The great oxygenation event is probably the most similar historical precedent for what could happen if we continue chemically sterilizing the oceans:
https://en.wikipedia.org/wiki/Great_Oxygenation_Event
We're in for a very tough time--I don't want to sow any seeds of defeatism or unnecessary cynicism, but we are no longer looking at a choice between catastrophe and a continuation of our normal way of life. We are looking at a choice between catastrophe and utter cataclysm. Between hundreds of millions of deaths and billions. This is going to eventually come to truly desperate measures. Humankind will not get through this without having to make some incredibly difficult and bloody decisions in the future. Every day we dawdle and ponder what to do makes the decision our descendants will get stuck with more and more horrific and Pyrrhic. It will eventually come to a point that the most rational decision is to start attacking industry and agriculture with force. That's terrible, but there will come a day when it's an act of basic self-defense. It will also lead to strife and death and famine for millions. But we no longer have the privilege of choosing an option that doesn't involve suffering and atrocity--we have missed our best chances and in the future we will be forced to choose based on the degree of harm, not whether or not there will be harm. The horrors of the 20th century won't even hold a candle to what we're currently laying out for our offspring. It's utterly shameful.
The world today is safer, more civilized, and more prosperous than it has ever been in history, but this is a false stability that we're borrowing against our own future to maintain. It's eventually going to come back to us with a shitload of interest.