Studies Renew Anxiety About Fading Ozone

It has been 12 years since UC Irvine chemists F. Sherwood Rowland and Mario Molina first warned that chlorofluorocarbons used as refrigerants and as propellants in aerosol cans could eventually destroy about 20% to 30% of the earth’s protective ozone layer.

That dire prediction, with all its potentially catastrophic implications, inspired a movement that led to a 1978 U.S. ban on aerosol sprays containing chlorofluorocarbons.

But in the years since Rowland and Molina first reported their findings, most other scientists have refined their calculations and steadily reduced the earlier estimates on ozone depletion. By 1984, the widely accepted estimate was an overall depletion of only 2% to 4% sometime in the next century.

‘Non-Issue’ Resurfaces

The threat of chlorofluorocarbons to the ozone layer had become a non-issue. Or so it seemed.

Now, new evidence is mounting to suggest that Rowland and Molina’s cautionary message in 1974 may have been understated. And it comes at a time when industrial production of chlorofluorocarbons is rising sharply.

The outlook, in Rowland’s words, has become “much more ominous.”

One recent U.S. study predicted as much as a 60% ozone depletion by the middle of the next century, if production of chlorofluorocarbons continues to grow by as little as 4.5% annually. Even if that growth rate dropped to 2.5% per year, the Environmental Protection Agency report said, ozone would be depleted by 26% by the year 2075. (Separately, the Rand Corp. recently estimated that total output of chlorofluorocarbons--better known by the trade name Freons--would grow by about 3% per year for at least the next 10 years.)

The EPA report came on the heels of the first direct indications of a gradual loss of protective ozone. Measurements conducted by satellite-borne instruments have shown that the concentration of ozone in the stratosphere over Antarctica has been slowly decreasing over the last decade and that the concentrations at certain locations last October were as much as 45% lower than they had been in the 1960s.

Increase in Ultraviolet Light

Destruction of Earth’s protective ozone will allow increased amounts of ultraviolet light from the sun to strike Earth’s surface, leading to an increase in the incidence of skin and other cancers and a decrease in agricultural productivity, among other effects.

Chlorofluorocarbons are inert. And because they do not burn or react with other chemicals, have no direct health effects and are not biodegradable, chlorofluorocarbons are ideal for use as a propellant in aerosol cans, as a refrigerant in air conditioners and refrigerators, as a solvent in the electronics industry, and as a foaming agent to produce a sponge-like texture in polymers such as those used for insulation.

But that very inertness causes chlorofluorocarbons to accumulate in the atmosphere, eventually rising to the stratosphere--the segment of the atmosphere, extending from about 9 to 30 miles above Earth’s surface, in which temperature increases with increasing height.

Ozone is a highly reactive, naturally occurring ingredient of the stratosphere, formed from oxygen by sunlight.

Screens Ultraviolet Radiation

Although the amount of ozone in the stratosphere is minuscule, about 3 parts per million, it screens out more than 99% of the invisible ultraviolet radiation in sunlight. High intensities of ultraviolet radiation are harmful to nearly all forms of life, and most scientists agree that life on Earth did not evolve until after the ozone layer was formed.

Rowland and Molina’s key contribution in the early 1970s was the recognition that chlorofluorocarbons in the stratosphere could be broken apart by sunlight. Their destruction releases highly reactive chlorine and fluorine atoms that can themselves destroy ozone, but which also undergo a series of reactions with other components of the atmosphere to form additional ozone-destroying chemicals.

This complex series of reactions--which involves at least 192 chemical reactions and 48 reactions stimulated by light--are poorly understood. It has been only recently that scientists have begun to obtain accurate rates for most of those reactions.

By sheer coincidence, most of those more accurate measurements have tended to lower the estimates of eventual depletion. “The (chlorofluorocarbon) industry jumped all over those low numbers” as a rationale to continue chlorofluorocarbon production, Rowland said in a recent interview. The public and the media were lulled into “a false sense of security,” according to Donald Wuebbles of the Lawrence Livermore Laboratory.

Chlorofluorocarbons are not the only chemicals that have an adverse effect on ozone concentration, although they are by far the most significant.

