When an unusually powerful El Niño struck in 1997, civil conflicts erupted across the tropics, from Sudan to Peru -- as floods, droughts and fires devastated crops, fisheries and livelihoods.
It wasn't an isolated case, suggests growing evidence that links El Niño's extreme weather with a spike in violent conflicts in tropical regions. As one of the strongest El Niño events in recorded history gains steam this fall, some experts are warning of the potential for more unrest to come – and the urgent need to take preventive action.
"Half the world's population is exposed to a higher risk of violence this year," says Solomon Hsiang, professor of public policy at Berkeley. "Now that we know what to expect, we shouldn't necessarily sit back and watch sparks fly. There are a lot of things we can do."
Collapses of entire civilizations have been linked to climate shifts, with examples that go back centuries. The Little Ice Age in the mid-1600s, for example, has been blamed for widespread wars and political crises that occurred around much of the world at the time.
More recently, scientists have focused on the devastating potential of El Niño, a phenomenon that happens every few years, when surface waters in the Pacific become warmer than normal. That extra heat rises, altering atmospheric circulation patterns and affecting weather conditions around the world.
El Niño events play out in various ways from region to region, often but not always bringing rain to the American West and mild conditions to the upper Midwest. But El Niño hits hardest and most predictably in the tropics, with rising temperatures and rainfall extremes that go both ways.
Already, this year's El Niño has sparked forest fires in Indonesia, drought in Ethiopia and a wimpy monsoon season in India, says Marc Cane, a climate scientist at the Lamont-Doherty Earth Observatory at Columbia University in New York, who created the first predictive model for El Niño.
Flooding is expected in Uruguay, southern Brazil and northern Argentina, while parched conditions are likely in most of Southeast Asia, Africa, the South Pacific and much of South and Central America.
Those weather extremes can be economically disastrous in the tropical world, where crops often fail when it's too hot, dry or wet. Farmers struggle to make a living. Food prices go up. And people who already spend a large portion of their income on food end up facing financial stress. Conflict, according to accumulating evidence, often follows.
Over a span of five decades from 1950 to 2004, for example, risks for violent conflicts doubled in El Niño years compared to La Niña years (when Pacific waters are colder than usual), found a 2011 analysis. Overall, concluded the research team, which included Hsiang and Cane, ENSO events played a role in 21 percent of all civil conflicts during that time.
In another analysis of 60 rigorous studies, Hsiang and colleagues reported in 2013 that the risk for conflict rises alongside progressively warmer temperatures and more extreme deviations from normal rainfall. Because these studies tapped into natural experiments in places that remain the same in most other ways from year to year, Hsiang says they make a convincing case that weather patterns are a major cause of strife.
Less clear is why, exactly, El Niño events might instigate unrest, though researchers have a variety of theories. One idea is that when people struggle economically, they may become more likely to join existing militia groups as a way to make a living. It's a trend frequently seen elsewhere, including in the United States, where the armed forces began to meet their recruiting goals as the job market declined after the 2008 recession.
There's also a known link between hot weather and all sorts of interpersonal violence, including road rage, fights over sporting events and assaults, perhaps because of physiological stress. "When it's hot, people get cranky," Cane says. "People shouldn't think that doesn't matter."
Not everyone agrees that the link between climate and violence is so simple. Many other political and economic factors influence the potential for conflict, argues John O'Loughlin, a political geographer at the University of Colorado, Boulder. In his view, studies that blame conflict on El Niño, climate change or other environmental factors miss the deeper context. Changing climate conditions, he adds, can also end up increase cooperation and boosting resilience.
"Certainly the effects of climate change can reduce resource availability and could have knock-on effects in terms of competition between communities and increased migration," O'Loughlin says. "But other factors such as institutional and government responses, land allocation procedures, the nature of livelihoods, and the local histories of cooperation can all mitigate these negative effects."
Weather is certainty not the only factor that triggers the onset of civil conflict. But because El Niño appears to play a clear role, Hsiang argues, it's important to start taking steps now to keep the peace this winter.
Preparing relief organizations for upcoming crop shortages is one way to help, he says. Among other strategies, international lending agencies like the IMF could temporarily relax demands for payments from tropical nations that are likely to be dealing with agricultural stress.
A redistribution of food on the global market could also make a big difference. Even as many people in the tropics struggle to feed their families during El Niño years, Hsiang says, total global food production increases because crops tend to do well in growing regions of the United States, where El Niño's usually bring cool and wet conditions.
"Food is not getting to people who need it most," Hsiang says. "People who are lucky enough to live further north should really have sympathy for folks who are going to endure the coming eight months of hardship in the tropics."