ON SEPTEMBER 26, 1974, a headline on the front page of The New York Times announced: “Tests Show Aerosol Gases May Pose Threat to Earth.” The trouble, the story said, began with synthesized chemicals called fluorocarbons. Some scientists believed that these manmade substances would destroy a large part of the atmosphere’s ozone layer. The results would likely be dire. No longer adequately shielded by ozone from the harshest of the sun’s ultraviolet radiation, human beings might suffer an epidemic of skin cancers. No one could say what might happen to the plants and animals of the earth, but it seemed clear that an altered ozone layer would mean a greatly altered planet.

Looking back eight years afterward, a scientist involved in research on ozone remarked, “An abstract scientific theory made the front page of The New York Times, and six months later it was a subplot on the Archie Bunker show.” The fluorocarbon theory proposed that global catastrophe could have a homely, even

frivolous source, and perhaps that is why the story traveled very widely. Practically everyone in the industrialized world used fluorocarbons, which had long been thought to rank among the safest of manmade chemicals. Colorless, nearly odorless, fluorocarbons are present in virtually every modern system of refrigeration. They are used as solvents and as foaming agents for making such items as disposable coffee cups and rigid insulation. In 1974, fluorocarbons propelled the contents from half of the roughly 2 billion aerosol cans being sold in America. The headlines focused on this last application. Was the world really going to end with the hissing of cans of deodorant? In contemplation of like questions, many commentators in America could not restrain their mordant glee. The prospects were just too wonderfully ironic. In the early days of the alarm, some scientists went before Congress and on the basis of meager evidence drew the worst of imaginable conclusions. One, for example, said that the continued production of fluorocarbons “could drive life on earth back to where it was hundreds of millions of years ago.” Hyperbole was rampant.

In this case, however, excessive zeal in relating bad news probably had some salutary effects. The notoriety of the fluorocarbon theory promoted investigation of the stratosphere, an atmospheric region that is very important to life on earth but that scientists had just begun to explore. The alarm also caused many Americans to stop buying aerosols, and by 1978, when Congress banned the use of fluorocarbons in all but a few sorts of spray cans, American industry had already substituted other propellants. Worldwide production of fluorocarbons had been doubling roughly every seven years. After the alarm, production began to decline, and has remained at a level some 18 percent below that of 1974. If there had been no general fright, production of the chemicals would certainly have grown far more quickly than knowledge about their effects on ozone. As it turned out, a margin of safety was probably bought, albeit at the expense of a thriving industry.

Scary stories often create, for a little while, a comforting illusion that the world has only one problem. Then they tend to disappear. By the end of the 1970s, news about the ozone layer had all but vanished from the popular press, but the effects of manmade chemicals onozone are still worth worrying about. However, the scientific issues have become increasingly complex and ambiguous. One of the authorities on the ozone layer, a chemist from Harvard named Michael McElroy, described the general problem in this way: “The atmosphere is the air shared by the entire living system, inhaled and exhaled on a planetary scale, and we are doing things to it which are readily detectable but the effects of which are very hard to predict.”

PHOTOGRAPHS OF EARTH taken from outer space have lost their novelty, but, like a psychoanalysis, they have irreversibly altered the view. An extraterrestrial picture taken as the sun appears over the limb of the earth reveals the earth as a dark, convex semicircle and, above it, space as a dark, concave one; in between is a shining sliver, a fingernail paring of light, which is the atmosphere. Seemingly huge and inexhaustible, it is in fact quite small.

Some ozone turns up in the lowest section of the atmosphere. There, it’s a manmade constituent of urban smog, an especially nasty one, which has a corrosive effect on the lungs. Higher up, however, in the stratosphere, which is the second layer of the atmosphere, ozone is beneficent. Oxygen exists in abundance in the atmosphere because of biological activity on the planet below. In the stratosphere, sunlight creates ozone out of oxygen. Stratospheric ozone is, therefore, a product of biological activity, and it also protects that activity by absorbing most of the sun’s ultraviolet radiation. Life could not have evolved on earth as it did without the layer of ozone above, and vice versa. The system is a lovely wheellife leads to ozone, ozone leads to lifeand perhaps a fragile one.

The ozone layer isn’t very far away; the stratosphere begins ten miles up and extends to a height of about thirty miles. While the ozone layer can be pictured as a shield, encircling the earth and absorbing the blows of ultraviolet radiation, it is hardly as sturdy as the term “shield” implies. This shield’s thickness varies greatly with both time and place: the lower the latitude, the thinner the column of ozone above, and there’s more of it in the spring than at any other time of the year.

