Do you believe in nuclear extinction? Of course you don't. We talk about the possible risk as rational people should but we don't give way to nightmares. Sen. Edward Kennedy said the other day that Russia and America have stockpiles that amount to nearly ''four tons of TNT for every man, woman, and child presently living on this planet.'' We reluctantly accept that and dutifully shiver. But a nuclear holocaust? That seems unthinkable.
One of the things that has changed recently in public life is that a respectable writer has come forward in a reputable journal and discussed the matter in detail. There is something faintly snobbish about the New Yorker, the sophisticated weekly which hides its table of contents in places you can't find, publishes only unrhymed verse you can't understand, and doesn't identify the knowledgeable people who write for it. It has an impressive quality, though, in educated discussion and has recently carried three lengthy articles, running to 187 pages, extracted from the forthcoming book by Jonathan Schell, ''The Fate of the World,'' which Alfred A. Knopf will publish shortly. There is something in the sober, understated, technical, detailed, and difficult exposition of Mr. Schell that is a lot more alarming than the cries of a speaker who raises his voice and shouts. It is almost impossible to doubt his dedication.
Time magazine supplied the melodrama that Schell ostentatiously left out: its cover-story headline (March 29) was a lurid warning called ''Thinking the Unthinkable: Rising Fears About Nuclear War.'' Simultaneously Newsweek featured the same theme: ''A New Outcry over Nukes.'' Yes, the European nuclear scare has leaped the Atlantic.
The Schell thesis is simple; it is defined in the opening paragraph: ''Since July 16, 1945, when the first atomic bomb was detonated at the Trinity test site , near Alamogordo, New Mexico, mankind has lived with nuclear weapons in its midst. Each year, the number of bombs has grown, until now there are some fifty thousand warheads in the world possessing the explosive yield of twenty billion tons of TNT, or one million six hundred thousand times the yield of the bomb that was dropped by the United States on the city of Hiroshima, in Japan, less than a month after the Trinity explosion. These bombs were built as 'weapons' for 'war' but their significance greatly transcends war and all its causes and outcomes. They grow out of history, yet they threaten to end history. They were made by men, yet they threaten to annihilate man.'' The paragraph runs on to the inevitable question at the end: how should we respond to the unique new nuclear peril?
The answer is -- nobody knows. The rational course is to learn the facts (so far as we are able) and not to grow hysterical. It fits into the zone of uncertainty, too, to reflect that even while we meditate a few more warheads are added to the world arsenal, each capable of blotting out a city. Fairly soon, probably, the bomb will not be confined just to a few major powers but will be available to other lands.
Everybody agrees that we should do something -- but what? Can we keep up this dynamic dance of destruction indefinitely? Common sense points to negotiations: trying to avoid simplistic solutions on one hand and dead-end deadlocks on the other. The thing Schell keeps insisting on is not merely the danger to cities but to the ozone layer, the biosphere, the ecosphere. Pinch yourself. We seem to be talking not only about civilization but all life. The threat is there.