Nuclear winter may kill more than a nuclear war

A regional exchange of relatively small nuclear weapons could plunge the world into a decade-long "nuclear winter", destroying agriculture and killing millions, according to a new study.Weapons experts to consider that small-scale nuclear exchanges are now more likely than the massive US-Soviet exchanges feared during the Cold War.In the 1980s, scientists calculated that such exchanges would put enough smoke into the atmosphere to shade the Earth from the Sun, causing a nuclear winter.Now scientists have re-calculated the likelihood of nuclear winter using modern, vastly improved climate models and a more likely modern scenario for small-scale nuclear war. Brian Toon, head of atmospheric and oceanic sciences at the University of Colorado at Boulder, and Alan Robock of Rutgers University in New Jersey, both in the US, predict less cooling than the 1980s modellers. However, they predict the cooling would last longer, with potentially devastating consequences.
by Debora MacKenzie

How Does Nuclear Winter Work?

A nuclear explosion is like bringing a piece of the Sun to the Earth's surface for a fraction of a second. Like a giant match, it causes cities and industrial areas to burn. Megacities have developed in India and Pakistan and other developing countries, providing tremendous amounts of fuel for potential fires. The direct effects of the nuclear weapons, blast, radioactivity, fires, and extensive pollution, would kill millions of people, but only those near the targets. However, the fires would have another effect. The massive amounts of dark smoke from the fires would be lofted into the upper troposphere, 10-15 kilometers (6-9 miles) above the Earth's surface, and then absorption of sunlight would further heat the smoke, lifting it into the stratosphere, a layer where the smoke would persist for years, with no rain to wash it out.
The climatic effects of smoke from fires started by nuclear war depend on the amount of smoke. Our new calculations show that for 50 nuclear weapons dropped on two countries, on the targets that would produce the maximum amount of smoke, about 5 megatons (Tg) of black smoke would be produced, accounting for the amount emitted from the fires and the amount immediately washed out in rain. As the smoke is lofted into the stratosphere, it would be transported around the world by the prevailing winds. We also did calculations for two scenarios of war between the two superpowers who still maintain large nuclear arsenals, the United States and Russia. In one scenario, 50 Tg of black smoke would be produced and in another, 150 Tg of black smoke would be produced. How many nuclear weapons would be required to produce this much smoke? It depends on the targets, but there are enough weapons in the current arsenals to produce either amount. In fact, there are only so many targets. Once they are all hit by weapons, additional weapons would not produce much more smoke at all. Even after the current nuclear weapons reduction treaty between these superpowers is played out in 2012, with each having about 2,000 weapons, 150 Tg of smoke could still be produced.
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