Water wars, conflicts over grazing grounds or simply hungry hordes from famine-stricken areas descending on the better-off. Increasingly one hears of such events or prospects around the world. Now the El Nino is being directly linked to large-scale violence.
The civil war and famine gripping the Horn of Africa is a typical example of what happens when a climate swing causes drought and overstresses an already fragile society, say the new study's authors.
The inquiry appears in Thursday's issue of the journal Nature.
"What it does show and show beyond any doubt is that even in this modern world, climate variations have an impact on the propensity of people to fight," said Mark Cane, a climate scientist at the Lamont-Doherty Earth Observatory at Columbia University in New York.
"It's difficult to see why that won't carry over to a world that's disrupted by global warming."
El Niņo, the periodic warming of the waters of the eastern tropical Pacific, gets its name - "the Christ child" in Spanish - because its onset is often observed just before Christmas. It boosts temperatures and cuts rainfall over a broad swath of the globe, tropics including Africa, the Middle East, India, Southeast Asia, Australia and the Americas, It is unleashed every three to seven years, doubling the risk of civil wars across many countries in the tropics.
El Niņo is one half of the Southern Oscillation, the other half being La Niņa, which conversely is a cooling phenomenon marked by more plentiful rainfall over the areas affected; the two together are known as Enso (El Niņo-Southern Oscillation).
The researchers compared more than half a century of Enso data with the history of conflicts in the tropics from 1950 to 2004 which killed more than 25 people in a given year. The data included 175 countries and 234 conflicts, over half of which each caused more than 1,000 battle-related deaths.
For nations whose weather is controlled by Enso, they found that during La Niņa, the chance of civil war breaking out was about 3 per cent, whereas during El Niņo, the chance doubled, to 6 per cent. (Countries not affected by the cycle remained at 2 per cent no matter what.)
Overall, the team calculated that El Niņo may have played a role in 21 percent of civil wars worldwide over the past 50 years, although they stressed that the study does not show that wars are started by weather alone.
"But if you have social inequality, people are poor, and there are underlying tensions, it seems possible that climate can deliver the knockout punch," said the study's lead author lead author Solomon Hsiang of Columbia's Earth Institute. "When crops fail, people may take up a gun simply to make a living."
It is poorer countries which appear to be tipped into chaos more easily by bad weather; that of rich Australia, for instance, is controlled by Enso, but the country has never seen a civil war. But on the other side, at least two countries, said Dr Hsiang "jump out of the data."
In 1982, a powerful El Niņo struck impoverished highlands Peru, destroying crops; that year, simmering guerrilla attacks by the revolutionary Shining Path movement turned into a full-scale 20-year civil war that still sputters today.
Similarly, forces in southern Sudan were already facing off with the domineering north, when intense warfare broke out in the El Niņo year of 1963. The insurrection flared again in 1976, another El Niņo year. Then, 1983 came a major El Niņo and the cataclysmic outbreak of more than 20 years of fighting which killed two million.
Dr Hsiang said other countries where festering conflicts have tended to blow up during El Niņos include El Salvador, the Philippines and Uganda (1972); Angola, Haiti and Myanmar (1991); and Congo, Eritrea, Indonesia and Rwanda (1997).
He noted that El Nino was an invisible factor -- though not the only one -- in driving intra-border conflict.
By causing crop losses, hurricane damage or helping to spread epidemics of water-borne disease, it amplified hunger, loss, unemployment and inequality, which in turn fuelled resentment and division.
Other factors that could affect risk and the outcome are the country's population growth and prosperity and whether its government is able to manage El Nino events properly.
"Even though we control for all of these factors simultaneously, we still find that there's a large and pervasive El Nino effect on civil conflicts," Hsiang said in a teleconference.
Although the current crisis in the Horn of Africa occurred beyond the parameters of the study, it was a "perfect example" of the hidden destruction of an El Nino.
"Forecasters two years ago predicted that there would be a famine in Somalia this year, but donors in the international aid community did not take that forecast seriously," said Hsiang.
"We hope our study can provide the international community and governments and aid organisations with additional information that might in the future help avert humanitarian crises that are associated with conflict."