity to study the human response to climate change given the region’s extreme climatic variability, incredible archaeological preservation and robust records, researchers said. In this study, researchers found that on average, for every 10-centimeter decrease in annual ice accumulation at the Quelccaya glacier, the likelihood of interpersonal violence more than doubled.


Quaternary Research




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Here is the article’s abstract and introduction.

Climate change intensified violence in the south-central Andean highlands from 1.5 to 0.5 ka

Published online by Cambridge University Press:  05 June 2023

Thomas J. Snyder and Randall Haas

TypeResearch ArticleInformation

Quaternary Research , First View , pp. 1 – 11


This is an Open Access article, distributed under the terms of the Creative Commons Attribution licence (, which permits unrestricted re-use, distribution, and reproduction in any medium, provided the original work is properly cited.Copyright

Copyright © University of Washington. Published by Cambridge University Press, 2023


The archaeology of the pre-contact Andes provides an ideal study of human responses to climate change given the region’s extreme climatic variability, excellent archaeological preservation, and robust paleoclimate records. We evaluate the effects of climate change on the frequency of interpersonal violence in the south-central Andes from ca. 1.5–0.5 ka (AD 470–1540) by comparing incidents of skeletal trauma observed among 2753 crania from 58 sites to rates of ice accumulation at the Quelccaya Glacier. We find that, in the highlands, the odds of identifying inter-personal violence increase on average by a multiplicative factor of 2.4 (1.8–3.2; 95% C.I.) for every 10-centimeter decrease in annual ice accumulation. Our statistical analysis does not detect a relationship between ice accumulation and interpersonal violence rates among coastal or mid-elevation populations. This disparity likely resulted from variable economic and sociopolitical strategies at different elevations. The failure of rain-fed agriculture during periods of drought and concomitant dissolution of organizing polities likely predisposed highland populations to socioeconomic stress and violent competition for limited resources. Conversely, diversity among lowland and midland economies may have buffered against the effect of drought.


Anthropogenic climate change has begun to create immediate problems for human populations, ranging from increased wildfire frequency to reduced growing seasons for staple crops (Skarbø and VanderMolen, Reference Skarbø and VanderMolen2016; Allen et al., Reference Allen, de Coninck, Dube, Hoegh-Guldberg, Jacob, Jiang, Revi, Masson-Delmotte, Zhai, Pörtner, Roberts, Skea, Shukla and Pirani2019). Many scholars and agencies predict that one of the primary consequences of rising global temperatures will be an increase in the prevalence of interpersonal violence (Anderson and DeLisi, Reference Anderson, DeLisi, Forgas, Kruglanski and Williams2011; Caruso et al., Reference Caruso, Petrarca and Ricciuti2016; Mares and Moffett, Reference Mares and Moffett2016; Levy et al., Reference Levy, Sidel and Patz2017; Robbins Schug, Reference Robbins Schug and Robbins Schug2020). Recent sociological studies observed upticks in direct interpersonal violence associated with rising global temperatures in modern contexts (Anderson and DeLisi, Reference Anderson, DeLisi, Forgas, Kruglanski and Williams2011; Mares and Moffett, Reference Mares and Moffett2016; Levy et al., Reference Levy, Sidel and Patz2017). For example, Mares and Moffett (Reference Mares and Moffett2016) note that global homicide rates increased by an average of 6% for each degree of increase in average annual temperature (Celsius).

The extent to which such relationships apply across variable ecological and cultural contexts remains poorly understood. Sociological studies and the human securities literature operate at limited timescales and within cultural contexts shaped principally by European colonialism and global markets (Robbins Schug et al., Reference Robbins Schug, Parnell, Harrod and Buikstra2019; Robbins Schug, Reference Robbins Schug and Robbins Schug2020; Rockman and Hritz, Reference Rockman and Hritz2020). The cultural breadth and temporal depth of archaeology provide a means of investigating the diverse ways that humans respond to climatological changes more broadly (Douglass and Cooper, Reference Douglass and Cooper2020; Burke et al., Reference Burke, Peros, Wren, Pausata, Riel-Salvatore, Moine, de Vernal, Kageyama and Boisard2021). Bioarchaeological research is uniquely situated to evaluate questions about the effects of climatological fluctuations on human behaviors. Life experiences manifest themselves in the skeleton through a process of embodiment, potentially reflecting disease, violence, and malnutrition, or lack thereof. Through various biological and cultural processes, the social and physical environments are transcribed on the skeleton (Walker, Reference Walker2001; Armelagos, Reference Armelagos2003; Sofaer, Reference Sofaer2006; Larsen and Walker, Reference Larsen, Walker and Larsen2010; Agarwal and Glencross, Reference Agarwal, Glencross, Agarwal and Glencross2011; Tung, Reference Tung2021).