Effects of Supersonic Jets

Supersonic jets, for example, fly in the stratosphere and release nitrogen oxides that can also destroy ozone. The industrial solvents carbon tetrachloride and methylchloroform also destroy ozone.

In contrast, some trace gases are beneficial to the ozone layer. Nitrogen oxides released into the lower atmosphere by subsonic jets, methane and carbon dioxide can all increase the concentration of ozone in the stratosphere. But the effects of all these chemicals--whether deleterious or beneficial--are small in comparison to those of chlorofluorocarbons.

As the amount of ozone in the stratosphere decreases, the amount of ultraviolet light reaching Earth’s surface increases. The size of that increase varies with a number of factors such as latitude and altitude. In general, the increase in radiation is about twice the decrease in ozone. Thus, a 5% decrease in ozone will produce a 10% increase in ultraviolet radiation at Earth’s surface.

For humans, a significant increase in radiation could have serious consequences, including a noticeable rise in cancer, partly as a result of damaged immune systems.

A 1982 National Academy of Sciences report predicted that every 1% decrease in ozone concentration will cause an additional 12,000 to 30,000 cases of skin cancer every year in the United States alone.

An increase in ultraviolet radiation could also cause a possibly significant reduction in the yield of many agricultural crops, damage to the eyes of certain species of cows, and the death of many aquatic organisms that live near the surface of the ocean.

Effect on Products

In addition, many consumer and industrial products will have a shorter useful life because most plastics are also degraded much more rapidly than normal in the presence of ultraviolet radiation.

And finally, an increase in ultraviolet radiation reaching Earth’s surface--as well as absorption of sunlight by chlorofluorocarbons themselves--could cause a warming of the atmosphere, accompanied by marked changes in climate, including the melting of parts of the polar icecaps and potential flooding of low areas along the coastlines.

In light of these undisputed potential consequences of ozone depletion, it has become increasingly crucial to determine exactly how much depletion of the ozone layer will result from continued chlorofluorocarbon release. And on that subject, there is continued disagreement.

The most recent official word is a new report from the National Aeronautics and Space Administration. Selected sections of the report were made public in the middle of January, but the complete report has not been released.

The draft NASA report projected that if chlorofluorocarbon release continued at the 1980 rate, the average amount of ozone above the earth would fall by 4.9% to 9.4% during the next century. The decrease in ozone would be 4% to 5% at the Equator, 8% to 9% at 40 degrees north latitude (the latitude of Philadelphia) and 9% to 14% at 60 degrees north latitude (Oslo and Leningrad). Those numbers are only a little lower than the predictions first made by Rowland and Molina.

‘Problem Hasn’t Changed’

“The problem hasn’t changed” since it was originally presented by Rowland and Molina, said Wuebbles of the Lawrence Livermore Laboratory, one of the scientists whose computer models formed the basis of the NASA conclusions. “Only the public’s perception of the problem has changed.”

The NASA estimates, furthermore, are based on a projection that release of chlorofluorocarbons will continue at the same rate as in 1980. “That’s silly,” said Wuebbles in a telephone interview. “In fact, production has grown about 10% in each of the last three years.”

The April, 1985, EPA report was the first to incorporate the increasing rates of production, although many scientists think its projections are too high. Wuebble’s own calculations show a smaller projected ozone depletion if chlorofluorocarbon output continues to rise, but the trend is the same.

“The catch in all these predictions,” Wuebbles added, “is that we don’t know what world production is.”

The Soviet Union and the Eastern European countries have never revealed what their chlorofluorocarbon production is, although most scientists suspect that it is large and growing.

Scientists hope that situation will change. The EPA report was prepared as a working paper for a March, 1985, conference in Vienna in which 43 nations debated whether international controls should be imposed on chlorofluorocarbon production.

The United States had hoped to persuade other nations to adopt an immediate ban on the use of chlorofluorocarbons in aerosols. The final agreement, however, simply committed the participating nations to study the problem further and to reconvene in 1987. The agreement also called on all countries to disclose their chlorofluorocarbon production.

Eastern Bloc Data

Because the Soviets signed that agreement, scientists were hopeful that the amount of chlorofluorocarbons produced in the Eastern Bloc will be made known, thereby improving the accuracy of calculations. To date, that data has not been disclosed.

