Full-scale nuclear war probably would cause such a severe and sudden climatic change that it could destroy agriculture in the northern hemisphere for a year or more, plunging 2.5 billion people into famine, a major study has concluded.

The study is the first large, formal attempt by scientists around the world to estimate the biological effects of "nuclear winter," the period of darkness and freezing that many atmospheric scientists say could result from a globe-girdling pall of smoke and dust produced by a major nuclear war.

"Sudan and Ethiopia today are probably far more representative of what the world would be like after a nuclear war than are Hiroshima and Nagasaki," said Mark A. Harwell, co-leader of the biologists from 30 countries who made the study.

"I think our study shows that the indirect effects of nuclear war have been grossly underestimated," Harwell said. "The potential deaths from indirect effects greatly exceed those from the direct effects of blast, fire and radiation."

The new study, released yesterday, was conducted under the auspices of the Paris-based International Council of Scientific Unions, a 54-year-old organization of scientific societies from 70 countries.

The study was carried out over the past three years by the council's Scientific Committee on Problems of the Environment (SCOPE). The effort, which involved nearly 300 scientists from about 30 countries, resulted in a two-volume report totaling nearly 900 pages.

The first volume, dealing with atmospheric effects and using more sophisticated computer models of the atmosphere than have been used before, confirms estimates that a nuclear war could trigger an abnormal freeze, plunging temperatures in much of the northern hemisphere to 35 to 70 degrees Fahrenheit below normal. The second volume focuses on the effect this would have on agriculture and ecosystems around the world.

Paul Crutzen, the West German scientist who first raised the possibility of nuclear winter in 1982 and who was an author of the new study, said the report should go far toward dispelling criticisms that the hypothesis is far-fetched.

But some skepticism remains.

"A lot depends on how much soot survives its first six hours in the atmosphere," said Joseph P. Knox of the Lawrence Livermore laboratory in California, noting that the soot can fall back to earth, join with water and make rain, be redistributed in the air or change its ability to reflect sunlight.

"All these things can move in the direction of reducing the severity of impact," and cutting the soot in the atmosphere to a third of the estimate, Knox said, can make a modest nuclear winter just a "transient chill, the kind of thing man has survived before in history."

The nuclear winter scenario is not based on extreme assumptions of what might happen. The SCOPE study assumes that a nuclear war would involve only half the nuclear warheads of the United States and the Soviet Union, and that their use would set fire to about 25 percent to 30 percent of the urban areas of North America, Europe and the Soviet Union. Rural and forest fires would add another few tens of millions of tons of smoke.

Fires ignited by the bombs would produce between 50 million and 150 million tons of smoke. While this is less than earlier estimates had assumed, the new study found that a larger share of the smoke -- about 30 million tons -- would consist of carbon, which is black and capable of absorbing sunlight. The carbon would come from burning stored oil, plastics and other petrochemical products.

The SCOPE study also assumed that rain would wash half the particles to the ground in the first day.

However, the study confirms earlier suspicions that much of the smoke would quickly rise above weather zones and enter the stratosphere, the highest layer of air. This is because as the particles absorbed solar heat, the air around them would warm up and rise long after the cities were no longer burning. Smoke that entered the stratosphere could remain aloft for a year or more.

The study shows that within days there could be dense patches of smoke that would block 99 percent of the sunlight, subjecting the land to freezes that could plunge temperatures 35 to 70 degrees below normal. Within a few weeks the patches would spread, blanketing the northern hemisphere enough to block out 90 percent of the light.

The worst freeze would be inland, since coastal temperatures would be moderated by the still-warm oceans. The study also suggests that a drought could follow for some weeks or months and normal weather patterns would be disrupted, eliminating the monsoon rains upon which much of Asia and Africa depend for agriculture.

Harwell said that while a severe freeze in summer obviously would destroy crops, even slight drops in temperature, by only 3 or 4 degrees, could be enough to shorten the growing season in northerly latitudes. This slight a temperature drop, which could continue for many months because of the smoke in the stratosphere, would prevent wheat being grown in Canada, the northern United States and much of the Soviet Union.

Harwell said that scientists assumed that the entire northern hemisphere would lose one year's crop production and that there would be no exporting of food. Under such circumstances, the United States and Canada would have enough food on hand to avoid major famine but might be unable to distribute it in the absence of fuel for trucks and trains.

Most other parts of the world would risk severe famine. Japan, even if not hit by a bomb, probably could keep no more than half its population alive in the absence of food and oil imports. India and Brazil could be hit even harder.

Harwell said famine would kill about 2.5 billion people, in addition to the 500 million who would die in the immediate effects of a war.