El Nino, a phenomenon that leads to droughts and hot weather some years, doubles the risk of civil war in 90 tropical countries, a new study suggests.

In fact, El Nino may help account for a fifth of conflicts worldwide during the past 50 years, said the study published online Wednesday in the journal Nature.

"What it does show beyond any doubt is even in this modern world, climate variation has an impact on the propensity of people to fight," said Mark Cane, a climate scientist at Columbia University who co-authored the report, during a press briefing organized by Nature.

Notable civil outbreaks in El Nino years 1982-83: Peru’s Shining Path movement’s guerilla attacks give way to full-scale, 20-year civil war; Outbreak of violence in Sudan that killed 2 million people

1991-92: Angola, Haiti and Myanmar see an increase in violence.

1997-98: Civil conflicts blow up in Congo, Eritrea, Indonesia and Rwanda Compiled by Solomon Hsiang and colleagues

"It is frankly difficult to see why that won't carry over to a world that is disrupted by global warming."

However, the researchers noted that the link between conflict and a cyclic climate phenomenon doesn't directly address non-cyclic long-term climate change.

The El Nino-Southern Oscillation or ENSO is a cyclic warming and cooling of the tropical Pacific Ocean. Its La Nina phase results in plentiful rainfall in affected countries, which form a belt that runs east-west and spans from just north of the equator down to Australia and the South Pacific. Every three to seven years, the El Nino phase causes unusually hot and dry weather.

The researchers tracked ENSO from 1950 to 2004 and correlated its effects to data on 235 conflicts in 175 countries over the same period.

They found that during El Nino years, affected countries faced, on average, a six per cent chance of civil war breaking out, compared to a three per cent chance in other years. Countries unaffected by El Nino always had, on average, a two per cent chance of civil conflict breaking out.

It is hard to conclusively say why the link occurs, but during times of drought, the increased food prices and unemployment may make joining armed groups more attractive, said Kyle Meng, a Ph.D. student, at Columbia's School of International and Public Affairs, who also co-authored the report. He added that humans have a greater tendency toward violence when temperatures are hotter.

The study found some evidence there were fewer conflicts in the year following El Nino, suggesting that El Nino may push conflicts earlier than they would have otherwise occurred, Meng said. The researchers concluded that some El Nino-linked conflicts are new and some would have happened anyway, though not necessarily at that time.

In general, conflicts tended to hit poorer countries, and the researchers suggsted that might be because they lack the resources to mitigate the effects of catastrophic weather. On the other hand, Australia, which is heavily affected by El Nino, did not have any civil conflicts at all during the 50-year study period.

Predictions can aid preparedness

Meng said that while the research findings do not make it possible to predict conflicts, they should allow national and international governments to prepare for the possibility based on climate predictions and avert some of the human suffering associated with civil conflicts.

Solomon Hsiang, lead author of the study, noted that forecasters predicted two years ago that there would likely be a drought this year in the Horn of Africa, where climate patterns tend to be the reverse of the rest of Africa with respect to El Nino.

"But donors to international aid groups did not respect that forecast and did not take that very seriously," he said.

Now, Somalia has been hit by a famine, and Al-Shabab, the armed militant Islamist group that controls much of the south, has been blocking humanitarian and allegedly preventing starving Somalis from fleeing to government-controlled areas.

Hsiang said he hopes the findings of the study will help motivate authorities to take future climate forecasts more seriously and to be more proactive.

Previous studies linking climate and civil conflicts have been controversial. A 2009 study led by Marshall Burke at the University of California Berkeley linked outbreaks of civil conflicts in Africa to year-to-year temperature variations. Following its publication, critics slammed the statistical methods used draw that connection.

Andrew Solow, a researcher at Woods Hole Oceanographic Institution in Woods Hole, Mass., called the statistical analyses used in Hsiang's paper "careful" in a review in Nature accompanying the paper. He added that relating complex human behavior to environmental factors is "invaluable" but need to be complemented by detailed studies showing why the link occurs.

In a Columbia University news release, Halvard Buhaug, a political scientist at the Peace Research Institute in Norway, said due to that missing information, the new study "fails to improve on our understand of the causes of armed conflicts… Correlation without explanation can only lead to speculation."