Phytoplankton are also responsible for 50-80% of atmospheric oxygen, so in the parent poster's scenario I think the loss of a breathable atmosphere is the most

Phytoplankton are also responsible for 50-80% of atmospheric oxygen, so in the parent poster's scenario I think the loss of a breathable atmosphere is the most likely nail in our species' coffin. Probably won't happen, probably not a great idea to push things to see how far we can let things go before it's catastrophic, though.