Nick Heath

A Blue Future For Global Warming

How rising carbon dioxide levels can dissolve the shells of sea creatures.

By now we’re familiar with apocalyptic visions of a scorched and flooded world ravished by global warming.  But this gloomy prognosis is now set to take a nosedive beneath the ocean waves.

Mussels in the Arcachon basin off the coast of south west France that are used as biological indicators of industrial and agricultural pollution.©Nick Heath

A European study has produced hard evidence that our profligate production of the greenhouse gas CO₂ (carbon dioxide) is likely to have far reaching consequences for life offshore. Its findings could spell disaster for the edible mussel and the pacific oyster, two species that play an essential role in the marine food chains and support a $10.5 billion global industry.

Researchers at several European universities examined how the mussels and oysters were able to produce the calcium carbonate they needed to make their shells as they increased the CO₂ in the atmosphere. They were particularly interested in how their development was affected by acidification, the process by which water becomes more acidic as it absorbs CO₂.

Every day over 25 million tons of CO₂ is soaked up by seawater, gradually making it more acidic. Marine pH levels are now dropping at a rate at least 100 times greater than any point during the last 600,000 years. The results of the study were stark: at levels of atmospheric CO₂ expected by the year 2100 – 740 parts per million – the speed at which shells were produced fell by 25pc in mussels and 10pc in oysters. Significantly they also found that mussel shells would dissolve when CO₂ in the atmosphere reaches 1800 ppm.

The results have prompted fears of irreversible damage to coastal ecosystems and the communities that have depended on them for centuries. The burgeoning worldwide shellfish market has grown by 8pc a year for the last 30 years. Production levels reached 12 million tons a year in 2002, 10.8pc of which were pacific oysters and 3.6pc of which were mussels. Beyond this, mussels and oysters also help to create habitats for other species, by controlling the flow of oceanic material and are the stable diet of several varieties of marine bird.

The study was carried out by Frédéric Gazeau, a scientist at the Netherlands Institute of Ecology, and his colleagues, including Jean-Pierre Gattuso, director of research at the Oceanographic laboratory at Villefranche-sur-Mer (CNRS/Université Pierre et Marie Curie). Mr Gattuso said that further studies were needed as a matter of urgency. “The impact on molluscs’ fishing and culture is difficult to assess because our publication is the first one on this topic and is lab-based,” he said. “Potential consequences, such as a longer time to reach commercial size, increased susceptibility to predators and increased mortality of early life stages, now need to be fully investigated.”

The researchers will now assess the long-term impact of the phenomenon by examining how easily mussels and oysters can genetically adapt to produce their shells in a more acidic environment. But real world evidence is less gloomy than this study suggests. Anecdotal accounts from UK shell fishermen, especially those on the Yorkshire Coast where the richest grounds are to be found, say that crab, lobster and mussel stocks are at their best for years.

How does acidification threaten shellfish?
Molluscs and other sea creatures rely on dissolved calcium and carbonate ions to produce their shells and skeleton. It is by disrupting the supply of carbonate ions that acidification stops normal shell production. Carbonate ions are normally so abundant in sea water it is said to be supersaturated.
But while the calcium concentration remains constant in seawater, increasing the acidity of the oceans leads to a greater concentration of dissolved CO₂ and fewer carbonate ions. By removing this raw material, it slows the speed at which molluscs produce their shells, as well as making them more prone to dissolution, early mortality and predators.

What is the wider threat?
A report by the Royal Society in 2005 has shown that ocean acidification threatens a vast range of marine habitats, with the most severe effects concentrated on coral reefs and in the Southern Ocean.Its report and previous studies have predicted that creatures such as corals, shellfish, sea urchins, phytoplankton, and starfish are likely to suffer the same difficulties in producing calcium carbonate skeletons and shells in increasingly acidic environments. It said that even the more conservative estimates of future CO₂ emissions could lead to corals becoming rare on the currently rich tropical and subtropical reef ecosystems, such as the Great Barrier Reef, by 2050. The Royal Society report found that planktons, the bedrock of several major food chains, may be unable to make their calcium carbonate shells by 2100.

Another consequence of increased CO₂ levels in the oceans, is that larger marine animals could find it harder to extract the oxygen they need from the seawater. Creatures such as squid are particularly susceptible, as they move by jet propulsion, which is very energy demanding and requires a steady supply of oxygen. Mr Gattuso said it was becoming apparent that acidification would have a widespread impact on marine habitats.

“The negative impact of ocean acidification on calcification of marine organisms has now been demonstrated in a large number of groups such as corals, macroalgae, phytoplankton, molluscs and echinoderms. The gravest consequences seem to be for coral reefs, the very existence of which entirely relies on calcification.”

The heat is on.
Ocean acidification is also bad news for climate change, as the absorption of atmospheric CO₂ by the oceans helps regulate greenhouse gas levels and stave off global warming. In the past 200 years the oceans have absorbed about half of the carbon dioxide produced by humans, which has been primarily generated by the burning of fossil fuels.

The world’s seas currently take up one tonne of this carbon dioxide for each person on the planet every year. But the ocean’s effectiveness as a carbon sink will decrease as acidification takes place and it is also thought that rising ocean temperatures would further reduce its ability to soak up CO₂. Mr Gattuso said there needed to be more studies into the effect on the ocean’s ability to absorb CO₂.

What can we do?
Various options have been considered to slow the acidification, including adding limestone to the oceans to make them more alkaline. But ultimately Mr Gattuso said that man would have to curb his CO₂ emissions if he wants to preserve the earth’s ancient aquatic habitats. He said: “The only way to reduce ocean acidification is to decrease the uptake of atmospheric CO₂ into the ocean, hence the concentration of CO₂ in the atmosphere. Reducing anthropogenic CO₂ emissions is the only way to achieve it.”

- April 2007

About the Author

Nick Heath is a newspaper journalist based in Norfolk with a keen interest in science.