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Ocean Carbon Feedback

Ocean ecosystems depend strongly on climatic conditions such as near-surface density stratification, ocean circulation, temperature, salinity, the wind field, and sea ice cover. In turn, ocean ecosystems affect the chemical composition of the atmosphere by the production of gases and aerosols. Most of ocean ecosystem components are sensitive to change with changing climate. Marine biota also influence the near-surface radiation budget through changes in the marine albedo and absorption of solar radiation (bio-optical heating). (reference)

Feedbacks between marine ecosystems and climate change are extremely complex. Increased surface temperatures and stratification may lead to increased photosynthesis and fixation of carbon dioxide while associated reductions in vertical mixing and overturning circulation may decrease the return of required nutrients to the surface ocean and alter the vertical export of carbon to the deeper ocean. The sign of the cumulative feedback to climate of all these processes is still unclear. Changes in the supply of micronutrients required for photosynthesis, in particular iron, through dust deposition to the ocean surface can modify marine biological production patterns. Ocean acidification due to uptake of anthropogenic carbon dioxide may lead to shifts in ocean ecosystem structure and dynamics, which may then alter the biological production and export from the surface ocean of organic carbon and calcium carbonate.

A more sluggish ocean circulation and increased density stratification, both expected in a warmer climate, would slow down the vertical transport of carbon, alkalinity and nutrients, and the replenishment of the ocean surface with water that has not yet been in contact with anthropogenic carbon dioxide. This narrowing of the bottleneck for anthropogenic carbon dioxide invasion into the ocean would provide a significant positive feedback to atmospheric greenhouse gas concentrations.

Chemical buffering of anthropogenic carbon dioxide is the quantitatively most important oceanic process acting as a carbon sink. Carbon dioxide entering the ocean is buffered due to scavenging by the carbonate ions and conversion to bicarbonate, that is, the resulting increase in gaseous seawater carbon dioxide concentration is smaller than the amount of carbon dioxide added per unit of seawater volume.

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