I know there would probably be an ash-based cooling period, but, how long would that last and then what happens? Is the post-apocalyptic desert even possible in a post-nuclear war world?

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    $\begingroup$ How severe was the war? Ash-based effects on Earth run from as "mild" as the eruption of Mount Tambora -- the Year Without A Summer -- to its being a major factor in the extinction of the dinosaurs. $\endgroup$ – Mary May 17 '20 at 4:08
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    $\begingroup$ It also depends on what kind of nuclear war it is. You get most ash from the firestorms caused by nuking urban targets. You get the least by nuking sparsely populated missile silos and military bases. So do combatants do counter-force attacks or counter-value attacks? $\endgroup$ – Ryan_L May 17 '20 at 4:33
  • $\begingroup$ Technically, WW2 was a nuclear war, because two nuclear weapons were dropped o the enemy. It depends on how many nuclear weapons are actually detonated. There is some concern that the American armed forced are so poorly maintained that half of the ICBM's would not be capable of being launched. Certainly many Russian nukes would fail before launch. It is estimated by some credible sources that about half of the cruise missiles America WANTED to send against Syria never made it for various reasons. $\endgroup$ – Justin Thyme the Second May 17 '20 at 19:58

We don't know.

There will be several factors which affect the climate in different ways. There is no empirical data on which will dominate.

  • How many weapons and where? Arsenals are much smaller than they were at the height of the Cold War, and we can assume that Moscow, Washington, and places like Minot AFB are still getting overkill.
  • Firestorms could blow ash into the higher atmosphere and cause a nuclear winter. That could be just a brief effect, a few years or a decade, or it could lead to increased snow/ice cover and increased albedo. That lowers the temperatures further, and a feedback loop occurs.
  • Or the firestorms could lead to deforestation in some fragile ecosystems, and runaway desertification.
    It could also be that the death of many humans reduces deforestation, instead.
  • In addition to direct climate effects, killing people and economies will also matter, but 200 years could be enough to get people back on their feet and emitting greenhouse gases as before.

Imagine you have a bucket full of water, with a couple of holes in it that cause a steady loss of water. There is also a water pipe above with slow leaks, steadily refilling it. The water level in the bucket is mostly stable. That's our climate. And then somebody gives the whole Rube Goldberg contraption a good shake. Nobody can predict if the water level will rise and drop after that. Only that the shake is a very bad idea ...

So, for your story/setting, select what you want and info-dump a bit of technobabble.

  • You want it cold? Nuclear winter killed the thermohaline circulation.
  • You want it hot? Firestorms killed the rainforests.
  • You want a mostly normal ecology with a few sick trees and animals? The northern hemisphere is like the Chernobyl Exclusion Zone.

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