But they can shift catastrophically into other modes
The cold, oxygenated seas full of life that we know, and evolved alongside, are not the only way earth’s oceans can be. Jarring changes in the balance of the ecosystem can turn the seas deadly to complex life.
With too little CO2 in the atmosphere, the oceans can freeze solid, from poles to equator – and did so many times in the earth’s deep history. It is only recently this has been known. The microscopic ancestors of current life had to spend eons cut off from the sun, probably taking energy from volcanic seeps on the ocean floor. Only gradual volcanic eruptions restored the CO2 balance and freed the seas for the explosion of multicellular life.
Yet for large, complex life – like fish, whales, and humans – it’s even more disastrous to have too much CO2. This global heating melts polar ice caps and shuts down ocean currents, causing oceans to stratify and become anoxic. Ocean heating melts methane ice on the sea floor, speeding up the heating. The heating waters hold far less CO2, and push more back into the air like a warming cola.
Yet the increasing CO2 load winds up, for the most part, dissolved in the ocean where it becomes carbonic acid. And in a more acidic ocean, calcium shells can’t form. In fact, the existing ones fizz away, exactly like an antacid tablet in your stomach. All the coral. All the sea shells. All the animals, large, and small which can’t live without them – gone.
Purple bacteria which generate toxic hydrogen sulfide bacteria rise from the depths of the oceans to claim the deep seas, bubbling up poison gas which can sweep the land.
This has happened in the past multiple times. Frozen seas, and toxic purple seas. Those possibilities lie in wait for CO2 imbalance.
This won’t happen while you’re alive. But it can be irrevocably set into motion while you’re alive. With “business as usual” we will cross CO2 thresholds leading to the dissolving of shells in the entire ocean. It has already begun at high latitudes. And as we pass into regimes in which positive feedbacks like burning forests, heating oceans, and methane dominate atmospheric heating, we will lose what control we have now.