2
THE OZONE HOLE
Our story starts high in the atmosphere, in the cold, dry region called the stratosphere. On the average, the atmosphere becomes gradually colder with increasing altitude. The atmospheric temperature at sea level, averaged over the entire Earth, is about 15 degrees Celsius, or 59 degrees Fahrenheit. As altitude increases, the temperature falls by about 6.5 degrees Celsius for each 1,000 meters in elevation (or about 3.6 degrees Fahrenheit for every 1,000 feet in elevation). Thus, if we stand at the summit of Mt. Everest, which is about 29,000 feet, or 9 kilometers, in altitude, the temperature is colder by some 59 Celsius degrees or 104 Fahrenheit degrees than at sea level. At this height, more than two-thirds of the materials making up the atmosphere are below us.
If we were somehow to leave the summit of Mt. Everest and rise even higher, the temperature would continue to decrease, until, at an altitude of about 11 kilometers (or 36,000 feet, or 7 miles), the temperature would first stop decreasing and then begin to increase with further gains in elevation. At the altitude where the temperature stops decreasing with elevation, we would be entering the stratosphere. The region below us, called the troposphere (from the Greek word for "overturning," because this region of air is kept well mixed by rising and falling air currents), contains all of our familiar weather and all human beings, except those temporarily in high-flying aircraft or spacecraft. The stratosphere, which extends up to an altitude of about 50 kilometers, or 30 miles, is cold, barren, and inhospitable. Yet it's essential to our well-being, because it's home to the ozone layer.
Ozone is a form of oxygen. Ozone, chemical symbol O3 , has three atoms of oxygen in every molecule. A molecule is simply an assemblage of atoms held together by chemical forces. An atom is a tiny particle, the smallest unit of an element that can participate in a chemical reaction. Most gases in the atmosphere occur in the form of molecules containing two or more atoms. Ordinary oxygen, a gas that's common in the atmosphere, is O2 , with just two oxygen atoms per molecule. The O2 form of oxygen is a million times more plentiful than ozone. Oxygen is the second most plentiful gas in the atmosphere. Most of the atmosphere consists of nitrogen; nearly all of the rest of it is oxygen. The two gases together constitute about 99% of the dry atmosphere. (It's helpful to speak ill terms of the dry. atmosphere, the part that's not water, because the concentration of water vapor is highly variable.) Ozone is actually very rare—there's hardly any of it, compared to the enormous amounts of ordinary oxygen in the atmosphere. Yet, ozone is critically important, as we’ll see, because of its remarkable ability to shield people and other living things from ultraviolet radiation, a component of sunlight that's harmful.
Whether ozone is helpful or harmful to us depends on where it is in the atmosphere. We'll come back to the harmful ozone later when we discuss smog; fur now, suffice it to say that ozone in the lower troposphere, near the surface of the Earth, is an ingredient in photochemical smog. It irritates the lungs, it harms plants, and it's a major part of the concern about air quality in many urban areas. But the ozone we talk about when we talk about the ozone layer, and the ozone hole, is in the stratosphere, far above the Earth's surface. The ozone layer is not a thin sheet like a rubber blanket. Instead, there's a range of altitudes, making up a region in the stratosphere, where ozone is slightly more plentiful than it is elsewhere. But even in this region, ozone is one of the rarest molecules. If you imagine a picture of how ozone is distributed with height, the peak in this picture—the altitude at which ozone is most plentiful—would be at about 25 kilometers, or 16 miles above sea level. So, when we talk about the ozone layer, we're talking about a rather deep stratospheric region, high above ground level. Even at those heights, the atmosphere is still composed mainly of nitrogen and ordinary oxygen, the latter with two oxygen atoms in each molecule. Ozone, with three atoms per molecule, is still a minor component, but it's more plentiful in the lower stratosphere than it is near the surface.
Ozone has been present for billions of years. It appears to have been essential to the formation and evolution of life on Earth, because of its
role in shielding the surface of the planet from harmful ultraviolet solar radiation. There apparently was no life on the continents before there was an ozone layer. Deep in the sea, early forms of life may have been protected by the water above.
Ozone concentration, the amount of ozone per unit of volume, also varies with latitude, or distance from the Equator. Ozone in the tropics, near the Equator, is typically much less concentrated than that at middle latitudes, between the Equator and the poles. Ozone concentration also changes with the seasons. In absorbing the sun's ultraviolet radiation and in using energy from this radiation to power chemical reactions, ozone also affects the temperature of the stratosphere. In fact, the increase of temperature with altitude in the stratosphere, noted above, is largely due to ozone.
Ozone occurs in the atmosphere because certain types of sunlight are able to break up the molecules of ordinary oxygen and leave free, single oxygen atoms floating around. These free atoms (chemical symbol O) sometimes combine with the ordinary oxygen molecules (O2 ) to form ozone (O3 ). There's a complicated chain of chemical reactions going on all the time in the stratosphere, in the course of which ozone is constantly being formed and also constantly being destroyed. In the absence of other reactions involving other chemicals, there's an equilibrium, so that the average ozone concentration doesn't change very much. The much-publicized story of the ozone hole is the story of how people have inadvertently and unsuspectingly tampered with that equilibrium.
Ozone is produced and destroyed at all altitudes, but its concentration varies, because ozone is moved about by winds as well as because of the chemical processes that produce and destroy it. Although the maximum concentration of ozone is found within the region we call the ozone layer, even there its concentration is only a few parts per million. So if you sampled a bucket full of molecules from the atmosphere in the ozone layer, only a few of them, typically ten or fewer molecules per million, would be ozone molecules. If you brought all the ozone in the atmosphere down to the surface, it could be contained in a layer about 3 millimeters thick. (A millimeter is about the thickness of the wire used in a paper clip; an inch is about 25 millimeters.)
In spite of how few ozone molecules there are in the ozone layer, they serve a vital function. Were it not for these ozone molecules, harmful radiation would reach the surface of the Earth. For human beings, the most serious immediate consequences would probably be damage to the
skin. Skin cancer is very closely related to dosage of ultraviolet (UV) radiation. Some wavelengths of UV radiation are more harmful than others, and ozone absorbs almost all of these especially dangerous wavelengths of radiation. If you like to speculate, you might consider that perhaps one reason why we and other living things have evolved in the way we have is because we had the good fortune to do so in an environment that was sheltered from harmful UV light, thanks to ozone. Thus, in our atmosphere, a few molecules per million have the remarkable property of shielding the whole lower atmosphere from UV radiation from the Sun.
