14.2.4.1 The marine carbon system

The marine carbon cycle plays an important role in the partitioning of CO₂ between the atmosphere and the ocean (Chapter 3, Section 3.2.3). The primary controls are the circulation of the ocean (a function of the climate system), and two important biogeochemical processes: the solubility pump and the biological pump, both of which act to create a global mean increase of dissolved inorganic carbon with depth.

The physical circulation and the interplay of the circulation and the biogeochemical processes are central to understanding the ocean carbon system and future concentrations of CO₂ in the atmosphere. In the ocean, the prevailing focus on surface conditions and heat transport has led to a comparative neglect of transport processes below about 800 m depth. For carbon cycle modelling, however, vertical transports and deep horizontal transports assume fundamental importance. The importance of the thermohaline circulation is obviously important (and insufficiently well understood; see Section 14.2.3.2) in moving carbon from the surface to deeper layers. Similarly, the regional distribution of upwelling, which brings carbon- and nutrient-rich water to surface layers, is poorly known and inconsistently simulated in models. The ventilation of the Southern Ocean provides an extreme, though not unique, example.

It has been pointed out by a number of modelling studies that if there were no marine biological system, then the pre-industrial atmospheric CO₂ concentration would have been 450 ppmv instead of 280 ppmv (Sarmiento and Toggweiler 1984; Maier-Raimer et al., 1996). Any complete model of the natural ocean carbon cycle should therefore include the biological system; however, most recent assessments of the oceanic uptake of anthropogenic CO₂ have assumed that the biological system would not be affected by climate change and have therefore only modelled the chemical solubility in addition to the physical circulation. This was based on the understanding that nitrate or other nutrients limit marine phytoplankton growth. There would therefore be no CO₂ fertilisation effect as has been suggested for terrestrial plants and that, unless there was a large change in the nutrient supply to the upper ocean because of a climate-induced shift in circulation, then no extra anthropogenic CO₂ could be sequestered to the deep ocean by the organic matter pump. More recently, a number of studies have suggested possible ways in which the organic matter pump might be affected by climate change over a 200-year time-scale (see Chapter 3, Sections 3.2.3.2 and 3.2.3.3). The main conclusion was that, because of the complexity of biological systems, it was not yet possible to say whether some of the likely feedbacks would be positive or negative. However, it is clear that our understanding of these issues needs to be improved.

Simulating the calcium carbonate system with a process-oriented model presents another level of complexity beyond simulating the organic matter formation-decomposition: the distribution of particular phytoplankton species (mainly cocco-lithophorids) must be simulated. The calcium carbonate pump, however, contributes relatively little to the vertical dissolved inorganic carbon (DIC) gradient compared to the organic matter and solubility pumps. The importance of this pump needs careful evaluation and its past (palaeo) role in the carbon cycle needs to be considered (see end of Chapter 3, Section 3.2.3.3).

In the ocean, models incorporating biology are relatively underdeveloped and incorporate empirical assumptions (such as fixed Redfield (nutrient) ratios) rather than explicitly modelling the underlying processes. As a result, present models may be unduly constrained in the range of responses they can show to changes in climate and ocean dynamics. A better understanding is required concerning the workings of nutrient constraints on productivity, the controls of nitrogen fixation, and the controls on the geographical distribution of biogeochemically important species and functional types in the ocean. To develop this understanding it will be necessary to combine remotely sensed information with a greatly expanded network of continuous biogeochemical monitoring sites, and to gather data on the space-time patterns of variability in species composition of marine ecosystems in relation to climate variability phenomena such as ENSO and NAO. (See Chapter 3, Sections 3.6.3 and 3.7).

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