In 1971, the U.S. Congress cut off funding for the SST prototypes, when it was argued that the pollution created by SSTs could reduce the stratospheric ozone content and increase the rate of skin cancer [1].
At about the same time it was proposed that human activities were already affecting the stratosphere in ways that had not been recognized before. Coupled to the rapid growth of the world's population, life-sustaining activities on the earth's surface, like cattle raising and rice growing, would raise the methane content in the atmosphere. Being long-lived, methane would percolate into the stratosphere and undergo chemical reactions there that affected the ozone layer on a scale similar to that of a fleet of SSTs [2] .
By 1974, Ralph Cicerone and Richard Stolarski, investigating the effects of rocket exhausts in the upper atmosphere, postulated that chlorine could destroy ozone by catalytic action [3]. At about the same time, Sherwood Rowland and Mario Molina discovered a much more effective method for ozone destruction, based on the long lifetime of CFCs [4]. Their work has provided the impetus for the current decision to phase out CFC production.
In the meantime, interest in the environmental effects of SSTs waned. Even though a small fleet of Concorde aircraft and numerous military planes were flying in the stratosphere, world attention focused on CFCs and other manmade halocarbons.
From CFCs to Skin Cancers
The links between the release of CFCs into the atmosphere and the most serious concern, an increase in the incidence of skin cancer, particularly malignant melanoma, are as follows:
Do CFCs Determine Stratospheric Chlorine?
An extreme view still prevails that CFCs, being heavy molecules, cannot rise in the atmosphere. But direct observations find CFCs to be well-mixed in the troposphere [5]. More important, the content of fluorine compounds in the stratosphere has been found to increase rapidly: Two independent sets of observations from mountaintops, both involving data on absorption of solar infrared by hydrofluoric acid (HF), conclude that the observed increase must be due to CFCs [6,7].
While the situation with respect to fluorine is quite clear, there has been a dispute about whether CFCs contribute appreciably to stratospheric chlorine. One group argues that natural sources would overwhelm any contribution from CFCs and other halocarbons, since volcanos and the ocean discharge more than 10,000 times as much chlorine into the atmosphere, mainly in the form of HCl and salt spray [8,9]. The opposing group, however, points out that almost all of this chlorine dissolves in water droplets and is quickly rained out of the atmosphere, while CFCs, being less soluble, can reach the stratosphere [10,11] .
The argument has become quasi-theological, with each side basing its arguments on faith in their own imperfect calculations. In principle, actual observations of stratospheric chlorine should settle the issue; but these have been conflicting as well. Early airplane observations by NCAR scientists Mankin and Coffey [12] , showed what many interpreted--and some still do [13]-- as an increasing trend of HCl between 1979 and 1982, caused by CFCs. But this time span is too short to establish a trend; nor are the data of high enough quality [14]. In addition, Mankin and Coffey themselves ascribe the observed increase in 1982 as due to the volcano El Chichon [15].
The matter seemed settled when Belgian scientist R. Zander published his results in 1987, showing no increasing trend in HCl [16] (while his HF data showed a 10 percent per year increase). The ready explanation would seem to be that natural sources of chlorine predominate in the stratosphere. But the situation reversed in 1991 when NASA researcher Curtis Rinsland and colleagues, using a similar methodology but a different mountain- top, did find an increasing trend for HCl of 5 percent per year; they therefore conclude that both natural and man-made sources contribute [7].
How Much Ozone Does Chlorine Destroy?
While chlorine can destroy ozone in the laboratory, the situation is much more complicated in the stratosphere (where most of the ozone is concentrated). First of all, most of the chlorine is bound up as hydrochloric acid (HCl) and must be put into the form of active chlorine, Cl or ClO, before it can attack ozone. Then too, other molecules, principally NOx and HOx, interfere with the simple catalytic chlorine process [17]. And finally, it has now become evident that heterogeneous reactions (on the surface of particles or aerosols) are more important than gas-phase reactions, at least in the lower stratosphere where ozone concen- tration peaks [18].
