Greenhouse Gases

CFC-Ozone Puzzle: Lecture

Speakers: F. Sherwood Rowland and Mario J. Molina
Series: The John H. Chafee Memorial Lecture on Science and the Environment
1st National Conference on Science, Policy and the Environment
Date: December 7, 2000
Location: National Academy of Sciences, Washington, DC


Rowland: Mrs. Chafee, the new Senator Chafee, Karim Ahmed and Richard Benedick. We are pleased to be here to honor one of the outstanding participants in our government, the kind of participant that makes scientists feel that it is worthwhile to work with them because you are dealing with someone who is trustworthy, honest and a fighter for what needs to be done.

Molina: Just like Sherry, I’m very honored to be able to present this lecture this evening. I remember when I first met Senator Chafee. I was quite a young scientist at that time. I was very impressed with him and remained impressed over the years, so it is a very big honor for me to be here tonight presenting this inaugural Chafee lecture.

Rowland: You’ve seen that there are two of us giving the lecture. So the only thing I want to say about it— what we plan to do is basically to describe alternate slides. So this will be a joint lecture. The only problem we have is the feeling that one has of coming to the national track championships in a relay race without having practiced the handoff. With that as background, we will start.


Rowland: This introductory slide is a copy of the notes that I took at a meeting in Ft. Lauderdale in January of 1972. The lecturer [Dr. Lester Machta] reported the results of some experiments by a man named James Lovelock, who had discovered a new component of the earth’s atmosphere that is, trichlorofluoromethane, one of the CFCs [chlorofluorocarbons]. He had measured it and found that it existed in both hemispheres. To a chemist, this raised a challenge. Here was a new compound that had not been in Earth’s atmosphere before, and the question was, “What will happen to it?”

Slide 1: Ft. Lauderdale, January 1972.


Fortunately, I had a research contract with the Atomic Energy Commission that gave me considerable freedom as to what research we would do. So, in 1973, I asked the question. I said we would like to study what would happen to this molecule in the atmosphere. I was delighted when an outstanding graduate student, a new post-doctoral from Berkeley, a Mexican scientist named Mario Molina, applied to my research group to study. I offered to him several possible research projects, one of which was the question of the chlorofluorocarbons.

Slide 2: FSR and MJM, with Dr. Luisa Molina in the foreground, University of California, Irvine laboratory, 1976.


Molina: So this was many years ago. To me at that time, it was really a fascinating question. I immediately picked that project, of course, the CFCs. What happens to them in the environment? It is an open-ended question. Why is it that we asked it? Well, we had the situation in human society, of industry using some chemicals and actually changing the chemical composition of the atmosphere. And that was happening on a global scale!

So I said to myself, this is the sort of situation that requires an answer. If human society is changing something important in the environment, human society should also find out whether there are any consequences. At the very least, I thought it was very bad manners just to release these chemicals without even knowing what would happen.

In fact, we learned later, Sherry and I, that at that time there were actually other groups of scientists who had asked a similar question because they were aware that these chemicals were in the environment, but they concluded that there was really nothing to worry about. These compounds are extremely stable. You can even breathe them and they were present at that time in the atmosphere at parts-per-trillion levels.

So why worry? Well, we reached a different conclusion.

Slide 3: CFC photolysis with high energy UV radiation in the stratosphere.


Rowland: The search started by asking what would happen to these molecules in the atmosphere. There are three common things that happen to most molecules. One is, if they form a colored gas, then it is decomposed by sunlight. The color tells you that it is intercepting the sunlight. Another thing that happens is for a molecule to dissolve in rainwater. That is the fate of hydrogen chloride, for instance, in the atmosphere. And the third is to be oxidized somewhere in the atmosphere.

But Mario went through this list very quickly and realized that chlorofluorocarbons were not soluble in water. They were transparent, and they didn’t react with any of the oxidizing agents in the atmosphere. This left as the only possibility something that we had both known all along—if they got up into the atmosphere far enough, and this would have to be in the middle of the stratosphere, then these compounds would run into ultraviolet light that is energetic enough to break them apart. And this would release the chlorine atom that is shown in the equation.

We had found out the answer to the question that was originally asked—the molecules would break up in the stratosphere. In fact, we calculated that for the upper molecule [CCl3F] it would take on the average about 50 years and for the bottom one [CCl2F2] an average of 100 years. And it would decompose with the release of chlorine atoms.

So, now there was another question. What would happen to the chlorine atoms?

Slide 4: ClOx catalytic chain.


Molina: This question had to do with what are the consequences of this chlorine atom release. It was not sufficient just to find out how these molecules were to be decomposed. What we realized at that time is that there is a process that involves very small amounts of certain species—free radicals like these chlorine atoms—the so-called catalytic process that could potentially affect the ozone layer.

We have learned from earlier work of our colleague Paul Crutzen and from Harold Johnston that we had this situation in the stratosphere. Very small amounts of certain compounds— levels measured in parts per billion or less—these small amounts of these catalysts could actually affect and control the larger amounts of ozone that are vital for our survival here on the earth’s surface. Ozone itself is present at the parts per million level.

So, here we had a situation where human activities could actually generate large amounts of these species in the parts per billion levels comparable to those natural levels of catalysts that control ozone. And as you see in these reactions, what happens is that there is a recycling—the chlorine and ClOx radicals are generated and regenerated.

We were actually not aware, when we first realized that these reactions could happen, that some colleagues of ours, Ralph Cicerone and Richard Stolarski, had several months earlier, in fact, come up with the same idea in terms of the catalytic effects of chlorine. But they had not associated chlorine with the CFCs. They were worried about chlorine from the space shuttle or chlorine from volcanoes.

