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Climate Change Decreases the Ability of Oceans to Soak up Carbon

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Posted January 8, 2015

According to the estimates of the Intergovernmental Panel on Climate Change, Earth’s oceans have taken up more than 90 per cent of the heat trapped by atmospheric CO2 and methane since the 1970s. That heat is later converted into organic carbon and stored on the ocean floor.

Water-borne CO2 is turned into organic carbon compounds by phytoplankton and stored on the ocean floor. Image credit: Alfred Wegener via Wikimedia, CC BY-SA 2.5.

Water-borne CO2 is turned into organic carbon compounds by phytoplankton and stored on the ocean floor. Image credit: Alfred Wegener via Wikimedia, CC BY-SA 2.5.

The higher the temperature, however, the less carbon can be effectively stored. And the less carbon is stored, the higher the temperature. Researchers from the University of Southampton warn that this positive feedback loop might become reality sooner than we thought before.

Their study in the Proceedings of the National Academy of Sciences measured how much carbon is stored via the “biological pump”. The latter term refers to the way marine phytoplankton uses sunlight and water-borne carbon dioxide for photosynthesis, whereby it is converted into organic carbon compounds. Once these microorganisms die, they sink to the ocean floor, taking the carbon with them.

What the team found was that warmer ocean temperatures correlate with lower amounts of stored carbon, which remains near the surface and escapes back into the atmosphere. If business continues as usual, this could turn into a vicious cycle.

“This would potentially result in reduced storage of carbon dioxide by the oceans, effectively acting as a positive feedback mechanism, with less atmospheric carbon dioxide being removed by the oceans,” said Dr. Chris Marsay, lead author of the study.

In order to determine how efficient the “biological pump” is at different temperatures, Pelagra sediment traps were employed. “Sediment traps act in a similar way to a rain-gauge. They typically consist of a funnel or cylinder that is held vertically in the water, with the top end open to catch material sinking through the water column and a sample cup connected to the bottom end to store the material for analysis,” explained Marsay.

These traps are superior to traditional traps in that they can be programmed to sink to particular depths, allowing researchers to collect a much wider sample set. This is the first study to use direct ocean measurements to make the link between temperature and sinking organic carbon.

Several of these traps were placed at four locations in the Atlantic and used to collect samples from various depths. Then the results were compared to the data of a similar study conducted in the Pacific.

The analysis clearly showed that where the water is warmer, the amounts of organic carbon are lower. This means that warmer temperatures dissolve phytoplankton and other microorganisms before they sink to bottom, letting the CO2 return to the atmosphere.

Marsay hopes to see more research using Pelagra traps in the future, which would help to determine the rates of carbon absorption in other water bodies around the world.

Sources: study abstract, zmescience.com, carbonbrief.org.

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