In 2016, scientists added CO2 to seawater over a Great Barrier Reef patch for 30 days; net reef calcification fell 34% under future-like conditions

In a 30-day test on Australia’s Great Barrier Reef, scientists added CO₂ to seawater and found the reef built 34% less limestone than on comparison days.

Where does the carbon dioxide from our cars and power plants go? Part of it ends up in the ocean, and scientists wanted to know what that does to a coral reef. According to a California Academy of Sciences press release, a team of researchers led by Rebecca Albright and Ken Caldeira tested this on Australia’s Great Barrier Reef. They bubbled carbon dioxide into a tank of seawater, pumped it onto the reef, and allowed it to flow across. The water was made as acidic as it is projected to become by the end of this century if emissions continue unchecked. The reef built up calcium carbonate more slowly.

A test on a real reef community

Coral reefs are built by living organisms, and rising ocean acidity makes that building harder. (Representational/Unsplash)

Many previous studies have mainly concentrated on single species or single organisms. This one aimed bigger. The team worked on the reef flats surrounding One Tree Island, a small coral cay in the southern Great Barrier Reef. It was the first ocean acidification experiment where researchers added carbon dioxide to seawater to make it acidic and then let it flow across a natural reef community, the press release said. That matters because reefs are complex communities of corals, algae, and other organisms. By studying the whole community, researchers can understand how acidification impacts the ecosystem and the coastal communities that rely on it.

The Great Barrier Reef, seen from above. The experiment took place at One Tree Island, a small coral cay at the reef’s southern end.

How the 30-day experiment worked

The paper says that net community calcification was 34% lower on the days with added carbon dioxide. That’s roughly a third less. The authors also mentioned a statistically significant decrease in calcification on each of the 20 experimental days. They say this is, to their knowledge, the first community-level estimate of this type from a controlled experiment in a natural setting. This is consistent with a 2016 study by Albright and Caldeira. In that study, the team made seawater less acidic, and the corals' ability to build their skeletons improved. That earlier study found evidence that acidification is already reducing reef calcification.

Why acidic water is hard on coral

When we burn coal, oil, or gas, we release carbon dioxide into the air. Some of it is absorbed by the ocean, where it stays for thousands of years. There it reacts with seawater and forms carbonic acid, which is harmful to reefs, shellfish, and other marine life. Reefs are particularly vulnerable because corals build their skeletons from calcium carbonate, a process called calcification, and that process gets harder as the water becomes more acidic. It’s like building a house when the bricks are harder to make.

Why this matters to you

Even if you live far from the coast, reefs still affect people in many ways. Reefs feed millions of people, protect coastlines, and generate billions of dollars a year in tourism, the Nature paper notes. Caldeira also pointed to fishing and tourism as sources of income for nearby communities. The results don’t mean every reef will respond the same way because this study only looked at one reef flat. There is also the added danger of warming due to climate change. But Albright said the findings are strong evidence that acidification will severely slow reef growth in the future unless greenhouse gas emissions are cut steeply and rapidly. Cutting emissions is something we can still work on.