Slide 5: Original ozone depletion paper by Molina and Rowland, Nature, June 28, 1974.


Rowland: This started out as an environmental scientific problem—that is, something where we were curious about what the situation was. But there was no thought at the start that this was an environmental problem in the sense that it was something that required people to be worried about it. However, as soon as we found out that there was a long chain reaction, and a global threat involved with it—then we realized that this was not something that you can start talking about in the newspaper and have it come out believable.

What we needed to do was publish first in the recognized scientific journals. The first paper that we had came out in June 1974 in Nature called: “The stratospheric sink for chlorofluoromethanes: chlorine atom-catalyzed destruction of ozone.” We actually thought that there would be a big reaction to this, but there was initially very little response. It wasn’t until a few months later that it began to be widely discussed in public.

Slide 6: New Times, March 7, 1975.


Molina: In hindsight, it was really not surprising that there was very little reaction initially. We were talking about this invisible gas rising in the atmosphere to affect an invisible layer that was protecting us from invisible rays. And with a title such as this one, it is not a surprise that not much happened.

But eventually the press indeed got interested in this issue and articles began to appear and, of course, it was in connection with spray cans, spray cans in American homes. There were on the average, I think, 30 or 40 spray cans per household at that time. And so it is the connection—just the idea that a lot of people pressing these little buttons inadvertently were actually polluting the planet—that eventually caught the attention of the press.

Slide 7: Chemical and Engineering News, FSR, September 23, 1974.


Rowland: At the time, the figure we had was 6 billion spray cans produced every year in the world, and in our home we had 15 of them. So my wife went around and threw them all out, and I thought, 15 down and 6 billion to go.

The first time the CFC-ozone story hit the public in a major way was at the American Chemical Society meeting in September of 1974 at which Mario and I presented papers on aspects of the chlorofluorocarbon problem, and then ventured into public policy. Both of us agreed we ought not to be releasing CFCs into the atmosphere. The value of the CFCs produced in 1974 was about 2 billion dollars, and the industrial efforts that depended upon that were estimated at 200 billion dollars by the industry. So they did not exactly leap to follow our suggestion that they quit putting it into the atmosphere.

Slide 8: Report of Rogers Subcommittee, House of Representatives: December 11-12, 1974.


Molina: Our initial preoccupation with the press was, of course, not just to seek publicity. We knew that was the way to get the attention of government, of the regulators.

So it was early on in the hearings by the Rogers Committee of the House of Representatives, in this case, that the government began to pay attention to this issue. That was indeed what happened. After the press responded, the public began to be concerned. And this initial idea that we had, we really had to be explicit about it. Is it enough for a scientist simply to publish a paper?

Isn’t it a responsibility of scientists, if you believe that you have found something that can affect the environment, isn’t it your responsibility to actually do something about it, enough so that action actually takes place? There was nobody else at that time that would actually fulfill that role. So that’s why Sherry and I, pretty consciously then, decided to take that additional step to make sure the government would actually pay attention.

Slide 9: Chlorine nitrate formation from chlorine oxide and nitrogen dioxide radicals.


Rowland: The government responded in two ways. One of them was to create a committee called the Inadvertent Modification of the Stratosphere [IMOS] Committee, with members from 14 agencies of the government. The second was to fund [jointly by NSF, NASA, NOAA, EPA and FAA ] a study to be done by the National Academy of Sciences to see whether there was validity to the questions about ozone depletion that had now come into the public press.

During 1975, the IMOS Committee made a report, which said that it looks as though this is quite a valid argument here, and, unless the National Academy of Sciences finds some reason that is different in the science, regulation will probably be required. The first problem that came up, that raised a question on the science, was raised by Mario and myself.

We had assumed that this molecule, chlorine nitrate, shown in the slide, would be in very low concentration in the atmosphere and wasn’t going to be important. Then we realized that this wasn’t actually true. We made some chlorine nitrate, which is not hard to make but is difficult to keep because it reacts very easily with water. We found out that its lifetime would be long enough that it could probably exist in the stratosphere.

The National Academy of Sciences had a report about ready to come out in April 1976 when we said that chlorine nitrate might be significant, and the Academy had to pull it back because the stratospheric calculations were thrown into disarray. This led to a delay of about six months in its coming out.

Slide 10: 1976 reports from two NAS committees.


Molina: I remember thinking at that time that it was sort of ironic because chlorine nitrate was a very esoteric molecule. I remember finding it in the old German scientific literature. But that is the way science works. The German scientists at first working with this chemical species really could not foresee at all that something like this, so esoteric, that actually human activities were going to be making many tons of this compound floating in the atmosphere.

So, the National Academy reports began to come out and these were, of course, very influential reports validating the science, or at least the idea that the CFCs would indeed reach the stratosphere and actually affect the ozone layer. And this CFC problem was something that certainly deserved further study, and quite possibly regulations by governments all over the world.

Slide 11: FSR and MJM near Logan, Utah, September 1976.


Rowland: The conclusions for these two volumes I can put in two words. The red volume said “yes,” and the blue volume said “but.” And most people said, “Can you shorten that?”

This is a picture taken in Utah just after a long-scheduled meeting that occurred just four days after those two volumes came out. Mario and I are shown in Logan, Utah. One of the consequences, one of the things that happened was this: when the National Academy of Sciences said: “yes, there is validity to this science,” then that took a certain weight off our backs. Because for the previous two years, it had been Mario, Ralph Cicerone, and I that would appear to do the testimony [to Congress and to state legislatures]. Now somebody else had said that if they are kooks they’re hiding it well, and the CFC-ozone problem became something that was very much of scientific interest.

Slide 12: FDA Commissioner Alexander M. Schmidt at press conference, October 1976.


