Most people are aware of the greenhouse effect and the effect that burning fossil fuels has on climate change and rising sea levels. However, a recent study published in Nature reports that rising levels of atmospheric CO2 may have another profound effect on the world's oceans: the acidification of seawater.1 A team of oceanographers, led by Dr James Orr, from the Climate and Environmental Sciences Laboratory at the French Atomic Energy Agency (CEA)2 in Saclay and an international team of scientists have been using computer models to study the effect of CO2 absorption by the oceans.3
The uptake of CO2 by the oceans will help to moderate future climate change, since over 25 million tons of CO2 dissolve in seawater every day. But this also increases the hydrogen ion (H+) concentration, reducing the pH of seawater, which is also known as acidification. These extra hydrogen ions then react with carbonate ions (CO32-) in the water, decreasing their concentration. This has serious consequences for the many marine organisms that use carbonate ions to make their shells and exoskeletons.
Orr and co-workers first calculated current carbonate ion content of the world's oceans from data collected in two ocean surveys. They then used computer models to calculate how future CO2 emissions would change this carbonate concentration. The model predicted that colder waters in the Southern Ocean and the subarctic Pacific Ocean would be affected more seriously than tropical waters. Worryingly, they found that quantities of aragonite, a form of calcium carbonate made by coral to build reefs, would fall below a critical level in Southern Ocean surface waters in as little as 50 years. By 2100, this deficiency could extend throughout the entire Southern Ocean and into the subarctic Pacific Ocean. This finding was unexpected: scientists had previously believed that adequate levels of calcium carbonate ions would persist in surface waters for hundreds of years.
The team went further; they also studied the effect of this acidification on individual marine organisms, in particular, the pteropod Clio pyramidata. Pteropods, a type of zooplankton, are winged snails, measuring several millimeters across, that swim through surface waters. Like coral, this species secretes aragonite to make its shell. The researchers compared the shells of pteropods that had swum in today's “non-corrosive” seawater with pteropods forced to swim in “corrosive” seawater with a composition similar to the one expected in 2100. In the corrosive water, the shells of live pteropods started dissolving within 48 hours. This is only one example—acidification would also undoubtedly affect coral reefs, particularly those in cold waters. The result would be a disaster for the marine ecosystem, as pteropods make up the basic food for organisms ranging from zooplankton to whales, and coral skeletons provide an essential habitat for fish, crabs and sea urchins. Obviously, commercially important species, including North Pacific salmon, mackerel, herring and cod, would be among those affected.
Although the impact of CO2 emissions on global warming and future climate change remains relatively unclear, Orr and his team are much more certain about the threat of ocean acidity and its effects on marine biodiversity. This pioneering work should go a long way in highlighting the sometimes unforeseen consequences of our reliance on fossil fuels.
1. J. Orr et al., “Anthropogenic ocean acidification over the twenty-first century and its impact on calcifying organisms,” Nature. 437 (7059): 681-6. 2005.
2. Laboratoire des sciences du climat et de l'environnement, Commissariat à l'énergie atomique. View web site
3. Scientists from the UK, Germany, Belgium, the US, Japan, Australia, and France.