The UV that we worry about most causes skin cancer, including malignant melanoma, a type of skin cancer that can be fatal if left untreated. UV has other effects, too. It causes premature aging of the skin, and it damages the eyes. It's thought to be connected to the formation of cataracts and may also cause damage to the retina. In addition, UV can suppress the human immune system, and it can injure other animals and plant life as well.
Our eyes are sensitive to the broad spectrum of colors in the rainbow— red, orange, yellow, green, blue, indigo, violet. But there are also colors we can't see: the colors on the other side of red, called infrared, and the colors on the other side of violet, called ultraviolet. If we had better eyes, we could see many more colors. Perhaps we wouldn't think of them as colors but simply different kinds of light. It's the ultraviolet light, the same light that causes skin cancer and cataracts, that breaks oxygen molecules apart. You can think of the process as one in which a small quantity of sunlight, called a photon, is absorbed by an oxygen molecule. The energy from the photon of ultraviolet sunlight is what breaks the molecule apart.
A solitary atom of oxygen, chemical symbol O, is rare because it's highly reactive chemically and, therefore, tends to combine with other molecules. Thus, oxygen atoms don't remain alone for long. By contrast, the everyday oxygen molecule, O2 , is not very reactive, so) it's plentiful rather than rare.
An ozone molecule, as we've said, contains three oxygen atoms, chemical symbol O3 . Why is there any ozone at all? Ozone occurs because ordinary oxygen (O2 ) is sensitive to sunlight, and sometimes breaks apart in a process called photodissociation, which is jargon for the breaking up of molecules due to absorption of solar radiation. The chemical reaction is one in which an ordinary molecule of oxygen, in the presence
of ultraviolet sunlight, breaks up into two free atoms of oxygen. The process occurs high in the atmosphere. Here's how we write the reaction:
O2 + ultraviolet radiation ® O + O
Notice that the reaction expresses a balance, in the sense that there are two oxygen atoms on the left of the arrow and two on the right. Oxygen atoms are not created or destroyed. Instead, they're simply rearranged. Some of the oxygen atoms that result from this photodissociation reaction soon recombine with ozone to form new oxygen molecules. The relevant reaction is:
O + O3 ® 2O2
The meaning of this reaction is that one free oxygen atom combines with one ozone molecule to produce two oxygen molecules. Again, there is a balance, with four oxygen atoms on both the left and the right sides, but in different combinations.
Not every oxygen atom from every oxygen molecule that gets broken up recombines. Still, an equilibrium is established between the breaking up of oxygen molecules into atoms and the recombining of the atoms into oxygen molecules.
Every now and then, on some occasions, a free oxygen atom, the product of photodissociation, will combine with an oxygen molecule to form ozone, O3 . We write the reaction like this:
O +O2 + M ® O3 + M
The "M" in this reaction is another molecule. Don't even think about it. Leaving it there mysteriously reminds us that what we're doing is exploring this subject at a level that's not as deep as we could. M is called a mediator or "third-body" molecule. For now it will suffice to say that the presence of the M molecule is one of the special circumstances required for the ozone-forming reaction to take place.
We've just said that ozone forms typically in the atmosphere when the oxygen molecule combines with an oxygen atom, the lone atom having come from the dissociation process. This raises an interesting question: Why doesn't the oxygen in the atmosphere end up being all ozone? If every now and then we get an oxygen atom and an oxygen molecule combining in the presence of a third-body molecule to form
ozone, why doesn't the atmosphere, as time goes on, contain less and less ordinary oxygen and more and more ozone? Why isn't there more ozone this year than last year, much more now than a billion years ago?
Because ozone can also be destroyed. The same ultraviolet radiation that causes the oxygen molecule to break apart can photodissociate the ozone molecule, breaking it apart into its components, ordinary oxygen and the lone oxygen atom:
O3 + ultraviolet radiation ® + O2 +O
Alternatively, the ozone molecule can combine with an oxygen atom to form two oxygen molecules, in the same reaction we saw earlier:
O+O3 ® O2 + O2
Again, notice that in all these reactions the total number of oxygen atoms involved must remain the same. In the last reaction, we started out with four atoms of oxygen, three from the ozone molecule and one from the single atom, and we ended up with four again, in the form of two ordinary oxygen molecules.
If we left the atmosphere alone, all of these reactions would go on continually at each of their various rates, depending on how much sunlight there was, what the temperature was, whether the third-body molecules were present, and what other chemical compounds were around, because these ozone and oxygen molecules and atoms combine with other things besides themselves. In the real atmosphere, there are lots of other things going on too.
In general, taking all that into account, you could imagine the atmosphere reaching equilibrium, a situation in which the total amounts of ozone and oxygen do not change. Indeed, it seems to do that. There's much more ordinary oxygen, even in the stratosphere, than there is of this relatively rare gas, ozone. Photodissociation, powered by the energy from sunlight, produces about a gigaton of ozone per year. A gigaton is one billion tons. Thus, although ozone is a relatively rare constituent of the atmosphere, there's still a lot of it, because the atmosphere is so vast.
In the atmosphere, the ozone is nearly all in the stratosphere. There are about three gigatons of it in the atmosphere right now. Ozone is always being created and destroyed: many gigatons are created each year, and many gigatons are also destroyed, through the processes
we've just discussed. But as you can see, you can change the amount of ozone that's there, either by producing more or by finding a way to destroy it faster than it's produced.
It's interesting that, from the point of view of people and other living things, there's both good ozone and bad ozone. Good ozone and bad ozone are the very same molecule, O3 ; they differ only in location in the atmosphere. Ozone in the stratosphere is good, because it shields us from ultraviolet radiation, in part because it's absorbing this ultraviolet radiation—destroying itself to save us, so to speak. Then there's the bad ozone, down here at the bottom of the atmosphere where we have to breathe it. It too absorbs ultraviolet radiation, but it also hurts plants, hurts your eyes, attacks many materials, and is poisonous if you breathe enough of it. That's why we worry about ozone in the lower atmosphere, particularly as a constituent of smog.
To further discuss ozone amounts, it helps to use a mathematical-shorthand notation called powers of 10. This is simply a technique for writing large numbers in a compact form. Ten with a superscript 1, or 101 , is shorthand for 10 itself. Ten with a superscript 2, 102 , is 10 times 10, which is 100. Ten with a superscript 3, 103 , means 1,000. You can see that the superscript, called an exponent, represents the number of zeros after the one. So the rule is, if you want to write a power of 10 the long way, the usual way, write a I and then as many zeros as are in the exponent. But you don't have to stick to powers of 10; you can write any number at all in this notation. For example, if you want to write 2,000, it's 2 × 103 If you want to write 2,690, it's 2.69 × 103 . If you want to write 26.9 quadrillion, you can write that as 2.69 × 1016 , or 269 and 14 zeros. That last number is pronounced, "two point six nine times ten to the sixteenth."
