Pakistan and India are next-door neighbors that are armed to the teeth with nukes and becoming absolutely infuriated with each other. What could possibly go wrong?

With political tensions between the two countries starting to reach a boiling point, a new study in the journal Science Advances has set out to calculate the potential cost of a nuclear war between Pakistan and India in the coming decade. Not only would a nuclear clash directly result in the deaths of millions of people, but it could also spark a full-blown global climate catastrophe and mass starvation, even if the war remains relatively localized to South Asia.

These two nations currently have a stockpile of approximately 140 to 150 warheads each, and that’s set to expand to 200 to 250 by 2025. This study works on the premise that Pakistan attacked urban targets in 2025 with 150-kiloton nuclear weapons, while India drops 100-kiloton nuclear weapons onto their neighbor's cities. For perspective, Little Boy, the nuclear bomb dropped on Hiroshima in 1945 by the USA, was a 15 kiloton bomb. Both India and Pakitan are densely populated, so these attacks could result in the immediate deaths of 50 to 125 million people.

Using observations from the Hiroshima bomb, they estimate that firestorms from the smoldering cities could release between 16 and 36 teragrams of soot into Earth’s atmosphere. This could be enough to produce a “Nuclear winter” effect. The abundance of black carbon and soot in the stratosphere would block out some direct sunlight from reaching the surface of the Earth, resulting in the cooling of Earth’s surface by 2 to 5°C. It would also reduce snow and rainfall by 15 to 30 percent. 

The drop in light and precipitant would take a heavy toll on the world’s plants, reducing the rate at which plants store energy as biomass by 15 to 30 percent on land and by 5 to 15 percent in oceans. As a result, global food production would significantly slump, potentially enough to threaten mass starvation.

Given the size of the stakes, it's unclear how likely a nuclear war between the two south Asian states would be. India has hinted that they might abandon their "no first use" policy when it comes to using nuclear weapons in light of recent tensions, while Pakistan has never followed any such policy. Pakistan has even suggested it's prepared to use nuclear weapons defensively in the event of a ground invasion.

It's worth remembering that this scenario is between two neighboring regions with a relatively small arsenal of nuclear weapons. The whole world has nearly 14,000 nuclear warheads spread between nine nations, many of whom have alliances and intensifying rivalries. To imagine the bigger picture, another study released last month detailed the potential effect of a nuclear war between the US and Russia, the two nations that possess the lion’s share of nukes. As their results showed, the effect would be even more dramatic.