Molina: The expectations we had had earlier eventually began to materialize. There were indeed regulations. You can read here that labels were required on sprays cans. In fact, it was the FDA commissioner, and also the EPA and the Consumer Product Safety Commission, that came up with this regulation.

As was to be expected, the first target of the regulation was spray cans. Spray cans, after all, were perceived not to be essential. And certainly less essential was the use of CFCs as propellants for these devices. There are other ways to deliver whatever you need to spray. In some sense, it was a victory because the US government began to take some action.

Slide 13: Substitutes for CFC aerosol sprays—roll-on, pump valve, hydrocarbon propellant.


Rowland: For instance, the substitutes that could be put in for CFCs as propellant gases include: having a pump spray where you used your finger to provide the pressure, and a roll-on, (over on the left) and then you have an “environmental formula” that substituted hydrocarbons as the propellant gases. There was a lot of argument about the banning of CFCs saying that they were very safe to use. I remember Mario saying that he had never heard of anyone swallowing a roll-on deodorant.

The overall problem, of course, was that in the United States at the time, about two-thirds of the CFCs were used as the propellant gases in aerosol sprays, and the US used about half of all the CFCs in the world. But the ban on CFCs as spray can propellants left out all the other uses: as refrigerants, in air-conditioning; using them as solvents, etc. The CFC problem was partially solved, but only for use as a propellant gas, and then only in the United States, Scandinavia and Canada, because the other countries didn’t follow suit on the aerosol propellant controls.

Slide 14: NAS Report 1982.


Molina: Here we have additional National Academy reports and updates. By that time the science began to be really validated. There were, of course, many remaining uncertainties. Indeed, the science process was moving ahead and, again, these National Academy reports really were extremely important.

Rowland: It is worth noting the title, “Causes and Effects of Stratospheric Ozone Reduction.” I point out that the word was “reduction.” What was happening was that the scientific community was saying, “All of these calculations have been done about the atmosphere but the only change that is included is the steady increase in the amount of chlorofluorocarbons.” But, in the real atmosphere, carbon dioxide is increasing too, and methane is increasing, too. They began to make more complex models. As soon as you began to have more complex models, then you get more spread on what was being predicted as eventual outcomes.

Slide 15: Dahlem Conference, Berlin 1982—Ralph Cicerone, Paul Crutzen, FSR and MJM (left to right).


Rowland: There were many scientific conferences, including this one in 1982 in Berlin called the Dahlem [a suburb of Berlin] Conference. This is Mario on the right, I’m sitting next to him, and then we have Paul Crutzen, the Dutch scientist working in Germany, with whom we shared the 1995 Nobel Prize, and on the left is Ralph Cicerone who had found the chlorine chain reaction with his partner, Rich Stolarski. The scientific community continued to work very hard on these problems, but the regulatory situation was not much changed.

Slide 16: NAS Report 1984.


Molina: We had an additional National Academy of Science report in 1984. Notice the change in wording here. The title doesn’t say “reduction” any more, but now uses “changes” here. Because of these complexities that Sherry was alluding to—the use of these more complicated models, incorporating more and more variables—there were calculations for some of these models raising the possibility of ozone levels increasing in the lower atmosphere as a consequence of the release of these compounds. Of course, this possibility only happened in some of these models and for some of the guesses about future inputs of other atmospheric gases.

Slide 17: Haruo Sato and FSR: HCl + ClONO2 data, as presented at a conference in Feldafing, Germany, June 1984.


Rowland: Back in 1975, when the National Academy was doing their initial study, one of the questions that was asked of me at a meeting in Snowmass, was, “Is there anything you can think of that would make ozone depletion worse?” I said, “Yes, if hydrogen chloride were to react with chlorine nitrate, then two chemical forms of chlorine, inactive forms that do not attack ozone, would be reacting to create an active form.” But there was no evidence for this reaction occurring.

Mario and I had tried an experiment with John Spencer in 1976. We mixed the two gaseous chemicals, hydrogen chloride and chlorine nitrate, and John ran them down to the infrared instrument at the end of the corridor, and found that everything had already reacted. But the experiment had lasted five minutes, and that was too slow. Certainly a reaction had occurred, but if the reaction had required all of five minutes, then it was not fast enough to be significant in the atmosphere. And at the time, we didn’t have equipment available to carry out the experiment more rapidly.

But now in 1984, when we started looking back at this possibility again, a Japanese post-doctoral associate, the late Haruo Sato, and I looked at hydrogen chloride and chlorine nitrate with a new piece of equipment [a Fourier-transform infrared spectrometer] which would allow separate measurements in one-second intervals. What Haruo found was that as soon as you added hydrogen chloride, the chlorine nitrate disappeared: gone in one second.

When we [with Don Wuebbles and Peter Connell of the Livermore Laboratory] put this finding into the model, it created a major loss in stratospheric ozone. If this reaction were happening in the atmosphere, and there was no evidence then that it was, then it would be serious because it could change the magnitude of eventual ozone loss quite substantially from 4 percent to 32 percent in the future.

Slide 18: October average ozone over Halley Bay, Antarctica 1957–1984.


Molina: By that time I was working independently of Sherry, but we had reached quite similar conclusions in our own laboratory experiments.

Something else happened around that time, a very, very important finding, but also very unexpected. Here we have measurements of ozone levels: the levels in Dobson units, the measure of the total amount of ozone that exists over the earth’s surface, but in this case, over Halley Bay in Antarctica. A scientist from the British Antarctic survey, Joe Farman, had been doing these measurements with his team since the International Geophysical Year in 1957. They realized early in the 1980s that something strange was happening with ozone. In the springtime, which is when the light begins to appear after the long Polar night, the levels were going down.