The theory of ozone depletion has been evolving steadily since 1971 when first water vapor, and then nitrogen oxides from SST exhausts were pinpointed as the proximate cause. Initial depletion estimates ranged as high as a 70 percent, but soon dropped to about 10 percent. By 1976, however, the theory reversed sign and predicted that NOX would cause an increase in ozone. By 1978, the theory again reversed sign as additional chemical reactions were found to be of importance (See figure). While CFCs have always been predicted to cause ozone depletion, the degree of depletion has been dropping year by year--until the 1985 discovery of the (unpredicted) Antarctic ozone hole, when theorists realized that they had to include heterogeneous reactions and stopped making predictions [19].
At present, the theory is in a rather uncertain state. To cite just two examples:
The main reason one is interested in the level of atmospheric ozone is its absorption of solar ultraviolet radiation in the UV-B region (280 to 320 nanometers). While the Antarctic ozone hole-- the temporary thinning of a portion of the ozone layer during the Antarctic spring (October)--is a genuine phenomenon, one cannot be sure as yet about the claim of a worldwide depletion of ozone. There is doubt about the quality of the data; Belgian researchers Dirk De Muer and H. De Backer have pointed out that the air pollutant sulfur dioxide can interfere with the ozone measure- ments. Since both gases absorb UV-B radiation in a similar way, changes in SO2 can be misread as changes in ozone. Therefore, as the authors point out, the decreasing trend of SO2 since the 1960s, as a result of pollution control in the United States and Western Europe, simulates a "fictitious" ozone trend [21].
Also, the "noisiness" of the record makes it difficult to establish the existence of any small, long-term trend. First, it is necessary to eliminate the large natural variations of global ozone, especially an 11-year cycle that correlates strongly with the number of sunspots. Unfortunately, getting rid of the solar- cycle effect is as yet an impossible task, given the shortness of the ozone record--some 35 years--and the virtual absence of any data on long-term variations of the solar far-UV radiation that produces ozone in the upper atmosphere. The claim of global ozone depletion also fails to meet a crucial test: its magnitude is found to depend on the choice of starting date and stopping date. If the natural variations had really been eliminated, then the trend should not be sensitive to the choice of time interval selected for the analysis [22].
There has, of course, been a determined search for a secular increase in UV-B to match the claimed global depletion of ozone. No such trends had been observed until publication in November 1993 of a startling increase in UV-B between 1989 and 1993 over Toronto, Canada [23]. University of California Prof. F. Sherwood Rowland, coauthor of the CFC/ozone depletion theory, immediately endorsed the Canadian study in the Los Angeles Times, saying, "Now, at last, we have good data to point to." But closer examination revealed that this "smoking gun" was smoke and little else. The authors had confused a short-lived increase at the end of their record--likely due to a severe weather disturbance, the "storm of the century" that hit the northeast in March 1993--with a long-term UV trend [24].
Fears about small changes in UV-B intensity may be ill- founded, in any case. To put matters into perspective: UV-B increases naturally by about 5000 percent between pole and equator, largely because of the change in the average angle of the sun [25]. Therefore, a 10 percent increase at mid-latitudes translates into moving just 60 miles to the south--hardly a source for concern.
The Skin Cancer Scare
Finally, much of the driving force behind the policy to phase out CFCs has been the fear of an epidemic of skin cancer, particu- larly malignant melanoma. But unlike basal and squamous cell skin cancers, which are easily cured growths caused by long-term exposure to UV-B, melanoma rates do not show the characteristic increase toward lower latitudes, where UV-B is strongest [26] . (European data on melanoma actually show an increase toward higher latitudes [27].) And indeed, recent laborato- ry experiments have now established that melanoma rates are not likely to depend on exposure to solar UV-B radiation.