Violence is a complicated topic and often difficult to define. The World Health Organization (WHO) broadly defines violence as “The intentional use of physical force or power, threatened or actual, against oneself, another person, or against a group or community, that either results in or has a high likelihood of resulting in injury, death, psychological harm, maldevelopment or deprivation” (Krug et al., Reference Krug, Mercy, Dahlberg and Zwi2002, p. 1084). Alternatively, Francis Galtung (Reference Galtung1969) conceived of violence as tripartite, consisting of direct violence, structural violence, and cultural violence. While structural and cultural violence can be subtle and invisible (Farmer, Reference Farmer2004), only direct violence takes the form of readily visible interpersonal assault (Galtung, Reference Galtung1969).

Within the archaeological record, evidence for the different types of violence can take numerous forms, including cranial fractures, paleopathological lesions indicative of metabolic stress, and stable isotope signatures indicative differential access to important foodstuffs (Walker, Reference Walker2001; Tung, Reference Tung2021). These forms of evidence often point to events of direct and structural violence among past populations (Klaus and Tam, Reference Klaus and Tam2009). Additionally, violence is not necessarily limited to events that affect living persons directly. One example of post-mortem structural violence includes the dissection of deceased individuals interred in cemeteries belonging to subaltern communities by medical schools in the nineteenth century (Nystrom, Reference Nystrom2014; Nystrom et al., Reference Nystrom, Sirianni, Higgins, Perrelli, Liber Raines and Nystrom2017). Within Andean contexts, another form of violence includes the sacrifice of human individuals by slitting the throat or strangulation—as is seen in Moche, Chimú, and Inka contexts (Verano, Reference Verano, Bourget and Jones2008; Faulbaum, Reference Faulbaum2011; Prieto et al., Reference Prieto, Verano, Goepfert, Kennett, Quilter, LeBlanc and Fehren-Schmitz2019).

As one of the most environmentally variable landscapes in the world, the South American Andes, presents an ideal opportunity to investigate how people respond and adapt to rapid climatological change and environmental variation. People have inhabited the extreme environments of the Andes for at least 13,000 years (Lindo et al., Reference Lindo, Haas, Hofman, Apata, Moraga, Verdugo and Watson2018; Contreras, Reference Contreras2022; Fiedel, Reference Fiedel2022). They rapidly developed an impressive suite of cultural and biological adaptations leading to a number of diverse archaeological cultures and traditions (Dillehay and Kolata, Reference Dillehay and Kolata2004; Silverman and Isbell, Reference Silverman and Isbell2008; Malpass, Reference Malpass2016; Lindo and DeGiorgio, Reference Lindo and DeGiorgio2021). To facilitate discussion of the diversity of archaeological cultures in the Andes, archaeologists have divided time periods into ‘horizons’ of widespread sociopolitical organization and relative cultural homogeneity, and culturally differentiated ‘intermediate periods’ between horizons (Rowe, Reference Rowe1962). This analysis is specifically concerned with the Andean middle horizon and late intermediate period.

