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A nuclear war would not only trigger a nuclear winter, but also severely damage the ozone layer, making recovery even harder. Now, a study has shown that a relatively small nuclear war between India and Pakistan could do just as much damage to the ozone layer as a larger nuclear war between the US and Russia.

“We want to emphasise that even a small-scale nuclear war can produce far-reaching global side effects beyond the conflict regions,” says at the University of Quebec in Montreal.

A nuclear war would devastate the areas where bombs or warheads explode, with the explosions, heat and radiation potentially killing many millions directly. The explosions and fires would be so large that huge quantities of smoke would be pumped into the atmosphere, blocking sunlight and causing global temperatures to plummet – a nuclear winter.

“There’s strong surface cooling in the first several years,” says Zhuo, who presented her team’s results at a meeting of the European Geosciences Union in Vienna last month.

For instance, a 2007 study estimated that a billion people could die of starvation as a result of a nuclear winter caused by a war between India and Pakistan.

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Recovery from a nuclear winter would be delayed by damage to the ozone layer in the stratosphere that blocks harmful ultraviolet light – volcanic eruptions and even large wildfires can also damage the ozone layer. High UV levels can harm plants as well as animals, meaning lower yields from farming even as temperatures recover.

Recent studies with advanced climate models suggest . So, concerned by the many conflicts around the world, Kuo and her colleagues decided to look at the possible consequences if one went nuclear. Drawing on estimates from previous studies, they modelled an India-Pakistan nuclear war that would release 5 million tonnes of soot into the atmosphere and a US-Russia war releasing 16 million tonnes. Unlike previous studies, they also took into account other pollutants such as organic carbon.

Their climate model suggests that air circulation patterns in the tropics would allow the pollutants from an India-Pakistan war to rise higher into the atmosphere, stay there longer and spread more widely around the world.

“The upward transport is stronger for the tropical cases,” Kuo says. So although the quantities of pollutants are smaller than from a US-Russia war, the effects on the ozone layer are actually greater.

The damage to the ozone layer would be greatest over the poles, similar to the situation caused by ozone-damaging pollutants known as CFCs. But there could be an increase in UV levels of up to 30 per cent even in tropical areas, the model suggests, with serious impacts on the health of people and wildlife.