Pamela Zurer's article (C&EN May 24) refers to me as a "persistent scoffer at the seriousness of ozone depletion," but nowhere does she explain my position. Here is some of the missing information.

1. The validation of the (otherwise quite plausible) theory that stratospheric ozone is being depleted globally by CFCs requires three separate stages of proof--with the burden on those who advocate hasty actions that will cost consumers tens of billions of dollars in the coming decade.
   1. First of all, are the data sound? The Ozone Trends Panel (OTP) used data from the ground-based network of Dobson ozone spectrometers to assert--in a March 1988 press conference--that northern-hemisphere ozone was being depleted by CFCs. In the April 20, 1992 issue of the Journal of Geophysical Research, however, D. DeMuer and H. DeBacker, label the depletion as "fictitious." Their claim--not refuted, so far--is that the ozone data are contaminated by UV absorption from tropospheric sulfur dioxide.

   2. Even if the data were sound, there is the problem of extracting a small trend from a short ozone record that has a natural variability up to some hundreds of times greater. Even with much averaging, one is left with a natural variation of a few percent, related to the 11-year sunspot cycle--while the claimed trend is only about 2 percent per decade. The correlation between sunspot number and ozone content is by no means perfect; besides, each sunspot cycle is unique and each ozone cycle is different. Any claim that a statistical analysis of a time-limited data set can remove such natural variations must be viewed with great skepticism. The OTP quotes a trend from 1969 (a sunspot maximum) to 1986 (a sunspot minimum!), but has the solar-cycle variation really been eliminated? Apparently not: the depletion trend is found to depend on the time interval selected for analysis [1].

   3. Finally, even if a long-term trend can be established, its cause cannot be assigned unless the mechanisms are understood and unless "fingerprints" support such a mechanism. For example, in view of known long-term trends in sunspot numbers, one would expect corresponding ozone variations of natural origin.

2. Granted that stratospheric ozone can be depleted by CFCs or by natural factors, are such depletions "serious"? It is generally agreed that the intensity of solar UV-B increases by 5000 percent in going from the poles to the equator, primarily because of the change in average zenith angle of the sun. Thus, a 10 percent increase in UV-B, as a result of a 5 percent depletion of ozone, would also be experienced just by moving 60 miles towards the equator (at mid-latitudes). While non-melanoma skin cancers increase towards lower latitudes (at least partly because of longer exposures and other changes in lifestyle), I am not aware of increases in cataracts, damaged immune systems, ecological problems, etc. at lower latitudes (or higher altitudes).

3. Ms. Zurer also claims that I have "retreated" from my assertion in 1989 that "volcanoes...contribute substantially to stratospheric chlorine and thus dilute the effects of CFCs." Any such statement about the relative effects of natural and human sources of chlorine must, of course, be based on data. Two studies by R. Zander et al, published in the prestigious Journal of Atmospheric Chemistry in 1987, claimed that the total columns of HCl and HF increased from 1977 to 1986 at rates of (0.75 + 0.2)% and (8.5 + 1)% per year, respectively. Since HF is ascribed entirely to CFCs, the much lower rate for HCl led many scientists--including me--to conclude that constant (i.e., natural) sources of stratospheric chlorine overwhelm the contribution from CFCs. The situation--and my own view--changed after 1991 when Curtis Rinsland and colleagues repeated Zander's measurements and reported increases for HCl and HF of (5.1 + 0.7)% and (10.9 + 1.1)% per year, respectively, for the period 1977-1990, suggesting CFCs as a major source. Rinsland et al conclude, however--and I tend to agree with them: "...in contrast to HF, there are significant natural as well as anthropogenic sources of HCl." [2]

4. I note in passing that the Montreal Protocol was signed in November 1987, and that production limits on CFCs were tightened in the years 1987 to 1991--during a period when published scientific data indicated that CFCs were not an important source of stratospheric chlorine.