The National Academy of Sciences, the nation's most prestigious scientific body, gave its seal of approval yesterday to the controversial theory that a nuclear war could drastically alter Earth's climate, plunging the Northern Hemisphere into a freezing darkness that has come to be called nuclear winter.

A nuclear exchange involving only half the world's arsenal, the academy said in a long-awaited report, could put enough dust and smoke into the atmosphere to blacken the sky for six to 20 weeks. If the war occurred in spring or summer, the loss of sunlight could cause temperatures throughout most of North America and Eurasia to fall by 18 to 55 degrees.

Though the report added little to the nuclear-winter scenario that individual scientists have been describing for more than a year, it strengthened the theory's technical foundation and gave it the scientific establishment's most visible stamp of authority.

The report was commissioned in 1983 by the Defense Department after scientists began speculating that a nuclear war might produce the same climatic effects that are believed to have occurred 65 million years ago when, evidence suggests, an asteroid or a comet collided with the Earth. That event is thought to have wiped out the dinosaurs, along with most other living species.

Although the academy's report did not discuss the effects of nuclear war on people, biologists familiar with the report said that even the minimum effects it suggests would be catastrophic for the human species.

For one thing, they said, the cold and the dark would destroy agriculture in the Northern Hemisphere for at least a year and could even kill deciduous forests that have not had time to prepare for a natural winter.

The report was prepared by a committee of 18 scientists appointed a year and a half ago by the academy's working arm, the National Research Council.

The committee included specialists in various areas of science from universities, government nuclear weapons laboratories and private industry. The chairman was George F. Carrier of Harvard University.

Carrier said his committee chose to study the effects of a hypothetical nuclear war using about half the weapons in the nuclear arsenals of the United States and the Soviet Union.

There are now about 50,000 nuclear weapons with a total explosive power of 13,000 megatons.

In the committee's hypothetical war, the two sides exploded 12,500 strategic weapons with a yield of 6,000 megatons and an equal number of the much smaller tactical weapons with a total yield of 500 megatons.

The committee also assumed that the weapons would be used against known military targets and against the 1,000 most populous cities in NATO and Warsaw Pact countries.

The report says the blasts would immediately raise anywhere from 10 to 24 million tons of dust that would be propelled into the stratosphere by the bombs' explosive force. Urban fires started by the blasts would produce between 20 and 650 million tons of smoke. Because the energy of fires is lower, the smoke would not rise as high in the atmosphere. In its temperature estimates, the committee settled on 15 million tons of dust and 180 million tons of smoke.

Within days, the report says, the vast clouds of dust and smoke would spread around the Earth, wreathing the Northern Hemisphere in a pall that would block more than 99 percent of the sun's light.

Temperatures would begin to drop immediately and within days could become catastrophic. If the war came during atmospheric conditions typical of April, the eastern half of North America would cool by about 18 degrees within six to 10 days, as would much of Europe and virtually all of Asia.

A nuclear exchange in July would be worse. Temperatures in the heartland of North America and Eurasia could drop as much as 55 degrees below normal as lesser degrees of cooling affect the rest of the two continents.

Carrier emphasized that these effects were calculated only for a specific hypothetical war and did not imply any threshold. He said that it was clear a smaller war would produce similar effects, but that there was no way to say how small a nuclear war would have to be to avoid a nuclear winter.

Carrier said it was difficult to estimate how quickly the black cloud might clear. Much of the clearing would depend on rain washing the particles of soot and dust to the ground.

But patterns of rainfall might be severely disrupted in an atmosphere where the sun's heat accumulates in the upper atmosphere instead of penetrating to the ground.

Richard P. Turco, a member of the committee and the lead author of an independent scientific report a year ago on nuclear winter, said the academy's report should quiet the controversy surrounding the issue.

"This legitimizes the problem," said Turco, an atmospheric chemist at a private consulting firm called R&D Associates in Marina del Rey, Calif. "It shows that this isn't some wild idea of a bunch of left-wing, liberal college professors. This was a balanced panel and we're saying there really is cause for concern."

Although the panel stressed that there were many uncertainties in its calculations, Paul Ehrlich, the Stanford University ecologist, said that this was almost irrelevant in terms of nuclear winter's effect on life. Ehrlich was not a member of the committee but he has written extensively on the biological effects of nuclear winter.

"A lot of the uncertainty in the debate is damped out by the extraordinary sensitivity of living organisms," Ehrlich said in an interview. "It only takes a drop of five or six degrees to wipe out the grain crop. People have got to understand that we all depend on photosynthesis the process by which plants capture solar energy to make carbohydrates and when you interrupt that, we're all in deep, deep trouble."

The report's findings were also endorsed by the American Institute of Biological Sciences, a federation of professional biological societies.