Pinatubo Validates Climate Model

LOS ALAMOS COMPUTER MODEL ACCURATELY PREDICTS GLOBAL CLIMATE EFFECTS OF PINATUBO ERUPTION

PUERTO VALLARTA, MEXICO, Jan. 20, 1997 - The world's gotten a little colder since the 1991 eruption of Mount Pinatubo in the Philippines.

But Los Alamos National Laboratory researcher Jim Kao said the eruption has helped validate the accuracy of a Los Alamos computer model of Earth's climate.

Kao, who works in Los Alamos' Atmospheric and Climate Sciences Group, today at the International Association of Volcanology and Chemistry of the Earth's Interior 1997 General Assembly in Puerto Vallarta, Mexico, presented information about how the Pinatubo eruption has helped deplete Earth's ozone layer, increase global cooling and validate Los Alamos' three-dimensional global climate model.

When the mountain erupted for a two-month period in 1991, with a major eruption on June 15, 1991, it spewed millions of tons of sulfur compounds into the atmosphere. About a month later, the sulfate aerosols had formed a wide band around Earth's middle latitudes. This, in turn, began a cycle of damage -- a cycle different than the damage to lives and property suffered by Filipinos who felt the mountain's fury firsthand.

"Sulfate aerosols play a very important role in the chemical reactions that help deplete Earth's ozone layer," Kao said. "In aerosol form, the sulfur dioxide mixes with water vapor and forms sulfuric acid, an important catalyst in ozone depletion reactions. Not only that, but when we get ozone depletion, we get a little bit of a cooling effect because ozone normally acts as a greenhouse gas by trapping heat from surface-emitted terrestrial radiation; it also absorbs direct sunlight. In addition, ozone depletion reactions involving sulfate aerosols work better at lower temperatures. So we can guess that volcanic eruptions might play an important role in ozone depletion worldwide."

To test the hypothesis, Kao and colleagues Xuepeng Zhao and Richard P. Turco of the University of California, Los Angeles, and Los Alamos collaborator Scott Elliott used the Los Alamos global climate model to show what effects the Mount Pinatubo eruption might have had on Earth's global climate.

The model predicted that the amount of sulfate aerosols released by the eruption was ideal to promote sustained ozone depletion reactions.

The model predicted that Earth's ozone layer would deplete by about 12 percent during the winter of 1992 and 1993. Satellite data collected that winter indicated that the ozone levels had depleted by about 15 percent.

But more than that, the Los Alamos global climate model predicted that a wind vortex encircling the Arctic -- which traps and segregates ozone and ozone-destructive chemicals during the winter months and gives rise to an uninterrupted period of ozone depletion -- would remain for a longer period than normal after the eruption.

During the winter, less sunlight reaches the pole and the area becomes cooler. Prevailing westward winds begin to circle the pole until a vortex surrounds a center of calm -- where ozone and the ozone-depleting chemicals can interact undisturbed. When the region warms during spring months, the vortex breaks, and the trapped air mass begins mixing with mid-latitude air masses.

"Since there was a depletion of ozone levels during the winter of 1992-93, and since ozone is a greenhouse gas that absorbs heat, we would expect to see the polar vortex last longer because the area inside the vortex would remain colder longer," Kao said. "During the winter of 1992-93, as the model predicted, we didn't see the polar vortex break until two weeks later than normal." Kao and his colleagues will adjust the model to account for actual observed ozone levels, atmospheric circulation and heating patterns, and chemical reaction rates.

"The ultimate goal is to develop a very accurate three- dimensional model including comprehensive chemistry that will help us understand the intricacies of the global climate," said Kao. "Fortunately, we had something as unfortunate as the 1991 eruption of Mount Pinatubo that we could use as a way to test and validate our model."

Funding for the global climate model and Kao and his colleagues' research comes in part from Los Alamos' Institute for Geophysics and Planetary Physics.

Los Alamos National Laboratory is operated by the University of California for the U.S. Department of Energy.

-30-

For more Los Alamos news releases, visit World Wide Web site http://www.lanl.gov/Internal/News/pressreleases