This century’s worst-case climate scenario isn’t global warming of 4 or even 5 degrees Celsius. It’s a nuclear winter that would trigger global cooling up to 12 or 13 degrees.

That would happen within weeks of the start of a nuclear war, as smoke from burning cities blotted out the sun. The result would be a massive famine as the ocean’s food chain collapsed and global crops failed.

In most scenarios, hunger would spread around much of the globe and kill hundreds of millions of people, said Alan Robock, a climatologist at Rutgers University and co-author of two new studies on agriculture collapse and ocean destruction. How bad it gets depends on the size of the nuclear exchange, but even a "smaller” nuclear war — say, between India and Pakistan — would cause enough global cooling to starve hundreds of millions. In a war that involved Russia and the U.S., which have more powerful weapons and larger stockpiles, the death toll would likely exceed half the world’s population.