The year is 2025 and militants have attacked India’s parliament, killing most of its leaders. New Delhi retaliates by sending tanks into the part of Kashmir controlled by Pakistan.

Fearing it will be overrun, Islamabad hits the invading forces with its nuclear weapons, triggering the deadliest conflict in history  and catastrophic global cooling.

This scenario was modeled by researchers in a new paper published on Wednesday, which envisaged more than 100 million immediate deaths, followed by global mass starvation after megatons of black soot block out sunlight for up to a decade.

It comes at a time of renewed tensions between the two South Asian rivals, which have fought several wars over the Muslim-majority territory of Kashmir and are rapidly building up their atomic arsenals.

They currently each have about 150 nuclear warheads at their disposal, with the number expected to climb to more than 200 by 2025.

“Unfortunately it’s timely because India and Pakistan remain in conflict over Kashmir, and every month or so you can read about people dying along the border,” Alan Robock, a professor in environmental sciences at Rutgers University, who co-authored the paper in Science Advances, said.

India’s Prime Minister Narendra Modi scrapped the autonomy of the part of Kashmir controlled by New Delhi in August, with his Pakistani counterpart Imran Khan warning the UN last week the dispute could escalate into nuclear war.

India has a ‘no first strike’ policy, but reserves the right to mount a nuclear response to any hit by weapons of mass destruction.

Pakistan has declared it would only use nuclear weapons if it could not stop an invasion by conventional means or were attacked first with nuclear weapons.

Based on their current populations and the urban centres that would be likely targeted, the researchers estimated up to 125 million could be killed if both countries expended the bulk of their highest yield weapons.

Around 75 to 80 million people were killed in World War II. This most extreme scenario would involve the use of 100 kiloton weapons, more than six times as powerful as the bombs dropped on Hiroshima.

A single airburst from such a bomb could kill 2 million people and injure 1.5 million — but most deaths would occur from the raging firestorms that followed suit.

“India would suffer two to three times more fatalities and casualties than Pakistan because, in our scenario, Pakistan uses more weapons than India and because India has a much larger population and more densely populated cities,” the paper said.

As a percentage of its urban population, though, Pakistan’s losses would be about twice those of India.

But nuclear Armageddon would only be the beginning.

“I hope our work will make people realise you can’t use nuclear weapons, they are weapons of mass genocide,” Robock said, adding that the paper lent more evidence to support the 2017 UN Treaty on the Prohibition of Nuclear Weapons. – AFP