Why does the writer say that the ocean taking in carbon dioxide "might sound like a good thing"?

For more than two hundred years, since the industrial revolution, people have been burning coal, oil and gas and changing the way land is used. As a result, the amount of carbon dioxide in the air has been rising.

The ocean takes in about thirty per cent of the carbon dioxide released into the air. That might sound like a good thing, and in one way it is, since it keeps some of the gas out of the atmosphere. But it changes the sea. When carbon dioxide dissolves in sea water, a chain of chemical reactions makes the water more acidic. This slow change is called ocean acidification.

The change matters most to creatures that build shells and skeletons. Oysters, clams, sea urchins and corals all use substances called carbonate ions as building blocks. In more acidic water there are relatively fewer of these, so building and keeping a shell becomes harder.

Even animals without shells can be affected. In more acidic water, some fish become worse at sensing the predators that hunt them. And when the creatures at the bottom of a food chain are in trouble, everything that eats them may be in trouble too.