As you read this, frigid air spirals slowly downward from the stratosphere into the winter darkness of the arctic, part of a complex process destroying the ozone layer that shields us from cancerous ultraviolet radiation. Scientists from Harvard and elsewhere are flying in and out of the vortex making measurements they expect will help them understand and eventually predict ozone losses in the Northern Hemisphere, and how this activity may be linked with global warming.
The exploration includes the first high-altitude reconnaissance flights over Russia since the Soviet Union shot down Gary Powers in a U-2 spy plane in 1960. The incident precipitated the collapse of a Cold War conference between the United States, the Soviet Union, Great Britain, and France. The flights are now being made by a National Aeronautics and Space Administration ER-2 aircraft, a civilian version of the U-2.
These sorties are part of the largest international effort to date to measure ozone in this hemisphere. The effort includes more than 350 researchers from the United States, Canada, Europe, Japan, and Russia; 19 of the researchers come from Harvard. The effort started last November, runs into mid-March, and goes by the awkward name of SAGE III Ozone Loss and Validation Experiment, or, simply, SOLVE.
“We want to learn enough about the polar vortex to predict whats going to happen there over the next decade,” says James Anderson, Weld Professor of Atmospheric Chemistry and one of two mission scientists directing the ER-2 flights. “Another critical question involves how conditions in the arctic connect to a loss of protective ozone over mid-latitudes, that is over the northern U.S., Canada, most of Europe, and much of Asia.”
Since 1970, springtime levels of ozone over these latitudes have been decaying at about 1 percent per year. Each 1 percent of decay produces a 2 percent increase in the amount of hazardous ultraviolet radiation reaching Earths surface. This radiation is thought to be associated with skin cancers, incidences of which are on the rise in the United States and several other countries.
“A causal link between the increase in skin cancers in the U.S. and the degree to which ozone has been depleted has not been established, but the depletion is clearly a public health concern,” Anderson notes.
Flying into the Vortex
Last month Anderson flew under the ER-2 in another instrumented aircraft as both planes entered Russian airspace. The one-engine ER-2, which flies higher than 70,000 feet or twice the altitude of a commercial jetliner carries only one pilot. Anderson and his colleagues flew in a DC-8 at much lower altitudes. His fellow passengers included a Russian general who had participated in the 18 months of negotiations it required to obtain clearance for such flights.
“It was a brilliant clear winter day as we flew over St. Petersburg,” Anderson recalls. He and his colleagues made measurements inside the vortex, then flew south of Moscow. “In previous years, we struggled to get into the vortex from Norway and other places; this time our goal was to find the edge and get out.”
When the vortex forms, its like “a large washing machine loaded with chemicals,” Anderson says. “No material crosses the edges, at which there occurs an extreme change in the concentrations of the chemicals.”
The more chemicals present, particularly in a highly reactive or free-radical form, the greater the destruction of ozone. The cycle starts in the stratosphere, which extends from about 30,000 to 180,000 feet above the surface. Every year, cold air begins to sink out of the stratosphere in winter. Earths rotation turns it into a counterclockwise spinning funnel of air.
Low temperatures and the presence of water vapor in the usually cloudless stratosphere trigger formation of ice clouds. The clouds alone do no harm, but they provide a base for making the chemicals that destroy ozone. (This ozone, thus, is not the same ozone that comes from vehicle and power-plant exhausts and forms part of the smog near the ground.)
Chemicals containing chlorine and bromine, which come from various spray-can propellants, refrigerants, solvents, and fumigants, rise into the stratosphere and collide at the surfaces of ice particles in the clouds. When sunlight strikes this mix, rapid reactions create new chemicals that are much more reactive. Sunlight-driven reactions among these chemicals convert ozone into ordinary oxygen.
Unlike ozone, ordinary oxygen does not absorb harmful ultraviolet from the sun, but lets the rays penetrate to sunbathers below. At the same time, nitrogen-containing compounds, which normally protect the stratosphere by limiting the concentrations of highly reactive chlorine and bromine, become neutralized. Once these chemical “brakes” fail, ozone destruction accelerates.
“You cant imagine a more dangerous, demonic mix,” Anderson comments. The same mechanisms create the much larger ozone hole over the Antarctic.
Seeking Early Warnings
“We know this happens,” Anderson continues, “but we dont understand it at a level that enables us to predict whats going to happen from year to year, or several years from now. We want to be able to forecast what the degree of ozone loss will be for specific levels of water vapor, temperatures, and chlorine/bromine concentrations.” Ozone watchers would like to be able to make predictions as good or better than El Niño forecasts, which provide enough warning for people to take protective actions.
SOLVE field work started last November with measurements made from planes, satellites, ground stations, and balloons that soar to more than 100,000 feet. Measurements made in November and December give researchers an accurate look at conditions before an ozone hole occurs. January and February explorations are providing details about the formation of clouds in the stratosphere and the amounts and movement of chlorine and other chemicals. In March, the sun returns to the winter-darkened arctic, and maximum ozone loss begins to take place.
“The vortex now is strong, colder than usual, and contains huge amounts of reactive chlorine, so it could be a very interesting year,” Anderson notes. But he makes no prediction. “A week from now everything could change,” he adds.
Steven Wofsy, Rotch Professor of Atmospheric and Environmental Science, leads a Harvard team that uses balloons to measure carbon dioxide in the atmosphere. The gas increases with time, so the less carbon dioxide it contains, the older the air. Determining the age of an air mass this way allows it to be tracked from place to place.
“This year, the stratosphere is colder than usual,” Wofsy notes. “That indicates the possibility of a big ozone hole forming, but we cant be sure. No ones offering a prediction.”
Contrary to what you might think, warmer temperatures near Earths surface lead to colder temperatures in the stratosphere. Global warming, particularly in the late 1990s, may therefore be linked to recent large ozone holes over both the arctic and Antarctic.
Water vapor and carbon dioxide in the air act like a greenhouse roof, trapping heat and radiating it back to the surface. Otherwise, that heat would escape into the stratosphere. Although carbon dioxide is often cited as the main greenhouse gas, water vapor actually blocks more heat.
“In this respect, water vapor is the dog and carbon dioxide the tip of its tail,” Anderson comments. “However, if the carbon dioxide content of air changes rapidly, the tail can wag the dog. Therefore, its important for our future to control carbon dioxide emissions.”
Carbon dioxide and the global warming to which it contributes could be one explanation for a cooling stratosphere and increased ozone destruction. “Theres no definite proof of this,” Wofsy admits, “but were concerned about it.”