Levels of global catastrophes: from mild to extinction
It is important to make a bridge between existential risks and other possible risks. If we say that existential risks are infinitely more important than other risks, we put them out of scope of policymakers (as they can’t work with infinities). We could reach them if we show x-risks as extreme cases of smaller risks. It could be done for most risks (with AI and accelerator's catastrophes are notable exceptions).
Smaller catastrophes play complex role in estimating probability of x-risks. A chain of smaller catastrophes may result in extinction, but one small catastrophe could postpone bigger risks (but it is not good solution). The following table presents different levels of global catastrophes depending of their size. Numbers are mostly arbitrary and are more like placeholders for future updates.
http://immortality-roadmap.com/degradlev.pdf
I fail to see the difference. Of course, if we take into account possible space colonies it will not be extinction... Also the article speaks about "which refers specifically to climate changes that cause a planetary body's water to boil off". I don't think it is survivable.
Also runaway global warming is (relatively) easy to prevent by the means of geo-engineering.
Most of the planet being uninhabitable means that there are still part of it that are habitable.