Science in Society Archive

I-SIS Special Miniseries
Save Our Oceans, Save Our Planet

Oceans Carbon Sink or Source

Do oceans remove carbon dioxide from the atmosphere or contribute to it? The answer is crucial for climate change. Dr. Mae-Wan Ho

NASA’s ocean watch

NASA’s SeaWiFS project (Sea-viewing Wide Field-of View Sensor)(SeaWiFS) has been watching the oceans twenty-four hours a day every day since September 1997 [1]. It provides quantitative data on the biological state of the global oceans through remote colour sensing. The colour in most of the world’s oceans varies with the concentration of chlorophyll and other plant pigments contained in the phytoplankton, the greater the concentration of plant pigments, the greener the water. As an orbiting sensor can view every square kilometre of cloud-free ocean every 48 hours, satellite-acquired ocean colour data are valuable for determining the abundance of ocean life and to assess the ocean’s role in climate change.

One big question the SeaWiFS project wants to answer is whether the oceans are a carbon source that adds carbon dioxide to the atmosphere, or a carbon sink that removes it from the atmosphere, which is crucial to monitoring climate change and taking appropriate action. The oceans not only contain 97 percent of all the water on earth, they are also the biggest carbon reservoir, and hence a major player in climate and climate change (Oceans and global warming, this series).

Passive and active carbon exchanges

Carbon dioxide in the atmosphere can dissolve in water, and the colder and more turbulent regions of the oceans tend to absorb carbon dioxide, while the warmer and less turbulent regions release it, carbon dioxide being less soluble in warm water than in cold water. In the early 1990s, the oceans were thought to be a net carbon sink [2], with the North Atlantic Ocean accounting for 60 percent of the carbon dioxide absorbed by the world’s oceans, which amounted to about 2 Gt a year [3].

The passive uptake and release of carbon dioxide is not as important, however, as the active uptake of carbon dioxide by the phytoplankton in photosynthesis and its active release in respiration by the whole community of marine organisms, which amount to about 100 Gt a year each way (Oceans and global warming, this series).

Phytoplankton consists of microscopic green algae that grow at prodigious rates in the surface layers of the oceans, feeding an enormous marine food web that has nine times the biomass of the terrestrial food web. A thriving phytoplankton population would tend to remove more carbon dioxide through photosynthesis than is returned through respiration by the entire community (phytoplankton plus zooplankton and other organisms living in the surface layers), and the ocean works as an effective carbon sink. Conversely, if respiration of the community were to exceed photosynthesis, more carbon dioxide would be generated than is fixed, and the ocean becomes a carbon source.

Monitoring how well the ocean’s plankton is doing is therefore very important for predicting climate and climate change

North East Atlantic a carbon source

Scientists at several Spanish universities used data from nine Spanish cruises conducted between 1991-2000 in the subtropical NE Atlantic to calculate gross primary production (photosynthesis) and respiration [4]. They found that two-thirds of the 33 stations investigated had respiration rate greater than photosynthesis. That meant the phytoplankton was not fixing carbon dioxide fast enough, and the oceans, or at least the northeast Atlantic Ocean could be a carbon source instead of a carbon sink.

In photosynthesis, carbon dioxide and water are combined to make sugar (carbohydrates) with the evolution of oxygen; conversely, oxygen is consumed to oxidise sugars back to carbon dioxide and water in respiration. So a convenient way to estimate the rates of photosynthesis and respiration is to measure the oxygen produced (in the light) and consumed (in the dark).

The researchers found that the average photosynthesis over all the marine stations in northeast Atlantic was 2 600 + 271 mg O2/m2/day, while the average community respiration was 3 821 + 276 mg O2/m2/day. Clearly, respiration rate was far in excess of photosynthesis. Additional evidence indicated that over the period of a year, respiration still exceeded gross production.

The study concentrated on the water column from a depth at which one percent of the sunlight has penetrated up to the surface with full sunlight, and did not include the respiration of organisms living at greater depths, where no photosynthesis could take place. If that were included, the deficit in gross production would be even bigger. The scientists estimated that 0.5 Gt of carbon is released per year just by the plankton community covering the 5.26 million square kilometres of the subtropical NE Atlantic.

Other evidence has come to light since indicating that the increase of carbon dioxide in the atmosphere and global warming are undermining the conditions of growth for phytoplankton, which has the potential to wipe out the marine biota at its very basis and to aggravate global warming (Shutting down the oceans, this series).

Article first published 25/07/06


  1. SeaWiFS Project NASA Goddard Space Flight Center,
  2. Acker J. The basics of ocean chemistry: carbon circulation, and critters, NASA Educational Workshop, SEWIFS: Ocean Chemistry,
  3. Solubility pump, Wikipedia,
  4. Duarte CM, Agusti S, Aristegui J, Gonzalez N, Anadón R. Evidence for a heterotrophic subtropical Northeast Atlantic. Limnol Oceanogr 2001, 46, 425-8.

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