Overall, the shield is very thin. The ozone in it represents about a hundred parts per billion of the total atmosphere. Were the ozone layer distributed uniformly around the earth and squeezed until it reached the pressure of the air at sea level, it would make a girdle only one eighth of an inch thick. Moreover, the molecules of ozone that make up the shield behave in an unstable fashion. They react with many other sorts of molecules, and, in reacting, turn into something else.

In the natural state of things, the shield of ozone is constantly broken down by chemical reactions and constantly recreated by one simple process, which begins with solar photons splitting oxygen atoms in two. An enormous amount of solar energyabout ten times as much energy as the human world consumes in a yeargoes into making the stratosphere’s annual ration of ozone. So for all practical purposes, the supply of ozone remains fixed. Ingenious mankind can’t significantly acid to the ozone layer. According to the theories of ozone depletion, however, mankind can accelerate the destruction of ozone and create someday a new balance of forcesand with it a dangerously thinner version of the thin ozone shield.

Not much that is sent from the earth into the air escapes the atmosphere. Most substances, in fact, don’t get beyond the lowest region, which cleanses itself efficiently, returning airborne substances to earth in rain. Fluorocarbons, however, are inert, which means that they don’t react with other chemicals. This property, which makes them safe inside homes, and thus has delighted industrial chemists, causes them eventually to escape the lower atmosphere and to travel intact to the stratosphere. There the fierce, relatively unfiltered sunlight breaks them into several different chemicals. One of these, atomic chlorine, belongs to a class of substances vividly named “free radicals,” and laboratory experiments have shown that atomic chlorine will act as a catalyst in transformations that destroy ozone. According to present atmospheric theory, one fluorocarbon molecule can end up destroying 100,000 molecules of ozone.

The scientists who propounded or endorsed this theory felt especially alarmed because their calculations suggested that it would take a long timenow thought to be on the order of fifty yearsfor a fluorocarbon molecule to make the round trip from release in the biosphere to eventual return to earth. Fluorocarbons could thus become the source of an awful problem. Industrycould continue to load up the atmosphere with these chemicals, and the ill effects could be disguised for many years.

OVER THE PAST EIGHT YEARS, scientists have named several other possible agents of ozone depletion. The most important of these are certain nitrogen compounds, which, like fluorocarbons, engage in catalytic reactions that transform ozone into something else. Before the fluorocarbon theory made the news, some scientists suggested publicly that a large nuclear war or a large fleet of highflying SSTs could inject into the stratosphere dangerous amounts of these chemicals. Somewhat later, other scientists reckoned that these nitrogen compounds are routinely delivered to the stratosphere by less dramatic means, both natural and manmade.

Nitrogen is the essential food for life on earth, and out of nitrogen life creates, among other things, a substance called nitrous oxide. Once airborne, this chemical behaves in the same fashion as fluorocarbons. Ultimately, it delivers to the stratosphere ozonedestroying oxides of nitrogen. In nature, explorers of the stratosphere discovered, the byproducts of nitrous oxide destroy more ozone than do any other chemicals. Nitrous oxide is a crucial force in the maintenance of ozone’s steady state. In nature’s conservative regime, the nutrition of the planet connects with the maintenance of a beneficent shield of ozone. But nitrous oxide also gets into the air through such familiar human activities as farming, power production, and transportation. In fact, the human contribution probably equals or exceeds the natural one. Fairly recent and very precise measurements show that the quantity of nitrous oxide in the atmosphere has been rising steadily. The implications for ozone are troubling.

The National Academy of Sciences issued its latest report on these matters in 1982. This document states that if the 1977 rate of fluorocarbon production should hold throughout the next century, fluorocarbons alone might reduce the shield of ozone by 5 to 7 percent. If the amount of nitrous oxide in the air should double during the next hundred years, fluorocarbons and nitrous oxide together might deplete the total amount of ozone by about 13 percent. The biologists' part of the report is thin. They don’t know how earth would fare bereft of that much ozone. Depletion of about 13 percentwould not snuff out life or drive people back to caves, but it would almost certainly lead to large increases in the least deadly of skin cancers. The list of possiblebut not at all certainconsequences makes for grim imagining. It includes increases in often lethal melanomas, interference with photosynthesis in plants and algae, disturbances of the immune systems of human beings and animals. One scientist has suggested that bees out on their pollinating rounds might lose their way in the unfamiliar light.

