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
While I don't agree with everything arctic news say and I see that they overhype some recent small changes in methane concentration, I heard the idea of runaway global warming before. I also think that artificial runaway global warming is possible if nukes will be used to destabilize methane of ocean floor. While answering previous comment I was surprised to find in wiki that if median earth temperature once rise to 47C, than water vapor greenhouse effect will push it until new stable state at 900 C. I still think that we should be more afraid of small probability (I estimate it in 1 per cent) of runaway global warming than of high probability conventional global warming (2-4 C).
The main thing about it is, in my opinion, connected with your notion in the second period in your post that we don't observe past runaway global warming. (Except small scale one like 55 mln. years ago.) One of the reason for that may be observation selection: there is no life on all planets with runaway global warming (Venus).
But more serious consequence of it may be that runaway global warming is long overdue and our atmosphere is in metastable condition where small influence may flip-flop it. I wrote an article about in before "Why anthropic principle stopped to defend us"? https://www.scribd.com/doc/8729933/Why-anthropic-principle-stopped-to-defend-us-Observation-selection-and-fragility-of-our-environment (It needs to be rewritten).
Points taken. Still, the PETM was as recent as 50 million years ago, when the sun was on the order of 99.5% as bright as it is now and the base state from which the temperature and atmospheric carbon rose (as quickly as we have measurements able to say - as in, it took less than 15k years and they cant be more specific than that and there's evidence that at least some places warmed ifaster) was one in which there were fewer eroding mountain belts to suck up carbon and more subduction zones to produce it and a geological equilibrium CO2 concentration on th... (read more)