6.3.2. Biological Processes
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.,
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 oceanfor 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.
6.3.3. Marine Carbon Dioxide Uptake
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.