[In the following review, Ackland offers praise for A Path Where No Man Thought.]

At their summit meeting in February, Presidents George Bush and Mikhail Gorbachev are scheduled to sign the Strategic Arms Reduction Treaty (START). That will be a good step toward disarmament and many people are bound to reckon that the threat of a global nuclear catastrophe has died with the cold war. But, in fact, the risk is far from gone.

In A Path Where No Man Thought Carl Sagan and Richard Turco, who were on the scientific team that devised the concept of nuclear winter, remind us that the risks of nuclear war, even of a relatively "small" one, are unacceptably high. Given the possibility of nuclear winter—the "darkening, cooling, enhanced radioactivity, toxic pollution, and ozone depletion" that would follow a nuclear holocaust—the authors note that the reliable prevention of nuclear war still "deserves by far the highest priority of all the entries on the policymaker's agenda" despite the lessened tension between the superpowers.

This book, regardless of its awkward title, is a valuable updating of the scientific and policy controversies that have surrounded the concept of nuclear winter since Mr. Sagan, Mr. Turco and three of their colleagues, Brian Toon, Tom Ackerman and Jim Pollack, introduced it in the journal Science in December 1983.

The original analysis of nuclear winter followed a 1982 study by Paul Crutzen and John Birks, published in the Swedish journal Ambio, in which they analyzed the atmospheric effects of the enormous quantities of smoke that would be generated by nuclear war. To this analysis the Turco and Sagan team added the idea that the smoke would lead to a severe temperature drop on the earth's surface. And they used dozens of different war scenarios to calculate the magnitude and duration of the cooling. Their most startling conclusion, which has been fundamentally upheld in subsequent studies by many other scholars, is that a nuclear war involving hundreds of warheads (in a world where more than 50,000 now exist) could bring on a nuclear winter, under certain conditions.

Here Mr. Sagan, a professor of astronomy at Cornell University and the creator of the television series Cosmos, and Mr. Turco, a professor of atmospheric sciences at the University of California, Los Angeles, repeatedly delineate where scientific facts end and considered speculation begins. "In this book, we do not claim that a given sort of nuclear war will inevitably produce a given severity of nuclear winter; the irreducible uncertainties are too large for that," they write. "What we do claim is that the most likely consequences of many kinds of nuclear war constitute climatic and environmental catastrophes much worse than the worst our species has ever encountered—and that prudent national policy should treat nuclear winters as a probable outcome of nuclear war."

The obvious, and unfortunately unrealistic, way to avert nuclear winter is to completely abolish nuclear weapons. But even though nuclear war can't be made impossible, Mr. Sagan and Mr. Turco argue that nuclear winter can be. This can be accomplished by reducing "the nuclear arsenals to levels at which threshold quantities of smoke cannot be generated, no matter how a nuclear war is 'fought' or who is in charge of the nuclear-armed nations."

Members of the progressive arms control community, such as the physicist Frank von Hippel, have long argued that nuclear arsenals should be drastically cut to some small number of invulnerable strategic weapons that would deter nuclear attack by insuring a devastating retaliatory strike. To this idea of a "minimum deterrence," Mr. Sagan and Mr. Turco add the goal of insuring against nuclear winter and thus come up with the term "minimum sufficient deterrence." They figure that the United States and the Soviet Union should have no more than 100 to 300 strategic warheads each, and that other states with nuclear weapons should reduce their arsenals too. The last third of the book focuses on how we might reach these goals.

The authors admit that their plans for minimum sufficient deterrence "are rough sketches only, intended to stimulate and encourage better artists and draftsmen." And their humility is warranted, for as they stray into the morass of weapons and strategies, more and more muck sticks to their arguments. For example, even while they advocate cutting strategic weapons to about 2 percent of their current number, the authors equivocate on calling for an immediate comprehensive nuclear test ban that would curtail new weapon development.

I was disappointed that only one of the book's 22 short chapters (each of which is supplemented by lots of sidebars) was devoted to the problems of proliferation. Given the increasing possibility of regional nuclear wars between rivals such as India and Pakistan or between various countries in the Middle East, there should have been a more complete discussion of the nuclear winter scenarios that would result from such conflicts. Another weakness of the book is the authors' propensity to understate the role that factors other than nuclear winter have played in changing the world's attitudes toward nuclear weapons and policies during the past seven years. Did the nuclear freeze movement have no part? Was the emergence of Mikhail Gorbachev so insignificant?

Such blemishes do not, however, negate this book's value and timeliness. It is an important reminder that much remains to be done before we can scratch the danger of catastrophic nuclear war from the top of the human agenda.