Their impact starts at .1%

Toby Ord: Carl Sagan suggested it could potentially lead to our extinction, but the current people working on this, while they are very concerned about it, don’t suggest that it could lead to human extinction. That’s not really a scenario that they find very likely. And so even though I think that there is substantial risk of nuclear war over the next century, either an accidental nuclear war being triggered soon or perhaps a new Cold War, leading to a new nuclear war, I would put the chance that humanity’s potential is destroyed through nuclear war at about one in 1000 over the next 100 years, which is about where I’d put it for climate change as well.

There is debate as to whether climate change could really cause human extinction or a permanent collapse of civilization. I think the answer is that we don’t know. Similar with nuclear war, but they’re both such large changes to the world, these kind of unprecedentedly rapid and severe changes that it’s hard to be more than 99% confident that if that happens that we’d make it through and so this is difficult to eliminate risk that remains there.

In the book, I look at the very worst climate outcomes, how much carbon is there in the methane clathrates under the ocean and in the permafrost? What would happen if it was released? How much warming would there be? And then what would happen if you had very severe amounts of warming such as 10 degrees? And I try to sketch out what we know about those things and it is difficult to find direct mechanisms that suggests that we would go extinct or that we would collapse our civilization in a way from which you could never be restarted again, despite the fact that civilization arose five times independently in different parts of the worlds already, so we know that it’s not like a fluke to get it started again. So it’s difficult to see the direct reasons why it could happen, but we don’t know enough to be sure that it can’t happen. In my sense, that’s still an existential risk.