At first, his team did not believe these data. They thought that perhaps something was wrong with their instruments. There was another piece of information that made them skeptical. There was a NASA satellite measuring ozone already globally, including Antarctica, and they had heard nothing unusual about the results of those measurements. As you can see, eventually in 1985, Joe Farman and his team published their results, and actually suggested that this very big change in ozone over Antarctica was a consequence, or was connected to, the release of CFCs.


Slide 19: Antarctic ozone, October 3, 1979. (Produced by Dr. Rich McPeters, NASA)


Rowland: This observation by the British brought the NASA satellite people back to look at their data. Actually, they’ve gotten sort of a bad rap about it. What they did was program their data to reject, but notify, that some unusually low ozone values were being recorded. If you are getting unexpected low values when the instrument is working at the very limit of its detection, then you put that aside, saying, maybe there is something happening there that is real, but maybe it is an instrumental problem, and we’ll have to go back and look at it carefully.

Then, when Farman published his work, they realized that the other explanation, that the numbers were real, that the ozone levels were very low, was very likely true and they looked again. This is the way their instrument [the Total Ozone Mapping Spectrometer on the Nimbus 7 satellite] recorded the ozone levels. With hundreds of thousands of ozone measurements, the numbers have been converted to a color code, and all of the data for the southern hemisphere on one single day are shown in the Figure. You can also see the outlines of the continents: South America, Africa, Australia, and Antarctica. The first slide is for October 5, 1979, and the next is for October 5, 1983, four years later.

Slide 20: Antarctic ozone, October 3, 1983. (Produced by Dr. Rich McPeters, NASA)


All you have to do is look at the color change to know that something very striking had happened in the four years in between the two measurements. The amount of ozone had decreased over all of Antarctica, not just Halley Bay. These observations raised a red flag for the whole atmospheric scientific community. Something startling was going on over Antarctica that had been tipped off by Farman’s measurements and then confirmed by the NASA satellite that it wasn’t just something at Halley Bay, but it was something the size of Antarctica; that is, over an area somewhat larger than the entire geographic area of the United States.

Slide 21: Two ClO altitude measurements over Antarctic region (Robert deZafra and Philip Solomon).


Molina: Because of these observations, scientific expeditions were organized to go to Antarctica. In the first one, the most important one, a number of scientists went to Antarctica to study what was happening. The team of Bob deZafra and Phil Solomon [unrelated to the NOZE Expedition leader, Susan Solomon ] were able to detect emissions at ground level from the key ozone-depleting radical, ClO, with an instrument, which also provides information about the altitude where it occurred.

We have their results you see in this figure. We already knew that some very significant changes in the ozone were happening, but now the changes actually could be attributed to the presence of chlorine catalysts, the very same catalysts we had been worried about initially. Except we had, of course, not specifically suggested that these catalysts would be abundant over Antarctica.

However, at that time, portable computer power was not particularly large, and they had to take their data back to the laboratory in the US and refine the calculations to obtain the altitude distribution shown here. Consequently these data did not receive as much pubic attention as they deserved in hindsight.

Slide 22: June 10, 1986 statement by Senator John Chafee, as quoted in Newsweek.


Rowland: In June of 1986, Senator Chafee held hearings on the questions of stratospheric ozone depletion, greenhouse effect and global climate change. This is the statement that he made at those hearings, “There is a very real possibility that man, through ignorance or indifference or both, has irreversibly altered the ability of our atmosphere to support life.” That was a very strong statement, a very powerful statement, coming from an influential senator. It was quoted not only in Newsweek but also later in U.S. News and World Report. The statement indicated that there would be support for regulations because of the concerns that had reached the senatorial level and had made an impression there.

Molina: When this depletion of ozone over Antarctica first became known, it wasn’t really clear at that time what was the cause. I remember we worried about it. We thought it was very likely that the CFCs, the man-made compounds, were involved. Of course, it also could possibly have a natural cause. We also realized later we hadn’t really covered all the bases, natural or man-made, we missed something that the press actually uncovered.

Slide 23: Ozone loss from UFO Aliens? (December 16, 1986).


Rowland: The scientific objection to this article is that it was not peer reviewed.

On the other hand, there were some genuine arguments, and a number of explanations. The meteorologists leaned toward meteorology, and the chemists leaned toward chemistry, and it became a bit acrimonious. This quotation is taken from the Atlantic Monthly in May 1987.

“I think that the atmospheric scientists who announced that the ozone hole was caused by CFCs made a very serious mistake. I’m amazed at how much people have lost their scientific objectivity because of political and funding pressures.”

Slide 24: Weather Versus Chemicals—Atlantic Monthly, May 1987.


There was a certain amount of war about everything that was going on at that time and it was necessary to have further expeditions to explore it. After the ground expedition to Antarctica in 1986, led by then 30-year-old Susan Solomon, she subsequently led another very successful one in 1987, and there was an aircraft expedition [based in Punta Arenas, Chile ] that went as well in 1987.

Slide 25: Statement by Robert Orfeo, Chemical Manufacturers Association, at Senator John Chafee hearing, June 11, 1986.


Molina: The war that Sherry was alluding to also extended to industry. Here we have a statement to Senator Chafee’s hearings in 1986, from Bob Orfeo, a representative of the fluorocarbon manufacturers. You can read it, but his conclusion really was that there was no justification for additional regulation at that time. Indeed, at that time, the problem of ozone depletion over Antarctica was already surfacing, but there again, the fact that it was possible that it was an entirely natural phenomenon was certainly a fact that industry emphasized quite a bit.

Slide 26: Time Magazine 1993.