The Andean middle horizon (MH), ca. 1.5–1.0 ka (AD 500–1000), saw the rise of the first states in the Andes—Wari and Tiwanaku (Isbell and Schreiber, Reference Isbell and Schreiber1978; Janusek, Reference Janusek2004Reference Janusek2008; Isbell, Reference Isbell, Silverman and Isbell2008; Nash, Reference Nash2019; Williams and Nash, Reference Williams and Nash2021). Previous research showed clear evidence for interpersonal violence during this period (Tung, Reference Tung2012; Arkush and Tung, Reference Arkush and Tung2013). The Wari Empire is thought to have engaged in numerous episodes of violent conquest, an aspect of their culture mirrored in the bioarchaeological and archaeological records through the prevalence of skeletal trauma during periods of expansion and militaristic iconography on Wari ceramics (Williams, Reference Williams2002; Schreiber, Reference Schreiber2004; Tung, Reference Tung2007; Arkush and Tung, Reference Arkush and Tung2013). Trophy heads are frequently found at sites in the Wari heartland, and isotopic evidence compellingly suggests that these trophy heads were the victims of conquered communities (Williams, Reference Williams2002; Tung, Reference Tung2008a; Tung and Knudson, Reference Tung and Knudson2008). Conversely, Tiwanaku traditionally has been thought to have exerted its sphere of influence more subtly, taking a “Zen road to statecraft” (Janusek, Reference Janusek2008, p. 287) with cultural hegemony diffusing via multi-regional exchange networks. However, recent bioarchaeological studies have begun to complicate the image of Tiwanaku’s peaceful hegemony, with some middle horizon archaeological sites within Tiwanaku’s sphere of influence revealing evidence of direct interpersonal violence (Torres-Rouff et al., Reference Torres-Rouff, Knudson, Pestle and Stovel2015; Becker and Alconini, Reference Becker, Alconini, Tiesler and Lozada2018; Blom and Couture, Reference Blom, Couture, Tiesler and Lozada2018).

The abandonment of Wari and Tiwanaku sites at the end of the middle horizon has been the subject of debate. Despite impressive monumental architecture and the wide-scale regional influence wielded by these states, their influence receded and vanished almost entirely by ca. 1.0 ka (AD 1000). Interestingly, this sociopolitical unravelling occurred slightly after the onset of the Medieval Climate Anomaly (MCA), a centuries-long global climate perturbation that occurred from ca. 1.05–0.70 ka (AD 950–1300) (Lüning et al., Reference Lüning, Gałka, Bamonte, García Rodríguez and Vahrenholt2019). Previous research demonstrated that the MCA broadly affected human behavior around the world. For example, bioarchaeological meta-analyses indicate that there was a sharp increase in violence among foragers throughout what is now the state of California during this period (Schwitalla et al., Reference Schwitalla, Jones, Pilloud, Codding and Wiberg2014; Allen et al., Reference Allen, Bettinger, Codding, Jones and Schwitalla2016). Closer to the study region, the MCA likely had consequences for Indigenous populations within the Amazon Basin, as indicated by increased rates of interpersonal violence (Neves, Reference Neves, Nielsen and Walker2009; Moraes and Neves, Reference Moraes and Neves2012) and site abandonment during this time frame (Riris, Reference Riris2019; Bush et al., Reference Bush, Nascimento, Åkesson, Cárdenes-Sandí, Maezumi, Behling and Correa-Metrio2021).

Within the Andes, some scholars argue that enduring drought caused by the MCA may have played a causal role in the decline of Wari and Tiwanaku (Binford et al., Reference Binford, Kolata, Brenner, Janusek, Seddon, Abbott and Curtis1997; Kolata et al., Reference Kolata, Binford, Brenner, Janusek and Ortloff2000). According to the model, this drought undermined Tiwanaku’s agricultural economy, which is adapted to relatively wet conditions adjacent to Lake Wiñaymarka on the Andean Altiplano. Drought-induced resource scarcity appears to have further jeopardized critical cycles of reciprocity and exchange between Tiwanaku elites and commoners (Janusek, Reference Janusek, Alt and Pauketat2019). The undermining of this inter-class trust destabilized the Tiwanaku sociopolitical and cultural system (Ortloff and Kolata, Reference Ortloff and Kolata1993; Binford et al., Reference Binford, Kolata, Brenner, Janusek, Seddon, Abbott and Curtis1997; Moseley, Reference Moseley, Oliver-Smith and Hoffman1999; Kolata et al., Reference Kolata, Binford, Brenner, Janusek and Ortloff2000; Janusek, Reference Janusek2008). Other scholars suggest that drought was unlikely to have affected agricultural productivity (Erickson, Reference Erickson1999), or that the temporal resolution of paleoclimatological reconstructions and radiocarbon do not permit strong claims regarding the effect of climate change on Tiwanaku’s sociopolitical stability (Marsh et al., Reference Marsh, Contreras, Bruno, Vranich and Roddick2021).

