( ASCII text )
Reprinted with permission of the author.
The following was written for The Global Warming Desk Reference published by Greenwood Press.
Bruce E. Johansen
Chloroflourocarbons (CFCs), initially raised no environmental questions when they were first marketed by Dupont Chemical during the 1930s under the trade name Freon. It was a time when such questions usually were not asked. At about the same time, asbestos was being proposed as a high-fashion material for clothing, and radioactive radium was being built into timepieces so that they would glow in the dark.
By 1976, manufacturers in the United States were producing 750 million pounds of CFCs a year, and finding all sorts of creative uses for them, from propellants in aerosol sprays, to solvents used to clean silicon chips, to automobile air conditioning, and as blowing agents for polystyrene cups, egg cartons, and containers for fast food. "They were amazingly useful," wrote Anita Gordon and David Suzuki. "Cheap to manufacture, non-toxic, non-inflammable, and chemically stable." (Gordon, 24) By the time scientists discovered, during the 1980s, that CFCs were thinning the ozone layer over the Antarctic, they found themselves taking on a $28-billion-a-year industry.
The ozone shield is important because it protects plant and animal life on land from the sun's ultraviolet rays, which can cause skin cancer, cataracts, and damage to the immune system. Thinning of the ozone layer also may alter the DNA of plants and animals.
By the time they were banned internationally during the 1980s, CFCs had been used in roughly 90 million car and truck air conditioners, 100 million refrigerators, 30 million freezers, and 45 million air conditioners in homes and other buildings. Because CFCs remain in the stratosphere for up to 100 years, they will deplete ozone long after industrial production of the chemicals ceases.
These human-created chemicals do more than destroy stratospheric ozone. They also act as greenhouse gases, with several thousand times the per-molecule greenhouse potential of carbon dioxide. What's more, the warming of the near-surface atmosphere (the lower troposphere) seems to be related to the cooling of the stratosphere, which accelerates depletion of ozone at that level. An increasing level of carbon dioxide near the Earth's surface "acts as a blanket," said NASA research scientist Katja Drdla. "It is trapping the heat. If the heat stays near the surface, it is not getting up to these higher levels." (Borenstein)
During the middle 1990s, scientists were beginning to model a relationship between global warming and ozone depletion. A team led by Drew Shindell at the Goddard Institute for Space Studies created the first atmospheric simulation to include ozone chemistry. The team found that the greenhouse effect was responsible not only for heating the lower atmosphere, but also for cooling the upper atmosphere. The cooling poses problems for ozone molecules, which are most unstable at low temperatures. Based on the team's model, the buildup o f greenhouse gases could chill the high atmosphere near the poles by as much as 8 degrees C. to 10 degrees C. The model predicted that maximum ozone loss would occur between the years 2010 and 2019. (Shindell, et. al., 589)
At about the same time, scientists were looking for reasons why the ozone layers over the Arctic and Antarctic were failing to repair themselves as expected following the international ban on production of CFCs. They began to suspect that global warming near the surface might be related to ozone depletion in the stratosphere. In 1998, the Antarctic ozone hole reached a new record size roughly the size of the continental United States. Some researchers came to the conclusion that, as Richard A. Kerr describes in Science:Unprecedented stratospheric cold is driving the extreme ozone destruction.... Some of the high-altitude chill...may be a counterintuitive effect of the accumulating greenhouse gases that seem to be warming the lower atmosphere. The colder the stratosphere, the greater the destruction of ozone by CFCs. (Kerr, 1998, 291)"The chemical reactions responsible for stratospheric ozone depletion are extremely sensitive to temperature," Shindell, et. al. wrote in Nature. "Greenhouse gases warm the Earth's surface but cool the stratosphere radiatively, and therefore affect ozone depletion." (p. 589) By the decade 2010 to 2019, Shindell, et al. expect ozone loses in the Arctic to peak at two-thirds of the "ozone column," or roughly the same ozone loss observed in Antarctica during the early 1990s. "The severity and duration of the Antarctic ozone hole are also expected to increase because of greenhouse-gas-induced stratospheric cooling over the coming decades," Shindell, et al. assert. (p. 589)
During the middle 1990s, scientists began to detect ozone depletion in the Arctic after a decade of measuring a growing ozone "hole" over the Antarctic. By the year 2000, the ozone shield over the Arctic had thinned to about half its previous density during March and April. Ozone depletion over the Arctic reaches its height in late winter and early spring, as the Sun rises after the midwinter night. Solar radiation triggers reactions between ozone in the stratosphere and chemicals containing chlorine or bromine. These chemical reactions occur most quickly on the surface of ice particles in clouds, at temperatures less than minus 80 degrees C. (minus 107 degrees F.)
Space-based temperature measurements of the Earth's lower stratosphere, a layer of the atmosphere from about 17 kilometers to 22 kilometers (roughly 10 to 14 miles) above the surface, indicate record cold at that level as record surface warmth has been reported during the 1990s. Roy Spencer of NASA and John Christy of the University of Alabama at Huntsville and the Global Hydrology and Climate Center, obtained temperature measurements of layers within the entire atmosphere of the Earth from space, using microwave sensors aboard several polar-orbiting weather satellites. They found that, despite significant, short-livved warming following the eruptions of El Chichon in Mexico in 1982 and Mt. Pinatubo in the Philippines in 1991, the stratosphere as a whole has been cooling steadily during the past fifteen years.