No important depletion of ozone has occurred so far, but that fact does not invalidate the theories, since they predict no measurable depletion yet. Chemists haven’t actually observed in the stratosphere all the steps along the postulated chains of manmade ozone depletion. They have learned a great deal about the chemistry of the place. All of it indicates that the theories implicating nitrous oxide and fluorocarbons are probably, as the scientists say, “qualitatively correct.” Prophecies about the quantity of ozone depletion have varied wildly over the past eight years, however, and not even the scientists who made them believe that the latest predictions are likely to hold. Those predictions, in any case, leave out several other important substances, probably the most important of which is carbon dioxide. Inclusion of carbon dioxide, as the National Academy’s report points out, leads to the intriguing but hardly comforting possibility that one threat to the planet’s environment may cancel out another.

THE STANDARD DIAGRAM of the carbonoxygen cycle, the one that every schoolchild studies, shows a person and a tree and a couple of arrows connecting them. One arrow leads from tree to person, indicating that plants release oxygen and that people and animals inhale it. The other arrow goes the other way, from person to tree, signifying the fact that people and animals exhale carbon dioxide, which plants in turn photosynthesize. That’s the general outline of the grand, circular dance. Within its circumference, and tangential to it, other dances proceed. A significant number of plants, including many algae, don’t return the carbon in them to the air when they die. Instead that carbon is burieddeep in marine sediments below the ocean floor, for instance. The circle closes eventually, perhaps 100 million years later, when the buried carbon is uplifted in a new mountain range or, as one scientist put it, “processed through a volcano.” Then the carbon that those plants inhaled, as it were, finally returns to the atmosphere as carbon dioxide.

Civilization has accelerated this slow but continuous portion of the carbon cycle by replacing forests with farms and cities, and especially by mining and burning buried carbon, which is fossil fuel. Civilization has been doing essentially what nature does, but doing it twenty times faster. Measurements taken since 1958 show that in only twentyfour years the amount of carbon dioxide in the atmosphere has increased by 6 percent, and there are estimates that it has risen by 20 percent since the Industrial Revolution. In the past 100 years, mankind has added to the atmosphere some 100 billion tons of carbon.

Some scientists began to feel concerned about the trend as long ago as 1938. Lately, many physicists, oceanographers, meteorologists, biologists, and chemists have turned their attention to a couple of difficult questions. They wonder how long it will take for carbon dioxide to double in the atmosphere. Estimates range from about fifty to several hundred years. They also want to know what a doubling would mean to the planet. The theoretical answer, the one with widest currency, holds that a doubling would intensify the socalled “greenhouse effect.”

The analogy is old and apt. Like the glass in a greenhouse, carbon dioxide lets solar radiation pass but inhibits the passage of infrared radiation, which is heat. Carbon dioxide in the air lets the sun heat the earth but it keeps some of that heat from traveling away from the earth and into space. It makes a sort of thermal blanket around the globe, and as that blanket thickensso most current theory holdsthe average temperature of the earth will rise.

Some scientists reckon that the West Antarctic ice sheet will slide into the sea. In the event, water levels would rise, at least high enough to cover the world’s coastal cities. Some have thought that the flood might come in as few as fifty years, but recent papers hold that it couldn’t happen in less than 200 years. Almost everyone agrees, however, that climates would be affected. America’s Corn Belt might need a new name, connoting infertility. Some nations might benefit, while others would decline.

The carbondioxide theory conjures up odd visions of caravans evacuating the seacoasts, of farmers gazing sadly out over parched fields, of rain forest springing up where once was permafrost. Substantiation is missing, but such imaginings are plausible and they make good copy. The chief villain of the theory is combustion of fossil fuel. Accordingly, the carbondioxide hypothesis represents an argument for both the nuclearpower industry and advocates of conservation and solar energy. For the fluorocarbon industry, it is a mixed blessing.

Scientists figure that accumulation of carbon dioxide in the air should, while it warms the earth, cool off the stratosphere. That idea cheers up the fluorocarbon industry, because, in cooling the stratosphere, carbon dioxide would slow down the catalytic reactions that destroy ozone. Intensification of the greenhouse effect would, in theory, ameliorate the effects of fluorocarbons and nitrous oxide. It might, said one of the authors of the National Academy’s report, “heal the ozone deficit.” One recent calculation, a stew of facts and theories, with carbon dioxide in the mixture, predicts no depletion of ozone over the next two centuries.

