Comment by mitthrowaway2

Worst case is warming oceans create a hypoxic environment where anaerobic bacteria thrive en masse and generate H2S over country-scale areas undersea. The chemocline breaches the surface and the ocean and atmosphere become poisonous to most complex life and agriculture, also stripping the ozone layer in the process, irradiating the surface. This is one mechanism posited for the end-permian mass extinction that eradicated most ocean and surface life, including the trilobites.

Not sure how long we'd survive such a scenario, even sheltering underground. But surely it couldn't happen to us.

(However, it now seems like the AI might get us first.)