Mounting evidence over the 20th and 21st centuries shows that interpersonal violence – from homicide to war – is increasing with climate change. Is it a recent problem or is it a persistent trait in human societies?
To answer this question, a team of researchers from the University of Utah studied how climate change and population pressure influenced homicide and war rates in the Nazca Highlands of ancient Peru. Their findings suggest that climate change has the potential to harm all populations, directly or indirectly, through destabilization and refugee crises. By focusing on resource availability per capita, the study provides a blueprint for understanding the conditions that foster violence and war in the past and in the future.
“We are interested in the conditions that systematically promote deadly violence,” said Weston McCool, postdoctoral researcher in the university’s Department of Anthropology and lead author of the study. “In doing so, we hope to take some first steps towards building science-based tools that can help mitigate conflict.
Published in the Proceedings of the National Academy of Sciences on April 21, In 2022, the team worked with local Peruvian archaeologists and community members to excavate ancient tombs that were on the verge of collapse. The assemblage of these graves was used to track levels of violence by examining weapon trauma on 270 human skeletons, 150 of which were directly radiocarbon dated. They combined this data with paleoclimate information that tracks local and non-local rainfall, as well as quantitative estimates of population growth based on the density of nearby radiocarbon-dated settlements.
PHOTO CREDIT: Weston McCool
Researchers descend to a fortified settlement atop a hill in Pukara.
Together, these data characterize long-term trends in lethal violence, climate, and demography over a 700-year window from CE 750 to 1450. During this period, people experienced dramatic climatic and demographic changes and wars chronicles.
Unlike many other studies, they found that violence and war culminated in an increasingly stable and supportive climate. This was counterintuitive until the authors incorporated demographic change.
“Favorable local climate change has improved conditions in the Nasca Highlands. At the same time, neighboring regions were experiencing droughts. The inhabitants of these regions migrated to the highlands during a population boom, which put considerable pressure on the agricultural economy. Increased competition for limited resources and arable land has led to chronic warfare,” McCool said.
The research offers a nuanced approach to examining how past variations in climate and demographics have driven changes in violence over time.
“Looking back in time allows us to see a greater range of variation than we can see in recent history,” said Brian Codding, associate professor of anthropology and second author of the study. “It matters because future climate change and population growth will push us beyond what we have experienced in our collective memory.”
As the threat of anthropogenic climate change looms, researchers seek to predict its impact on people. Many are studying how people have responded to past climate changes that have been far more dramatic than what was observed in the 20th century. It can provide insight into how people are coping, or failing to do so, as the climate becomes increasingly unstable over the coming decades.
PHOTO CREDIT: Weston McCool
A view showing the difference between irrigated and non-irrigated land.
These findings also add weight to a growing theory that violence and broader social instability are driven, at least in part, by reduced resource availability, which results from the interplay between climatic and demographic dynamics.
The implications of this research are clear: in a globalized world, climate shocks in one region can have negative effects on others. “When climate change causes mass migration, even places spared initial climate impacts can see increased violence and wider social instability,” McCool said.
Other study co-authors include Utah anthropology doctoral students Kenneth B. Vernon and Kurt M. Wilson; Peter M. Yaworsky, a former anthropology student from Utah, now a postdoctoral researcher at Aarhus University in Denmark; Norbert Marwan of the Potsdam Institute for Climate Impact Research; and Douglas J. Kennett of the University of California, Santa Barbara.