In a unique study, published in the July 1993 Proceedings of the National Academy of Sciences, Dr. Richard B. Setlow and colleagues at the Brookhaven National Laboratory, Long Island, New York, tackled the problem of the cause of malignant melanoma. They conducted their experiment on specially bred hybrid fish that are extremely sensitive to melanoma induction. Groups of such fish were exposed in the laboratory to radiation in narrow wavelength bands in the UV-B and UV-A region. In this way, the researchers measured the "action spectrum" (sensitivity of melanoma induction as a function of wavelength). They concluded that in natural sunlight, 90-95 percent of melanoma induction may be caused by wavelengths greater than 320 nanometers--the UV-A and visible regions of the solar spectrum [28]. But UV-A is not absorbed by ozone, and therefore, melanoma rates would not be affected by changes in the ozone layer.
Ozone Policy and the Future of the SST
If Setlow's results are confirmed, then much of the fear associated with ozone depletion disappears, and along with it much of the rationale for the phaseout of the chemicals encompassed by the Montreal Protocol and other international agreements. This includes efforts to eliminate the production of CFC refrigerants, halon fire extinguishers, methyl bromide fumigants, carbon tetrachloride solvents, and other important chemicals.
But policy is seldom driven by rational science. We should note, for example, that the Montreal Protocol was signed in November 1987, and that production limits on CFCs were tightened in the period 1987 to 1991, when published scientific data indicated that CFCs were not an important source of stratospheric chlorine. Nevertheless, the fear was propagated in a Washington, DC, March 1988 press conference that the Antarctic ozone hole would expand to cover the globe [29].
Another example: The precipitous U.S. government decision in 1992 to accelerate the CFC phaseout, advancing it from the year 2000 to 1995, followed yet another NASA press conference, this one propagating the fear of an Arctic ozone hole covering populated regions of North America--the infamous "hole over Kennebunkport" [30].
These hasty efforts to withdraw a whole range of widely used chemicals from the economy are already causing serious adjustment problems. Substitutes will eventually be found, though much testing will be necessary to establish their safety and effective- ness. In addition to inevitable costs caused by dislocations and uncertainties, there is the huge direct cost, estimated at over $130 billion in the U.S. alone, of replacing or retrofitting capital equipment that cannot accept the CFC alternatives [31] . With the health and environmental consequences of ozone depletion still unproven and long-term global depletion itself in question, the American public may well balk at the imposition of a $1300 burden on the average household.
The difficulty with the present directions of policy is that it cannot easily accommodate changes in the underlying science. Setlow's melanoma research is a prime example of this. Thanks to the Montreal Protocol, an extraordinarily expensive effort is now underway to prevent a possible future depletion of ozone and a rather minor increase in UV-B radiation. The best policy may be simply to educate the public to the dangers of excessive exposure to the sun.
It will be interesting to see whether the new scientific results and a closer scrutiny of the old ones will force a re- examination of existing policies. Chances are they won't. Once policies are set--and especially if they are sanctified by international treaties--they do not change, even if the original conditions no longer hold.
It is difficult to tell how all this will affect a future supersonic transport program. On the one hand, the fears of an increase in melanoma skin cancer are clearly irrational and contrary to the scientific evidence. Further, it is not clear whether a fleet of SSTs will increase or offset any ozone deple- tion. On the other hand, activists will certainly construe a go- ahead for the SST as a breach in the wall, presaging a relaxation of the ban on CFCs and other halocarbons.
My general conclusion, based on a quarter century of involve- ment in the ozone controversy, is that policies should not be applied too hastily and might well benefit from a firmer science base. Furthermore, policies should be flexibly constructed so as to accommodate to a science base that inevitably undergoes change as new discoveries are made [32]. While lip service is often paid to these principles, in practice they are outweighed by the precautionary principle ("We must act now, even if we are not sure that this policy will do us any good") and by the "public choice" paradigm ("Policies self-reinforce and entrench themselves as they build up constituencies"). The unfortunate outcome may be an unconscionable waste of resources, a consequent loss of public trust, and a real setback to the environmental effort.