Ocean Acidification Explained: What It Is and Why It Matters

Ocean acidification is sometimes called "the other carbon dioxide problem." While most climate news focuses on warming, the same carbon dioxide that heats the planet is also changing the chemistry of seawater. This shift is measurable, well understood in its basic chemistry, and important for many creatures that build shells and skeletons.

The ocean as a carbon sink

Every year, people release billions of tonnes of carbon dioxide by burning fossil fuels, making cement and clearing forests. Not all of it stays in the air. Plants on land absorb some, and the ocean absorbs a large share. According to NOAA, the ocean has taken up about 30 percent of the CO2 released by humans since the start of the industrial era.

That has been a big help in slowing global warming. Without the ocean's uptake, CO2 in the atmosphere would be considerably higher than the more than 420 parts per million measured today. But this service comes at a cost to the ocean itself.

The chemistry in plain English

When carbon dioxide dissolves in seawater, a short chain of chemical reactions follows:

1. CO2 combines with water to form carbonic acid, a weak acid (the same one that gives fizzy drinks a slight tang).
2. Carbonic acid releases hydrogen ions. More hydrogen ions means a lower pH, which means more acidic water.
3. Many of those extra hydrogen ions then combine with carbonate ions already in seawater, turning them into bicarbonate.

That last step is the one that matters most for sea life. Carbonate ions are a building block that many marine organisms use to make calcium carbonate, the hard material in shells and coral skeletons. When CO2 rises, there are fewer carbonate ions available, so building and maintaining shells takes more energy.

Understanding pH

The pH scale runs from 0 to 14. A pH of 7 is neutral, below 7 is acidic, and above 7 is basic (alkaline). Seawater has a pH of about 8.1, so it is slightly basic.

The scale is logarithmic, which means each whole number step represents a tenfold change in hydrogen ion concentration. So a drop of 0.1 pH units, which sounds tiny, corresponds to roughly a 30 percent increase in acidity. That is why scientists take a seemingly small change seriously.

How scientists measure it

Long-term monitoring stations track ocean chemistry directly. One of the best known is the Hawaii Ocean Time-series station near Hawaii, which has measured seawater since the late 1980s and complements the famous Mauna Loa air record. Its records show seawater CO2 rising in step with atmospheric CO2 while pH steadily declines. Similar records exist near Bermuda and the Canary Islands.

NOAA's Pacific Marine Environmental Laboratory carbon program and international partners also use moored buoys, research cruises and sensors on commercial ships to map changes across the oceans.

Effects on shellfish

Oysters, clams, mussels and scallops build their shells from calcium carbonate. Young shellfish are especially vulnerable because they need to build shells quickly in their first days of life.

One well-known example comes from the US Pacific Northwest. In the late 2000s, oyster hatcheries in Oregon and Washington suffered major die-offs of oyster larvae. Scientists traced the problem to deep, naturally CO2-rich water welling up to the coast, made more corrosive by the added CO2 from the atmosphere. Hatcheries responded by monitoring seawater chemistry and adjusting the water they use, which shows that adaptation is possible in some cases, though it costs money and does not help wild populations.

Tiny free-swimming snails called pteropods, sometimes called "sea butterflies," are another concern. They are an important food source for fish such as salmon, and studies have found signs of shell damage in pteropods in some regions with more corrosive water.

Effects on corals

Coral reefs are built by tiny animals that secrete calcium carbonate skeletons. Lower carbonate levels can slow coral growth and make reef structures weaker and more prone to erosion. For corals, acidification adds to the bigger and more immediate threat of marine heat waves, which cause bleaching. You can read about that in Coral bleaching explained.

Wider effects and what is still uncertain

Not every species responds the same way. Some seagrasses and algae may grow faster with more dissolved CO2. Some fish show changes in behavior or senses in laboratory experiments, though results have been mixed and some findings have not held up when repeated. Scientists are still working out how effects on individual species add up across whole food webs, and how acidification combines with warming and falling oxygen levels.

The IPCC Special Report on the Ocean and Cryosphere concluded that ocean acidification will continue as long as CO2 emissions continue, and that its effects will be greater under higher emissions. Under high-emission scenarios, surface pH is projected to fall by a few more tenths of a unit by 2100.

What can be done

The main solution to ocean acidification is the same as for climate change: reducing CO2 emissions. Unlike warming, acidification is driven almost entirely by CO2 itself, so cutting other gases such as methane does not address it.

Local actions can also help ecosystems cope. Reducing nutrient pollution from farms and sewage helps, because that pollution can make coastal waters more acidic. Protecting seagrass meadows and kelp forests may offer small local buffers. And monitoring helps fisheries and hatcheries prepare.

To understand why CO2 is building up in the first place, see What is the greenhouse effect?, and for practical steps, What can one person do?

The bottom line

The ocean has absorbed a large share of humanity's carbon dioxide, slowing global warming but changing seawater chemistry. The basic chemistry is clear and measured: pH is falling and carbonate is becoming scarcer. The full ecological consequences are still being studied, but shell-building animals and coral reefs are among those most at risk.