Although the potential drivers of Wari sociopolitical collapse have not been systematically investigated, bioarchaeological evidence demonstrates that the immediate aftermath of collapse was a violent time within the Wari heartland of the Ayacucho Basin, perhaps indicating a backlash against elite groups perceived as being responsible for the difficult times (Williams, Reference Williams2002; Finucane et al., Reference Finucane, Valdez, Calderon, Pomacanchari, Valdez and O’Connell2007; Tung, Reference Tung2012). The period after the collapse of Wari and Tiwanaku, and before the rise of the Inka Empire, is termed the late intermediate period (LIP), ca. 1.0–0.6 ka (AD 1000–1400), and is frequently characterized as a period of unrest and violence suggesting something of a cultural “collapse” across the MH–LIP transition (Covey, Reference Covey2008; Arkush and Tung, Reference Arkush and Tung2013).

Sociopolitical ‘collapse’ is not a simple process. Nor is ‘collapse’ a particularly fitting word to describe the destabilization of the Wari and Tiwanaku as sociopolitical hegemonies during the Andean Middle Horizon when one considers that these cultural traditions persisted, albeit in different forms. Resilience theory presents a more apt framework for thinking about the nature of societal reorganization (van der Leeuw and Redman, Reference van der Leeuw and Redman2002). Borrowed from ecology, resilience theory posits that human societies function similarly to natural systems—moving through adaptive cycles over time (Redman, Reference Redman2005). These cycles include exploitation, in which rapid expansion is emphasized; conservation, characterized by the slow storage of energy and material in increasingly organized structures; release, what archaeologists broadly understand as collapse; and finally, reorganization in which human societies develop new social systems and methods of adaptation (Holling and Gunderson, Reference Holling, Gunderson, Gunderson and Holling2002). Importantly, although the process is cyclical, reorganization entails movement to new socioeconomic forms that retain some characteristics of earlier ones. Within this framework, the people living towards the end of the Middle Horizon within the Wari and Tiwanaku hegemony likely separated themselves from their spheres of influence in something of a release phase. This is exemplified in the Moquegua Valley where a new cultural style of settlement, known as Tumilaca, emerged during the end of the Middle Horizon. Associated with Tiwanaku style architecture but lacking Tiwanaku iconography, these individuals were descendants—cultural, biological, or both—of those who abandoned Tiwanaku colonies (Sutter and Sharratt, Reference Sutter and Sharratt2010).

The hypothesis that environmental conditions may have affected the frequency of interpersonal violence in the Andes is supported by recent quantitative research by McCool et al. (Reference McCool, Wilson and Vernon2022a), who identified a positive relationship among interpersonal violence, altitude, and altitudinal variation. They argued that the increased marginality of high-altitude environments, caused by hypoxic conditions and extremely low temperatures, reduced the carrying capacity of the environment and motivated violent competition over scarce resources.

In a more region-specific study, McCool et al. (Reference McCool, Codding, Vernon, Wilson, Yaworsky, Marwan and Kennett2022b) found that violence rates in the Nazca highlands were, counter to expectation, greatest during periods of elevated precipitation. The authors hypothesized that the increased precipitation may have acted as a pull factor—drawing more people to the area than the environment could support (McCool et al., Reference McCool, Codding, Vernon, Wilson, Yaworsky, Marwan and Kennett2022b). These findings conflict with findings from other world regions indicating increased violence associated with increasingly dry conditions (Allen et al., Reference Allen, Bettinger, Codding, Jones and Schwitalla2016; Schwindt et al., Reference Schwindt, Bocinsky, Ortman, Glowacki, Varien and Kohler2016). The Nazca case shows that human responses to climate change can be highly variable with micro-ecologies and socio-political contexts influencing outcomes within macro-ecological conditions.

The study presented here offers a systematic evaluation of the hypothesis that drought and climatological instability drove interpersonal violence throughout the south-central Andes. Thus, the study presents a meso-scale analysis that spans multiple cultural regions within the south-central sub-region of the Andes. Following on these previous findings, we hypothesize that the highest incidences of cranial trauma in the south-central Andes occurred during periods of drought and climatological instability, particularly during the LIP. We therefore expect to observe that, on average, increased rates of violent trauma in the highlands corresponded to decreased and more variable ice accumulation rates. Additionally, we evaluate the relationship between climate and violence proxies across an elevational gradient in order to assess the extent to which these dynamics apply in different ecological contexts.