Rowland: Time magazine later published a description of the timeline that was involved in the stratospheric ozone situation. In 1985, the first international action was led by Ambassador Benedick. The Vienna Convention arranged that it would be possible to establish regulations, but it did not call for regulations at that time. But then in 1987, the Montreal Protocol was signed just in advance of the results that were obtained from the two 1987 expeditions carried out flying over Antarctica from South America and on the ground in Antarctica itself.

Slide 27: Mark Alan Stamaty Cartoon, Village Voice, 1987.


Molina: So, we have this argument among the scientists and this argument with industry, but also this argument with the government.

It turns out the Secretary of Interior Hodel, at that time, had some other ideas, mainly that it was perhaps possible to protect ourselves with sunglasses from the harm of ultraviolet radiation that would penetrate in larger amounts if ozone were depleted in ways that did not require any real changes in the industry. Actually, the press had a field day. They had elephants, lions, and zebras all wearing sunglasses.

Slide 28: ER-2 Flights over Antarctica, 1987.


Rowland: The key experiment that firmly convinced many people involved the simultaneously measurement of ozone loss and the presence of very high concentrations of chlorine oxide, the smoking gun of chlorine attack on ozone. But I think it is worth describing a little more about the circumstances of this experiment. What it required was sending an airplane with a single pilot flying from Punta Arenas about 3,000 miles down over Antarctica and back. It was an ER-2, a single engine airplane that evolved from the U-2 that Gary Powers was flying over the Soviet Union when he was shot down in 1960. There are no alternate airports. If its engine conks out, then that is pretty much the end of the story.

These experiments were critically dependent on an exceptionally courageous and dedicated group of pilots, who flew for hours toward the polar darkness, knowing that there was no hope for survival in case of engine failure. The experiments are all automatic; the pilot turns on the equipment at the proper time, the instruments record the data, and when the plane comes back the scientists examine what they have found. You don’t know until the plane has returned and people have had a chance to look at the data.

On the first flight everything seemed to have worked well. Mike Proffitt’s instrument to measure ozone worked throughout and showed less ozone over Antarctica than outside the Polar vortex. The key instrument for measuring chlorine oxide was working very well until the plane got over Antarctica where the temperature in the stratospheric polar vortex is very cold, about minus 85 degrees Centigrade, or minus 120 degrees Fahrenheit. In the intense cold, the instrument quit working. And then as the plane emerged from the Polar vortex, the temperature went up and the instrument came on again. Well, needless to say, the pilots who were risking their lives on every flight were not enthusiastic about further trips if the key chlorine oxide instrument was not going to work.

This instrument was newly created by Jim Anderson’s group from Harvard especially for these flights. The only in-flight testing that had been possible was on the “ferry” flights that took the ER-2 to South America. [Actually, “newly created” is an understatement. The instrument went from hardware sketches in January to unassembled parts in May, a marginally operating flight system in July, and included software development under the ER-2 wing in driving rain during stops in Panama and Puerto Montt, Chile.] The person who had programmed the instrument flight computer that both controlled the instrument and recorded data in flight was an exceptional undergraduate, Norton Allen, who joined the research group after graduation and had then accompanied the group to the flight base in Punta Arenas, Chile, a base used by the Chilean Air Force as a strategic outpost.

After the first flight, Allen wrote a new watchdog routine overnight for the flight computer that surveyed all communications between the instrument and the computer. The next time they flew, the same thing happened. Everything worked fine until they got into the Polar vortex, and then the chlorine oxide instrument quit again. After the ER-2 returned, and the data had been analyzed, the computer program indicated where the problem lay. [As the temperature dropped through -85 degrees Centigrade, a main status line buried in the spinal column of the instrument opened up, a result of metallic contraction at the site of a microscopic defect in the solder joint in the center pin of a connector. The evidence was unmistakably captured and stored in the instrument’s memory.]

Prior to the third flight, the connector was isolated, tested to failure at low temperature with liquid nitrogen and then replaced. On the third flight the instrument then performed flawlessly, and the results shown on the left side of the figure for the 23rd of August came from that flight. What they found was that there was a lot of chlorine oxide over Antarctica, but little or no change in the ozone. However, the change in ozone had always been observed during September and into October. This was late August, the end of the southern winter, and the sun had just barely begun to penetrate to the latitudes over Antarctica.

On September 16, as the flights continued, they again found lots of chlorine oxide, but now two-thirds of the ozone had gone away during the intervening three weeks. This evidence from the whole series of flights persuaded almost everyone in the scientific community that the cause of the loss of ozone over Antarctica could be attributed to chlorine, and that the presence of all this chlorine could be attributed to compounds put into the atmosphere by mankind.

Slide 29: Chlorine chemistry with clouds over Antarctica.


Molina: By that time, we were also carrying out additional laboratory experiments, and we really understood well why it is specifically over Antarctica that we would have this huge effect on ozone. It was related to the very low temperatures that exist at those latitudes. There is very little water in the stratosphere. It is very dry. But if it gets sufficiently cold, that even that little bit of water actually condenses, and you get clouds, ice clouds. And these ice clouds provide those surfaces that promote the reactions that Sherry alluded to before. You get chlorine activation making this catalysis very efficient. You also remove the nitrogen oxides that interfere with the chlorine chain by reacting with chlorine oxide.

We were able to carry out some experiments in my laboratory in collaboration with my wife, Luisa, in which we demonstrated the presence of another compound that we had not considered before, namely chlorine peroxide, the combination of two ClO radicals to form Cl2O2. And we came up then, with additional chemical explanations that were specific to Antarctica, a region where ozone has not been made, but it could be destroyed extremely efficiently by these reactions. In fact, the time scale of a few weeks coincided very, very well with the observations of Jim Anderson.

Slide 30: NASA—WMO Ozone Trends Panel, 1988.


