SAN FRANCISCO -- U.S. and European scientists warned here yesterday that mammoth volcanic eruptions have been occurring somewhere in the world on an average of once every 100 years, and when the next one strikes it is likely to cause enormous disruptions to modern life.
For example, were the Laki eruptions in Iceland in 1783-84 to occur today, the ash clouds could stop air travel on the most favored North Atlantic routes for months and cause tremendous crop damage in Europe, according to reports made on the opening day of the annual meeting of the American Geophysical Union.
For 30 years, since smaller eruptions near Bali, Indonesia, scientists have been measuring the effects of sulphur dioxide ejected by volcanoes on climate around the world.
Combining with water vapor, the sulphur compounds slowly change in the stratosphere into sulphuric acid and aerosols that can cool large areas thousands of miles away for two or three years and even contribute to depletion of the ozone layer.
It has now been established that such large eruptions as El Chichon in Mexico in 1982 and Pinatubo in the Philippines in 1991 can be responsible for reducing temperatures by 1 or 2 degrees in very large regions.
A Canadian researcher, Amir Shabbar, outlined yesterday how five separate eruptions in this century -- located from Siberia's Kamchatka Peninsula to the Caribbean -- apparently altered the weather in Canada, lowering temperatures by as much as 5 degrees in the eastern part of the country.
But these eruptions, including that of Katmai in Alaska in 1912, El Chichon and Pinatubo, were not nearly as large as some in recent centuries, and their effects were relatively minor, researchers said.
Alan Robock, the University of Maryland meteorologist who coordinated yesterday's discussions, identified the Kelud eruption in the South Pacific in 1453, at Tambora in Indonesia in 1815, and at Krakatoa in Indonesia in 1883, as well as the Laki eruption, as fitting the type large enough to cause catastrophic effects at large distances. The Krakatoa eruption was in the mammoth category.
Thorvaldur Thordarson of the University of Hawaii noted that Laki had ejected an estimated 147 megatons of sulphur dioxide in the 1783-84 eruption. This compared to just 30 megatons by Pinatubo in 1991, a volume considered quite large.
Beginning June 16, 1783, a cloud of ash and sulphur was noted in Western Europe and by July of that year it had extended as far east as Moscow and Syria, Mr. Thordarson noted. "The sun was blood red and could be observed with naked eyes at noon. The glow of dusk lasted well into the night, equal to a full moon."
Acrid odors, dry decomposed droppings of sulphur compounds and damages to many vegetable crops whereas also experienced, he reported.
Kevin Pang of Pasadena's Jet Propulsion Laboratory, for example, told the meeting that after Kelud exploded in the New Hebrides in 1453, now called Vanuatu, heavy snow fell in southern China, normally a temperate area, for 40 days.
After the Krakatoa eruption, many New Yorkers were under the misapprehension that Staten Island was aflame.
"Volcanoes can impact global climate far more significantly than is generally realized," said Hans Graf of the Max Planck Institute of Hamburg, Germany. "Fortunately, really huge eruptions are very rare events."
Scientists yesterday also heard that Alaska's immense Bering Glacier is on a rampage, speeding up in recent months rather than slowing as expected.