WHAT is the connection between all those dinosaurs that mysteriously came to an end 63 million years ago (at the boundary of the Cretaceous and Tertiary Periods) and the meeting at Geneva in a few weeks of Soviet Foreign Minister Andrei Gromyko and Secretary of State George Shultz? Why, simply, that a lot of geologists now think that dinosaurs were done in by a sudden shift of climate - the kind of thing that might transpire if an asteroid hit the earth - and what could happen today if the two big nuclear powers (that's Russia and the United States) went after each other in an all-out war. Preposterous? Don't use that word any more when we have piled up 18,000 strategic weapons in the world today.
We are, in fact entering Act 2 of the Nuclear Age - after inventing the infernal thing and living with it for 40 years after exploding it at Alamogordo, N.M., July 16, 1945. we have only recently faced the realization in the past two or three years that we can extinguish life with it on earth. Begin Act 2.
It's called ''Nuclear Winter.'' The point is that after considering nuclear war chiefly in terms of blasts, concussions, and explosions, the focus of popular thought is finally shifting to a more devastating possibility - changes in climate and environment. Biological life will suffer, the scientists say soberly.
Surely there must be a mistake. Surely these are alarmist fears by imaginative, liberal intellectuals! I first ran into the argument on what was called the ''Conference on the Long Term Worldwide Biological Consequences of Nuclear War'' on Oct. 31 a year ago in Washington at the Sheraton Washington Hotel. There were more than 500 participants plus 100 media representatives. The gathering was linked by satellite with a similar meeting in Moscow under auspices of the Soviet Academy of Sciences. The leaders on either side were at the top of their professions. They spoke soberly and scientifically. They avoided ideological arguments. What they said was that there could be no winner in a nuclear war.
Prominent in the Washington meeting (and an earlier one under the auspices of the American Academy of Arts and Sciences at Cambridge, Mass.) was Dr. Carl Sagan of Cornell University. Prominent, too, was the so-called TTAPS group (an acronym of the initials of five leading environmentalists). A common feature was the careful tone of the findings. I should say it was rather stodgy, if anything , until you analyzed what they were trying to say. Much of this has now been put together under the title ''The Cold and the Dark: The World After Nuclear War'' (Norton, $12.95). It is signed by four authors, of whom the first is Paul R. Ehrlich (Stanford University). Never was a threat to the world presented so quietly. In brief, says a preface: ''The biological scientists left the Cambridge meetings in general agreement that these effects on the hemisphere (nuclear explosions) could be devastating to a degree previously unforeseen, and they concluded that they could not rule out the possibility that the long-term biological effects of nuclear war could cause the extermination of humankind and most of the planet's wildlife species.''
This brightly printed book, ''The Cold and the Dark,'' has a kind of Who's Who of scientific names attached to it, and it is introduced by a preface, a foreword, and an introduction. Nobody can sidle up to the subject of man's possible extinction except obliquely, but they all reach more or less the same conclusions. Dr. Donald Kennedy, world famous biologist and president of Stanford University, put it like this: ''What our most thoughtful projections show is that a major nuclear exchange will produce among its many plausible effects, the greatest biological and physical disruptions of this planet in its last 65 million years - a period . . . more than 100 times the life span of our species so far.''
The point they make, that now seems accepted by answering Russian scientists, is that an exchange would pour smoke, dust, and soot into the heavens, plunging much of the globe into night for months. The result would be a nuclear winter - cold enough to kill crops and animals in forests. Changes of climate would hurt both belligerents and nonbelligerents. Can anything be done at the forthcoming Geneva talks to restore sanity?