The El Nino climate cycle, which spreads warm, dry air around the globe every four years or so, doubles the risk of civil wars in 90 tropical countries, researchers reported on Wednesday.
El Nino climate cycle spreads warm and dry air that causes drought in Thailand in 2006
And because El Nino patterns can be predicted up to two years in advance, scientists suggest their findings could be used to help prepare for some conflicts and the humanitarian crises they cause.
Historians and climate specialists have noted signs that changes in climate sent past societies into conflict and decline, but this is the first study to quantify the link between El Nino's heat, the droughts that follow, and upheaval in countries that bear the brunt of it.
Between 1950 and 2004, one out of every five civil conflicts were influenced by El Nino, scientists reported in the journal Nature.
El Nino starts as a large patch of warm water in the tropical Pacific and influences global climate and weather across much of Africa, the Middle East, India, Southeast Asia, Australia and the Americas.
This pattern can cause large crop losses and increased risk of natural disasters like hurricanes and the spread of infectious diseases, study co-author Kyle Meng of Columbia University's Earth Institute.
"These events can lead to increases in income inequality... and a labour market effect," Meng said in a telephone briefing. "These lead to increased unemployment, which makes the opportunities to fight a little bit more attractive."
Responding to El Nino with violence
It is also often more difficult for governments to enforce laws during severe weather events, he said.
Peru in 1982 and Sudan in 1963, 1976 and 1983 showed remarkable links between El Nino patterns and civil unrest, the researchers found. Other countries with a strong link between violence and El Nino include El Salvador, the Philippines and Uganda in 1972; Angola, Haiti and Myanmar in 1991, and Congo, Eritrea, Indonesia and Rwanda in 1997.
The researchers focused on internal civil conflicts because since 1950, these account for 80 to 90 percent of all conflicts.
Some 40 percent of the conflicts that occurred would probably have happened anyway, but the stresses of El Nino made conflict more likely, and sometimes made it happen earlier. Poorer countries were at the greatest risk, the scientists said.
"It's the poorest countries that respond to El Nino with violence," said study co-author Mark Cane, a climate scientist at Columbia's Lamont-Doherty Earth Observatory.
To get their findings, the researchers correlated El Ninos from 1950 to 2004 with civil conflicts that killed more than 25 people in a given year, including 175 countries and 234 conflicts, over half of which caused more than 1,000 battle-related deaths.
Countries with weather controlled by the El Nino cycle had a 6 percent chance of civil war breaking out when the hot, dry El Nino pattern was in force, the scientists reported. That was double the 3 percent chance of internal conflict when the cooler, wetter La Nina pattern prevailed.
The study does not predict how the El Nino cycle might change in a warmer world, but Cane said climate change might make the Earth "more El Nino-like."
"What (the study) does show, beyond any doubt, is that even in this modern world, climate variations have an impact on the propensity of people to fight," Cane said. "And it is frankly difficult to see why that won't carry over to a world that is disrupted by global warming."