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Mounting evidence across the 20th and 21st centuries shows that interpersonal violence—from homicide to warfare—increases with climate change. Is this a new problem or a persistent trait in human societies?
To answer this question, researchers from the University of Utah examined how climate change and population growth influenced homicide and war rates in the Nasca Highlands of ancient Peru. Their findings indicate that climate change could have the potential to directly or indirectly harm all peoples through destabilization, refugee crises, and other means. The study focuses on the per capita resource availability and provides a blueprint to understand the conditions that encourage violence and war in both the past and the future.
“We are interested in what conditions consistently promote lethal violence,” said Weston McCool, postdoctoral researcher in the U’s Department of Anthropology and lead author of the study, “By doing so, we hope to take some initial steps towards building science-based tools that may aid with conflict mitigation.”
PublishedProceedings of the National Academy of the Sciences, 21 April 2007., 2022, the team worked with local Peruvian archaeologists and community members to excavate ancient tombs that were about to collapse. The assemblage was used to examine the level of violence in these tombs by looking at the weapon-based trauma on 270 human skulls. 150 of these skeletons were radiocarbon dated. These data were combined with paleoclimate information to track local and non-local precipitation. They also used this information to calculate population growth based upon the density of nearby radiocarbon-dated communities.
PHOTO CREDIT Weston McCool
Researchers descend to Pukara’s fortified hilltop settlement.
These data combine to show the long-term trends in lethal violence and climate over a 700-year period, from CE 750 up to CE 1450. People witnessed dramatic climate and demographic change, and chronic warfare.
Contrary to other studies, violence and war peaked in a more stable and favorable environment. This was counterintuitive until demographic change was included.
“Favorable local climate change improved conditions in the Nasca Highlands. At the same moment, droughts were occurring in neighboring areas. These people migrated to the highlands as part of a population boom. This put significant pressure on the agricultural economy. Increased competition for limited resources and arable land led to chronic warfare,” McCool said.
The research provides a nuanced way to examine how past variation of climate and demography has led to changes in violence over the course of time.
“Looking back in time allows us to see a greater range of variation than what we can see in recent history,” said Brian Codding, associate professor of anthropology and second author on the study. “This is important because future climate change and population growth will push us outside what we’ve experienced in our collective memory.”
Researchers are trying to predict how anthropogenic global climate change will affect populations, as the threat of anthropogenic global warming looms. Many are studying how people have reacted in the past to climate change that was much more severe than that seen in 20th century. This information can help people understand how to cope with climate change in the future.
PHOTO CREDIT Weston McCool
A view that shows the difference between irrigated or non-irrigated land.
These findings add weight and support a growing theory that violence is caused by a reduced resource availability. This is a result of interaction between demographics and climatic factors.
The implications of this research are clear—in a globalized world, climate shocks to one region can negatively affect others. “When climate change provokes mass-migrations, even places spared by initial climate impacts may see increases in violence and broader social instability,” McCool said.
The study also includes co-authors from Utah, including Kurt M. Wilson and Kenneth B. Vernon, Utah anthropology doctoral student; Peter M. Yaworsky (Utah anthropology alumnus), who is now a postdoctoral researcher at Aarhus University. Norbert Marwan at Potsdam Institute for Climate Impact Research; Douglas J. Kennett at University of California Santa Barbara.
