India-Pakistan nuclear war would kill a billion, cause agricultural crisis in rest of world

Links to a couple of related articles. Here's a quote from one:

Earlier studies have suggested that such a conflict would throw five million tonnes of black soot into the atmosphere, triggering a reduction of 1.25°C in the average temperature at the earth's surface for several years. As a result, the annual growing season in the world's most important grain-producing areas would shrink by between 10 and 20 days.

Helfand points out that the world is ill-prepared to cope with such a disaster. "Global grain stocks stand at 49 days, lower than at any point in the past five decades," he says.

Then there's the ozone destruction.