In 1957, civil war broke out in then Burma (now Myanmar) and Oman. In 1965, similar conflicts erupted in Burundi, Chad, the Dominican Republic, Indonesia and Peru. In 1997, the Comoros, Congo, Eritrea, Niger and Rwanda also saw battles over national power that each killed more than 25 people. What do all these years and countries have in common? First, they are all tropical countries, strongly affected by the weather pattern known as El Niño, a warming of equatorial waters in the eastern Pacific Ocean that occurs cyclically every three to seven years and generally makes the climate hotter and drier throughout the tropics. Secondly, these were all years in which the global climate was enduring an El Niño.

In fact, research published August 24 in Nature now demonstrates a link between El Niño years and civil war in 93 tropical countries. (Scientific American is part of Nature Publishing Group.) "Since 1950, one out of five civil conflicts have been influenced by El Niño," says economist Solomon Hsiang of Columbia University, lead author of the research. "This represents the first major evidence that global climate is a major factor in organized patterns of violence."

Starting with data on conflicts that killed more than 25 people, as compiled by the Center for the Study of Civil War to include 175 countries and 234 civil wars in the last six decades or so, the researchers mapped out how many of these disputes occurred in years with an El Niño weather pattern. They found that the risks of civil war breaking out in a tropical country during an El Niño doubled. Then, running a comparative simulation in which such El Niño weather patterns did not occur, the researchers determined that the hotter, drier conditions helped stoke 48 civil wars that did not occur in the modeled El Nino-free world. "Even in this modern world, climate variability has an impact on the propensity of people to fight," says climate modeler Mark Cane of Columbia University. "When people get warmer than comfortable they get irritable and they are more prone to fight."

That is one hypothesis, at least, for why this effect would be seen, along with the fact that El Niño conditions are associated with declines in crop yields across the globe, which may contribute to conflict. It is also clear that El Niño alone does not inspire civil war; it just ups the risk. And it is evident that rich tropical countries, such as Australia, do not see this linkage between warmer and drier conditions and civil war. Hsiang likens the finding to ice patches on roads in winter: the ice patches increase the likelihood of car crashes, just as El Niño conditions exacerbate underlying tensions in a given society. "The state of global climate can determine whether conflict is more or less likely," he says.

That could be bad news as the global climate is changing in a generally warmer direction thanks to increasing concentrations of greenhouse gases, like carbon dioxide, in the atmosphere. "It is frankly difficult to see why that won't carry over to a world that is disrupted by global warming," Cane says. "If these smaller, shorter-lasting and, by and large, less serious kinds of changes associated with El Niño have this effect, it is hard to imagine that the more pervasive changes that come with anthropogenic climate change are not also going to have negative effects on civil conflict.”

Certainly, history has been influenced by climate changes, ranging from the drought-driven collapse of Mayan society to the partially climate-induced rising food prices that helped to touch off the more recent "Arab spring." In fact, diminished agricultural production as a result of climate variability matches up with fluctuations in the frequency of war, mass migrations and even regime change in the historical record. "It seems to us that the climate-war association is not only valid in the past, but also valid in present days," says geographer David Zhang of the University of Hong Kong, who was not involved in this study but has analyzed world history and found linkages between war and temperature change in previous published research.

Ultimately, because El Niño patterns can be predicted as much as two years in advance, the world might have forewarning of the potential for increased conflict. "At a minimum, national governments and international institutions should be ready for such events," says economist-in-training Kyle Meng of Columbia University, who also worked on the latest study.

But Meng and Hsiang's work cannot explain the root causes of such conflicts, merely showing a correlation between the outbreak of civil war and El Niño events, so it may not prove a useful guide. "Correlation without explanation can only lead to speculation," says political scientist Halvard Buhaug of the Peace Research Institute of Oslo.

And, of course, forewarned does not mean forearmed. Researchers predicted the present famine in the Horn of Africa more than two years ago and yet nothing was done. "It was not until the famine and violence actually began that donors began supplying the resources needed," Hsiang notes.

Nonetheless, Hsiang's research does seem to show that, for the more impoverished countries of the tropics, the linkage between climate and conflict has yet to be broken—even if it may have been in richer countries that are more technologically insulated from such shocks, thanks to air conditioning, food surpluses and other signs of wealth. As Hsiang says, "In the modern world, we still depend on climate to a very large extent."