Not all of the carbon dioxide released into the atmosphere stays there.
About 30% of it gets absorbed by the ocean.
One of the things you can measure about a liquid is its pH (power of hydrogen) – how many H+ ions are in the liquid, and what effect that has on the liquid’s properties. If a liquid has a lot of H+ ions, it’s acidic with a pH below 7. If a liquid doesn’t have very many H+ ions, it’s alkaline (or basic) with a pH above 7. Regular water has a pH of very close to 7 or exactly 7.
Oceanic salt water is slightly basic, with a pH slightly above 8. But then carbon dioxide (CO2) gets absorbed into the ocean, some of which turns into carbonic acid (H2CO3, which is CO2 combined with H2O), where it can lose lose its hydrogen ions into the larger ocean and raise the acidity of the ocean.
“What’s the big deal? It’s just a few hydrogen ions! What’s the harm in that?”
Other than the part where massively changing the chemical composition of the ocean doesn’t sound like a good idea, there is explicit harm in ocean acidification. Namely, carbonate becomes less abundant for organisms like coral, shellfish, and oysters to use. Even worse, many of these organisms are seeing their shells and structures dissolving, other organisms that rely on an alkaline ocean will have their life cycle disrupted, and this will only worsen as more carbon dioxide is dissolved into an acidic ocean.
If this goes on for long enough, and the oceans get acidic enough, then you run the risk of mass ocean extinctions. The new pH of the oceans doesn’t have to kill every organism that relies on carbonate or an alkaline ocean, just enough of them to cause damage up and down every ocean ecosystem and food chain – and damage to all of the people that rely on the oceans for food.
The science of ocean acidification is still developing, and I’m not going to claim to be an oceanographer. In the end though, this is yet another reason to stop putting carbon dioxide in the atmosphere – and one of the problems we would need to solve even if climate change wasn’t real.