Big volcanic eruptions have had a cooling effect on Earth's climate, but the Icelandic event is too small to provide any such respite from man-made global warming, scientists said Friday.

The benchmark cooling event of the past 20 years was in 1991, when Mount Pinatubo erupted in the Philippines . It cooled Earth's surface by 0.5 degrees Celsius over the next year, enough to offset the impact of greenhouse gases from 1991 to 1993.

A smaller cooling episode occurred in 1980, when Mount St. Helens in the U.S. state of Washington blew its top, an event that while impressive disgorged only a tenth of the material of Pinatubo.

The chill comes from a simple formula: The volcano spews out a huge amount of fine volcanic ash and sulfur dioxide, which are transported into the high altitude layer known as the stratosphere. There, chemistry and physics combine, creating a fine layer of whitish particles that for months or years are blown around the Earth and reflect some of the Sun's rays rather than let the radiation reach the ground.

"Basically, it's like putting a reflector shield over the windscreen of your car--you're stopping the inside of the atmosphere getting too warm," said Colin Macpherson of Durham University , northeastern England . But he and others said the eruption at Iceland 's Eyjafjallajokull glacier so far was too small, didn't have sufficient sulfur content and its plume circulated at too low an altitude to make an impact.

Any effect will be "very insignificant," said Scylla Sillayo of the World Meteorological Organization in Geneva .

"At the moment, we are looking at something that's about 100 times smaller than Mount St. Helen's. At the scale it's at now, it's relatively unlikely that it will have any perceptible effect on climate," explained Kathryn Goodenough of the British Geological Survey.

Emmanuel Bocrie of the French weather service, Meteo France , said: "With Pinatubo, ash rose to 18,000 meters, in the tropics.

"It isn't at all the same situation in this case, where the plume is at an average of 6,000 meters, with peaks of 11,000 meters. In addition, this is in a zone of the atmosphere [the troposphere] where there are powerful winds, which have a huge dispersing effect."

The scientists said the Eyjafjallajokull eruption could have a regional effect on Europe 's climate--but only provided it went on for a matter of years.

"Back in the 1780s, a large eruption in southern Iceland went on for a couple of years and generated a lot of sulfur," said Macpherson. "It caused a smog that was really rather nasty, there was a failure of crops as a result of acid rain, the air quality was really very poor. But that took the better part of two years to cause that kind of effect and certainly we are not looking at anything like that right now."

Another is a theory--but no more--that the present eruption could touch off a far larger, neighboring volcano called Katla. From 1821-1823, the present volcano erupted on and off, and at the end of the cycle, Katla erupted.

"People have suggested there may be links between them, through fissures, but it's important to emphasize there is no proof the one would trigger the other," Goodenough said.

Volcanic cooling is in any case only a temporary brake on the effects of manmade greenhouse gases, blamed for stoking dangerous change to the climate system. Earth's surface temperature rose 0.74 Celsius from 1906-2005, the United Nations Intergovernmental Panel on Climate Change says. In the last half-century of that period, the warming doubled, to around 0.13 Celsius a decade.

Another downside is that the volcano material in the stratosphere damages Earth's ozone shield against ultraviolet radiation.

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