But holding chlorofluorocarbon production to current levels may not be an adequate solution, scientists say.

The modest estimates of a 5% to 10% depletion of ozone if worldwide chlorofluorocarbon production remains constant represent “a precarious balance” between a dramatic decrease in ozone concentrations in the upper atmosphere and an equally large increase at lower altitudes, according to Herbert Kaufman of the University of Pittsburgh.

The reason for this altered distribution is that most of the reactions that destroy ozone tend to occur in the upper stratosphere, while those that restore it occur lower in the atmosphere. In one scenario calculated by Wuebbles, for example, the overall depletion of ozone totaled only 2%, but that value was the net result of a 30% depletion in the upper stratosphere and a 10% increase in the lower atmosphere, where there is more ozone.

‘Impact on Climate’

The significance of such changes in ozone is not clear. “But we suspect it could have a major impact on climate,” Rowland said.

He and others are very concerned about future depletions of ozone because they think they can already begin to see some.

Nearly all the calculations predict that the ozone layer should have been depleted by about half a percent by now. Because of normal fluctuations in ozone concentration, however, such a depletion would be too small to be reliably detected.

“In fact,” says Donald Heath of the Goddard Space Flight Center in Greenbelt, Md., “we have seen perturbations of the ozone layer an order of magnitude larger than the predictions indicate.”

From 1978, when the Nimbus 7 satellite carrying Heath’s instrument was launched, to 1984, Heath observed an average yearly ozone loss of about 1.5% at latitudes corresponding to the United States. At higher latitudes, the yearly loss averaged 3%.

“Some of that loss was clearly due to the effects of the eruption of the Mexican volcano El Chicon in 1982, and some is due to the normal loss of ozone when sunspot activity reaches a maximum,” Heath said. “But the loss is still greater than we can account for by any mechanism.”

Even more surprising was the observation that the concentration of ozone in the stratosphere directly over the South Pole has dropped sharply every October, in the Antarctic spring, creating a “hole” in the ozone layer. In 1985, the ozone concentration over the South Pole was about 45% lower than concentrations in the rest of the world.

Findings Corroborated

“We thought something was wrong with our instrument because the ozone value was so low,” Heath said. The investigators then discovered a May, 1985, paper in the journal Nature by scientists at the Natural Environment Research Council in England. They reported much the same observation using instruments on the ground and in balloons.

“We’re now convinced the data is right,” Heath said.

Scientists are at a loss to explain precisely what is happening in the Antarctic. “The most likely thing is that many atmospheric reactions involving (chlorofluorocarbons) are taking place in the long darkness of winter and creating light-sensitive molecules,” Rowland says. “When the light returns in the spring, the high concentrations of these molecules react rapidly to reduce ozone.”

Interestingly, similar effects have not been observed in the Arctic.

But “the most ominous aspect of the ozone depletion in the Antarctic,” Rowland argued, “is that none of the models people are now using show that at all. The chemistry’s not there. . . . The model problem was already severe six to eight months ago, but with the Antarctic hole we’ve got terrible problems.”

There may also be other new factors that can worsen the situation, Rowland added. Many scientists think that certain types of bromine-containing chemicals, such as those used in fire extinguishers, can also destroy ozone.

Scientists from the Max Planck Institute of Aeronomy in Lindau, West Germany, reported in 1985 that the concentration of these chemicals in the upper atmosphere has grown 20% annually for the last three years. The effects of these chemicals have also not been included in the models.

The debate about the effects of chlorofluorocarbons and other chemicals will obviously continue for a long time. But Rowland is concerned that time may be running out. “We were promised an early warning” about destruction of ozone, he said, “but we didn’t expect 45%.”

EFFECTS OF FREON

Normally, nearly all ultraviolet (UV) rays produced by sunlight are absorbed by the natural ozone layer in the stratosphere.

Freon, a man-made substance used in refrigerators, destroys ozone, weakening Earth’s protective shield and allowing UV rays through.

Allowing more UV rays through could have catastrophic health effects:

Incidence of skin cancer could increase dramatically.

Agriculture could suffer losses.

Increased heat could melt polar ice caps and cause flooding.