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 have read somewhere that the main difference is that now we have 10 times more accumulated methane in the form of methane hydrates on ocean floor and permafrost. Methane is strong green house gas, like 100 times stronger than CO2. But as it lives only 6 years in atmosphere, the main difference would be not its amount, but the speed which which it could be realised from permafrost. This depends of speed of CO2 accumulation (which is now very high, while the total level is still low). The last thought is mine conclusion from what I read.
Geochemical situation also changed as we had several ice ages which are very good for methane hydrates accumulation and also because geography of Arctic ocean changed.
Anyway, I think that we runaway global warming may be easily prevented by artificial nuclear winter or some cloud seeding, if situation will be urgent. But AI situation is more complex.