Washington — There will be no winner in a nuclear war, and the nation that precipitates it commits suicide. This is the conclusion of top American biologists attending a conference here on what the world would be like after a nuclear war.
The group - including Carl Sagan of Cornell University; Donald Kennedy, president of Stanford University; and about 50 other natural scientists - concluded the two-day meeting by talking, via satellite, with their Soviet counterparts. Both groups agreed that earlier scientific studies underestimated the dangers to planetary life from nuclear war.
The American biologists concluded in a statement: ''It is clear that the ecosystem effects alone, resulting from a large-scale thermonuclear war, could be enough to destroy the current civilization in at least the Northern Hemisphere.''
In a hotel conference room here, with satellite images projected on triple screens, the American audience heard Evgeny Velikhov, vice-president of the Soviet Academy of Scientists, declare through a translator that the nuclear arms stockpile ''must be destroyed before it kills the human race.
''The only conclusion here is that nuclear arms cannot be weapons of war or tools of politicians. They are suicide,'' Mr. Velikhov said. The satellite teleconference was described as the first of its kind between scientists of the superpowers. Each side agreed to omit ideology.
The session here seemed symbolic in establishing a chain of communication among scholars at a time of intensifying political disagreement between the two nations.
In a separate development, Soviet President Yuri Andropov responded in a letter to the International Physicians for the Prevention of Nuclear War, which has campaigned for a nuclear weapons freeze and a test-ban treaty. James Muller of Boston, representing the 72,000-member organization, met with the Soviet ambassador to the United States, Anatoly Dobrynin, to receive the reply. In it Mr. Andropov declared: ''We are prepared for radical solutions. It is up to the other side.''
The exchange, which brought immediate charges that Moscow is trying to use the natural scientists to spread Soviet propaganda, illustrated the difficulty of trying to keep scientific and political ideology separate. The group here counters that it feels strongly that physical dangers of nuclear war have been understated.
One participant at the convention was Paul R. -Ehrlich, a biology professor at Stanford. He described the after-effects of an all-out nuclear war: ''When you turn off the lights, turn down the temperature to 40 degrees below zero in the middle of July, and turn up the radiation, you would have an unprecedented biological disaster. We could not preclude the extinction of Homo sapiens (mankind).''
Does it play into Moscow's hands to emphasize the dangers of nuclear war? he was asked. Like others here, Dr. Ehrlich argued that the situation must be faced and that the scientists are only relaying findings based on laboratory or astronomical conclusions.
Dr. Sagan, director of the laboratory for planetary studies at Cornell and recipient of Pulitzer, Peabody, and NASA awards, shares these views.
''Scientists initially underestmated the effects of nuclear explosions,'' he says. ''What else have we overlooked? Even small nuclear wars can have devastating climatic effects, enough to generate an epoch of cold and dark. It is not yet too late. We can safeguard the planetary civilization and the human family if we choose.''
The American group also recorded its view that global stresses ''from a large-scale nuclear war of 10,000 megatons'' would be much as same ''as from a much more limited 500-megaton-or-smaller war in which cities were targeted.'' A megaton equals a million tons of TNT.
Emerging from the recent discussions is a new picture of the dangers of nuclear war - not only would there be direct hits and demolition of cities, but also alterations in the atmosphere and what biologists call the ecosystem.
According to the report: ''The possibility exists that the darkened skies and low temperatures would spread over the entire planet. . . . (In this event) species extinction could be expected for most tropical plants and animals and for most terrestrial vertebrates of north temperate regions, a large number of plants, and numerous freshwater and some marine organisms.
''It seems unlikely, however, that even in these circumstances Homo sapiens would be forced to extinction immediately,''