HAS OZONE STARTED TO DISAPPEAR?

Ozone, skin cancer and proposed US participation in a treaty banning chlorofluorocarbons (CFCs) are back in the news, perhaps because of Interior Secretary Hodel's alleged remarks about using hats, sunglasses and sunscreen lotions to prevent skin cancer rather than signing international treaties. Such a remark would have been utterly justified, had he made it, but since the report appeared in The New Republic, it was not likely to be true. It was, in fact, shamelessly distorted as admitted even by the ultra-liberal Tim Wirth, with whom I am afflicted as my senator, after shooting off his mouth before checking the facts.

But media mendacity is old hat; instead, let me suggest a way in which a layman can judge the ozone layer without having to take anybody's word (not mine, either). The reader will only need to check a minimum of facts, though he will have to do something that does not come easily to many people: use his own head.

I have previously explained why layers in the high atmosphere are self-healing, moving to higher or lower densities when there is a change in the cause that creates them in the first place. But this is not something a layman could devise for himself, so this month let me take another tack and ask a fundamental question:

Compared to what?

And we will look for the usual answer: compared to nature doing the same thing without man's (perceived or real) intervention.

Surely by now we have enough experience with this method from radioactivity. In his beautiful, just published book (reviewed on p.4), Edward Teller includes a lecture given 27 years ago, where he objects to the misleading measure of "maximum permissible radia-tion dose," because it makes people think, incorrectly, that if they got 20~o of it, they have gone 20% of the way toward getting cancer. The measure he suggested instead is the fraction of the average natural radioactive background.

Analogously, is there anything in nature that changes the ozone layer without man's doing? You bet there is. The ozone layer is ultimately caused by solar radiation, and its concentration varies distinctly with both latitude and season, as shown in the figure on the next page.

[The curves are taken out of a textbook published in 1951 which I happen to own. Far more accurate data are available now; however, we will use only percentage changes, and these cannot have changed much since then. The ozone layer, incidentally, was discovered around 1880.]

The actual units of the scale on the left do not matter, just look at the change in percent. Between April and September, the ozone concentration above Denver (or New York City or Naples, latitude 40 degrees) changes by 25%.

In May and June, the difference in ozone concentration above Miami (26 degrees latitude) and Vancouver (49 degrees latitude) is 24%.

All of this applies to measurements made in the same year, for superimposed on latitude and season is the solar activity-cycle with its average period of 11 years.

And what percentages due to other causes are we talking about? A total of 0.5% per year since 1978, at least half of which is at-tributable to normal and known causes. As for the CFC effect, if any, Science (10 Jul 87) states "it might have been between 0.1 or 0.2% per year [since 1978] at most according to the latest consen-sus, but it could also have been zero over that time;" and in typical Science fashion, that statement appears under the remarkable headline "Has Stratospheric Ozone Started to Disappear?"

But 0.2% per year makes 20% per century, you may say. Not so: the (suspected) decrease is itself decreasing year by year, so linear extrapolation is not justified.

Please note that in this comparison the layman need check out no more than that the figure of ozone dependence on latitude and season shown in the figure is not concocted out of thin air; this he can do by consulting any textbook on the higher atmosphere (or geophysics or meteorology) in a college library.