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Higher temperatures lead to 'substantially more violence'

As climate change takes hold, the link between hot temperatures and human aggression could make for more crime and civil conflict. But we still need to understand why, says Stanford researcher Marshall Burke.

DW: You’ve been working on a study looking into

climate change

and its impact on the occurrence of conflict in different regions of the world. But how does an increase in temperature lead to conflict?

Marshall Burke: The main explanation coming from economics has to do with what happens in a really bad year - a year where there is a drought or it's really hot. What we know about the economics is that in a bad year, agricultural productivity declines.

Another explanation comes from psychology. Psychologists have studied for a long time the relationship between hot temperatures and human aggression, and what they find over and over is humans are really bad physiologically at dealing with hot temperatures. When it gets hot out, we become more aggressive, and we become worse at making decisions. They've shown that in the lab, and now we think we've shown in broader-scale data, that this actually leads to substantially more violence.

What types of conflict did the study look at? And what climate factors were considered?

We looked at interpersonal conflict, things like aggravated assault and murder, and at group level conflicts, things like riots and civil wars. On the climate side, we looked at a broad range of climate variables - but most of the focus was on changes in precipitation or changes in temperature.

What were the findings of the study?

Looking back through time, and looking at various settings around the world, what we find is a consistent relationship between hotter-than-average temperatures and increases in the likelihood of conflict. Of the 24 studies that we were able to analyze, all 24 showed a positive relationship between temperature and violence.

We found in the tropics that a 1 degree Celsius increase in temperature is associated with a more than 20 percent increase in these large-scale civil conflicts.

A state trooper walks by a grieving student as students and family members reunite at Shoultes Gospel Hall, following a school shooting at Marysville-Pilchuck High School, in Marysville, Washington October 24, 2014. (Photo: REUTERS/Jason Redmond)

Higher temperatures were linked with more violent behavior in countries around the world

Could you give us some examples of climate-induced conflict?

You see it in crime statistics in the US: so you talk to police chiefs and they've noticed that crime actually spikes on really hot days. What we've been able to do with this study, and what others have done in studies that we've reviewed, is to show this very explicitly using lots of data on different cities in the US and different types of crime. It also shows up in these broader-scale conflicts around the world - but particularly in

sub-Saharan Africa

, where we've seen a lot of civil conflicts over the last two to three decades. Again, what we see there is that the likelihood of these conflicts is much higher in hot years.

What I think is surprising is that the relationship was as strong and consistent as it was across time and different countries of the world.

What added stresses does climate change place on societies?

It'll depend on what kind of society you're in. If you're in a society that's largely dependent on agriculture for your income, we know increases in temperature are going to hurt agricultural productivity, and so this will be one of the most direct stresses of climate change. If you're in a place like the US, most people are not farmers. But what research has shown is that it doesn't matter if you're farming or not, people are just unhappier and they get along less well on a hot day. So there will also be this direct effect of heat stress on human physiology.

This data comes from a working paper, so you will be continuing your research. What needs to be looked into next?

What we think is the main research need and the main place research should go is to really try to understand why changes in temperature, in particular, are linked to conflict. So what we see is this strong relationship over and over between hotter-than-average temperatures and increases in conflict. But there are many different stories that could link these two things, and understanding exactly what's going on is really import for policy-makers who want to intervene and break this link between temperature and conflict.

Right now we don't have a great understanding of the precise mechanisms that link these two things, we have a lot of hypotheses, so our sense is where research should go is to test these different hypotheses and really get at the mechanism that links temperature to conflict.

Marshall Burke is assistant professor at the

Environmental Earth System Science Center

and the

Center on Food Security and the Environment

at Stanford University in California. He is co-author of the working paper on climate and conflict, which was released by the US-based

National Bureau of Economic Research

this October.

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