Rowland: Starting in 1985 there had begun to be international assessments and measurements to replace the series of National Academy studies. In late 1986, Bob Watson of NASA put together a group called the Ozone Trends Panel, which involved initially about 25 scientists in the main panel, and, perhaps, 125 after sub-panels were formed. In 1988, the Ozone Trends Panel made its report as shown here. This volume did not come out until later, but the initial press conference was held on March 15, 1988 and essentially said that the cause of the loss of ozone over Antarctica was chlorine-containing compounds, especially the chlorofluorocarbons.

Slide 31: Decline in ozone over northern hemisphere, as reported in the New York Times, March 16, 1988.


Molina: This graph was prepared by the New York Times from the Ozone Trends Panel material, indicating very clearly that observations had shown ozone being depleted and thinned, not just over Antarctica, but actually also in the northern hemisphere winter where it also gets very cold. The depletion doesn’t reach the same extent found in Antarctica, but the very important consequence of these observations is the fact that the phenomenon of ozone loss was not really confined to the deep southern latitudes.

Slide 32: NASA—WMO 1988 report of winter-time loss of ozone in northern hemisphere (derived from Ozone Data of the World, ODW).


Rowland: The starting point for this determination was a series of ozone measurements that had been made for many years. In fact, people had been asking since 1974, “Is there any evidence for ozone losses over the United States or Europe?” And, for 11 years, statisticians had been doing very elaborate calculations and had always concluded that no evidence for any ozone loss had been detected. This new effort started with a graduate student of mine named Neil Harris, an Englishman, who now is back in England. He first did some calculations for the long record since 1931 of ozone measurements at the permanent station in Arosa, Switzerland.

He divided the record into 1931-1969 and 1970-1986 and compared the before/after averages for each calendar month, and found that there had been less ozone over Arosa in the winter months after 1970 than before. Then, as part of a subgroup of the Ozone Trends Panel, we extended this to all of the ozone-measuring stations with records for at least 22 years, that is, for the length of two solar sunspot cycles [because it was known that ozone levels varied a little with sunspot activity]. These calculations were simply the average over the winter months for one 11-year period, and then for another period of 11 years, subtracting one from the other. Every station in the northern hemisphere north of 30 degrees N latitude had shown less ozone in the second 11-year period than in the first.

The statisticians had missed this because they had assumed that if there were any ozone loss that it would be uniform all through the year. And it had been known for many years that the summer months had much less natural variation in ozone, so, if the summer had much less natural variation, then obviously, you should look at the times where a change would most easily be detectable. What the Ozone Trends Panel showed was that there was clearly a wintertime loss, and no significant evidence at that time for a loss during the summer. These calculations were the first evidence that ozone had been lost over heavily populated latitudes of the northern hemisphere.

Slide 33: John Gille: We’ve Found the Corpse.


Molina: The scientific evidence was really accumulating by that time. Here we have a statement from by a colleague of ours at the Ozone Trends Panel press conference on March 15, 1988, “We’ve found more than the smoking gun. We’ve found the corpse.” So, no surprise, ten days later after all of these findings really became evident, the Dupont Company, the largest manufacturer of CFCs, changed their mind and decided they would no longer manufacture these compounds.

The scientific evidence was extremely clear. I believe that was a very important turning point for the chemical industry. As many of you might know, the chemical industry, the Dupont Company among others, has a very, very different attitude towards environmental problems nowadays. In fact, they pride themselves now on being environmentalists.

Slide 34: Prime Minister Thatcher attacks Prince Charles over protection of stratospheric ozone.


Rowland: The ozone question also was international. Prime Minister Margaret Thatcher called a meeting on “Saving the Ozone” in London early in 1989. On Tuesday, Prince Charles had spoken to the delegates, saying that the British government was acting too late, and the next day the Prime Minister said, no, we are doing it at just the right time. Around that time, Bob Watson, who had chaired the Ozone Trends Panel , born in England but now a naturalized US citizen, was present at a meeting in England at which Prime Minister Thatcher was listening to the arguments, and found out that a very long time scale, stretching to a century or more, was involved. She then said, “Time is not on our side. We must act now.” I think the UK policy on ozone depletion changed on that night.

Slide 35: Projected tropospheric chlorine concentration for various regulatory options.


Molina: This graph illustrates these time scales. We have here the levels of chlorine expected to be present in the atmosphere, including projections from the year 2000 on, depending on what society does. Without any regulations we would expect a continuing, very significant increase, the green line in the figure. The original version of the Montreal Protocol was relatively weak. It only called for a partial limitation [50% reduction] to the production of CFCs, but when the scientific evidence began to accumulate, the Protocol was strengthened, first in London in 1990, and then in Copenhagen.

The international agreement reached there in 1992 called for a complete ban in the production of CFCs by industrialized countries by the end of 1995, with developing countries having some allowance to continue limited production of these compounds. You can see the very long time scale over most of the 21st century to return to the pre-1975 chlorine concentrations.

As Sherry showed initially, these compounds, the CFCs, remain in the environment for a very long time. They must be essentially inert for them to survive unchanged in the lower atmosphere long enough to get into the stratosphere. So we’re talking about recovery times that are measured in many decades, even if the production stops, so the prediction is that the ozone hole over Antarctica will not disappear until the middle of this century.

Slide 36: Recalibration of R-B meters explains an apparent 11 percent drop in ozone.


Rowland: When you ask, what are the possible consequences of the loss of ozone, the prime consequence for humans is more ultraviolet radiation penetrating to the surface of the earth, especially ultraviolet-B radiation, which are the wavelengths that can be absorbed in the skin and damage DNA there. This ultraviolet-B radiation is the cause of human skin cancer. So increasing exposure to UV-B was a major concern.