I have a deep purpose for wanting you to know about this number 2.69 × 1016 . It relates to something called the Dobson unit, named after Gordon Dobson, an Oxford physicist who was a pioneer in studies of atmospheric ozone. In 1926, Dobson designed a simple instrument to measure stratospheric ozone amounts from the ground, and it was he who created the first global ozone-monitoring network, locating his instruments throughout the world. The instrument, called a spectrophotometer, works by measuring solar radiation in different wavelengths. With knowledge of the radiation-absorbing properties of gases like ozone, these observations can be converted into measurements of ozone amounts.
If there happen to be 2.69 × 1016 molecules of ozone in the atmospheric column above a particular square centimeter of the surface of the Earth, then the abundance of ozone in that column is said to be 1 Dobson unit, or 1 DU. But the typical ozone concentration in the atmosphere is much greater than 1 DU. In fact, an average column of atmosphere above 1 square centimeter of the surface contains several hundred times more than 2.69 ÿ 1016 molecules of ozone. Thus, the average concentration of ozone in the Earth's atmosphere is several hundred DU. In general, ozone is most abundant at high latitudes, near the poles, and least abundant in the tropics, near the Equator. Scientists measure this abundance by the number of ozone molecules in an imaginary column, expressed in DU. A typical average value of ozone abundance is around 250 DU near the Equator, rising to above 300 DU in the middle latitudes. Values above 400 DU can occur near the poles.
In fact, the total number of molecules of ozone on the planet is about 4 × 1037 , which is four with 37 zeros after it—a very large number.
Nonetheless, ozone is rare. If you counted all the molecules of everything in the air—remember, it's 99% nitrogen and ordinary oxygen— you'd come up with about 1044 molecules. This is a great deal more than the number of ozone molecules. The rule for multiplying powers often is to add the exponents. For example, 1038 times I million (which is 106 ) equals 1044 , because 38 + 6 = 44. Thus, 1044 (the number of molecules in the atmosphere) is more than a million times greater than 4 × 1037 (the number of molecules of ozone), so there are more than a million molecules of air (nitrogen, oxygen, etc.) for every molecule of ozone. To put it another way, the concentration of ozone in the atmosphere is less than one part per million.
So, when we talk about the ozone "hole," that's really a figure of speech. It's not as if there were a rubber sheet of ozone floating around in the stratosphere and somebody tore it or punched a hole in it. The term is used to dramatize the thinning of something that's naturally very rare. When we talk about the concentration of ozone over Antarctica falling from 300 Dobson units down to only 150 Dobson units, we mean half of it is gone. What once was one part per million has become one part per two million.
There is less ozone in the stratosphere today than there was a few decades ago, due in part to the inventiveness of one industrious scien-
tist, Thomas Midgley, Jr. Born in 1889 in Beaver Falls, Pennsylvania, Midgley grew up in Columbus, Ohio, and studied mechanical engineering at Cornell University. Though his greatest achievements were in industrial chemistry, as a chemist he was largely self-taught. At the age of 33, while working at General Motors Research Corporation, he discovered the value of tetraethyl lead as a gasoline additive. This substance substantially raised the octane rating of gasoline, so that compression ratios in automobile engines could be higher and automobiles could thus be more powerful. It was a remarkable invention and a great spur to the fledgling auto industry.
Only much later did we learn that lead in gasoline, in paint, and in other substances has a deleterious effect when it gets out into the environment. Lead has now been banned from many applications, for this very reason. Many countries today require catalytic converters in the exhaust systems of cars to facilitate reactions that destroy the harmful pollutants in the exhaust gases. If lead gets on the catalytic converter, the catalyst becomes less efficient in helping the reactions. So to help the catalytic converter do its job, we have switched to unleaded gasoline. The story of lead in gasoline has many parallels with that of the chemicals that cause the ozone hole. What initially appeared to be a technological breakthrough, with many benefits to humankind, later turned out to have unanticipated side effects that were so severe that they outweighed the benefits.
Having succeeded with tetraethyl lead, Midgley became a hero at General Motors, and he was put to work to develop a safe refrigerant. The chemicals previously used in refrigerators—sulfur dioxide, ammonia, and methyl chloride—were toxic or flammable or both. Refrigeration was dangerous. In 1928, Midgley invented a class of chemicals called chlorofluorocarbons. (We'll return to this class of chemicals later, to examine in some detail what they are, and to learn something about their chemistry.)
Chlorofluorocarbons, or CFCs, are miracle chemicals. They're nontoxic and noncorrosive. Chemically, they're almost inert; that is, they tend not to react with anything. They make superb refrigerants, and it turns out that they have many other industrial uses as well. They're solvents for cleaning parts in the electronics industry, including circuit boards like those that go into computers and television sets. They serve as the cooling fluid in most existing automobile air conditioners. Millions of tons of CFCs have been produced. Du Pont and other
companies made many millions of dollars manufacturing them. Commercially, CFCs have been a great success.
But—and this is a big "but"—they destroy ozone. For that reason, they're now being banned, by international agreement.
So Thomas Midgley, Jr., an authentic superstar of industrial chemistry, developed at least two substances that had tremendous practical value. He made a lot of money for his employers, but, unbeknownst to him or anyone else at the time, his inventions turned out to be terribly dangerous when released in large quantities into the environment. Both tetraethyl lead and chlorofluorocarbons are being banned, and we may live to sec the day when nobody makes either of them anymore. Meanwhile, they're out there.
Midgley, by all accounts, was an energetic, gregarious, cheerful man. His accomplishments earned him many honors, including election to the presidency of the American Chemical Society, a remarkable odyssey for a man with no formal education in chemistry. But in 1940, he was diagnosed with polio. As his health deteriorated, he grew despondent, and in 1944 he committed suicide. Even in death, the inventive Midgley was ingenious. He had rigged up a harness to help himself get out of bed, and he intentionally strangled himself in it. It was a tragic and sad end to the life of a remarkable scientist, but at least he was spared the knowledge that his inventions have brought the environment great harm.
The CFC industry flourished for decades. And then, 30 years after Midgley's death, CFCs were implicated in ozone destruction. Two chemists at the University of California, Irvine, were analyzing stratospheric ozone and its sensitivity to other chemicals that might be found in the stratosphere. Mario Molina and Sherwood Rowland, in a paper published in 1974, theorized about the existence and nature of a special molecule, called a catalyst, that might be important in the chemistry, of ozone destruction.
