Scientists say there's a link between climate and violent conflict.
A statistical analysis of civil conflicts between 1950 and 2004 found that in tropical countries, conflicts were twice as likely to occur in El Nino years. The analysis appears in the journal Nature.
El Nino occurs when there is unusually warm water in the Pacific Ocean near the equator. But it affects weather patterns in tropical countries around the globe.
"Half the world's population experiences a completely different climate regime," says lead author Solomon Hsiang, a researcher at Princeton University.
In most places, El Nino means the weather gets warmer and drier, sometimes for years. The opposite occurs with La Nina conditions, which occur when waters in the Pacific become unusually cool.
Hsiang and his colleagues looked at 93 tropical countries and 82 other countries to see whether the El Nino-La Nina cycle affected civil conflict, which they defined as a new dispute between the government and an organized group that results in at least 25 battle-related deaths.
In La Nina years, there was a 3 percent chance that a tropical country would have a civil conflict, Hsiang says. "When the global climate shifts into its relatively hotter and drier El Nino state," he says. "The rate of conflict jumps — it actually doubles all the way up to 6 percent."
But not all countries were affected by El Nino, Hsiang says.
"The countries that are most sensitive to El Nino are the poorer countries," he says. "Wealthier countries such as Brazil or Australia don't exhibit civil conflicts."
There's also no effect in countries such as Libya, which are outside the tropical belt, Hsiang says
Fighting Over Resources?
Historians have long used anecdotal evidence to suggest that climate has played a role in many conflicts. For instance, the French Revolution occurred during an unusually hot summer, and a drought coincided with violent conflicts in Rwanda in the 1990s.
In 2009, a controversial study suggested a link between local temperatures and the likelihood of conflicts in Africa. But many scientists felt the study had statistical weaknesses.
The new study is far more robust, says Andy Solow, a statistician at the Woods Hole Oceanographic Institution in Massachusetts who wrote a commentary that accompanied the study.
It's still not entirely clear why El Nino conditions would lead to conflict, Solow says. But he says it's easy to think of reasons the two might be related.
"People don't just go to war because the weather changes," Solow says. "The effect of weather on human behavior is presumably operating through resource scarcity or food scarcity or something like that."
That could have been a factor in both the French Revolution and violence in Rwanda. In both cases, hot, dry weather had caused food shortages.
And Solow says El Nino is well known to affect crop yields in tropical countries.
"A lot of these countries are poor and mainly agricultural," he says. "As climate conditions change, that can put stress on the agricultural system in those countries — also possibly on water resources and other resources. And that may lead to conflict."
Researchers still need to figure out whether that's really what's going on with El Nino in the tropics, Solow says.
Even without knowing precisely how El Nino affects violence, it may be useful to consider climate when trying to anticipate conflicts in certain countries, Hsiang says. For instance, the current troubles in Somalia are occurring at a time when climate conditions are contributing to food shortages.
But Hsiang says being able to anticipate trouble doesn't necessarily mean other countries should step in to prevent conflict.
"Some conflicts are important," he says. "So it's not up to us to say that the conflicts per se should be stopped."
The goal should be anticipating the needs of innocent bystanders who suffer in these conflicts, Hsiang says.