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
Why is runaway global warming at "total extinction"/level 5 and not at the other levels?
Because it is "runaway" )). This means that temperature would rise really high like on Venus. Some scientists think that this is possible because methane and water vapor are also greenhouse gases, and quick release of methane from arctic floor may be the start of this process. See more here: http://arctic-news.blogspot.ru/