A major eruption doubles the chance that an El Nino will be unleashed in the following winter, according to the research, which is published on Thursday in the British scientific journal Nature.

The research is the first to supply statistical flesh to the theory that volcanic fallout may affect the world's climate system, spewing out billions of tonnes of fine ash that lingers in the upper atmosphere, reflecting back solar heat.

Climate experts led by Brad Adams of the University of Virginia looked at so-called geological indicators -- dust preserved in polar ice cores as well as tree rings and coral growth that reflect sudden changes in the climate -- and compared this with the dates of major known eruptions from 1649 onwards.

They found a "significant, multiyear El Nino-like response" that kicked in just after big volcanic activity in the tropics.

"The results imply roughly a doubling of the probability of an el Nino event occurring in the winter following a volcanic eruption," they write

El Ninos occur in cycles that vary from three to 11 years, when the sea surface temperature in the western tropical Pacific Ocean is warmer than usual.

The prevailing east-west trade winds die, causing a huge buildup of warm water in the western part of the ocean.

This has effects on climate that can reverberate around the southern hemisphere, inflicting snowfalls and landslides in South America, drought in southern Africa, a weak hurricane season in the Atlantic and forest fires in Indonesia.

The shift in weather is so abrupt that crops and fish migrations are hit, having a dramatic effect on human life.

Adams' team believes that the fine shroud of volcanic ash kick-starts the process.

A thin layer of "aerosolised" particles hangs in the stroposphere, causing an overall cooling of the planet's atmosphere of a few tenths of a degree Celsius (about half a degree Fahrenheit).

But the effect regionally is different.

There is a complex interaction between atmospheric temperatures and vast, circulating masses of water. Athough the rest of the world cools, there is a slight warming in the tropical zone of the Pacific, fuelled in part by a convection of warm water from elsewhere.

That small rise is is enough to trigger an El Nino, which is highly susceptible to small changes in sea surface temperatures.

According to the study, the El Nino usually lasts for the first three years after a big tropical volcanic eruption, and then goes into reverse, with the so-called El Nina phenomenon, for the three years after that.

But the researchers add a big caveat: eruptions themselves are not the only factor. Man-made global warming -- the spewing out of greenhouse gases by the burning of fossil fuels -- is also likely to play a role.

"Volcanic eruptions, such as that of Mount Pinatubo [in the Philippines] in 1991 may have a larger effect on Earth's climate than previously thought," says Shanaka de Silva of the Department of Space Studies at the University of North Dakota, in a commentary.

"If they influence the [El Nino] cycle as proposed, then explosive volcanism is a vital catalyst in global climatic interconnections, and a major player in Earth's climate system."

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