A catalyst is a substance that stimulates a chemical reaction, whether by enabling it or accelerating it, without itself being changed. In Rowland and Molina's theory, an ozone molecule combines with another molecule, the catalyst, in such a way that the ozone is broken up into two entities, an ordinary oxygen molecule and the leftover oxygen atom (the third atom from the ozone molecule), which combines with the catalyst. If we call the catalyst X, the reaction looks like this:
O3 +X ® OX + O2
Chlorine (chemical symbol Cl) works well as a catalyst in this reaction:
O3 + Cl ® ClO + O2
But things don't stop there. If they stopped there, the chlorine wouldn't be called a catalyst, because it wouldn't be emerging unchanged. It started out not connected to anything, but now it is combined with an oxygen atom to form chlorine monoxide (ClO).
The next thing that happens is that the CIO molecule combines with a free oxygen atom. The result is a molecule of ordinary oxygen, and the Cl atom is freed up again, like this:
O+ClO ® O2 + Cl
Where might the oxygen atom (O) come from? One source is the photodissociation of ozone described earlier:
O3 + ultraviolet radiation ® O2 + O
These last three reactions constitute a typical catalytic cycle. If we sum them up, we get:
2O3 ® 3O2
We started out with two ozone molecules and ended up with three ordinary oxygen molecules. We needed the chlorine molecule to form the intermediate compound ClO, but only temporarily, because the CIO molecule itself was soon broken up. That's why Cl is called the catalyst; it escapes unscathed.
Incidentally, you can't just take two ozone molecules, an O3 and an O3 , and expect them to break up into three O2 's:
O3 + O3® O2 + O2 + O2
That would seem to be a possible reaction; you've got six atoms in the two ozone molecules on the left and six atoms in the three ordinary. oxygen molecules on the right, so it's balanced. But this reaction doesn't happen. Ozone molecules are stable; they don't spontaneously break up. But in the presence of a special molecule—a catalyst—one of the ozone molecules can break up to form a molecule of regular oxygen.
The third, leftover oxygen atom then combines with the catalyst, temporarily, and the molecule thus formed effectively combines with another ozone molecule to produce two oxygen molecules, leaving the catalyst just as it started out.
Thus, although this last simple reaction doesn't occur, nevertheless, in the presence of the catalyst, something equivalent does. We start out with two ozone molecules and we end up with three oxygen molecules. In the process, we first create and then destroy the molecule involving the catalyst. And in the process, we destroy ozone.
Certain molecules work especially well as catalysts. For the reaction above, one of these is chlorine. A single chlorine atom can go through this cycle thousands of times before it finally combines more permanently with something else. The chlorine atom encounters an ozone molecule, breaks it up, temporarily forms a molecule of chlorine monoxide (chlorine and oxygen), then breaks up that molecule in the presence of another ozone molecule to form two more oxygen molecules and tree up the chlorine atom. Then the chlorine atom does all this again and again and again.
Chlorine can combine with other things, but the enormous power of this reaction, the frightening aspect of it, is that one chlorine atom can go through this catalytic cycle repeatedly. You don't need a brand-new chlorine atom to start it off. An individual chlorine atom, which just a few seconds ago went through this cycle and destroyed two ozone molecules, can next destroy two more ozone molecules, then two more and two more and so on. So if you add chlorine to the stratosphere, you create the potential for destroying great amounts of ozone very quickly. A typical chlorine atom can apparently go through this destructive catalytic cycle some 1,000 to 10,000 times before something else happens to it.
Scientists had been studying catalytic cycles for ozone destruction for a long time, but the role of CFCs was not recognized until the theoretical breakthrough by Molina and Rowland. Other scientists had concentrated on chemistry involving nitrogen oxides from the exhausts of a hypothetical fleet of supersonic aircraft. Chlorine was also considered, but the main source of chlorine was thought to be exhaust from the space shuttle. Molina and Rowland realized that CFCs could provide the chlorine that would serve as the catalyst that could destroy atmospheric ozone. They recognized that the CFCs Midgley had invented in
1928, which had been continually accumulating in the atmosphere ever since, were slowly making their way into the stratosphere.
In hindsight, it's clear that what Midgley did, quite inadvertently, when he invented chlorofluorocarbons was to create a highly efficient way to transport chlorine up into the stratosphere. CFCs, the simple molecules composed of chlorine and carbon atoms, or chlorine, fluorine, and carbon atoms, are chemically almost inert. That's why they're so good for refrigerants. That's how Midgley made his employers at General Motors rich. The new miracle refrigerants displayed none of the disadvantages of their predecessors. Thomas Midgley had invented a substance that's impervious to almost everything—until it gets up into the stratosphere. There, unfortunately, conditions are just right for ultraviolet radiation to break apart the CFC molecules and free up chlorine atoms.
CFCs are really fairly simple molecules, just slightly more complicated than ozone and oxygen. Chemically, they resemble methane, the main component of natural gas. Methane, as its chemical symbol CH4 indicates, has one atom of carbon and four atoms of hydrogen. CFC molecules are like methane molecules, but with each hydrogen atom replaced by either a chlorine atom or a fluorine atom.
There are several different CFC molecules, none of which occurs naturally. You wouldn't find CFCs anywhere in the atmosphere if people hadn't put them there. How do they get there? CFCs enter the atmosphere when old refrigerators are discarded and eventually leak, when the refrigerant in automobile air conditioners is replenished and the job is done carelessly, with no attempt to capture and recycle the CFCs, or when the insulation in refrigerators, which also contains chlorofluorocarbons, breaks down. Although CFC molecules are even rarer than ozone, there are millions of tons of them in the atmosphere now. Their concentrations are measured in parts per billion, not parts per million as in the case of ozone, but the atmosphere is so immense that just a few of these molecules in a sea of oxygen and nitrogen molecules are still millions of tons.
One of the chlorofluorocarbons, CFC-11, not only is used as a refrigerant but was for a long time the preferred propellant in aerosol spray cans. In the United States, aerosol spray cans containing Freon were banned in the late 1970s. CFC-11 was the Freon in those spray cans. (Freon, incidentally, is a trade name used by Du Pont; it's easier to say than "chlorofluorocarbon eleven.") Another chlorofluorocarbon, CFC-12, has been used in aerosols, refrigerants, and air conditioners.
CFC-113 is a solvent, used especially in the electronics industry for cleaning circuit boards. These are the main CFCs. In the 1980s, they were being produced at the rate of several hundred thousand tons per year. Closely related compounds called halons are used in fire extinguishers.
