In 2008, researchers at the University of Minnesota discovered something funny about a stalagmite sourced from a cave in northwest China. The nearly five-inch long piece of calcium carbonate contained layers recording thousands of years of the region’s rainfall history—as stalagmites had shown in the past—but these coincided with something else. Where the stalagmite showed intense drought or weak rains, history showed the collapse of the Tang, Yuan, and Ming dynasties. Where it showed steady, strong rain, dynasties grew and consolidated power.
"The climate acted as the last straw that broke the camel’s back," paleoclimatologist Hai Chang told Scientific American then.
The weather recorded in the stalagmite corresponded with changes in the global climate—at the time the Tang Dynasty fell, records show worldwide warming. But the University of Minnesota team hasn’t been the only one to recognize this phenomenon—a group of researchers from Princeton and Berkeley analyzed the similar, longitudinal studies of 190 other researchers and concluded that major climatic changes raise rates of interpersonal and intergroup violence.
It was a massive undertaking. Solomon Hsiang, lead author of the study, says the team worked for 18 months to re-interview study authors, gather all of their data, and allow the disparate puzzle pieces to connect. "The entire community has been working on this problem for a decade now," Hsiang said. "The overarching conclusion is that across the literature there’s been a lot of studies and they’re actually getting very similar results."
So Hsiang and his team quantified those results. What they found is that, on average, one standard deviation away from normal climatic conditions raised rates of interpersonal violence by 4% and intergroup violence by 14%. "It looks like most of the inhabited world is going to have two to four standard deviations warmer, and even more than four in parts of the tropics. We think that this suggests that interpersonal and group level conflict could become exacerbated by future climate changes if populations don’t adapt," Hsiang said.
The researchers’ work bolsters hypothetical scenarios drummed up by thinktanks and intelligence studies over the past several years. Tom Donilon, President Obama’s National Security Advisor, spoke extensively on the connection between climate change and national security at Columbia University earlier this year, and in 2010, the Department of Defense released a report warning that climate change "may act as an accelerant of instability or conflict, placing a burden to respond on civilian institutions and militaries around the world."
Hsiang and his colleagues’ research rounds up that instability by numbers. Whether his team was looking at incidence of county-by-county rape rates in the United States or Kenyan inter-group conflict, they noted strong correlations between increased violence and increased temperatures. The same held for Hindu-Muslim riots in India and rainfall loss.
"For a long time, there’s been a debate in the community trying to understand: Is there an effect of climate on conflict? We think we can say quite confidently yes, there is a very strong relationship," Hsiang notes. "Moving forward, we want to understand why is there this relationship. Similar to where medical researchers were in 1930s, we could tell smoking was the proximate cause of lung cancer, but researchers couldn’t tell you why. We’re kind of in the same position now, only several decades later."
There are several theories that seek to explain the phenomenon the Berkeley-Princeton paper outlines. One posits that the culprit is urban migration: When agricultural conditions worsen and people move to cities, pressure on urban resources creates tension. Another has to do with social and economic inequality. For example, one of the studies the Berkeley researchers analyzed showed that rainfall patterns could predict when landless farmers would invade wealthy landowners’ turf. Other theories look at individual psychology, and whether climatic shock may influence a person’s level of aggression or ability to make decisions.
This means that climate change holds consequences far beyond environmental devastation. "One of the big motivations of the study is to help us think about future climate change, and what that might look like," Hsiang says. "We want to understand the mechanisms so people can adapt."