A study came out in Science magazine in 1988 that had followed for a number of years, several ultraviolet B measurement instruments that really weren’t designed and calibrated to handle data year after year after year. But that study was published and showed no change in UV-B radiation over the U.S. Then another research group later went back and looked at the data, and concluded that these instruments were not good for measuring absolute trends over a decade on a continental scale.

There were serious changes in network management and flaws in calibration. For instance, this group did some statistical analysis and concluded that with one of the instruments a significant change had happened during a particular short interval of time. It looked as though there was a sudden UV-B change by 11 percent. They then visited the actual location, and found the cause of this 11 percent drop in UV that couldn’t be accounted for. This is an instrument that sits out in the open collecting UV-B from all angles, and an antenna had been built nearby that partially shielded it. This change hadn’t been recorded, and the calibration change taken into account.

This kind of investigation took care of the concern about why it was that we weren’t getting more UV-B radiation if the amount of ozone was decreasing. We were receiving more UV-B. It was just that this was a flawed measurement.

Slide 37: UV-B measurements at Ushuaia, Argentina, Spring 1991.


Molina: One of the questions that was commonly asked, and is still commonly asked, is, “If this ozone hole and the increasing ultraviolet radiation occur over Antarctica, perhaps we shouldn’t worry about it that much.” I can mention a couple of reasons why that is not the case. It is indeed the case that not many people live in the Antarctic continent.

But first of all, to me, it is symbolic that the place where these chemicals are having the largest effect is as far as possible from the sources, way out in the southern hemisphere. So we are truly having a global problem. Second, if any of you have been to Antarctica, what is really impressive in the oceans surrounding the continent is that they are teeming with life. So even though there are not many humans, it really is a very important portion of our planet in terms of ecosystems.

Changes there really have repercussions for life anywhere in our planet. If we affect their ecosystem by increasing the levels of ultraviolet radiation, that is something to worry about, and indeed there are experiments to show that this is the case.

But in fact we have to worry about people themselves because occasionally this ozone hole and the Polar vortex reach over the southern tip of South America, and over cities such as Ushuaia in Argentina where instrumental measurements show the increased levels of ultraviolet-B radiation. When you contrast the red with the green line, at wavelengths around 295 mm, the intensity increases by two orders of magnitude. The corresponding change in ozone by almost a factor of two from 189 to 355 Dobson units over just a few days is very, very large. But the increase in intensity in the ultraviolet-B wavelengths is very much greater. It is a very non-linear phenomenon.

Slide 38: Maximum irradiances at three locations, 1998.


Rowland: The current measurements now show just how much change has taken place. The data in this slide were taken with three comparable instruments, one in San Diego, California, one in Palmer (which is in the Antarctic Peninsula) and one at the South Pole. For each instrument, the measured UV-B wavelength intensities have been weighted by the erythemal index which is how your skin responds in terms of sunburn. For each location, the data represent the most intense UV-B day experienced during that entire calendar year.

What the figure shows is that on the most intense day in the Antarctic Peninsula, the ultraviolet radiation was 25% greater than on the most intense day in southern California. And the South Pole was not too far behind, because the South Pole sits in the Antarctic ozone hole for many weeks. So what we have done is produce a situation in which UV-B radiation can be on occasion very high in Antarctica, stretching out into places like Ushuaia and Punta Arenas in Chile, reaching levels normally associated only with the tropics.

Slide 39: Decline in atmosphere CFC-11, 1989 to 1998.


Molina: We could ask next, “Is the Montreal Protocol working?” The answer is, “Yes,” and this is an extremely important precedent, this international agreement. This figure shows the measured levels of one of the CFCs, CFC-11, as a function of the calendar year—measurements made, in this case, by NOAA (the National Oceanic and Atmospheric Administration). The data show the concentration increases in the earlier years as expected from the steady industrial production.

They then level off in anticipation, of course, of the prohibitions of the Montreal Protocol against further CFC release. In the red curve at the bottom of the figure, you can see the growth rate begin decreasing even before the Protocol became active, and then actually going negative, the rate of loss from the atmosphere now exceeds the slight further losses from residual leaking in older installations.

We can actually measure, then, in the atmosphere itself, a leveling off of these chlorine-containing compounds. In fact, there is another compound, methylchloroform that is also regulated by the Montreal Protocol and is much shorter-lived than the CFCs. Its concentrations have already decreased very substantially from the peak levels measured in the early 1990s. The Montreal Protocol really has been a very successful international agreement.

Slide 40: FSR and MJM at Nobel Press Conference, Stockholm, December 1995.


Rowland: We have skipped ahead here to December 1995. Mario and I together with Paul Crutzen spent a week in Stockholm. This photograph was taken during the press conference there. It is a signal to us that time is passing, not only in years, but here tonight. We want to give you a quick run down on what is happening now because there has been some information in the press about the Antarctic ozone hole this year. We’ll have a few quick slides here and then we’ll be ready to close up.

Slide 41: Balloon-measured ozone over South Pole, July 28 and September 29, 1999.


Molina: This figure shows the ozone profiles and again this is over Antarctica. The green line is temperature, and you can see the temperature being very low around 20 kilometers above the earth’s surface. The blue line is a normal-looking ozone profile in mid-winter [August 28, 1999] that is where we expect the ozone to be, but after several weeks [September 29, 1999] of this ozone being exposed to chlorine, you can see that over a certain altitude range, actually more than 99% of the ozone disappears.

That is why it is labeled “ozone hole.” Ozone is essentially gone there, and that again points out the importance of low temperatures, which is where these clouds form, and, it is, indeed, a very spectacular phenomenon. At lower altitudes where there are no clouds, the ozone is not affected.

