Volcanoes 'unleash' El Nino

2003-11-20 10:29

Paris - Volcanoes are a prime cause for El Nino, the climate phenomenon that can catastrophically disrupt weather patterns across the Pacific and beyond, a study says.

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 fall-out 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.

Significant

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 build-up of warm water in the western part of the ocean.

Dramatic effect

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.

But the effect regionally is different.

There is a complex interaction between atmospheric temperatures and vast, circulating masses of water. Although 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 enough to trigger an El Nino, which is highly susceptible to small changes in sea surface temperatures.

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.

- AFP