Well, a full nuclear exchange between the U.S. and Russia would end civilization as we know it. Though it is unlikely to wipe out the human race entirely, we'd be reduced to paleolithic population levels and it's really hard to predict how things would develop from there. A nuclear exchange between the U.S. and China, or between India and Pakistan on the other hand would cause a decades-long crisis, but wouldn't outright destroy civilization in the way a U.S.-Russia exchange (with 10x-100x more nukes) would.

I would personally be more worried about another global pandemic and/or biowarfare/bioterrorism. An infection with the virility of SARS-CoV-2 but the case fatality rate of Ebola or MERS would end civilization. What'll keep you up at night is realizing that this is almost at the point where a biology grad student could engineer such a thing in their dorm room.

It should be noted that people who think about these things are also worried about AI existential risk. Personally I think that while there is reason to have some concern, it doesn't hold a candle to the above two risks.

I've forgotten what the method was but it is within the realm of possibility today for a single wealthy individual to destroy the world at their whim.

IIRC it didn't require any fancy future tech or bombs. It just required the will with current tech. Like sending up 10000 balloons full of a certain kind of existing chemical into the upper atmosphere.

IIRC the article was about governments deciding they need to take planet wide effecting geo hacking steps for global warming but what if they're wrong. Following some of those steps it was clear that a modestly rich individual could implement some of them on their own.

I can't remember which website or podcast I heard this from. It could have been 80000 Hours or Making Sense or Slate Star Codex