happened before and the post-war apocalypse.

In the worst case, mass fires consume everything  within tens of thousands of square kilometers,  

killing hundreds of millions within  hours. But the worst part comes  

after that - nuclear war could trigger a  nuclear winter that might kill billions,  

maybe even completely collapsing our civilization.  How does it work and what would it look like?

Fire Causes Winter

When a nuclear weapon is detonated, a bubble of  gas hotter than the sun is forced into existence,  

so hot that everything within kilometers  immediately begins to burn. The terror bubble  

expands rapidly, filling the sky over its target,  creating a devastating shockwave that causes most  

of the immediate destruction. Basically you  break a lot of stuff and set it on fire – and  

in the worst case this turns into a firestorm that  consumes everything and everyone on the ground.

Right after the explosion a gigantic mushroom  cloud rises over the destruction like a demon  

throning over its perverse work, but in  the following hours a far more deadly  

cloud forms. The fire burning cities,  forests or fields, heats up so much air  

that it creates its own micro climate and  wind system. Hot air and hot smoke rise,  

pulling in fresh air from the surroundings,  and fresh oxygen stoking the flames even more.  

This creates an updraft and forms  a colossal pyrocumulonimbus cloud  

that carries the soot and aerosols from  the flames high into the stratosphere.

Under normal conditions, the soot rising  from a big fire is quickly washed out by rain.

But a pyrocumulonimbus cloud can  reach altitudes well above the height  

where rain clouds form. Once above the tropopause,  

there is simply no weather to remove soot from  the atmosphere, so it can stay aloft for years.

If this happens to a single city, it's a tragedy,  but a fairly local one. But in a full scale  

nuclear war, warring nations following the cold  logic of mutually assured destruction, could use  

hundreds or even thousands of nuclear weapons all  at once, creating hundreds of fire storms, sending  

up to 150 million tons of soot, a cube the size  of a skyscraper, directly into the stratosphere.

In the next few days and weeks the soot  begins to blanket the earth at high altitudes,  

absorbing light high above the ground  and preventing sunlight from reaching  

the surface . This is not like science fiction  where the sky turns dark and the sun disappears.  

Winter is what happens when just a little less  sunlight hits the ground – and now suddenly a  

lot less sunlight gets through. Yesterday the  world was normal. Today nuclear winter begins.

Winter Causes Hunger

How bad nuclear winter would be is still an active  area of research. It all hinges on one thing:  

How much stuff will burn really hot? How many  firestorms would be caused by the heat of the  

explosions? This depends on many factors, from  the materials a city is made of, to the time of  

the year, if a forest is nearby and so on. So just  keep in mind we are working with some assumptions.

Here is the good news: Nuclear winter is  not permanent, and definitely no new ice age.

The effects on the climate only last as  long as the soot remains in the atmosphere,  

which is at most a decade or so until it  clears out and temperatures normalize.

The bad news is that this causes almost  immediate climate change within a few  

weeks – it disrupts our climate system  faster than any living being can adapt to.

In this new climate our seasons are suddenly  all wrong. Winters are much longer, summers  

shorter and colder – or gone altogether. This  also means less evaporation over the oceans,  

which means less rain and maybe large scale  droughts. This is bad because our food eats the sun.

Without good summers and enough rain,  growing seasons shrink or even collapse.

The majority of humanity lives in  an area called the midlatitudes,  

a strip of land that has the ideal temperature  for our species – not just because it's not  

too hot or cold, it's also where the plants  we eat grow best. The vast majority of the food  

we eat stems from a few highly efficient  crops, that are mostly produced in a few  

very agriculturally productive regions, like  the US Great Plains or Ukraine. From these  

bread and rice baskets of the world, they  get traded and shipped around the world.

In the worst case of a full scale nuclear  war the temperatures in the midlatitudes  

will probably stay below freezing for  several years. Nothing at all can grow  

under these conditions and the world's  breadbaskets would suddenly turn empty. 

If food production crashes, the world's food  producers would very likely ramp up prices or  

even stop selling food to other countries – if  they're still able to farm their fields at all.

