Scientists have found another thing to blame on the climate demon El Nino: civil strife in poor tropical countries.
A new study released Wednesday finds a significant increase in unrest during the years of an El Nino, which is a regular climatic event that tends to warm up and dry out tropical regions.
"When people get warm and uncomfortable, they get irritable, they are more prone to fight," said Mark Cane, a professor of Earth and climate sciences at Columbia University and co-author of the study.
"People do like to fight and El Nino conditions help."
The flip side of El Nino, a La Nina, is also the most peaceful time for these tropical countries, the research found. Meteorologists predict the world is heading into another La Nina.
The researchers say the increase in civil unrest during an El Nino is so noticeable that there is more than just a link, but a partial cause. They say that El Nino influenced 48 of 234 civil wars or uprisings between 1950 and 2004.
For example, they point to internal strife in Chad, Congo, Eritrea, Indonesia, Rwanda, Myanmar and Niger during a strong El Nino in 1997.
The weather effects of an El Nino, which warms the central Pacific Ocean, are felt the most in about half the world, chiefly in the tropics, not including the United States or Europe. And it's in those mostly poor nations where the scientists noticed El Nino's affect on internal conflict.
In those affected countries, an El Nino makes the weather significantly warmer and drier, according to the study in the journal Nature. La Nina tends be cooler and slightly wetter in those regions.
Using internationally accepted figures for civil unrest, the study authors calculated that the annual risk of conflict during an El Nino year is 6 percent in the affected tropical countries. During a La Nina year, it is only 3 percent. For countries not in the affected region, the annual conflict risk is steady at 2 percent.
There are exceptions and they seem to be related to a country's economic health. For example, Australia gets major El Nino impacts, but it doesn't have a jump in internal conflicts, said lead author Solomon Hsiang, an international affairs and environmental policy researcher at Princeton University.
"We're not trying to explain all the conflicts in the world. What we are trying to show is that the global climate does play a major role where previously people didn't believe that," Hsiang said.
Historian Thomas Homer-Dixon of Canada's University of Waterloo said the research makes sense, noting there are classic cases in ancient history where weather was a factor in wars and downfalls of civilizations. He wrote the book, "Environment, Scarcity and Violence."
Homer-Dixon, who wasn't part of this study, said the new statistics-based analysis jibes with his own firsthand research into causes of internal violence in countries in the past 20 years.
Although this study looks back and is about El Nino, there are lessons to be learned for the future with man-made global warming, Cane and Homer-Dixon said.
"It's frankly difficult to see why (the climate link with conflicts) won't carry over into a world that is disrupted by global warming," Cane said.