This one still keeps me up at night, especially the figure on the 6th page.

https://web.archive.org/web/20180513182952/http://burro.case...

The short summary of this hypothesis is that the ocean develops hypoxic zones, anaerobic bacteria boom, and eventually the ocean starts releasing masses of poisonous H2S gas that wipes out most life on land (and strips the ozone layer for good measure).

They speculate that this might have been a mechanism behind the "great dying" at the end of the Permian. I'm sure the thinking has advanced in the last 20 years, but whenever people ask what the worst-case scenario for global warming could be, my mind drifts back to this.

Your url doesn't work, I can't read the article