Yet fluorocarbons themselves intensify the greenhouse effect, and, obviously, no one can take comfort in a vision that has one cataclysmic series of events preventing another. If the carbondioxide theory and its dire corollaries prove to be correct, then presumably the nations of the earththe threatened ones, at any ratewill want to do something about it. Then, of course, civilization will still have to contend with the problems of fluorocarbons and nitrous oxide. What’s more, the calculations that predict no depletion of ozone suggest that myriad industrial and agricultural practices have created a cozy balance among pollutions of the stratosphere. Such an accidental balance could not endure.

Calculations that throw fluorocarbons, nitrous oxide, and carbon dioxide into one computer program treat the theories as if all of them were true. In fact, atmospheric scientists haven’t proven any of the theories to their satisfaction, and in almost every paper on the stratosphere an ominous phrase “missing chemistry” appears. Scientists, in other words, worry that their models of the stratosphere may leave out something important.

IN 1975, SEVERAL scientists told Congress that they would answer all the crucial questions surrounding ozone and fluorocarbons within about three years. Seven years later, important mysteries remain. James Anderson is a thirtyeightyearold professor of chemistry at Harvard and one of the most persistent and adept of the explorers of the stratosphere. Asked why the research drags on, he replied, “The air is thin up there.” For example, a substance called the hydroxyl radical controls a large part of the chemistry of the stratosphere, but that substance exists up there in a quantity on the order of one part per trillion. It is as scarce, Anderson explained, as a drop of vermouth diffused in an Olympicsized swimming pool full of gin.

For the past decade or so, Anderson and many others have been attempting to measure precisely the concentrations of a host of chemicals that aren’t much more prevalent in the stratosphere than that drop of vermouth. The concentrations of these chemicals also vary widely in time and place. As for the reactions that need to be observed, some are very hard to see. “Chemically speaking,” says Anderson, “the stratosphere is a gigantic lowtemperature flame.” In the stratosphere, as in a candle’s flame, labyrinthine networks of chemical events occur. Substances appear and disappear rapidly, some so quickly that for a time there was doubt that they would ever be seen in action.

Anderson developed a technique for catching glimpses of even the most evanescent of the stratosphere’s chemicals. He has incorporated this technique into a number of small laboratories, hung those labs from large balloons, and sent them to the stratosphere. (His latest contraption is like a gigantic fishing rig; the balloon rises to the top of the stratosphere and then, by remote control, the laboratory hanging from it is reeled up and down through the stratosphere on a string twenty kilometers long.) Anderson is one of those who collect the facts and figures that theoreticians insert into their models of the planet. He feels very optimistic about the prospects for his quest. “Within five to ten years,” he says, “the stratosphere will become the first reasonably complex natural system that we will understandunderstand well enough to predict its behavior into the next century.”

Against that cheerful prospect, as Anderson concedes, stands the fact thatthe stratosphere is really very simple when compared with the other natural systems that affect it. The stratosphere seems forbidding enough, but the enormous tasks begin when carbon dioxide and nitrous oxide are thrown into the models. Anyone can make a reasonable guess about the amount of fluorocarbons that might rise to the stratosphere over the next century. Human industry makes fluorocarbons. Whatever quantities the factories produce are the quantities that eventually ascend to the stratosphere. Scientists now feel certain that none get lost along the way. No such simple equation applies to nitrous oxide or to carbon dioxide. Human beings and nature use nitrogen, and some of that nitrogen eventually turns up in the atmosphere as nitrous oxide. But the process is complex. Along the way, nitrogen is processed through the biosphere, and scientists don’t know in detail how the biosphere responds to increases in nitrogen. They can’t say how much nitrogen will lead to how much new nitrous oxide. They measure increases in nitrous oxide only after the fact. They can’t predict increases with any certainty. To gain prophetic power over nitrous oxide, they must learn a great deal more than they know now about how a very large part of the biosphere operates. The same injunction applies to theories about carbon dioxide.

To foretell manmade alterations in the atmosphere, and the consequences of those alterations, scientists must first understand how the world works in its natural state, and today some students of the atmosphere express a sense of urgency about this endeavor. Some fear that within a couple of decades human industry will have altered the atmosphere so much that it won’t be possible to distinguish what is natural from what is manmade.

Other mysteries bear on those mysteries, and these, too, look formidable. Is methane increasing in the atmosphere? Methane, it turns out, is a major source of the very scarce hydroxyl radical. So scientists need an answer to the question about methane. To get it they have to consider a number of sources, including termites in the tropics and the world’s many herds of cattle, which, improbably enough, produce important amounts of methane gas.

