Marine biota have an important role in shaping climate. Marine biological processes sequester CO₂ and remove carbon from surface waters to the ocean interior through the settling of organic particles and as ocean currents transport dissolved organic matter. This process, which is called the biological pump, reduces the total carbon content of the surface layers and increases it at depth. This process may be partially offset by biocalcification in reefs and organisms in the open ocean, which increases surface layer CO₂ by reducing bicarbonate alkalinity.

Projected global warming through the 21st century is likely to have an appreciable effect on biological processes and biodiversity in the ocean. A rise in temperature will result in acceleration of biodegradation and dispersal of global organic pollutants (petroleum and chlorinated hydrocarbons, for example). This process would promote their removal from the photic zone of the ocean, as has been demonstrated by Tsyban (1999a) for the Bering and Chukchi Seas.

Physiochemical and biological processes regulate uptake and storage of CO₂ by oceans. The Arctic Ocean, for instance, is an important CO₂ source in winter and sink in summer (Tsyban, 1999b). Climate change is expected to affect the processes that control the biogeochemical cycling of elements. Uptake and storage of CO₂ by the ocean via the biological pump therefore may change. Any changes that do occur are expected to feed back into the carbon cycle (Ittekkot et al., 1996).

Photosynthesis, the major process by which marine biota sequester CO₂, is thought to be controlled by the availability of nutrients and trace elements such as iron (de Baar et al., 1995; Behrenfeld et al., 1996; Coale et al., 1996; Falkowsky et al., 1998). Changes in freshwater runoff resulting from climate warming could affect the inputs of nutrients and iron to the ocean, thereby affecting CO₂ sequestration. Impacts are likely to be greatest in semi-enclosed seas and bays.

Climate change can cause shifts in the structure of biological communities in the upper ocean—for example, between coccoliths and diatoms. In the Ross Sea, diatoms (primarily Nitzshia subcurvata) dominate in highly stratified waters, whereas Phaeocystis antarctica dominate when waters are more deeply mixed (Arrigo et al., 1999). Such shifts alter the downward fluxes of organic carbon and consequently the efficiency of the biological pump.

The oceans are estimated to have taken up approximately 30% (with great uncertainties) of CO₂ emissions arising from fossil-fuel use and tropical deforestation between 1980 and 1989, thereby slowing down the rate of greenhouse global warming (Ittekkot et al., 1996). An important process in the oceans is burial of organic carbon in marine sediments, which removes atmospheric CO₂ for prolonged time periods. Studies of the Southern Ocean by Caldeira and Duffy (2000) have shown high fluxes of anthropogenic CO₂ but very low storage. Model results imply that if global climate change reduces the density of surface waters in the Southern Ocean, isopycnal surfaces that now outcrop may become isolated from the atmosphere, which would tend to diminish Southern Ocean carbon uptake.

Using models of the effects of global warming on ocean circulation patterns, Sarmiento and Le Quéré (1996) analyzed the potential for changes in oceanic CO₂ uptake. They found that a weakening of the thermohaline circulation could reduce the ocean's ability to absorb CO₂ whereby, under a doubled-CO₂ scenario, oceanic uptake of CO₂ dropped by 30% (exclusive of biological effects) over a 350-year period. In simulations with biological effects under the same CO₂ conditions and time frame, they found that the oceanic uptake was reduced only by 14%. Confirmation that a collapse of global thermohaline circulation could greatly reduce the uptake of CO₂ by the ocean has been reported by Joos et al. (1999).

Sarmiento et al. (1998) modeled carbon sequestration in the ocean with increasing CO₂ levels and changing climate from 1765 to 2065. They found substantial changes in the marine carbon cycle, especially in the Southern Ocean, as a result of freshwater inputs and increased stratification, which in turn reduces the downward flux of carbon and the loss of heat to the atmosphere.

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