Shifts in climate are strongly linked to human violence
around the world, with even relatively minor departures from normal temperature
or rainfall substantially increasing the risk of conflict in ancient times or
today, according to a new study by researchers at the University of California,
Berkeley, and Princeton University.
The results, which cover all major regions of the world and
show similar patterns whether looking at data from Brazil, China, Germany,
Somalia or the United States, were published in the
By amassing more data than any prior study, the authors were
able to show that the Earth's climate plays a more influential role in human
affairs than previously thought.
What the research
The study data covers all major regions of the world and
show similar patterns of conflict linked to climatic changes, such as increased
drought or higher than average annual temperature. Examples include spikes in
domestic violence in India and Australia; increased assaults and murders in the
United States and Tanzania; ethnic violence in Europe and South Asia; land
invasions in Brazil; police using force in Holland; civil conflicts throughout
the tropics; and even the collapse of Mayan and Chinese empires.
The new study could have critical implications for
understanding the impact of future climate change on human societies, as many
global climate models project global temperature increases of at least 2
degrees Celsius over the next half century.
Although there has been a virtual explosion in the number of
scientific studies looking at how climatic impacts shape human conflict and
violence, especially in in recent years, the research stems from disparate
research fields ranging from climatology, archaeology and economics to
political science and psychology.
"What was lacking was a clear picture of what this body
of research as a whole was telling us," said Solomon Hsiang, the study's
lead author, who was a postdoctoral fellow in Science, Technology, and
Environmental Policy at Princeton during the research project and is now an
assistant professor of public policy at UC Berkeley's Goldman School of Public
"We collected 60 existing studies containing 45
different data sets and we re-analysed their data and findings using a common
statistical framework. The results were striking."
The latest study adopted a broad definition of conflict and
used the latest research methods to re-evaluate what they found to be the most
rigorous quantitative studies released since 1986 to examine aspects of climate
such as rainfall, drought or temperature, and their associations with various
forms of violence.
To determine if a link between climate and conflict existed
at multiple levels of social organization, the UC Berkeley-Princeton researchers
looked at whether evidence of a linkage was consistent within each of three
broad categories of conflict:
- Personal violence and crime such as murder, assault, rape,
and domestic violence;
- Intergroup violence and political instability, like civil
wars, riots, ethnic violence, and land invasions;
- Institutional breakdowns, such as abrupt and major changes
in governing institutions or the collapse of entire civilizations.
They found that all three types of conflict exhibit
systematic and large responses to changes in climate, with the effect on
intergroup conflict being the most pronounced in percentage terms. The authors
found that conflict responded most consistently to temperature, with all 27 out
of 27 studies of modern societies finding a positive relationship between high
temperatures and greater violence.
New approach to
A central contribution of the study was to develop a method
for comparing results around the world, because the nature of climatic events
differs across locations. The authors' new approach was to convert climate
changes into location specific units known to statisticians as standard
"We found that a 1 standard deviation shift towards
hotter conditions causes the likelihood of personal violence to rise 4% and
intergroup conflict to rise 14%," said Marshall Burke, the study's co-lead
author and a doctoral candidate at Berkeley's Department of Agricultural and
"For a sense of scale, this kind of temperature change
is roughly equal to warming an African country by 0.4°C (0.6°F) for an entire
year or warming a United States county by 3°C (5°F) for a given month. These
are moderate changes, but they have a sizable impact on societies."
"We often think of modern society as largely
independent of the environment, due to technological advances, but our findings
challenge that notion," said study coauthor Edward Miguel, the Oxfam
Professor of Environmental and Resource Economics and director of the Center
for Effective Global Action (CEGA) based at UC Berkeley.
Just the beginning
The researchers said that exactly why climate affects
conflict and violence is the most pressing question for future related
"We're in the same position that medical researchers
were in during the 1930s: they could find clear statistical evidence that
smoking tobacco was a proximate cause of lung cancer, but they couldn't explain
why until many years later. In the same way, we can show that climatic events
cause conflict, but we can't yet exactly say why," said Hsiang.
"Currently, there are several hypotheses explaining why
the climate might influence conflict. For example, we know that changes in
climate shape prevailing economic conditions, particularly in agrarian
economies, and studies suggest that people are more likely to take up arms when
the economy deteriorates, perhaps in part to maintain their livelihoods."
No single answer
Burke said there are very likely multiple mechanisms at
play, since no single theory explains all of the evidence.
"The studies showing that high temperature increases
violence crime in the U.S. and other wealthy societies seems to suggest that
physiological responses are important, too, with very short-run exposure to
heat contributing to more aggressive and violent behavior," he said.
Nonetheless, in all cases, Burke noted "the collected evidence shows that
humans across the globe have proven poorly equipped to deal with exposure to
While the study finds strong evidence that climatic events
may be a cause of conflict, the researchers stressed that they are not claiming
that climate is the only or primary cause of conflict, cautioning that conflict
dynamics are complex and remain poorly understood.
"Say you look at data on car accidents," offered
Hsiang, "and you see that they become more likely on rainy days. Does that
mean that rain is the only factors responsible for accidents? Of course not.
Driver error ultimately causes accidents but rain can make it much more likely.
Similarly, violent conflicts might occur for a variety of reasons that simply
become more likely when climatic conditions deteriorate."
Facing a hotter
"Our results shed new light on how the future climate
will shape human societies," said Burke. The findings of the study suggest
that a global temperature rise of 2 degrees Celsius could increase the rate of
intergroup conflicts, such as civil wars, by over 50% in many parts of
"It's possible that future societies might figure out
better ways to cope with hot temperatures and variable rainfall, and indeed we
hope that they will," said Miguel. Nevertheless, the authors argue that it
is dangerous to assume that people will cope well to extreme climate when they have
not done so historically.
"We need to understand why climate changes cause
conflict so we can help societies adapt to these events and avoid the violence.
At the same time, we should carefully consider whether our actions today are
making our children's world a more dangerous one," said Hsiang.