Each weapon detonated could kill as many as 700,000 people, the study said.

The global cooling could drop temperatures to levels not seen since the last Ice Age.

“This is a war that would have no precedent in human experience.”

As many as 125 million people would die within days if India and Pakistan waged a nuclear war, according to a scientific study released Wednesday.

That's more than the death toll during all six years of World War II.

“Such a war would threaten not only the locations where bombs might be targeted but the entire world,” said study co-author Alan Robock, a professor of environmental sciences at Rutgers University.

In the weeks and months after the explosions, a global climate catastrophe could follow, triggering mass starvation as crops failed across the planet.

“This is a war that would have no precedent in human experience,” said study lead author Brian Toon, an atmospheric scientist at the University of Colorado–Boulder. Toon has worked on nuclear war scenarios for decades, and was part of the team that coined the term "nuclear winter" in the 1980s.

That's the term used to describe a period of extreme cold that could follow a large-scale nuclear war between the U.S. and Russia.

Using computer simulations, scientists theorized a war that could occur between India and Pakistan in 2025. The two countries, which both place a claim and have waged several wars over the Kashmir territory, could have a combined 400 to 500 nuclear weapons by then, up from about 300 today.

Each weapon detonated could kill as many as 700,000 people, the study said.

The research also found that fires ignited by exploding nuclear weapons could release up to 36 million tons of soot (black carbon) in smoke that would rise into the upper atmosphere, spreading around the world within weeks.

This would block out sunlight, cooling the global surface by up to 9 degrees and reducing precipitation by as much as 30%, which would threaten food shortages and mass starvation around the world. The cooling could drop temperatures to levels not seen since the last Ice Age.

The study was published in the peer-reviewed journal Science Advances.

In a related editorial, Science Advances' deputy editor Kip Hodges said that now, unlike during the Cold War when only a few nations could start a nuclear war, nine countries possess a total of nearly 14,000 nuclear warheads.

Hodges said that when it comes to India and Pakistan, the deteriorating relationship between these neighboring countries puts south Asia – along with the rest of the world – at risk.

“Nuclear weapons cannot be used in any rational scenario but could be used by accident or as a result of hacking, panic or deranged world leaders,” Robock said. “The only way to prevent this is to eliminate them.”

Toon said that "hopefully, Pakistan and India will take note of this paper. But mostly, I'm concerned that Americans aren't informed about the consequences of nuclear war."