Countering the A-bomb threat; Author argues world safety must begin with public awareness of choices ahead

''The Fate of the Earth'' is a book I'm urging everyone I know to read - not maybe, not sometime, but without fail, now.

Already the subject of intense interest and debate even before it's on bookstore shelves, this earnest, compassionate study of nuclear weapons transcends the ordinary measures of literary merit to become an event of global significance.

Former Vice-President Walter F. Mondale is one of several public figures who have praised the book. On the ''CBS Evening News'' Bill Moyers said it could play a pivotal role, like that of ''Silent Spring,'' in focusing public attention on a matter of the utmost importance. Sen. Alan Cranston (D) of California asked Schell to summarize his ideas for circulation among members of Congress.

The case Schell lays before us suggests that this subject deserves our prayers and our most persistent and conscientious efforts at solution. For a solution, he predicts, will demand a revolutionary new recognition of the oneness and interdependency of life on earth, the unworkability of war as an adjunct to diplomacy, and the need of a new political structure to replace our system of sovereign states - immense challenges, but ones Schell feels can be met.

The common sense of his appeal, first published two months ago as a New Yorker magazine series, cuts through our distraction and apathy. It could kindle the public sentiment needed to slow or halt the steady buildup of doomsday devices.

Schell's 244-page volume, to be published April 26 by Alfred A. Knopf at $11. 95, is the newest in a vast proliferation of books about nuclear questions. Publishers Weekly recently counted 130 such books, more than 100 of which have appeared since 1980.

Yet no other nuclear book speaks as clearly or as sensitively to the layman, or encompasses the cultural, moral, and spiritual as well as the technological and political considerations that Schell's does.

Of course, we don't need any book to tell us that the possibility of nuclear war - intentional or accidental - confronts us daily. We have only to keep up with the news. Although we don't like to think about it, we also realize there would be no ''winner'' in a large-scale nuclear exchange; both sides would experience devastation unparalleled in history.

But can we really appreciate what that means? How many people, for instance, would survive? Would shelters or evacuation help?What would we find when - and if - we climbed out of our bunkers after an attack? And how much time would our society need to recover?

These questions have been debated ever since the impact of the A-bomb was perceived, but too much of the discussion suffers from the hermetic quality Schell finds in military and ''think tank'' studies. ''Once the 'strategic necessity' of planning the deaths of hundreds of millions of people is accepted, '' he writes, ''we begin to live in a world in which morality and action inhabit two separate, closed realms. All strategic sense becomes moral nonsense, and vice versa. . . .'

In the first of the book's three sections, Schell explores the basic questions from the standpoint of an intelligent, inquisitive, and sensitive journalist. Drawn on five years of research and interviews with prominent scientists, his conclusions are far from reassuring.

Mankind's peril, he explains, results from knowledge of how to free the tremendous energy that binds atomic particles. Humanity can't escape that knowledge. Einstein's E EQUALS mc2 tells us what we get. The energy available in , say, a fleck of uranium dust is equal to its mass multiplied by the speed of light (186,000 miles per second) squared - power monstrously larger than anything humanity has known in the past. Einstein's was a discovery that ''altered the relationship between man and the source of his of life, the earth, '' Schell reminds us. The fruits of that discovery have been turned into weapons that might be resorted to in a moment of anger, confusion, or frustration of national aims.

The world's current nuclear stockpile, estimated at 17,000 to 20,000 megatons of explosive power, would produce a cumulative blast a million times as powerful as the one that destroyed Hiroshima, a city of 340,000, in a few seconds on an August morning 37 years ago.

The energy released in a nuclear blast, Schell explains, comes in four forms: (1) a thermal pulse that, among other things, melts glass and metal and exhausts the air's supply of oxygen; (2) an electromagnetic pulse that disrupts electric power and communications well beyond the blast area; (3) a blast wave that razes buildings and trees; (4) radiation that contaminates the environment, the food chain, and potentially the whole ecosphere.

