It only seems like nonsense because you're missing an important piece of the science.
Acidification is not caused by how much CO2 there is in the water. It is caused by a rapid increase in CO2 levels. We are dumping CO2 in very quickly, and so are acidifying the water.
But given time, oceans will mix down the to bottom. At the bottom it will encounter very large stores of calcium carbonate. As that dissolves, it renders the water no longer acidic. As that mixes back to the top the rest of the ocean becomes less acidic. This mixing process is estimated to take on the order of 1000 years. Therefore the long term the oceans can handle all of the CO2 we're dumping into them and much more. Increasing the long-term average CO2 of the atmosphere will not make the oceans acidic.
The problem is that this mixing process is too slow to help shellfish living near the surface today. Sure, the ocean winds up at a good ph. But it will be acidic for several centuries. And that is unprecedented. In fact there is no record of any event since the Permian-Triassic extinction over 200 million years ago that featured such rapid acidification as the oceans face today. And perhaps not even that one. We estimate that several times as much CO2 was dumped into the atmosphere as what we're releasing now, but it probably was not dumped in such a short time period. And that event wiped out an estimated 90% of all marine species.
To piggyback onto this comment, it really is a matter of rates, of chemical kinetics. The current increase in CO2 from fossil fuel burning and related human activities has occurred at a rate that is geologically instantaneous. There is no real precedent for this. CO2 has indeed been higher in the geologic past (e.g., Cretaceous maximum, early Paleozoic prior to vascular plant systems), but these increases are likely tied to very leisurely evolving cycles of plate tectonics and enhanced volcanic CO2 fluxes. Higher pCO2 during these periods produced warmer, wetter climates that drive enhanced rates of silicate weathering. The result? High pCO2, somewhat lower pH, yes, but also higher carbonate alkalinity in seawater as well, which gave rise to higher, not lower, saturation states for marine carbonate minerals, and are associated with higher rates of biomineralization (e.g. Steven Stanley's periods of 'hypercalcification'). So it's really a matter of the balance of rates. "Natural" rates of C oxidation via the slow uplift of buried organic carbon are far slower than what we have produced via fossil fuel burning of coal, hydrocarbons. In this process, silicate weathering acts as an essential thermostat through the feedback to chemical weathering of crustal silicates. This has been understood for quite awhile.
Acidification is not caused by how much CO2 there is in the water. It is caused by a rapid increase in CO2 levels. We are dumping CO2 in very quickly, and so are acidifying the water.
But given time, oceans will mix down the to bottom. At the bottom it will encounter very large stores of calcium carbonate. As that dissolves, it renders the water no longer acidic. As that mixes back to the top the rest of the ocean becomes less acidic. This mixing process is estimated to take on the order of 1000 years. Therefore the long term the oceans can handle all of the CO2 we're dumping into them and much more. Increasing the long-term average CO2 of the atmosphere will not make the oceans acidic.
The problem is that this mixing process is too slow to help shellfish living near the surface today. Sure, the ocean winds up at a good ph. But it will be acidic for several centuries. And that is unprecedented. In fact there is no record of any event since the Permian-Triassic extinction over 200 million years ago that featured such rapid acidification as the oceans face today. And perhaps not even that one. We estimate that several times as much CO2 was dumped into the atmosphere as what we're releasing now, but it probably was not dumped in such a short time period. And that event wiped out an estimated 90% of all marine species.