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.