Cool heads likely won’t prevail in a hotter, wetter world

Should climate change trigger the upsurge in heat and rainfall that scientists predict, people may face a threat just as perilous and volatile as extreme weather — each other.

Researchers from Princeton University and the University of California-Berkeley report in the journal Science that even slight spikes in temperature and precipitation have greatly increased the risk of personal violence and social upheaval throughout human history. Projected onto an Earth that is expected to warm by 2 degrees Celsius by 2050, the authors suggest that more human conflict is a likely outcome of climate change.

The researchers analyzed 60 studies from a number of disciplines — including archaeology, criminology, economics and psychology — that have explored the connection between weather and violence in various parts of the world from about 10,000 BCE to the present day. During an 18-month period, the Princeton-Berkeley researchers reviewed those studies’ data — and often re-crunched raw numbers — to calculate the risk that violence would rise under hotter and wetter conditions.

They found that while climate is not the sole or primary cause of violence, it undeniably exacerbates existing social and interpersonal tension in all societies, regardless of wealth or stability. They found that 1 standard-deviation shift — the amount of change from the local norm — in heat or rainfall boosts the risk of a riot, civil war or ethnic conflict by an average of 14 percent. There is a 4 percent chance of a similarly sized upward creep in heat or rain sparking person-on-person violence such as rape, murder and assault. The researchers report that climate-change models predict an average of 2 to 4 standard-deviation shifts in global climate conditions by 2050.

Establishing a correlation between violence and climate change now allows policymakers and researchers to examine what causes it and how to intervene, said lead author Solomon Hsiang, who conducted the work as a postdoctoral research associate in the Program in Science, Technology and Environmental Policy in Princeton’s Woodrow Wilson School of Public and International Affairs.

“We think that by collecting all the research together now, we’re pretty clearly establishing that there is a causal relationship between the climate and human conflict,” Hsiang said. “People have been skeptical up to now of an individual study here or there. But considering the body of work together, we can now show that these patterns are extremely general. It’s more of the rule than the exception.

“Whether there is a relationship between climate and conflict is not the question anymore. We now want to understand what’s causing it,” Hsiang said. “Once we understand what causes this correlation we can think about designing effective policies or institutions to manage or interrupt the link between climate and conflict.”

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