All of these compounds arc now known to cause ozone depletion, the destruction of ozone in the stratosphere. CFC-11, in particular, is responsible for nearly half of the observed ozone depletion. CFC-12 and CFC- 113 are the two next most important of these compounds. Because CFCs arc very useful and arc the foundation of a substantial industry, the world has not acted as fast as it might have to ban their production. At one point, Professor Rowland was chagrined at the slow pace of the international effort to outlaw CFCs. "What's the use of having developed a science well enough to make predictions," he said, "if in the end all we're willing to do is stand around and wait fur them to come true?"
In 1974, when Molina and Rowland wrote about the potential for ozone destruction, it was pure theory. It was intelligent conjecture, a scientific hypothesis they thought up as theoretical chemists using their knowledge of chemistry to figure out how the compounds involved would behave under the physical conditions of the stratosphere.
For a long time following their publication, Molina and Rowland's speculations were considered mere scientific theorizing and were ignored or disparaged by many people—including, as you might imagine, the industry that very profitably made the chlorofluorocarbons. These people discussed the subject in much the same terms that some elements of the tobacco industry have adopted in speaking about the possible health effects of tobacco smoking. You can have a lot of statistical evidence that links cigarettes and cancer, but in the absence of more conclusive "proof," there is still room fur interested parties to deny a cause-and-effect relationship.
Eventually, of course, more and more people became convinced that smoking leads to lung cancer, and the consciousness of nearly everyone has now been raised. Today you can't smoke on a domestic airplane flight in the United States, or in many airports, restaurants, and other public places. Bans on smoking arc also being legislated in other countries.
Similarly, in the case of CFCs, the evidence became more and more persuasive. Following the lead of Molina and Rowland, other scientists developed more theories, conducted more laboratory experiments, and became more and more convinced that CFCs might in fact be reducing the amount of ozone in the stratosphere. Still, during the 1970s, frag-
mentary evidence and conflicting viewpoints abounded in the scientific community, and even more so in the popular press. Furthermore, there was no incontrovertible evidence to show conclusively that CFCs actually do deplete stratospheric ozone. That evidence came to light in the 1980s, from a most unlikely place.
Dramatic proof that ozone depletion actually occurred was found in Antarctica. The discoverers were a group headed by a British scientist named Joseph Farman, who had traveled to Antarctica, in the southern winter, to a terribly desolate place called Halley Bay, to measure ozone. Again, this was pure research, performed not so much to prove or disprove Molina and Rowland, or anyone else, but because Farman and his colleagues, like many other scientists, were using Antarctica as a great outdoor laboratory to observe a number of processes that can go on in the environment. In fact, Farman and his group, members of the British Antarctic Survey, had been measuring ozone from Halley Bay for nearly 25 years. Their basic instrument was the spectrophotometer invented in 1926 by Gordon Dobson, a design that had survived essentially unaltered for more than half a century. In truth, the longevity of the instrument symbolized much about the field of ozone measurement: it was a rather dull scientific backwater, populated by a handful of devotees like Farman. All that was about to change.
In 1981, Farman noticed something most unusual during the spring, which starts in late September in the Southern Hemisphere. At the beginning of spring, the ozone amounts over Halley Bay in Antarctica decreased sharply and then seemed to recover, some months later. Ozone was first disappearing and then reappearing. The following year, the same thing happened. The decline was substantial—at first some 20% below the normal Antarctic springtime value of approximately 300 Dobson units, and much more severe in later years.
Farman was a cautious scientist, and he hesitated to publish his data. He wasn't sure of their significance, and he wanted to observe another year. He actually suspected that his trusty Dobson meter might have malfunctioned, and he installed a new one in 1982. The new one corroborated the low springtime ozone levels. Finally, in 1985, Farman and his colleagues published their results and conjectured that CFCs might be responsible for the loss of ozone.
Surprisingly, the depletion this time was not just a few percent, which might have been expected on the basis of Rowland and Molina's theory. It was almost 50%. Half of the ozone in the southern stratosphere over
Antarctica simply disappeared in the spring and then reappeared again. If this remarkable phenomenon was actually due to human intervention, then, for the first time in history, people had substantially changed the chemical composition of Earth's atmosphere.
Another actor enters the play at this point: NASA, the National Aeronautics and Space Administration. In late 1978, NASA had launched Nimbus 7, a satellite carrying instruments that measured many properties of the Earth, not only meteorological ones like cloud cover and reflectivity but also ozone. In particular, the Total Ozone Mapping Spectrometer, or TOMS, aboard Nimbus 7 represented a great advance in spaceborne ozone-measuring capability. TOMS measured ozone amounts over the entire globe while the satellite completed an orbit of the planet every hour and a half, passing over the North and South poles. The TOMS data from Nimbus 7 were radioed back to Earth and carefully archived on tapes.
NASA's record in ozone research is superb, and the TOMS data set is among NASA's greatest triumphs. The TOMS instrument on Nimbus 7 provided data from 1978 well into 1993, and these data are a primary source for research on the magnitude and severity of the ozone hole. Yet credit for the discovery of the ozone hole goes to Joseph Farman and his colleagues, using 1920s ground-based instrument technology. Why? Because of an easily understandable mistake in processing the TOMS measurements.
Before the NASA data were analyzed by scientists, they were processed by a computer program. You can't do satellite work without computers, because the data are so voluminous, made up of millions upon millions of numbers. Because the satellites are measuring all the time, and their radios are on all the time, the data come down to Earth continuously, and the numbers pile up. In fact, it's been said that trying to learn something from satellite data is like trying to drink from a fire hydrant.
So, before the data get analyzed by people, they get analyzed by computers. Part of the computer program is a "quality-control" check to make sure the data are good, that the instrument isn't malfunctioning and the data aren't being garbled in the radio transmissions. The quality control performed on the Nimbus 7 TOMS ozone data included comparing the readings with a range of numbers that was thought to be typical for ozone, such as might be measured under normal conditions.
NASA scientists had set maximum and minimum values of plausible ozone amounts. The numbers they chose were 650 and 180 Dobson
units. Since nobody had dreamed that an ozone hole might exist, nobody told the computer that numbers lower than 180 might be real, rather than the result of defective instruments. In going from the outside of the ozone hole to the inside, Nimbus 7 measured values of ozone that were too low when compared to what the computer had been programmed to expect. As a result, the quality-control program invalidated perfectly valid data. Thus, the program designed to make sure the data were good actually delayed the discovery of the ozone hole.