It's easy to calculate how many people can be  alive on earth – you take all the calories we  

can produce and divide them by what the  average person needs to survive. If you  

have more people than calories, then within a few  weeks you don't anymore. Humanity has only a few  

weeks' supply of crops and food, not enough  to survive this drastic drop in production.

But the climate is not the only issue: modern  industrialized agriculture is a complex affair  

that relies on functioning supply chains to  provide unthinkable amounts of industrially  

produced fertilizer and chemicals to kill weeds  and vermin. Massive numbers of specialized modern  

machinery is plowing, sowing, harvesting  and distributing. After a nuclear war,  

especially if the countries that produce the  food were part of the nuclear exchange, there  

may simply be no more fuel, fertilizer or machine  parts, because there are no more oil refineries,  

ports and other essential infrastructure left,  damaging global food production even more.

Ok, so now that we set the stage, let's look  

at what science says about the  actual wars that could happen.

Actual Nuclear War

Today there are two main conflicts that scientists  think about when making calculations of nuclear  

winter: a nuclear war between India and  Pakistan and one between the US and Russia.

The most likely smallish nuclear exchange would  be fought today between India and Pakistan,  

with relatively low yield weapons. Even  in a pretty mild nuclear war like this,  

the immediate explosions would  kill around 27 million people,  

which is horrible enough. In just a few  hours, more people would die than in all  

of World War 1. The ensuing fires would not cause a nuclear winter,

but a mild 'nuclear autumn'. But even this would disrupt the climate,  

and thereby global agriculture, enough to  starve up to 250 million people worldwide.

Unfortunately India and Pakistan are in an  arms race and have been increasing the number  

and power of warheads in their arsenal. The next  stage of escalation would be war with hundreds of  

nuclear weapons, the bombs and fires destroying  many major population centers and killing over  

100 million people. A war on this scale would  cause a nuclear winter that would damage global  

agriculture enough to cut the available calories  for humanity in half. The number of people

that starve to death would be as high as 2 billion.

One in four humans alive today.

The worst-case scenario is a full scale global  war between NATO nations and Russia – or China,  

which also continues to build its nuclear  arsenal. In a war between a former,  

future and current superpower, thousands  of nuclear weapons could be detonated.

In a scenario with around 4400 nuclear weapons,  360 million people would perish right away. We  

have no other event to even compare the death  and destruction to. It's like humanity dropping  

an asteroid on itself. The nuclear winter  that follows such an apocalyptic war would  

tank human calorie production by as much  as 90%. Not only would almost all of our  

agriculture take an immediate and deadly  hit, the climate would take at least a  

decade to recover. Because a war like this would  specifically hit the world regions that produce  

most of the food for humanity – recovery will  be much, much harder than with other conflicts.

Within two years the global death toll from  starvation could rise to about 5 billion.  

In mid latitude countries like Russia, China,  Canada, the U.S. and much of Europe only a few  

percent of the population might survive.  Humanity will never be the same again.

While nowhere is truly safe, some nations in  the southern hemisphere may fare well enough  

to endure, while the rest of the world collapses.  All the nuclear weapon states are in the northern  

hemisphere. So a few countries like Australia, New  Zealand, and Argentina may be able to endure for  

a bunch of different reasons. Their nuclear winter  would be milder, they have a lot of livestock that  

would not be as affected as crops. So they would  probably stop exporting food and focus on keeping  

their own people alive – assuming they aren't  invaded for their food by other starving nations.

It's safe to say that the world would become  extremely unpleasant for a long time and it's

impossible to know how many people will have  died when the nuclear winter ends. In the worst  

case human civilization could collapse  and the survivors would be thrown back  

thousands of years, slowly trying to recover  a world full of scars and graves. Eventually,  

when they've rebuilt civilization, would  they ever build nuclear weapons again?

We know for sure that we need to do  anything we can to make sure nuclear  

war never happens. This video was supported  by Open Philanthropy – if you want to know  

what YOU can do to reduce the risk for  nuclear war you can either support expert  

organizations or become a citizen expert  yourself and learn more. We've compiled a list  

of further reading and expert recommendations  in the infobox and our sources doc for you.

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