For about forty years, the entire human species lived one bad phone call away from extinction. We got lucky. Repeatedly. But let's say we didn't. Let's say it's 1983, and the warnings on the Soviet early-warning screens aren't a false alarm this time. Let's say someone gives the order.
The first hour
At the peak of the Cold War, the United States had approximately 31,255 nuclear warheads. The Soviet Union had roughly 39,197. Not all were deployed. Not all were strategic. But enough were aimed at each other's cities, military bases, submarine pens, and missile silos to destroy both nations several times over.
A full exchange begins with the ICBMs. Soviet SS-18s (the ones NATO called "Satan," which tells you something about the mood) launch from silos in Siberia. American Minuteman IIIs leave their holes in Montana, Wyoming, and the Dakotas. Flight time across the pole: about 30 minutes. That's how long the other side has to decide whether to retaliate before their own missiles are destroyed on the ground.
They always retaliate. Every simulation, every war game, every analysis concluded the same thing. If you see missiles inbound, you launch yours before they arrive. It's called "launch on warning" and it means there is no scenario where only one side fires.
Both sides also have submarine-launched ballistic missiles. American Ohio-class boats lurking in the Atlantic and Pacific. Soviet Typhoon-class submarines under the Arctic ice. These are the true doomsday weapons because they can't be found and destroyed in a first strike. Even if every silo on one side is obliterated, the submarines ensure retaliation. That was the whole point.
What a nuclear weapon does to a city
A single 800-kiloton warhead (a common yield for Soviet strategic weapons in the 1980s) detonated above central London would kill approximately 870,000 people instantly. The fireball, about two kilometres across, would vaporise everything in the City and most of Westminster. Out to five kilometres, the blast wave collapses every building. Out to ten, third-degree burns on exposed skin. Out to fifteen, windows shatter and flying glass becomes the weapon.
London wouldn't get one warhead. Soviet targeting doctrine assigned multiple warheads to major cities and military targets to ensure destruction even if some missiles failed. Estimates suggest London would receive between four and eight warheads. Moscow would receive a similar allocation from the American side.
Now multiply that across every major city in NATO and the Warsaw Pact. Washington. New York. Chicago. Los Angeles. Moscow. Leningrad. Kiev. Every major military installation, every port, every air base. Roughly 500 targets in the US alone. Comparable numbers in the USSR.
Within sixty minutes of the first launch, somewhere between 500 million and one billion people are dead.
The fire
Nuclear weapons don't just blast. They burn. The thermal pulse from a large warhead ignites everything flammable within a radius of about 15 kilometres. Every building, every vehicle, every tree, every person.
In a city, these individual fires merge into a firestorm. A firestorm creates its own weather. The heat rises so violently that it sucks air inward from every direction at hurricane speeds, feeding the fire with fresh oxygen. Temperatures at the centre exceed 800°C. Nothing survives a firestorm. Nothing.
After Hiroshima, researchers found the shadows of people burned into concrete walls. The people themselves were gone. And Hiroshima was a 15-kiloton weapon. The 1980s warheads were fifty times more powerful.
Hundreds of major cities burning simultaneously creates a problem that goes well beyond the cities themselves.
Nuclear winter
In 1983, the same year the Cold War nearly went hot for real, five scientists (including Carl Sagan) published a paper describing what would happen to Earth's climate after a full nuclear exchange. They called it TTAPS, after their surnames. The scenario they described was nuclear winter.
The firestorms loft millions of tonnes of soot and ash into the upper atmosphere. This black carbon absorbs sunlight before it reaches the ground. Global temperatures drop. Not by a degree or two. By 15-25°C in continental interiors. For years.
Growing seasons collapse worldwide. Photosynthesis slows drastically. Crop yields fall by 90% or more for at least a decade. Sunlight reaching the surface drops to a fraction of normal levels.
The people who survived the bombs, the firestorms, and the fallout now face starvation. And not just in the combatant nations. Nuclear winter is global. A farmer in Argentina, a fisherman in Indonesia, a herder in Kenya. None of them launched or were targeted by a single warhead, and all of them are starving because the sky went dark.
Recent modelling from Rutgers University (published in 2022 in Nature Food) estimated that a full US-Russia nuclear exchange would kill about five billion people through famine alone. Five billion. On top of the billion or so killed directly.
The fallout
Nuclear detonations produce radioactive fallout. For ground bursts (used against hardened military targets like missile silos), the explosion scoops up soil and irradiates it. This material rises in the mushroom cloud and then drifts downwind, settling as radioactive dust over thousands of square kilometres.
Fallout from a single ground burst can deliver lethal radiation doses to areas hundreds of kilometres downwind. Now picture thousands of detonations across the Northern Hemisphere. The fallout zones overlap. There is nowhere in Europe or North America that isn't contaminated to some degree.
Iodine-131 accumulates in thyroid glands. Strontium-90 settles in bones. Caesium-137 contaminates soil for decades. Cancer rates among survivors would be catastrophic. And with no functioning hospitals, no supply chains for medication, no power for diagnostic equipment, cancer becomes a death sentence again for virtually everyone who develops it.
What's left
The Southern Hemisphere survives the initial exchange largely intact. Australia, New Zealand, most of South America, sub-Saharan Africa. No warheads fall there. But nuclear winter doesn't respect the equator. Temperatures drop globally. Crops fail globally. Supply chains that connected the world economy are gone because the world economy is gone.
These nations would face the impossible task of sustaining their populations without imports, with reduced sunlight, with millions of refugees from the north (those who managed to get on boats or planes before infrastructure collapsed), and with no international order to coordinate anything.
Some would manage. Probably. Humans are difficult to kill off completely. We survived the Toba supervolcanic eruption roughly 74,000 years ago, though genetic evidence suggests our population may have dropped to as few as 10,000 breeding individuals.
But civilisation as a continuous project? The accumulated knowledge, infrastructure, institutions, and trust that allow eight billion people to coexist on one planet? That's gone. Rebuilding it took us about 10,000 years the first time, starting from the agricultural revolution. There's no guarantee the second attempt goes faster. The easily accessible coal, oil, and metal ores that powered the Industrial Revolution have already been extracted. A rebuilding civilisation would have to skip straight to renewable energy and recycled metals, which requires a level of technical knowledge that may not survive the collapse.
Why this matters now
The Cold War ended in 1991. But the weapons didn't go away. Russia still has approximately 5,580 warheads. The United States has about 5,044. That's more than enough. The Rutgers study's worst-case famine scenario assumed only a fraction of current stockpiles being used.
We tend to think of nuclear war as a historical anxiety, something our parents worried about. But the weapons are still there, still on alert, still minutes from launch. The Bulletin of the Atomic Scientists set their Doomsday Clock to 89 seconds to midnight in January 2025. It was the closest it had ever been.
Forty years of the Cold War produced at least a dozen moments when miscalculation, technical failure, or human error nearly triggered the exchange. Stanislav Petrov in 1983. The Norwegian rocket incident in 1995. Able Archer 83. Each time, one person or one small group made the right call. We've been running that lottery for decades, and we've won every time so far.
So far is doing quite a lot of work in that sentence.