After Farman's publication appeared in 1985, NASA went back to the data archive, where the raw data used as input to the quality-control program were still stored. It's just good scientific practice to archive raw data, however flawed they might seem, and NASA deserves credit for doing that in this instance. NASA reprocessed these data and found that the TOMS instrument aboard Nimbus 7 had indeed seen the ozone hole before Farman did, but NASA scientists, with misplaced faith in their computer program, had at first failed to realize what the satellite was trying to tell them. There's a lesson here.
Our understanding of the ozone hole today is that it's caused mainly by the catalytic effect of chlorine, which gets into the atmosphere in the form of CFCs. The CFCs are gradually mixed globally in the atmosphere and may take decades to reach the stratosphere. Once there, they're decomposed by ultraviolet sunlight, releasing the chlorine that eventually begins attacking the ozone. Initially, however, the released chlorine forms relatively stable compounds with hydrogen and nitrogen.
A key element in the process by which chlorine attacks ozone is a bizarre kind of cloud called the polar stratospheric cloud (PSC). Although the lower stratosphere is quite dry, with water vapor present in only a few parts per million, it can get so cold above Antarctica (78 degrees below zero Celsius or 108 degrees below zero Fahrenheit) that PSCs can form from nitrogen compounds and water. The tiny ice crystals in these PSCs provide the sites on which chlorine is liberated from its less reactive forms. The resulting molecular chlorine (Cl2 ) is itself quickly decomposed by ultraviolet sunlight, freeing up atomic chlorine (Cl), which is the catalytic culprit in destroying ozone.
Thus, the surfaces of the little crystals in these very special clouds in the cold, dark Antarctic stratosphere are the sites on which chlorine from CFCs finally becomes reactive atomic chlorine. Although there are several ozone-destroying processes, this seems to be the main one.
But why Antarctica? After an armada of people descended on the South Pole in response to Farman's discovery—flying planes, running computer models, observing the continent from satellites, doing laboratory experiments—much was learned, and we think we've learned what's unique about the place. The first clue is that it gets cold enough there, probably colder than anywhere else on Earth, for these special clouds to form in the stratosphere. The second clue is a vortex of winds circling constantly around the South Pole every winter. This swirling mass of air keeps the Antarctic stratosphere isolated from neighboring air masses during the winter, which allows the destructive chemical reactions to act on a limited local stock of ozone that cannot be replenished by ozone from other latitudes.
As winter ends, the vortex breaks down, allowing relatively ozone-rich air from lower latitudes to reach the Antarctic. We don't know if this springtime recovery will always occur in the future. Each year observed so far has been different in some ways from other years. We don't know whether the depletion process will become more intense, because so many things arc changing. But as the CFC concentrations continue to increase, the abundance of chlorine in the atmosphere will also increase. And if sufficient chlorine accumulates in the stratosphere, the winter depletions of Antarctic ozone could become so severe that ozone levels might not recover in spring.
Also ozone affects temperature. Everything in the atmosphere, after all, is connected to everything else. The temperature in the stratosphere, in Antarctica and elsewhere, is largely determined by a radiative balance involving two gases: ozone and carbon dioxide. The essential notion of this balance is simple: warming occurs because ozone absorbs sunlight, and cooling occurs because carbon dioxide radiates heat away. Thus, the temperature of this region of the atmosphere will change if there's a change in the relative abundances of these two gases.
As carbon dioxide increases, which it's doing because we're putting more of it into the atmosphere, and as ozone decreases, which it's doing because we're helping to destroy it, the stratosphere theoretically should be cooling. There is some evidence—we can't be sure of it yet—that globally the lower stratosphere is cooling. If the stratosphere continues to cool, we may find that conditions in the Arctic come more and more to approximate those in the Antarctic. Conditions that favor the formation of polar stratospheric clouds, for example, may begin to occur in the Arctic.
But there's a fundamental difference between the North and South polar regions: the vortex. In Antarctica the polar vortex keeps strato-
spheric ozone isolated, but no such condition exists in the Arctic. And it's hard to imagine anything like the Antarctic vortex forming anywhere else. So, is this vortex essential to forming an ozone hole? We don't know with certainty, but we think it is. If clouds, however, rather than a vortex of winds, are the essential component of ozone loss, we may see an ozone hole in the Arctic after all. Some ozone loss over the North Pole, as well as over the entire Northern Hemisphere, has already been observed.
At the same time, Molina and Rowland's original idea that ozone destruction would be a gradual global process is not dead either. The global amount of ozone has apparently decreased. Thus, not only have we the dramatic phenomenon of the Antarctic ozone hole to ponder, but the ozone layer everywhere may be in danger of dying—by a slower, more insidious form of depletion.
Meanwhile, in the 1970s, a young scientist named Veerabhadran Ramanathan who had just finished his Ph.D. at the State University of New York at Stony Brook also became interested in chlorofluorocarbons. He's now at the University of California, San Diego, at Scripps Institution of Oceanography.
Ramanathan's interest in CFCs was focused not on their effect on ozone, but on their role in the greenhouse effect. This is the process by which water vapor, carbon dioxide, and other gases warm the atmosphere by absorbing some of the Earth's radiation that would otherwise escape to space. The greenhouse effect keeps the planet some 33 degrees Celsius, or 59 degrees Fahrenheit, warmer than it would otherwise be. Many people, including Ramanathan's Ph.D. thesis advisor, told him that this was not a good subject to be working on, that it was obvious that CFCs were unimportant to the greenhouse effect. Fortunately, he ignored them and persisted in working on the theory. As it turned out, he was right. Science is often like that, and creative scientists who follow their own hunches often discover important things. Joseph Farina, before he became famous for discovering the ozone hole, was sometimes chided for his dedication in returning each year to Halley Bay to carry on the boring task of measuring stratospheric ozone.
Ramanathan's research provides a connection between two of the topics that concern us: ozone depletion and an increase in the greenhouse effect. CFCs, the same gases implicated in the destruction of the ozone layer, also add to the greenhouse effect. According to Ramanathan's theoretical calculations, CFCs trap heat in the same way that gases such as water vapor, carbon dioxide, and methane do. Although
we can't prove it, many scientists think the Earth today is a little bit warmer than it would have been if CFCs had never been invented and allowed to accumulate in the atmosphere. In general, the level of scientific uncertainty is greater in the case of climate change due to an enhanced greenhouse effect than it is in the case of ozone depletion. The science of the greenhouse effect and climate is inherently more complex, and as yet there's no climatic analog to the ozone hole, a dramatic and undeniable piece of evidence that the planet has changed because of human activity. But from a theoretical standpoint, Ramanathan's work is unassailable: CFCs are greenhouse gases.
