Experts Warn of Widespread Disruption by Volcanoes : Climate: Huge eruptions would release gases and ash, damaging crops and affecting air travel routes. Such events occur about once every 100 years, scientists say.

U.S. and European scientists warned here Monday 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 major North Atlantic routes for months and cause tremendous crop damage in Europe, according to reports made here 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 sulfur dioxide ejected by volcanoes on climate around the world.

Combining with water vapor, the sulfur compounds slowly change in the stratosphere into sulfuric acid and aerosols that can cool large areas thousands of miles away for two or three years and contribute to depletion of the ozone layer.

It has 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 one or two degrees over very large regions.

A Canadian researcher, Amir Shabbar, outlined here Monday how five separate eruptions in this century--located from the Kamchatka Peninsula in Siberia to Alaska to the Caribbean--had apparently altered the weather in Canada, lowering temperatures by as much as five degrees in the eastern part of the country.

But these eruptions, including Katmai in Alaska in 1912, El Chichon and Pinatubo, were not nearly as large as some in recent centuries, and their effects relatively minor, researchers said.

Alan Robock, the University of Maryland meteorologist who coordinated Monday’s discussions, identified the Kelud eruption in the South Pacific in 1453, the eruption at Tambora in Indonesia in 1815 and one at Krakatoa in Indonesia in 1883, as well as the Laki eruption, as large enough to cause catastrophic effects over large distances.

Thorvaldur Thordarson of the University of Hawaii noted that Laki ejected an estimated 147 megatons of sulfur dioxide in the 1783-84 eruption. This compared to just 30 megatons by Pinatubo in 1991, which was considered quite large.

Beginning June 16, 1783, a cloud of ash and sulfur was noted in Western Europe and by July of that year, it had extended as far east as Moscow and Syria, 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 deposits of sulfur compounds and damage to many vegetable crops were also observed, he said.

Many Europeans soon became aware that a volcano in Iceland was responsible for these phenomena. But in earlier times, a volcano could erupt and people affected thousands of miles away would not know the cause.

Kevin Pang of Pasadena’s Jet Propulsion Laboratory told the meeting that after Kelud exploded in 1453 in the New Hebrides, now called Vanuatu, heavy snow fell in southern China, normally a temperate area, for 40 days, and the people there were mystified as to the cause.

The eastern Roman Empire capital of Constantinople was about to fall to Muslim Turks that year, and the red sunsets resulting from the eruption thousands of miles away were so dramatic that the populace in the besieged city thought that fires had been set all around them by their attackers.

Similarly, after the Krakatoa eruption, many New Yorkers came under the misapprehension that Staten Island was aflame until they were informed differently by news reports.

“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.”

But one uncertainty led to a momentary angry exchange here Monday. That was whether a volcanic eruption can cause an El Nino, an unusually warm current in the Pacific Ocean that triggers climate changes in North America.

When Robock said some research he and associates have done indicated that the El Chichon eruption may have triggered the largest El Nino of the 20th Century, a Florida State University meteorologist, James J. O’Brien, rose in the audience to challenge him.

“El Nino can be predicted and this one was certain a year before El Chichon erupted,” O’Brien said. “You’re lucky I’m not reviewing your paper.”

“I’m very glad you’re not reviewing it,” Robock said.