I'm just not in the mood to devote a lot of time to what sounds (to me) like hyperbole: If the earth stabilised at +8 degrees celcius I find it hard to understand why the entire earth becomes literally uninhabitable.
Here is a well-written article (shared by another HNer here) on one plausible run-away event. But the same way as upon exerting more and more force to a branch it will first have linear changes going on, there will come a point where it will break.
Some possible runaway events include the aforementioned loss of clouds which in turn won’t reflect back that much sunlight (which currently can account for 30-70% of sun light not hitting the ocean water), melting ice will free lots of trapped CO2, changing climates may alter currents and winds causing very large-scale changes to certain areas (and might also affect cloud formation), etc. We are in a big, closed jar and that 8C change may not be a problem locally, it is that much more heat trapped globally.
But one should add that life is unlikely to die out in even the worst predictions (life, finds a way), but you still don’t want to find yourself in the situation where you will have to literally hunt down other humans because they may have food, or try to migrate with 100s of millions of other agitated, hungry people to nations which won’t be friendly to you because they themselves have enough problems to begin with.
If you want to think of a way it could be possible, though, think of those areas that would be habitable, realize that they would be highly contested, are mostly presently permafrost, and consider your own ability to both protect and survive on melted tundra and reproduce there, given you will starve in a single season without food.
I actually did consider that first - that maybe at +8c humans will kill each other, but that's just pure speculation.
And, sure, food is necessary daily and constantly, but the situation you outline isn't an overnight thing. Hell, it isn't even a 100-year change, +8c is a lot longer.
That gives you plenty of time to move, and at +8c, as long as it is stable at +8c, you'd still have enough arable and livable land to ensure that humanity is nowhere close to extinction.
So if it isn't lack of land, and it isn't the temperature range that causes human extinction, what does?
But if you add the fact that antibiotics will be in short supply, novel zoonotic diseases will ramp up, and most knowledge of how to survive in the wild has been lost and probably can’t be reconstituted in a couple of violent generations, I can see how it could come about.
There is also the social and psychological impacts of that scale of loss, and there are some other sort of scary indicators you could lean on (Calhoun rat experiments) to indicate that there are limits to what animal populations will reproduce through.
I’m not worried about it — we seem like a pretty robust system from my perspective. But it is interesting to think about abstractly.
> At one degree - the world we are already living in - vast wildfires scorch California and Australia, while monster hurricanes devastate coastal cities. At two degrees the Arctic ice cap melts away and coral reefs disappear from the tropics. At three, the world begins to run out of food, threatening millions with starvation. At four, large areas of the globe are too hot for human habitation, erasing entire nations and turning billions into climate refugees. At five, the planet is warmer than for 55 million years, while at six degrees a mass extinction of unparalleled proportions sweeps the planet, even raising the threat of the end of all life on Earth.