Half of all our oxygen comes from the ocean, by a CO2-oxygen exchange cycle not as completely understood as we would like. Climate change is only one risk from excess CO2 in the air. Ocean acidification is another. It alone could be reason enough to reduce the burning of fossil fuels.
The ocean is actually slightly alkaline; just becoming a little less so, but life is sensitive to the pH of the water in which it lives. This pH change does not do coral reefs any good, and in the tropics they are the base of the marine food chain near the ocean surface. Given enough time, some of them might adjust to decreasing pH and re-bloom, especially if not otherwise stressed, but that is uncertain.
Just as serious is the effect of increased acidity on the ability of zooplankton to form shells. If they can’t absorb calcium to form shells, they will weaken, and a lower zooplankton population spells trouble. They are the base of the food chain throughout much of the non-tropical ocean. Such a bottom up extinction is more serious than losing some big mammals on earth’s surface (the mammoth is long gone, and earth survived). But take away its food chain and nothing survives, and life on land depends on life in the oceans.
A different route to bottom up extinction was a big reason to fear the Antarctic ozone hole. Remember that? It’s out of the news, but still there, not growing, but not shrinking.
Freon and other chlorofluorocarbons (CFCs) were used as cleaning agents, as aerosol propellants and in air conditioning units. After Freon was discovered to be a potent catalyst of ozone destruction over the Antarctic, its use was severely curtailed by the 1987 Montreal Accord, and phased out by 1995. Now the major catalyst of ozone depletion is nitrous oxide from use of hydrocarbons. Although not as potent as Freon, it is generated in much higher quantities, some from combustion, so engine exhaust emissions are tightly regulated. A much bigger source of NOx is excess use of nitrogen (ammonia based) fertilizer, but since it has become essential to crop yields, this is quite a dilemma.
Depletion of ozone in the upper atmosphere allows ultraviolet rays (UBV-B) to reach earth’s surface which damages all life, including plankton in the sea. During the spring ozone hole above Antarctica, scientists have noted a 6-12% drop in plankton population. In tests, phytoplankton photosynthesis dropped by almost 65% after only one hour of exposure to UV-B light, important because much of the CO2-Oxygen interchange in the oceans depends on phytoplankton photosynthesis.
Phytoplankton are the bottom of a food chain that supports commercial fisheries, but plankton ecosystems have a big role in oceanic cycles that convert CO2 to oxygen. Inflicting global-scope damage on them carries a high risk to all life, including human. That’s why restraining the ozone hole is such a big deal.
There are many more threats. The chains of causality that create them are not easy to unravel. For example, increasing CO2 in seawater decreases sound absorption, particularly at lower frequencies characterizing the sounds of wind, waves, and rain. Although vaguely understood, this sound propagation is a biological signaling system. Worriers fear that “increasing the noise” in the ocean could disrupt oceanic life with who knows what consequences.