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In the 1980s, climate scientists in Russia and the U.S. theorized that all-out nuclear war between the superpowers would result in a "nuclear winter," as smoke from the atomic explosions blackened the sky and sent summer temperatures plummeting below freezing — killing crops and eventually starving all those who survived the initial explosions. Now that the risks of an all-out U.S.-Russian exchange have diminished, scientists are looking at the climactic effects of regional nuclear war — and the predictions are still sobering.

Alan Robock, a Professor in the Department of Environmental Sciences at Rutgers University who participated in the original nuclear winter research, recently completed a study on the results of a nuclear war between India and Pakistan. He spoke with TIME from his office in New Brunswick, New Jersey. ( See pictures from the aftermath of the Mumbai terror attacks.)

Tensions between India and Pakistan have been high recently. If they escalated to all-out nuclear war, what would be the effect to the global climate?

We looked at a scenario in which each country used 50 Hiroshima-sized weapons, which they are believed to have in their arsenals. That's enough firepower to kill around 20 million people on the ground. We were surprised that the amount of smoke produced by these explosions would block out sunlight, cool the planet, and produce climate change unprecedented in recorded human history.

Your study predicts mass cooling. With all the heat and radioactivity of the explosions, why wouldn't nuclear war warm the planet?

It has nothing to do with the radioactivity of the explosions — although that would be devastating to nearby populations. The explosions would set off massive fires, which would produce plumes of black smoke. The sun would heat the smoke and lift it into the stratosphere — that's the layer above the troposphere, where we live — where there is no rain to clear it out. It would be blown across the globe and block the sun. The effect would not be a nuclear winter, but it would be colder than the little ice age [in the 17th and 18th centuries] and the change would happen very rapidly — over the course of a few weeks.

Would you be able to see the smoke?

The sky would not be blue. It would be grey.

And what would the results be for humanity?

We calculated that there would be a shortening of the growing season in the mid-latitudes — that includes Europe and America in the Northern Hemisphere — by a couple of weeks. The growing season is defined as the period between the last frost in spring and first frost in the fall. Some crops that need the whole growing season would not reach fruition and there would be no yield. Others would grow more slowly and produce a small yield. In addition there would be less precipitation and it would be darker, also damaging yield. You compound that with [the shutdown of] the current global network of food trading — countries would likely stop shipping food and focus on feeding their own populations — and it's a big crisis. We don't have the resources to do detailed analyses on the impacts of crops in different farming regimes but this suggests it could be a very serious problem.

How confident are you that your modeling is correct?

We used ModelE, designed by NASA's Goddard Institute for Space Studies, and one of the models used to produce the results of the Intergovernmental Panel on Climate Change (IPCC). The model does an excellent job of simulating climate change that resulted from volcanic eruptions in the past. That gave us confidence. What's more, a group repeated the calculations for the Pakistan-India scenario with a different model at the National Center for Atmospheric Research in Boulder, Colo., and the results almost exactly agreed. Their research showed how the smoke from the fires would open up holes in the ozone, which would cause even more problems for humanity. We'd like other people to test the calculations with their models, but we're pretty confident that they'll get the same answer.

So we get a clue of the climatic effects of nuclear war from volcanic eruptions?

Yes. 1816 was known as the "year without summer." It followed the Tambora Volcano eruption in Indonesia in 1815. It was sudden climate change on a similar scale, and it resulted in a severe famine in Europe, food riots and mass emigrations. Volcanic aerosols have a lifetime of about a year in the stratosphere. The lifetime of soot from nuclear fires is about five years. It's obviously much harder for a society to recover from such an extended cooling.

Some scientists, most notably Freeman Dyson of The Institute for Advanced Study in Princeton, have stirred controversy by arguing that nuclear weapons are a more urgent environmental threat than global warming. Do you agree?

Yes. If India and Pakistan engaged in nuclear war, they would use about 0.3% of the global nuclear stockpile. And still the effects on the climate would be dramatic. Our calculations on nuclear winter from the early 1980s have been confirmed by modern climate models. And fundamentally the situation hasn't changed — even with reduced stockpiles there still exists enough weapons to cause nuclear winter. That's something that maybe people don't realize.

I think we have to solve the problem of the existence of all these weapons before we have the luxury of worrying about global warming.

See TIME's special report on the environment.

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Breaking News, Analysis, Politics, Blogs, News Photos, Video, Tech Reviews - TIME.com

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In the 1980s, climate scientists in Russia and the U.S. theorized that all-out nuclear war between the superpowers would result in a "nuclear winter," as smoke from the atomic explosions blackened the sky and sent summer temperatures plummeting below freezing - killing crops and eventually starving all those who survived the initial explosions.

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Are Nuclear Weapons Useful? | Carnegie Council for Ethics in International Affairs

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Technology goes out of existence over time. So why do we still have nuclear weapons after the Cold War?

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They don’t make the world safer because they can abliderste a city/town in an instant.

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They make the world safer by M.A.D. Which makes countries afraid to start a war with someone with them.

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The radiation leasts for several years- decades.