Carbon dioxide dissolving into seawater makes it more acidic. That is chemistry, not opinion.

What it means

Carbon dioxide and water make carbonic acid, which is the same reaction that puts the fizz and the bite in a soda. In the ocean it lowers the pH and, more importantly, uses up the carbonate that corals, oysters and tiny floating snails need to build shells. Those animals then have to spend energy on construction that used to be cheap, which is a cost paid at the bottom of a food web that everything else stands on.

The claim comes from NOAA.

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