Thus, even if we weren't worried about CFCs because of their role in destroying ozone, even if ozone in the stratosphere didn't matter to us and skin cancer weren't a problem, we'd still have to be concerned about CFCs today because they increase the greenhouse effect. In fact, molecule for molecule, they're much more powerful in this respect than carbon dioxide and some of the other gases that we'll learn about when we discuss the greenhouse effect in detail. Thus, banning CFCs for the purpose of preserving the ozone layer has the nice side effect of slowing the rate at which we're increasing the greenhouse effect.
The melancholy story of ozone depletion will have, I hope, a happy ending. We don't know that ending yet, but most of the recent developments are promising.
As the evidence piled up, it became harder and harder to deny that chlorofluorocarbons were responsible for ozone loss. Fortunately, there was the appealing prospect of a technological fix: it seemed possible to manufacture other chemicals that would be good refrigerants, solvents, and propellants without contributing to this undesirable effect on ozone. Thus, in a remarkable and dramatic diplomatic effort, an international agreement was reached to phase out the manufacture of CFCs, because they had been shown to be the "smoking gun" in the story of the Antarctic ozone hole.
An international conference was held in Montreal, Canada, in September 1987. The resulting agreement, the Montreal Protocol, ratified by 57 countries, put in place a global framework for phasing out the production of CFCs and halons on an agreed-upon schedule. It had become clear that although CFCs were the main culprit, other compounds also contributed to the destruction of ozone. Halons, used in fire extinguishers, for instance, introduce bromine into the stratosphere, and bromine has an even greater potential for the destruction of ozone
than does chlorine. In fact, molecule for molecule, halons are ten times more powerful ozone destroyers than CFCs. Fortunately, there are fewer of them around.
In the years following the 1987 meeting, new scientific evidence came to light suggesting that ozone depletion might be worse than had been anticipated, and in June 1990, 120 countries met in London to strengthen the agreement for phasing out CFCs and related chemicals. The negotiators at Montreal had foreseen exactly this possibility, and a provision for subsequently subjecting the terms of the treaty to modification had been included in the Protocol. One new development was confirmation that the ozone loss over Antarctica was indeed duet to CFCs. A second persuasive factor was the detection of ozone loss over heavily populated parts of the Northern Hemisphere.
One of the results of the London meeting was to accelerate the timetable for the phaseout. The Montreal Protocol, for example, called for a 50% reduction of CFC production by 1998. In London this timetable was accelerated: these chemicals were to be totally phased out by the year 2000, in some cases earlier.
The negotiators at the London meeting were environmental ministers, from agencies such as the Environmental Protection Agency (EPA) in the United States. The meeting of these ministers had been preceded by meetings of technical experts who were to advise the ministers on the feasibility of various timetables and on the costs that would attend the phaseouts. When the technical experts arrived, the draft documents were full of square brackets surrounding phrases that were contentious, or numbers that had to be decided upon—reductions of 50%, or 75%, or 100%, by 1995, or 2000, or 2010. There were, in fact, 190 sets of square brackets in the draft document.
One person at the conference was a 17-year-old Australian. Her name was Susannah ("Zanny") Begg, a member of the Australian Conservation Foundation Youth Delegation, a young woman with a flair for words. In her address to the conference she said to these ministers and technical experts, "Our fate lies in your square brackets." (We'll hear more from Ms. Begg shortly.)
Among the countries attending the London conference were some that hadn't ratified the Montreal Protocol. It became clear that, among other disputes, there was an issue of equity between developed countries and developing countries. China and India, for example, with large populations but relatively low degrees of economic development and
standards of living, were reluctant to embrace a treaty that foreclosed many of their options for development. In effect, they said to the developed countries of the West, "You have already benefited from air conditioning and refrigeration that's free of corrosive and toxic chemicals, and from a semiconductor industry that uses CFCs as solvents. We haven't yet realized those benefits. Why does the industrialized West now dictate to the Third World that it must behave in a more environmentally responsible way than you yourselves have done? You caused the damage in the first place." These issues were negotiated, and a number of imaginative solutions were reached.
One of the major achievements of this 1990 meeting in London was to set up an international fund into which the developed countries would contribute. The United Nations has a set of rules for determining the proportion of total contributions for each country, dictating that large, rich countries pay more than small, poor countries. The United States and the United Kingdom were the major contributors. In addition to moneys pledged in Montreal in 1987, representatives at the London meeting reached agreement that there would be a three-year budget of $360 million over the period between 1991 and 1993. This figure would be increased by $40 million when China ratified the Montreal treaty, and by another $40 million when India joined the ozone-protection club. The London agreements were signed by 93 countries. India and China, which had not signed the Montreal Protocol, ultimately did sign the London agreement.
The purpose of the fund is to provide technical assistance to countries that need it to develop CFC substitutes and to implement them. For example, you can't just instantaneously trade in all your old refrigerators or air conditioners and get new ones. It's not a simple matter of taking one substance out and putting another substance in. Changes have to be made, and changes cost money. Developing a new technology is expensive, as is making it widely available. So, although the amounts of money may not seem very large, the pledges were a step in the right direction. Countries recognized the disparity between the effects that an agreement like this would have on the developed world and those it would have on the less-developed world.
There seems to be general agreement that more than one step is needed. First, we must phase out the harmful chemicals, the chlorofluorocarbons and related compounds. We must also design alternative chemicals and be as sure as we can that these new alternatives don't themselves have
deleterious side effects. We need to make sure that present-day Thomas Midgleys aren't making breakthroughs that our grandchildren will discover are harmful. And we have to do a good job of monitoring changes in the ozone layer.
Despite the Montreal agreements, there are more CFCs in the atmosphere now than ever before, because they've been accumulating for so long and because they're still being produced. CFCs take many decades to decay naturally. So, although the rate of production has gone down following the Montreal and London agreements, the total amount in the atmosphere is still very large, and still growing.
The Montreal Protocol and its subsequent update in London are very hopeful steps. Certainly they bear watching and will have, we hope, analogs in other environmental problem areas where solutions aren't so clear-cut.
I mentioned that 17-year-old Zanny Begg has a flair for words. Here's some more of her address to the London conference:
We are speaking on behalf of the young people of Australia. We are here because we have a right to be involved in these decisions.
Over the past week of negotiations, we have been watching you. It has been at times fascinating, at times confusing, at times horrifying. We have had to keep reminding each other that what is actually being debated here is the future of the ozone layer. This debate has been largely guided by short-sighted commercial gains and national self-interest. There has been more concern for semantics than substance.…
The scientific imperatives are clear. Only an immediate end to the use of ozone-depleting chemicals will truly reflect the urgency of the situation. Even if we do this, it will be a further 60 years before the Antarctic ozone hole is repaired. We came to the conference with this knowledge, but it seems you did not. Your diplomatic compromises are compromising our future.
