Climate Change and Violence

An analysis of 60 studies finds that warmer temperatures and extreme rainfall lead to a rise in violence.

By Ed Yong | August 1, 2013

WIKIPEDIA, DAVID MONNIAUXIn the coming decades, the world’s changing climate could herald a rise in violence at every scale—from individuals to nations, from assault to war—according to a comprehensive new analysis of the link between climate and conflict.  

Analyzing data from 60 earlier studies, Solomon Hsiang from the University of California, Berkeley, found that warmer temperatures and extremes in rainfall can substantially increase the risk of many types of conflict. For every standard deviation of change, levels of interpersonal violence, such as domestic violence or rape, rise by some 4 percent, while the frequency of intergroup conflict, from riots to civil wars, rise by 14 percent. Global temperatures are expected to rise by at least two standard deviations by 2050, with even bigger increases in the tropics.

“The paper is remarkably strong,” said Thomas Homer-Dixon, an environmental and political scientist at the University of Waterloo in Ontario, who was not involved in the study. “[It means] the world will be a very violent place by mid-century if climate change continues as projected.”

Hsiang, together with UC Berkeley colleagues Marshall Burke and Edward Miguel, focused only on studies that provided the strongest evidence for a causal connection. “The ideal thing would be to take two identical Earths, heat one up and watch how conflict evolves,” said Hsiang dryly. “We can’t do that, so we looked for these natural experiments.”

The researchers ignored any studies that compared levels of conflict between different countries, which also differ in their history, culture, and politics. Instead, they focused on data that revealed how violence rises and falls in a single place as climate changes.

For example, crime statistics in the United States reveal that the number of rapes, murders, or assaults increases on a hot day. Civil conflicts in the tropics become twice as frequent during the hot and dry years caused by El Niño events. Farmers in Brazil are more likely to invade each other’s land if they have a particularly wet or dry year. And Chinese dynasties all collapsed during long dry periods.

The team analyzed these studies and more using a common statistical framework to control for any biases on the part of the individual authors. Together, the data sets stretch back to 10,000 BC, and cover all major world regions. They represent the collective efforts of more than 190 researchers working across varied disciplines, from psychologists looking at the effects of temperature on aggressive behavior to archaeologists studying levels of violence in the ancient civilizations.

Despite this diversity, “we were shocked at how well the results from all these fields lined up,” said Hsiang. “Given how some people had been talking, we thought they’d be all over the map,” but the data consistently showed that temperature and rainfall affect violence, across locations, times and disciplines.

“This is a contested area and the convergence of results in this meta-analysis represents a significant step forward,” said Neil Adger, an environmental geographer at the University of Exeter, who was not involved in the study. He notes that responses to climate change, such as “widespread growing of biofuels or displacing populations to build dams” could exacerbate any increased propensity for conflict. “The impacts of climate change will factor large in the future, especially if the world is already fractured and unequal,” he said.

David Zhang, a geographer from the University of Hong Kong who was not involved in the study, said the results were robust and important. However, he noted that most of the data sets cover the last century, and the effects of climate on conflict, may have differed across centuries of human history. Hsiang acknowledges this gap, but said that a few century-spanning data sets have found similar trends.

“Another obvious criticism is that older societies may not be a good analogue for modern ones,” Hsiang added. For example, technological advances might help us to adapt to changing climate more effectively than past generations.

Hsiang also emphasized that climate is just one of many factors that influence the frequency of conflict, and that his study does not address why such a link between conflict and climate exists. Shifts in climate could change the availability of important resources like water or crops, leading to failing economies, weaker governments, and more incentives to fight or rebel. They could also lead to mass migration, rapid urbanization, or growing inequalities.

“We now want to understand what the underlying mechanisms are,” said Hsiang. “If we understand them, we could come up with policies that could decouple climate and conflict, which might help society to adapt. That’s a good reason to push the research ahead.”

S. M. Hsiang et al., “Quantifying the influence of climate on human conflict,” Science, tbc, 2013.

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Avatar of: Ron1


Posts: 1

August 2, 2013

This research confuses correlation with causation. Social instability is most likely the outcome of long standing economic/political stress mediated by contagion and social networks. See interesting discussion at

Avatar of: Kathy Barker

Kathy Barker

Posts: 44

August 2, 2013

Both climate change and irresponsible governments (and many other factors) can lead to shifted or depleted resources, and increase conflict.

The best thing about this article is that scientists are working to prevent violence and war. That is a good place to be.

Avatar of: FrancisBacon


Posts: 14

August 2, 2013

How bad is this science! I dont know where to start. Someone mentioned that its database is from a previous century. What about the effect of population crowding? There are so many other caveats. What a waste of endeavour! And then to spruik its merits.... I am gobsmacked but not surprised.

This study illustrates what is majorly wrong with today's science funding model, which is predicated on translational ends as they are applied to the received beneficial economic and political ends.

Avatar of: James DeMeo

James DeMeo

Posts: 1

August 3, 2013

That high temps and heat waves often add to violence potentials has been known for years, but to connect that to the CO2 theory of global warming is to mix politics with science in unconscionable ways.  In fact, a long ignored causal factor is related to deserts and the outbreaks of hot desert air into moister regions in the creation of heat-waves. That brings not merely hot air but stultifying atmospheric stagnation and haze, dust particles and other offal that elicits biological irritation.  The effects over very long periods, especially for those living within deserts, can be generational and lead to more violent societies in total. 

And one of the largest cross-cultural studies on that issue, of violence around the world, showed the harsh deserts of Saharasia  -- the Sahara plus Middle East plus Central Asia -- had the highest violence potentials of any region on the planet.  Of course, this was totally un-PC from a variety of viewpoints.  Look up the book of the same term "Saharasia"  or Saharasia dot org.

Avatar of: Knights


Posts: 1

August 14, 2013

I guess it really must have been hot during the crusades!

August 14, 2013

Grossly oversimplifying complex problems or phenomena is all the rage these days. It gets politicians elected and climate scientists funded.

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