A nuclear detonation from one of today’s more powerful weapons would cause a fatality rate of 80 to 95 percent in the blast zone stretching out to a radius of 4 kilometers — although “severe damage” could reach six times as far.

But it isn’t just the immediate deaths we need to worry about — it’s the nuclear winter. This is when the clouds of dust and smoke released shroud the planet and block out the sun, causing temperatures to drop, possibly for years. If 4,000 nuclear weapons were detonated — a possibility in the event of all-out nuclear war between the US and Russia, which hold the vast majority of the world’s stockpile — an untold number of people would be killed, and temperatures could drop by 8 degrees Celsius over four to five years. Humans wouldn’t be able to grow food; chaos and violence would ensue.

Hundreds of nuclear weapons are ready to be released within minutes, a troubling fact considering that the biggest threat of nuclear war may be an accident or miscommunication. A few times since the 1960s, Russian officers (and, in 1995, the president) narrowly decided not to launch a nuclear weapon in response to what they’d later find out were false alarms.