[I also posted this in Environment/Anthropocene]
This occurs to me every time Mr. Thom mentions the looming Methane [or methane-ethane] and to my knowledge he never puts teeth in it. So [besides epic flooding, land loss, mass migration, invasive species, etc] why would global warming be bad?
wikipedia : "Anoxic Event" :
also PETM [Paleocene-Eocene Thermal Maximum cited by Mr. Thom]
[Wignall, Paul B.; Richard J. Twitchett (24 May 1996). "Oceanic Anoxia and the End Permian Mass Extinction". Science. 5265 272 (5265): 1155–1158. Bibcode:1996Sci...272.1155W. doi:10.1126/science.272.5265.1155. PMID 8662450.]
These links give a caution that the End-Permian injury is repeated now in the Anthropocene at maybe 20,000 times the speed [200 years of Industrial Revolution to 2,000,000 years of Siberian volcanic outgassing].
Oceanic acidification [from pH 8.2 downward] and CO2 saturation might well kill today's corals and then generally today's species [oceanic and terrestrial].
[Bacteria and some Archaea, usually photosynthetic and anaerobic, thrive in these man-made conditions and can again change the huge ocean production from oxygen to poison gas.
Off West Africa this has occurred recently from overfishing [sardines if I recall] and the resulting overgrowth then die-off of algae at the ocean floor. Local extinction of fish meant a murderous increase in bushmeat killing [including chimpanzees -- people want protein!].
In few : we are voracious and ignorant. We strive to force the ocean to switch from producing dioxygen [O2] from phytoplankton to producing hydrogen sulfide [H2S] which would kill us and most any living thing.