The Defense Department has acknowledged that atomic warfare might trigger "significant" weather changes, but concluded that the possibility of a "nuclear winter" helps to justify a new generation of weaponry and the "Star Wars" missile defense system.

In a report, "Potential Effects of Nuclear War on the Climate," the department conceded for the first time that nuclear war would unleash clouds of dust and smoke that might obstruct the sun and lower temperatures.

But the report said that further research is needed to predict the severity and territorial scope of climatic changes likely to be caused by nuclear war or to confirm the grim "nuclear winter" scenario of certain scientists who say the Northern Hemisphere would be plunged into freezing darkness regardless of who launched the attack.

"Even with widely ranging and unpredictable weather, the destructiveness for human survival of the less severe climatic effects might be of a scale similar to the other horrors associated with nuclear war," the 17-page report said.

Nevertheless, while partially endorsing a theory of nuclear freeze advocates, the agency came up with a different remedy. It said said the "most direct way" to prevent such climatic catastrophe is to strengthen deterrence against a Soviet nuclear strike by modernizing the U.S. strategic force.

"There are those who argue, in effect, that we no longer need to maintain deterrence as assiduously as we have because the posited prospect of catastrophic climatic effects would themselves deter Soviet leadership from attack," the report said. "We strongly disagree and believe that we cannot lower our standards for deterrence because of any such hope."

Plugging President Reagan's "Star Wars" space defense system, the report said its aim of destroying nuclear missiles before they reach their targets "may provide a greater mitigating effect on atmospheric consequences" of nuclear war than arms control agreements.

The report was delivered Friday night to Congress, which had requested it after a group of eminent scientists released evidence in 1983 that a nuclear exchange involving a fraction of the world's nuclear arsenal would cause global climatic catastrophe.

It had been generally argued by the new theory's proponents that, if it were proven true, major shifts in civil defense policy and nuclear strategy could result.

Most frequently mentioned is the idea that, if both sides suffer atmospheric chaos as result of a nuclear attack, a first strike might be ruled out as self-defeating even for the aggressor.

Proponents of the "nuclear winter" scenario said yesterday that while they were gratified by the partial recognition of the concept, they were puzzled how defense planners could use the report to support the campaign for new nuclear weapons systems.

"It would be far more prudent to make sure there were so few nuclear weapons that no misunderstanding or madman could trigger a nuclear winter," said astronomer Carl Sagan, an author of the nuclear winter scenario.

Sagan said it is "sad" that the Pentagon could agree with the basic theory of climatic disaster while using it as a defense for Star Wars. He added that "if 1 per cent of the Soviet missiles came through the net, you'd have a nuclear winter."

Two congressional sponsors of the study -- Sen. William S. Cohen (R-Maine) and Rep. Timothy E. Wirth (D-Colo.) -- said they were disappointed that the department failed to give more than passing treatment to certain issues, including civil defense. The report said until more is known about the climatic impact of nuclear war, it is "impractical" to spend money to defend the civilian population against possible disaster.

"All the Pentagon has done is use this as a soapbox for defending 'Star Wars,' " Wirth said. "Clearly, the obvious response is to reduce the number of nuclear weapons, not increase them."

The report said that while temperature and meterological changes could be "severe," there is "large uncertainty about the extent of those effects -- too large to influence the thrust of U.S. defense policy, which is maintain a strong enough arsenal of nuclear weapons to deter Soviet attack."

Moreover, the report said, there is no assurance that Moscow would realize the possibly suicidal climatic effects of a first strike.