> Not sure about that on a couple-of-hundred-years-from-now time-scale.
It's really not possible. Even if you took all of the oxygen and carbon on earth and converted it to CO2 (which is not even remotely possible) you still wouldn't get Venus. Venus's atmosphere is almost 100 times denser than earth, and it's nearly 100% CO2. Venus is also closer to the sun, which isn't the deciding factor, but it certainly helps keep Venus nice and toasty warm.
Look, earth is 4 billion years old, and life has existed for about 3.9 billion of those, give or take a few hundred million years. Environmental catastrophes have come and gone and life has persisted through all of them. But civilization has only existed for 10,000 years or so, and those 10,000 years have been climactically stable. Unusually stable. It is far from clear that civilization can survive climate change. That experiment has never been done.
> it's possible that 90% of humanity dies
Yes, that is definitely possible. Maybe even likely if we stay on the current trajectory.
> you still wouldn't get Venus. Venus's atmosphere is almost 100 times denser than earth, and it's nearly 100% CO2
That extremely high amount of CO2 in the atmosphere of Venus is not the original cause but a consequence of runaway warming. At a few hundred degrees C, the carbon dioxide bound in rocks starts to sublimate into the atmosphere. For example, limestone would go CaCO3 -> CaO + CO2. 10% of all sedimentary rock is limestone so one can see how that leads to a Venus-like atmosphere.
That said, I agree that a Venus scenario doesn't seem to be possible for Earth right now. In this paper published in Nature, even 3000 ppm of CO2 (i.e. all fossil fuels burned) was calculated not to trigger such a thing. It would require more like 30,000 ppm, which is not really breathable atmosphere anymore:
Not sure about that on a couple-of-hundred-years-from-now time-scale.
> 2. Homo sapiens is almost certainly not going to go extinct.
I agree. But it's possible that 90% of humanity dies, mainly because of starvation after the oceans become lifeless.
I also think there are going to be massive refugee streams in the order of more than 100 million people.