Ocean Acidification: The Other Carbon Dioxide Problem

Carbon dioxide does not only warm the air. About 30 percent of the CO2 released into the atmosphere ends up in the ocean, and as the amount in the air rises, so does the amount in the sea. Once it dissolves, it changes the water's chemistry — slowly lowering its pH over a long period of time. That long, slow shift is what scientists call ocean acidification.

Where the carbon dioxide comes from

For more than 200 years, since the industrial revolution, burning fossil fuels and changing how land is used have raised the concentration of carbon dioxide in the atmosphere. The ocean takes up part of that increase, which is the main cause of acidification.

What happens in the water

When seawater absorbs CO2, a chain of chemical reactions follows. The result is more hydrogen ions in the water, which makes it more acidic, and fewer carbonate ions relative to what was there before.

That second effect is the one that matters most to sea life. Carbonate ions are a basic building block of shells and coral skeletons. When they become scarcer, animals that build shells and other structures from calcium carbonate find it harder to make them and to keep them intact. The list of these calcifying organisms is long:

• oysters and clams
• sea urchins
• shallow-water corals and deep-sea corals
• calcareous plankton

Not only animals with shells

The changing chemistry reaches animals that build no shells at all. Some fish are worse at detecting predators in more acidic water. And because these animals are links in food webs, a risk to them can become a risk to the whole web.

Why it matters to people

Ocean acidification is happening across all of the world's oceans, including coastal estuaries and waterways. Many economies depend on fish and shellfish, and people around the world rely on food from the ocean as their main source of protein — which is why a change in the chemistry of seawater matters far beyond the sea.

Sources

Based on "What is Ocean Acidification?" from NOAA's National Ocean Service, a work of the United States government in the public domain.