Forget about the geopolitcal issues, which will indeed be massive. What about food production?
In 2010 a single heat wave triggered the Arab spring revolts a couple of months later.
> The wildfires destroyed one-third of Russia's wheat harvest. The Russian government refused to export the rest of its harvest and set a grain export ban to fight inflation. This led to extremely high food prices, which led to panicking on the global markets. Many experts including from the International Food Policy Research Institute say that the Russian wildfires in summer 2010 played a leading role in triggering the Arab Spring starting in 2010, especially in Egypt's case. Higher food prices helped to make oppression, poverty, and corruption under autocratic leaders even more aching for the local population. Also, as Russia was not even a member of the WTO at the time, banning exports was not a problem.
https://en.wikipedia.org/wiki/2010_Russian_wildfires
We are approaching a world were these kind of problems with food production will be globalized and will affect everything but specially the global economy.
While the "dark ages" were hardly dark and ignorant, the collapse of the Roman empire meant advanced road building and concrete were lost for 1500 years. So there is some historical precedent here.
It’s not like every book in every library would or could disappear.
If you took someone who worked in a steel mill I bet they would have no idea how to produce steel from first principles. They know what their job entails, they know which bits of equipment need serviced an maintained but they don't know how to get from rocks to bar stock.
Similarly if you had a perfectly working steel mill and you cut the supply of coke or iron ore few would know how to get it up and running again. If you cut the supply of the alloying elements you might get a passable mild steel but not one to the exact specifications you require.
I think it's probably surprising how few vital links you have to cut before life as we know it breaks down.
I mean some obviously do https://goodmenproject.com/featured-content/world-climate-ch...
here is someone who denies that it is an existential threat but provides a lot of nice links to people saying it is https://www.climatechangenews.com/2019/06/04/i-denier-human-...
Even worst case predictions for climate change don't imply the human race will be threatened. But what we will see is massive geopolitical disruption as previously habitable regions become uninhabitable. What happens when coastal Myanmar is underwater or southern India experiences 130 degree days on a regular basis? Climate change is going to make itself felt over the coming decades as increases in migrant crises, regional conflicts, and famines. (Not to mention economic loss as, e.g., the agricultural viability of large areas changes.)