What is known about the effects of these forces - and unfortunately our knowledge is far from complete - leads Schell to conclude that very few people on a targeted continent would survive the initial effects of a superpower's all-out attack; shelters or evacuation would be fruitless; anyone who lived through an all-out barrage by staying in a shelter would eventually emerge into a desert, in which everything flammable would have been consumed; such a person then would face the task of reinventing an economy at the basic level of growing one's own food and making one's clothing; finally, a survivor without protective gear would be unable to stay outside for more than 10 minutes at a time, because the atmosphere's ozone layer, depleted by oxides of nitrogen, would no longer screen out the fatal components of sunlight.

Schell's ultimate conclusion is that a large-scale nuclear exchange could result in the extinction of mankind. Not all scientists would agree. A recent study of the National Academy of Sciences, for instance, says otherwise. In fact , no one knows for sure. But the consensus seems to be that significant use of the weapons on hand would certainly destroy civilization as we know it.

Part 2 of Schell's book deals with human values, the meaning of possible extinction, and the differences between a catastrophe like the Nazi Holocaust and the potential loss of the entire human species. Schell also considers the shadow cast by the nuclear threat on our daily lives.He notes that humanity has never been confronted with the possiblity that its actions could prevent future generations from being born. And he speculates on ways in which the nuclear threat may influence our family and political lives, our art, and our literature.

In Part 3, Schell examines the choices we face, if we're to lessen or remove the nuclear threat. He stops short of proposing any political solution of his own but suggests that the task of finding one needs to be put at the top of mankind's agenda.

He argues that the ''invention of nuclear weapons'' made it no longer possible ''for violence to . . . . break down the opposition of the adversary; it can no longer produce victory and defeat. . . .''

He challenges the doctrine that huge, balanced weapon stockpiles will forever deter weapon use. He also insists that we need to devise a new political framework to replace war in the settling of disputes, and that our system of sovereign states is outmoded in the nuclear age.''

The self-extinction of our species,'' he writes, ''is not an act that anyone describes as sane or sensible: nevertheless it is an act that, without quite admitting it to ourselves, we plan in certain circumstances to commit.'' We might be galvanized into it by events, as we nearly were during the Cuban missile crisis of 1962, or we might blunder into it through a defective computer or human error.

Must the world live indefinitely under this threat? Schell doesn't think so. We can unmake nuclear weapons, he says, and we can fashion a new system to replace national sovereignty. But to do so depends on all of us. ''Social revolutions . . . . usually cannot occur unless they are widely understood and supported by the public.''

Some critics have faulted the book for being hard to follow, or merely belaboring the obvious, or betraying an utter lack of sophistication.

These criticisms are unwarranted. Though at times philosophical and repetitive, the book certainly won't overtax the powers of ordinary readers. The latter charges are grossly unfair, if one assumes that policy statements amount to more than posturing and that world leaders would indeed use their weapons if the situation became dire enough. In this case, the public needs to appreciate the magnitude of the choices and sacrifices involved and become more thoughtfully and vocally engaged in arms-limitation considerations. And this is a book that speaks to the public.

Schell anticipates and answers the charge of naivete in a passage that eloquently sums up the two opposing points of view:

''People seem to have decided that our collective will is too weak or flawed to rise to this occasion. They see the violence that has saturated human history , and conclude that to practice violence is innate in our species. They find the perennial hope that peace can be brought to the earth once and for all a delusion of the well-meaning who have refused to face the 'harsh realities' of international life - the realities of self-interest, fear, hatred, and aggression. They have concluded that these realities are eternal ones, and this conclusion defeats at the outset any hope of taking the actions necessary for survival. Looking at the historical record, they ask what has changed to give anyone confidence that humanity can break with its violent past and act with greater restraint. The answer, of course, is that everything has changed. ... To the old truth that all men are brothers has been added the inescapable new truth that not only on the moral but also on the physical plane the nation that practices aggression will itself die. . . .''

We owe Schell a debt of gratitude for launching a public debate that is long overdue and for which we all may be better prepared now than we were 30 years ago. This book will have a special place on my shelf - or rather not on the shelf, but in the hands of anyone I can persuade to read it.