Steve Hipskind, atmospheric and chemistry dynamics branch chief at NASA's Ames Research Center, Moffett Field, California, has been quoted as saying that chlorine atoms use clouds as "a platform" to destroy stratospheric ozone. (Arctic Region, 4) Clouds form more frequently in the stratosphere at lower temperatures. Ice crystals, which form as part of polar stratospheric clouds, assist the chemical process by which ozone is destroyed. CFCs' appetite for ozone molecules rises notably below minus 80 degrees C. (minus 107 degrees F.), a level that was reached in the Arctic only rarely until the 1990s. During the winter of 1999-2000, temperatures in the stratosphere over the Arctic were recorded at 118 degrees F. or lower (the lowest on record), forming the necessary clouds to allow accelerated ozone depletion.
As Dennis L. Hartmann, et al. explain:The pattern of climate trends during the past few decades is marked by rapid cooling and ozone depletion in the polar lower stratosphere of both hemispheres, coupled with an increasing strength of the wintertime westerly polar vortex and a poleward shift of the westerly wind belt at the Earth's surface....[I]nternal dynamical feedbacks within the climate system...can show a large response to rather modest external forcing....Strong synergistic interactions between stratospheric ozone depletion and greenhouse warming are possible. These interactions may be responsible for the pronounced changes in tropospheric and stratospheric climate observed during the past few decades. If these trends continue, they could have important implications for the climate of the twenty-first century. (Hartmann, et al., 1412)Ozone depletion has been measured only for a few decades, so these researchers caution that they are not entirely certain that rapid warming at the surface is not caused by natural variations in climate, which is powerfully influenced by the interactions of oceans and atmosphere. "However," they conclude, "It seems quite likely that they are at least in part human-induced." (Hartmann, et al.,1416) Hartmann and associates also raise the possibility that the poleward shift in westerly winds may be accelerating melting of the arctic ice cap, part of what they contend may be a "transition of the Arctic Ocean to an ice-free state during the twenty-first century." (Hartmann, et al., 1416). A continued northward shift in these winds also could portend additional warming over the land masses of North America and Eurasia, they write. (Hartmann, et al., 1416)
The connection between global warming, a cooling stratosphere, and depletion of stratospheric ozone was confirmed in April, 2000, with release of a lengthy report by more than 300 NASA researchers as well as several European, Japanese, and Canadian scientists. The report found that while ozone depletion may have stabilized over the Antarctic, ozone levels north of the Arctic circle were still falling, in large part because the stratosphere has cooled as the troposphere has warmed. The ozone level over the some parts of the Arctic was 60 per cent lower during the winter of 2000 than during the winter of 1999, measured year over year.
In addition, scientists learned that as winter ends, the ozone-depleted atmosphere tends to migrate southward over heavily populated areas of North America and Eurasia. "The largest ultraviolet increases from all of this are predicted to be in the mid-latitudes of the United States," said University of Colorado atmospheric scientist Brian Toon. "It affects us much more than the Antarctic [ozone `hole']." (Borenstein)
Ross Salawitch, a research scientist at NASA's Jet Propulsion Laboratory in Pasadena, Calif. said that if the pattern of extended cold temperatures in the Arctic stratosphere continues, ozone loss over the region could become "pretty disastrous." (Scientists Report, 3-A) Salawitch said that the new data has "really solidified our view" that the ozone layer is sensitive not only to ozone-destroying chemicals, but also to temperature. (Stevens, A-19) "The temperature of the stratosphere is controlled by the weather that will come up from the lower atmosphere," said Paul Newman, another scientist who took part in the Arctic ozone project. "If we have a very active stratosphere we tend to have warm years, when stratosphere weather is quiescent we have cold years." (Connor, 5) New research indicates that global warming will continue to cool the stratosphere, making ozone destruction more prevalent even as the volume of CFCs in the stratosphere is slowly reduced. "One year does not prove a case," said Paul Newman of NASA's Goddard Space Flight Center in Greenbelt, Maryland. "But we have seen quite a few years lately in which the stratosphere has been colder than normal." (Aldhous, 531)
"We do know that if the temperatures in the stratosphere are lower, more clouds will form and persist, and these conditions will lead to more ozone loss," said Michelle Santee, an atmospheric scientist at NASA's Jet Propulsion Laboratory in Pasadena and co-author of a study on the subject in the May 26, 2000 issue of Science. (McFarling, A-20) The anticipated increase in cloudiness over the arctic could itself become a factor in ozone depletion. The clouds, formed from condensed nitric acid and water, tend to increase snowfall, which accelerates depletion of stratospheric nitrogen. The nitrogen (which would have acted to stem some of the ozone loss had it remained in the stratosphere), is carried to the surface as snow.
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