The ocean generates 50 percent of the oxygen we need, absorbs 30 percent of all carbon dioxide emissions and captures 90 percent of the excess heat generated by these emissions.
Slow changes, a return to a Cretaceous-style climate, etc. are a very different story than an "overnight" exponential and unstoppable Venusification of the planet.
Slowly rising sea levels in Miami vs one day you wake up and can't breathe anymore. Very different situations.
>An anoxic event describes a period wherein large expanses of Earth's oceans were depleted of dissolved oxygen (O2), creating toxic, euxinic (anoxic and sulfidic) waters.[1] Although anoxic events have not happened for millions of years, the geologic record shows that they happened many times in the past. Anoxic events coincided with several mass extinctions and may have contributed to them.[2] These mass extinctions include some that geobiologists use as time markers in biostratigraphic dating
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.
As the arctic ice will disappear we will not just loose a few shiny white objects, but we will loose a heat buffer, to the tune of 334 kJ/kg ... times 18 000 cubic kilometers. That is 6 x 10 ^ 21 Joules.
Check this graph (note the zero is absolute):
https://en.wikipedia.org/wiki/Measurement_of_sea_ice#/media/...
We are going to witness the loss of a huge thermal buffer in a few decades at most.