Wars and civil conflicts are twice as likely to occur during years affected by the El Niño climate-warming phenomenon, a study suggests. The naturally recurring weather system, which boosts temperatures and cuts rainfall over a broad swath of the globe every three to seven years, doubles the risk of civil wars across many countries in the tropics, shows a remarkable statistical analysis by American scientists.

In fact, the researchers suggest, El Niño may help account for a fifth of worldwide conflicts during the past half-century, although the exact mechanism remains unclear. The study, in the journal Nature, claims to be the first demonstration that the stability of modern societies relates strongly to the global climate. Its disquieting implication, in the context of continuing man-made climate change, is that the world may be facing more turbulent times still.

At its heart is El Niño, the periodic warming of the waters of the eastern tropical Pacific, given its name – "the Christ child" in Spanish – because its onset is often observed just before Christmas. El Niño leads to warmer conditions and droughts across many parts of the tropics including Africa, the Middle East, India, Southeast Asia, Australia and the Americas.

It is one half of the Southern Oscillation, the other half being La Niña, which conversely is a cooling phenomenon marked by more plentiful rainfall over the areas affected; the two together are known as Enso (El Niño-Southern Oscillation).

The researchers, from Columbia University in New York, 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 Niña, the chance of civil war breaking out was about 3 per cent, whereas during El Niño, 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 Niño 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 Solomon Hsiang, the study's lead author. "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 Niño 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 Niño year of 1963. The insurrection flared again in 1976, another El Niño year. Then, 1983 came a major El Niño and the cataclysmic outbreak of more than 20 years of fighting which killed two million.