Climate Shifts Drove Wars in 90-Plus Nations Since 1950, Study Shows

In the past two decades scientists have concluded that climate shifts helped drive many of history's biggest conflicts—from the collapse of the Mayan civilization around 800 AD to the French Revolution beginning in 1789.

But the impact of climate on violence in modern societies, which are considered more technologically and politically adept at dealing with chaotic weather, remains controversial.

Studies in the past few years linking conflict to warmer temperatures or drought have been both dismissed and defended by the scientific community. But new findings from researchers at Columbia and Princeton Universities aim to clear up the confusion.

According to the results, which will be published tomorrow in the journal Nature, nearly 21 percent of civil conflicts that took place in 175 tropical countries since 1950 have been driven by the warmer, drier climate patterns associated with the El Nino cycles.

Most of the fighting happened in the 93 nations considered to be highly influenced by the El Nino-Southern Oscillation, or ENSO.

"It is a really enormous effect, much larger than anyone, including myself, expected," Solomon Hsiang, a climate and development researcher at Princeton and lead author of the paper, told SolveClimate News. "It means we have to seriously rethink how important the climate is to the stability of modern populations."

The study found that damage caused by El Nino to countries' agriculture—and, therefore, to their economies—may be the major conflict trigger, though researchers warn that the exact reasons for why climatic variations spark violence remain unknown. El Nino can also create conditions for disease outbreaks and natural disasters like tropical cyclones, the study said. Extreme and prolonged heat and dry weather may also agitate populations, making them more prone to aggression.

The analysis is the first hard evidence linking climate changes to modern-day violence over time and across regions, said Marshall Burke, a climate and food security researcher at Stanford University, who was not involved in the study. "Past studies looked only at the effect of local weather variations on conflict," he said in an interview.

The findings might begin to help guide societies toward strategies for coping, Burke added. "The fact that ENSO is itself somewhat predictable makes their findings policy relevant. If we think an El Nino is coming, then governments in regions susceptible to variations could put in place measures to try to reduce the risk of conflict in that year."

Hsiang said the results could assist governments and aid organizations to avert crises like the one being felt today in the Horn of Africa, which climate forecasters predicted two years ago.

As of this week, nearly 12 million people in Djibouti, Ethiopia, Kenya, Somalia and Uganda are believed to be suffering from food shortages spurred by drought, according to the United Nations Food and Agriculture Organization. Humanitarian groups have reported an increase in violence as food insecurity spreads.

Modeling El Nino and War: The Results

The new research got its start in the summer of 2009, after Hsiang read the book "Late Victorian Holocausts: El Nino Famines and the Making of the Third World" by Mike Davis, a University of California, Riverside professor, as part of his Columbia University dissertation research.

Intrigued by Davis's assertion that El Nino cycles were responsible for famine and conflicts in China, India, Vietnam and Brazil in the late 1800s, Hsiang wondered whether the climate cycle was having a similar effect on modern societies.