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We have to distinguish between end-of-human-world events and, as the NASA boffins put it, and end-of-modern-civilisation events.
End-of-world events include stuff like a massive asteroid collision, a rampant new disease, global nuclear war.
End-of-modern-civilisation events would be of lesser scale - typically, drought and famine, civil war on a large scale, financial and economic meltdown, large-scale national wars, gradual decline through uncontrollable diseases.
I think NASA is talking about the second. And probably about a convergence of man-made and natural catastrophes to have the necessary effect.
A civil war that would have huge ramifications outside national borders would be a 2nd American civil war. And I don't see one as unlikely. There are just so many touchpapers - even that old favourite: religion - for civil unrest to scale itself into armed conflict. And, let's face it, the American are the most heavily-armed population on the planet, so it would not be a slow escalation or a small war.
Peak oil is far less likely. Oil consumption is slowing. The planet still has vast reserves exploitable by new methods. New exploration reveals new deposits. Even a place like New Zealand is being scoured for deep-sea oil fields. Besides which, alternative energies are becoming more popular. Led, funnily enough, by the Chinese. I would expect a gradual but successful transition from fossil fuels to new energies.
But the most likely seems to be the degradation of the ability of modern medicines to control disease. Led by the already-growing disease immunities to antibiotics.
That or a similar "disease" scenario developed purposefully by the cyber warfare fraternity. The opportunity for miscalculation and unintended consequences in that arena are huge.