Scientists may soon answer the remaining questions about fluorocarbons to nearly everyone’s satisfaction, but even if that happens in five years, the investigation will have taken more than a decade of intense, well-financed research. If banning fluorocarbons in aerosols proves an insufficient remedy, an obvious, ultimate solution exists: quit producing fluorocarbons altogether. But the industrialized world has already largely given up the most dispensable uses of those chemicals. Total elimination of them would mean altering throughout the world almost all systems of cooling and refrigeration. Against such an eventuality, Du Pont has spent seven years and millions of dollars looking for another refrigerating medium, but hasn’t yet found a satisfactory one. Any complete solution would require international agreement, but so far nations other than America have taken the fluorocarbon theory seriously in inverse proportion to their domestic industries' investments in the chemicals.

Fluorocarbons are, nevertheless, a tractable problem compared with nitrous oxide and carbon dioxide. Those problems, if they are real, stem from a worldwide appetite for the most basic things—food and energy. It hardly seems realistic to think of cutting off either problem at its source.

WARNINGS ABOUT SPRAY cans seem to have initiated an era of alarms about the atmosphere. Respectable scientists, as jealous of their good reputations as Victorian maidens, nowadays offer propositions that one would expect to find in the tabloids or in the Bible. Even the scale of these hypotheses invites disbelief. The atmosphere is, however, the air shared by all living things, and modern civilization has already begun to alter the composition of the air, significantly and rapidly. About that, there is no doubt. The measurements are precise and unequivocal. The possibility does exist that some not-too-distant generation will witness the immersion of cities, or will have to carry umbrellas out into a sunshine that only the hardy cockroach is certain to enjoy. It might not turn out that way, but it seems incautious to allow these experiments in global chemistry to continue, and merely await the results.

Michael McElroy, the theoretical chemist, has been involved in the issue of ozone depletion from the beginning; he helped bring attention to the threat of nitrous oxide, and he wrote part of the National Academy’s latest report on ozone. McElroy was appointed a full professor at Harvard at the age of twentynine, and ranks as one of the most important scientists studying the atmosphere. One afternoon, in a ruminative moment, he offered a prescription for the problem of carbon dioxide. McElroy’s scheme involves no less than engineering of the atmosphere. Even small engineering projects can lead to a long chain of problems and repairs. This was global engineering, and it isn’t easy to imagine organizing the world in a grand, cooperative effort that doesn’t promise equal benefits for all. It isn’t easy to imagine even getting started on a plan like McElroy’s, but then again, it isn’t easy to imagine Manhattan under water.

"I really think that the biggest and most immediate problem is carbon dioxide,” McElroy said. “I don’t think you can go out and say, ‘Hey, let’s stop burning oil, let’s stop burning coal,’ because with the best will in the world you can’t make the switch in less than twenty-five or thirty years. But if you can control carbon dioxide you in principle control climate. And I don’t think that’s out of the question; and the way to control carbon dioxide in principle is to control a managed biosphere.

"Now, we do that on a small scale already. Take agriculture. If people had had to provide an environmental-impact statement for agriculture a hundred years ago, it would never have passed. If you proposed to take 10 percent of the land area of the earth and to control what it did, runoff and so on, well, you could never have written the statement. I think that we already do affect the earth on a global scale and I think we could go a bit further. I've been looking at what that entails. We've released—by burning fossil fuel over the past hundred years—about 100 billion tons of carbon that we have mined and kicked upstairs. If you want to pull 100 billion tons of stuff out of a diffuse global system, that’s a hell of a job. It would take quite a big vacuum cleaner. The only way you could manage it would be by using the biosphere. You try to grow more trees, which will then bind up that carboi. and bring it back into the biosphere. So you do a calculation. And it turns out that the amount of area you require is about the amount we use for agriculture. But that gets you into competition for land, with farmers and oil companies and so on. So you say, in principle can I grow forests where there’s now desert? I don’t see why not, if you really wanted to do it. If it were a global commitment. It might involve towing icebergs to the Sahara from Antarctica, or it might involve desalinizing seawater using nuclear power. But, in principle, it could be done.”

McElroy paused. “Now, if you do it, you're then managing a different planet. But that involves all kinds of controversial questions. Managing it for whose benefit? And I don’t want to get involved in that question. But in principle you could do it, or something like it.”

"In principle you could do it,” McElroy repeated. “And perhaps also, perhaps we’ll have to.”