Slide 42: Ozone trends versus altitude over Northern Hemisphere showing ozone loss at 40 kilometers.


Rowland: The measurements in the atmosphere in mid-latitudes show not only an ozone loss down around 20 kilometers, but there is also an ozone loss up around 40 kilometers which is the location for ozone loss in our prediction that we made originally back in 1974. That is where the main gas phase reactions occur both in the models and as observed in the atmosphere.

Slide 43: Low ozone levels over northwestern Europe, November 30, 1999 (No data obtainable from dark polar regions).


Molina: This figure shows measurement of substantial loss of total ozone in the northern hemisphere measured by satellite measurements, indicating that, as stated before, that ozone loss is not constrained to the southern hemisphere. Indeed, it is worrisome that significant depletions will occur in the future and the losses depend on whether you have a cold year or a warm year in the stratosphere.

Slide 44: Southern Hemisphere ozone, October 3, 2000.


Rowland: This figure shows that this year’s ozone hole which you have probably heard announced as having the greatest area formed very quickly. This is the ozone hole during the first week in October.

Slide 45: Ozone Hole Daily Minima 2000.


Rowland: If one looks at how deep the ozone hole was this year, then what you see in this figure is the minimum value recorded on successive days throughout the period from August to December. The gray region is the day-to-day range of minimum values recorded during the years from 1979 through 1992; the central line is the daily mean value for all of these minima. The red line shows the daily minima for 1999, and you can see that in September it was deeper than ever early, but when it reached the deepest part when the minima were about the same as in previous years.

In the end, the hole wasn’t any deeper in 1999, but it got there sooner. The same thing happened in 2000, as shown by the little crosses in the figure. In September, the minima in total ozone were even lower than in 1999, but this time the recovery was much quicker with much higher minimum ozone values in December 2000 than one year earlier.

Slide 46: Southern Polar O3, October 20, 1998.


Molina: On this day, the elongated, off-center ozone hole reaches to the southern tip of South America. The people living in Punta Arenas, Chile are really worried when ozone levels drop down to something like 170 Dobson units. The local government really wants everybody to wear sunglasses on days with such low amounts of ozone overhead, but they cannot afford them. Perhaps it is the responsibility of developed nations to help these countries cope with the new situation; after all, almost all of the CFCs came from the northern hemisphere.

Slide 47: Southern Polar O3, December 4, 2000.


Rowland: The situation actually is that you can now see yesterday’s ozone values [and all of the other satellite data back to November 1979] directly on the Internet. This figure shows the data for Monday of this week. The ozone hole is already gone for the year 2000.

Slide 48: Southern Polar O3, December 4, 1999.


Rowland: This is the way it looked one year ago, December 4, 1999.

Slide 49: Daily estimate of O3 over Antarctic ozone hole area, 2000.


Rowland: The next figure shows the area of the ozone hole, defined as the region with ozone values below 220 Dobson units. The symbols here for daily area measurements are the same as those used earlier for daily minima: gray area for the daily area values from 1979 through 1992, central line for the median values over that time period, red line for 1999, and small crosses for the year 2000.

You can see that the low ozone values did take up a very big area—30 million square kilometers, or 12 million square miles (the area of the United States is 3 million)—but then plunged sharply this year and the hole disappeared in mid-November.

This is meteorology that is involved here. In 1999, the region of low ozone over Antarctica lasted into early December; this year it disappeared two or three weeks earlier. The behavior of the Antarctic ozone hole was different in 1999 and again in 2000, but the magnitude of the loss at its deepest and the area of loss at the time of the lowest minima were not very much different in terms of the overall change.

Slide 50: CO2 Measurements over Mauna Loa, Hawaii 1958–1995.


Molina: We will move on now. These are measurements of atmospheric carbon dioxide concentrations since 1958, Dave Keeling’s measurements, made near the peak of the volcano Mauna Loa in Hawaii. These are historic measurements that show another situation similar to CFCs; namely, in steady increases over time in the concentration of carbon dioxide in Earth’s atmosphere, clearly attributable to human activities. This increase, of course, has consequences for the climate. There are indeed some scientific uncertainties about how our climate system will respond, but the measurements are there, and changes are under way.

Slide 51: Senator George Mitchell's statement at Senator John Chafee hearing, June 1986.


Rowland: This is the last slide and the next to last comment. “What is missing in the federal effort is action. The problem of global warming brings another round of scientists before us decrying the folly of waiting until it is too late to prevent irreversible damage.” That could have been a statement made, I think, anytime in the last several years, but it was actually made by one of the junior members of the Senate at those hearings Senator Chafee held in June 1986, more than 14 years ago.

The statement was made by George Mitchell, who was then Senator from Maine, not yet at that time the Majority Leader of the U.S. Senate. This was a time when the Senator from Maine, the Senator from Montana, Max Baucus, Senator Chafee from Rhode Island and the Senator from Vermont, Bob Stafford—Senators from both sides of the aisle—were working together on the problems of the environment.

Molina: I want to finish with a quote from Senator John Chafee, also from those hearings in 1986. Keep in mind this is almost 15 years ago, and I quote.

If we were masters of the world, we would do something about carbon dioxide. But we are not. We can’t tell the Soviets what to do or the Chinese. But it seems to me that is not an excuse for no action at all on the part of the United States. That is why I find fault with the view that if we take action, the Europeans may not. But that is not a call to inaction, it seems to me. We ought to do what we can and set an example.

Rowland & Molina: Thank you.


This is a chapter from CFC-Ozone Puzzle: Environmental Science in the Global Arena (Lecture).
Previous: Opening Remarks  |  Table of Contents  |  Next: Questions and Answers




Environment, N. (2012). CFC-Ozone Puzzle: Lecture. Retrieved from


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