We have a fight to demand a safe future for ourselves and all generations to come. …what is required is a fundamental change of attitudes, values, and lifestyles, particularly in the developed nations. We insist that over the next three days, you make decisions which reflect intergenerational equity and an active concern for the environment. Remember that we will inherit the consequences of your decisions. We cannot amend the Montreal Protocol. You can! You will not bear the brunt of ozone depletion. We will!
We demand that you think in the long term. Even the best proposals on the table now sanction an unacceptable increase in the chlorine loading of the stratosphere. Your rhetoric is not being matched by your action. Will you condemn us to a future fraught with skin cancer, eye cataracts, immune deficiency, depleted food sources, and a vanishing biodiversity?
At the moment we are afraid. Do not leave our generation without hope. Our fate lies in your square brackets. You are making history. Have the courage to save the ozone layer.
These are eloquent words, but not everybody agrees with them. There are those who have long felt that anything short of an immediate ban on ozone-depleting chemicals is an inadequate measure. There arc others who think gradually weaning the world from CFCs and related chemicals is both preferable and adequate. These people believe that the consequences of reduced ozone are bearable and less burdensome than the economic costs of instantaneously eliminating these chemicals. Firing the debate is the fact that these issues are not black and white. You can't ask people to make radical changes in areas that have enormous economic and social implications without a great deal of debate and a broad spectrum of opinions. I've offered a couple of samples of opinion here, and you may certainly have your own.
The trend among governments recently has been to accelerate the timetable for banning the chemicals that destroy ozone. In November 1992, another conference was held in Copenhagen, attended this time by representatives of 87 countries. They agreed to move up the deadline for a total phaseout of these chemicals from the year 2000, which had been agreed to in London, to 1996. In the United States, the government established December 31, 1995, as the deadline for ending the production of virtually all chemicals that arc known to destroy ozone.
You might ask whether there are other chemicals that we could make and intentionally release into the atmosphere to counteract the process that leads to the destruction of ozone. Not much research with that particular aim is going forward, but scientists would be sensitive to potential applications of this type if they turned up as by-products of other research. In any case, many scientists arc skeptical of planetary engineering, of trying purposefully to alter the environment on a large scale, because of the risk of damage posed by unforeseen effects. We'll come across this problem again when we talk about the greenhouse effect.
There are many proposals for planetary engineering being thrown about, but I'm leery of notions like that. I liken them to taking a pill you've prescribed for yourself, one whose properties you don't fully understand, to cure a disease that hasn't been properly diagnosed or evaluated. I'm worried about piling error on top of error. We didn't
know what chlorofluorocarbons would do for 46 years after Midgley invented them in 1928. It was in 1974 that Molina and Rowland pointed out, in the paper they published that year, that in the stratosphere, chlorine dissociated from CFCs might be trouble. Two decades later, we continue to learn more about how CFCs behave, and we continue to learn more about how the ozone hole is created. So I think it's dangerous to hunt for an easy out in the form of a "technological fix." It's a little bit, it seems to me, like saying, "Well, I eat too much fat, and I drink too much alcohol, so I think I'll try to find a pill that somehow keeps my arteries from clogging up and prevents my liver from being damaged, instead of modifying the way I live."
Incidentally, trying to pump gigatons of chemicals into the upper atmosphere intentionally would not be easy anyway. In fact, for me, one of the most comforting things about proposals for large-scale planetary engineering is that they're usually not feasible.
The Earth always changes, often for reasons we don't know. Ice ages just come and go, as a dramatic example. Their comings and goings aren't caused by people. In the same way, the amount of ozone in the atmosphere might change slowly over many years, due to natural changes in the processes that create and destroy the ozone.
It's true that ozone is created by photodissociation of ordinary oxygen by ultraviolet sunlight. It's also true that in the natural atmosphere, ozone is destroyed by the processes that we described, and by a few other processes as well. And it's true that on balance—at least in recent years—there are at any given time about 3 billion tons of ozone in the atmosphere. In a given year, about a third of it cycles through the processes of creation and destruction; that is, about 1 billion tons are created and I billion tons are destroyed, yielding an approximate equilibrium. But over very long periods of time that could change, owing to natural changes in the system, as well as to human activities such as the releasing of CFCs. For example, volcanoes can produce gases that could alter the chemical composition of the atmosphere, and this would happen naturally. But natural changes take place over a much greater time span than do the changes we've made to the ozone layer. Our changes, occurring in only a few years, were caused by adding certain chemicals to the atmosphere and thus producing the catalysts for the destruction reaction.
It's ironic, by the way, that some airplanes may actually have added to the ozone layer. There you have an example of inadvertent planetary.
engineering. An airplane, if you think about it, is a big engine burning fossil fuels (jet fuel is very like kerosene) and producing exhaust that goes out the tailpipe and makes smog. There's smog in the high troposphere and lower stratosphere where airplanes fly. You can see it when you fly. Just as the tailpipes of cars help to produce ozone in Los Angeles, so the tailpipes of airplanes help to produce ozone in the high troposphere and the low stratosphere, about 10 miles up. It may very well be that the net result of our inadvertent tampering with the atmosphere through the operation of aircraft is to increase slightly the amount of ozone. Of course, that's been more than outweighed by the inadvertent decrease produced by CFCs.
During the 1991 Northern Hemisphere fall, in October and November, which corresponds to the Southern Hemisphere spring, there was a relatively severe Antarctic ozone-loss episode. A year later, the ozone loss over Antarctica was greater still, and evidence of loss in the Arctic and in lower latitudes has also been found. So, we continue to learn as we go along. One characteristic of global-change problems is that they often seem to demand action before all the scientific results are in.
As the Montreal Protocol and subsequent international accords have demonstrated, the world has decided to act. The world has agreed to remove chlorofluorocarbons from industrial production and to find substitutes for them.
Because they last so long in the atmosphere, typically about a century, the CFCs in the atmosphere are still increasing. Indeed, even after production has been phased out altogether, we will have to wait many decades for most of the CFCs to disappear. They're out there in the atmosphere right now. The molecules we put out there in 1970 are mostly still there, and the ones put out there in 1950 are mostly still there, and even some of Thomas Midgley's original molecules from before 1930 arc still there. They continue to accumulate. Thus, we now appear to be inexorably committed to substantial increases in ultraviolet radiation for decades to come, with ensuing health and ecological effects. The consequences of producing CFCs for over half a century cannot be evaded by agreeing not to produce any more. The atmosphere remembers our past behavior. There are limits to the compassion of the forgiving air.