"Extinction" is a stricter outcome than “existential catastrophe", so I would imagine the odds this century are even lower than 0.1 percent. In other words: don't resign yourself to the extinction of humanity in your lifetime - it's highly unlikely, and you're just going to be feeding the defeatism that is so tempting when external events are so seemingly negative.
As sibling commenter to this post notes though, I would imagine that mass unrest, famines, death etc. in less-developed parts of the world will occur, with knock-on effects for the developed world.
The idea that all the humans will die off in the lifetime of anyone who is alive today, from something slow moving like climate change, is pretty far fetched.
Some places will be more habitable, like Russia.
That said I think it will cause immense socio-political chaos and mass death in our lifetimes.
The very same thing start on Earth w/ climate change. Temp raises slowly, more CO2 (and more importantly, CH4) in atmosphere. Temp reises more. Atmosphere is hotter, can absorb more H2O vapors as well. Now, when you start hitting first threshold, hydrates (CH4) will be released from shallow ocean bottoms, more CH4 in atmosphere hotter.. cycle accelerates hitting runaway (exponental growth). KABOOM, earth might be doomed.
Its simplified model, not accounting for huge vulcanos on polar, with might explode and make nuclear winter with will block the heating. Complicated stuff, thats why we need scientists :)
