system
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human
ELI5: How a nuclear war would destroy the planet

Whenever the issue of nuclear war comes up in movies, books etc. it is mentioned that nuclear war would cause human extinction or destroy the planet entirely. I don't see how this could be the case - if, say, Russia and America were to go to war, how would a nation like Nigeria be affected? Is this true, or is it just exaggeration?
gpt
JakeSteam's points are not entirely true.
  The reason why life could easily die on the surface of the earth after a nuclear war is twofold.

   The first is dust/ash.  This is called 'nuclear Winter.' There is ample available evidence that even small volcanos, and the smoke and dust they kick up can cause freezing, and snowing effects on other continents.  The exchange of a nuclear event would cause multiple times the amount of that dust and would exceed multiple times the ash that all of the volcanoes on earth could produce.  
  The extent of the world snowing for ten years would cause almost all biological systems the world over to die.  Some things, probably only things that don't exist connected to the surface, would survive.  How the ecosystem could function after ten years of snow, if at all, is anyone's guess.  Mass starvation is guaranteed, survival of the human race is not.

  The second problem is radioactive particles.  Microscopic radioactive particles would be scattered worldwide.  Ingesting even the smallest dust speck of plutonium or cobalt-60 in your lungs will cause internal burning at that location for the rest of your life, which won't be long, as you're getting an internal sunburn every day from that point forward.  The radiation in your lungs will decay for thousands of years.  Your body, once it ingests a heavy metal, cannot remove it, just like how mercury poisoning cannot be removed without extreme means with ketones and the like.  Those won't be available en masse, and are uncertain with radioactives.
  In short, you breathe in radioactive dust, you burn on the inside for the rest of your life, however short that may be.  We're talking weeks, maximum.

  We, as a species, spent endless, endless hours of resources making weaponized fissionable materials, grinding down with centrifuges the tiniest fractions of a percentage of amounts of materials, that are naturally occurring, literally distilling and spinning them 24/7 for years, and years, and years, until we get a weaponized product.  It is an abomination against nature that we would do this, nobody should have ever thought that it was necessary, but here we are.  We've spent endless hours, and then we expect to release it to the world, and this is what we get.

  We made the world's most deadly, perpetual, natural, and endless poison in plutonium and other weapons grade fissionable materials.  A bomb of this like will do damage into generations beyond ours.
  This will go down as the stupidest mistake mankind ever made.