Atlantic Ocean

Carbon Dioxide Flux

Presently, the net movement of carbon dioxide into the Atlantic and other oceans is from atmosphere to water. The oceans are an important sink, or storage reservoir, for carbon dioxide, holding as much as 60 times the amount of carbon dioxide as the atmosphere. More than three quarters of anthropogenic (human-sourced) carbon will eventually be stored in the oceans as carbon dioxide or as various carbon-containing ions and compounds. In the Northern Hemisphere, the North Atlantic is believed to be the largest reservoir for carbon. The effectiveness of the Atlantic Ocean as a carbon sink varies naturally over time.

Part of this variability is related to a phenomenon known as the North Atlantic Oscillation (NAO). The NAO refers to periodic movement of atmospheric mass and subsequent changes in the atmospheric pressure difference between two semi-permanent [Page 80]pressure systems that form over the Atlantic: the Icelandic Low and the Azores (or Bermuda) High. Changes in the NAO induce changes in sea surface temperature and mixing of layers of water. A strong NAO index (a relatively large difference in pressure between the two systems), tends to create warmer water and shallower winter mixing. Carbon dioxide is more soluble in water at cooler temperatures and under pressure. Thus, the carbon dioxide concentration of ocean water increases with depth. Deeper convection, or mixing of surface waters, increases the oceans efficiency as a sink for carbon dioxide.

Mixing also affects the nutrients available for biological activity, which has a seasonal effect on the carbon flux in the oceans. Carbon dioxide is used at the ocean surface by photosynthesizing organisms, and it is incorporated into shells of organisms in the form of carbonate, most importantly, calcium carbonate. A small portion (but still a large mass) of these organisms sinks to the ocean bottom, where much of the planet's carbon is stored. At the surface, the amount of carbon dioxide is generally at equilibrium with that in the atmosphere. Thus, as atmospheric carbon dioxide increases, so does the concentration in the surface water of the ocean. Once the ocean absorbs carbon dioxide, it combines with water to form carbonic acid and a series of acid-base reaction products, thereby lowering pH. It has been calculated that ocean pH has decreased by 0.1 units since the Industrial Revolution. This decrease in pH is expected to continue as the concentration of atmospheric carbon dioxide rises.

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