Ocean Biotic Feedbacks with Centennial Climate Change

Our ability to predict the impacts of global warming is limited by a number of key uncertainties, significant among which is the role of biotic feedbacks (IPCC, 2001). The response of biota in the surface ocean is particularly pertinent and still not well understood. However, the potential for multiple feedbacks between climate, ocean circulation and mixing, and photosyn-thetic primary production has been manifestly evident for some time (Falkowski et al., 2000; Gildor and Follows, 2002). Indeed, the oceans are estimated to have taken up approximately 30% (with great uncertainty) 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).

Although the ocean biota compartment is estimated to contain only around 3GtC, the flux from the dissolved inorganic reservoir to the par-ticulate organic phase (carbon uptake through primary production) is around 10GtCyr_1 (Siegenthaler and Sarmiento, 1993). Thus the size of marine biota carbon reservoir is much smaller than the fluxes in and out the reservoir. Elsewhere in the global carbon cycle, the reservoirs are much larger than the fluxes. This implies that any changes in the activity of this reservoir can mean substantial changes in the fluxes to related reservoirs. An especially important flux in the oceans is the burial of particulate organic carbon in marine sediments, which removes atmospheric CO2 for prolonged time periods. Fig. 1 schematizes the major carbon reservoirs and flux directions in the global carbon cycle.

Carbon Reservoirs
Figure 1: Schematic diagram of the major reservoirs and flux directions of the global carbon cycle.

Photosynthesis, the major process by which marine biota sequester CO2, is largely controlled by the availability of macronutrients and trace elements such as iron (de Baar et al., 1995; Behrenfeld et al., 1996; Coale et al., 1996; Falkowski et al., 1998). Changes in freshwater runoff or increases in aeolian dust transport resulting from climate warming could change the inputs of nutrients and iron to the ocean, thereby affecting CO2 sequestration.

Climate change can also 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). Changes to ocean stratification could impact species composition and alter the downward fluxes of organic carbon and consequently the efficiency of the biological pump.

Several coupled atmosphere-ocean models have been used to project the effect of climate change on marine biota (Sarmiento et al., 1998; Joos et al., 1999; Gabric et al., 2003; Pierce, 2003). These models include some or all of the processes associated with carbonate chemistry and gas exchange, physical and biological uptake of CO2, and changes in temperature, salinity, wind speed, and ice cover. They account for simple changes in biological productivity, but not for changes in external nutrient supply, or changes in the biogeography of planktonic species, which is a major deficiency as they thus cannot simulate more complex biological feedbacks (Gabric et al., 2003).

The range of model estimates of the climate change impact is dependent on the choice of scenario for atmospheric CO2 and on assumptions concerning marine biology. At high CO2 concentrations, marine biology can have a greater impact on atmospheric CO2 than at low concentrations because the buffering capacity of the ocean is reduced (Sarmiento and Queré, 1996). Although the impact of changes in marine biology is highly uncertain and many key processes are not included in current models, sensitivity studies can provide approximate upper and lower bounds for the potential impact of marine biology on future ocean CO2 uptake. A sensitivity study of two extreme scenarios for nutrient supply to marine biology gave a range of 8-25% for the reduction of CO2 uptake by mid-21st century (Sarmiento et al., 1998). This range is comparable to other uncertainties, including those stemming from physical transport.

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