How do volcanoes affect the weather?

April 25, 2000
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No, Chicken Little, the sky is not falling — although tons of debris and ash are an unwelcome sight to residents of Japan’s northernmost island, Hokkaido.

While the world waits to see if Mount Usu will grow in stature and strength, volcano experts are viewing this volcanic activity as nothing more than a normal disruption which will produce little or no changes in the Earth’s climate.

In fact, Usu is just one of several volcanoes that are currently active, all of which pose no threat to the Earth’s stratosphere.

“There are around 700 active volcanoes in the world, with 25 eruptions each year,” said Brian Toon, professor of Atmospheric and Oceanic Science at the University of Colorado. “Some are under the ocean — none of which are of any importance.”

Volcanic experts agree that the force of the eruption has a far greater affect than the size of the eruption. For example, the 1991 eruption of Mt. Pinatubo on the island of Luzon in the Philippines had a profound effect on the Earth’s climate because particles of ash and sulfur penetrated the stratosphere, forming a cloud that remained above the Earth for approximately two years.

In the case of Mt. St. Helens in Washington, the tremendous explosion was not strong enough to send debris past the troposphere (which varies in thickness from 5 to 10 miles) into the stratosphere (with widths up to 30 miles).

“Mt. St. Helens was more of a sideways explosion than a straight-up one,” explained John Wallace, Professor of Atmospheric Sciences at the University of Washington. What the world saw was a devastatingly powerful eruption that took many lives and caused a tremendous amount of property damage.

“You could put the Earth’s population in the hole blown off the top of Mt. St. Helens,” Toon added.

Should we worry?

If another eruption occurs like Mt. Pinatubo, then Earth may pass through a short period of cooler summers and milder winters. A global warming trend will continue after a brief disruption due to the presence of sulfur and other light-absorbing chemicals in the stratosphere.

“The Mt. Pinatubo eruption had a strong effect on global weather,” said Georgiy Stenchikov, professor of Environmental Sciences at Rutgers University. “The next summer the average temperature was a half-degree cooler and the winter had warmer temperatures by as much as three to four degrees. It was the strongest disruption by a volcano in the 20th century.”

A more startling example occurred in the 1815 eruption of Tambora, a volcano in Indonesia. For the two years following the eruption, world temperatures drop-ped from 2° to 4.5°F. Historians tabbed 1816 as the “year without a summer” in New England.

That type of effect will not happen as a result of any recent volcanic activity. Volcanoes with less disruptive force, like Usu and St. Helens, only expel ash and debris into the troposphere.

“If it [debris] is down in the troposphere, it is taken out and dissipated more rapidly by rainfall,” explained Melissa Free, researcher for the National Oceanographic and Atmospheric Administration.

She added that the height of a volcano doesn’t equate to a higher release of debris. “The eruption has to be strong enough to push material above the troposphere into the stratosphere.”

Once material is trapped in the stratosphere, the particles form a cloud which cannot be dissipated by rain. “Stratosphere clouds can hang around for a year or two,” said Toon. “This veil of sulfuric acid particles are a lot like smog.”

Effect on the ozone layer

While any negative long-term affects of eruptions on the Earth’s ozone layer are minimal, there is reason to closely watch the correlation.

“Whenever you change the dynamics of the stratosphere, you can change the ozone layer a bit,” said Toon. “Temporary effects from an eruption cool the Earth, heat the stratosphere, and lead to ozone loss. But it’s like hitting the Earth with a hammer — the response is unpredictable.”

The thinning of the Earth’s ozone layer has contributed to global warming, which has been a hot topic of discussion. A continued warming trend is often halted, briefly, by a disruption in the ozone layer. John Wallace said that the reduction of the ozone layer usually happens during the two years after an eruption.

Both “1991 and 1992 [after Pinatubo] were noticeably cooler, but an upward trend in temperatures started again in 1993,” he said. “Pinatubo produced enough cooling to negate the warming trend of the past 50 to 100 years.”

Is there another Mt. Pinatubo, or worse yet, a Tambora in our future? Volcanic eruptions are difficult to predict, but technology has made it easier to track them.

Wallace said that current eruptions are easier to follow thanks to the emergence of satellite tracking. He noted that the sulfuric-ash cloud from Mt. Pinatubo was watched closely thanks to views from satellites circling the Earth.

“We pretty much have a handle on all eruptions,” he said.

Wallace noted that warming trends caused by the atmospheric release of CFCs — used as refrigerants and aerosol spray propellants — is more of a concern than volcanic eruptions. “Volcanoes are only minor players in the drama of global change.”

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