Carbon cycle feedbacks

Carbon cycle feedbacks are processes that respond directly to increasing atmospheric

CO2, resulting in a change of the net landair or sea-air exchange of CO2. For example, the efficiency with which the ocean can absorb CO2 at the surface is related to how much CO2 can be converted to DIC. The measure of this is called the Revelle factor (RF) as given by Eq. 3.2 (Revelle and Suess, 1957):

The RF of surface ocean waters varies from 8-9 in the subtropical gyres to 13-15 in the higher latitudes. Figure 3.7 shows the change in DIC concentration of the modern surface ocean in response to a uniform increase in pCO2 of 10 ppm, plotted as a function of RF. It also shows that waters with low RF (~9) are four times more efficient at taking up CO2 (ADIC) than waters with very high RF (~15). The RF of ocean waters is controlled by the distribution of the DIC species, including the pH of the ocean.

As the ocean takes up anthropogenic CO2, the pH of the water decreases and the RF increases. With the anthropogenic CO2 estimates of Sabine et al. (2004b), the global average RF of surface waters today appears about one unit higher than the pre-industrial values. Thus, the surface ocean today is less efficient at taking up CO2 than the pre-industrial ocean providing a positive feedback. According to Fig. 3.7, the significance of this effect will vary depending on locations. Changing the RF by one in the high latitudes will have less effect than changes in the subtropics with relatively low RF. A further insight of these processes and their proper representation in ocean carbon models is important for understanding the ultimate long-term storage of anthropogenic CO2 in the ocean.

Inorganic carbon thermodynamics are reasonably well understood, but some carbon cycle feedbacks, particularly those involving biological processes, are not well understood. One example of this is the effect of anthropogenic CO2 on organisms that produce calcium carbonate (CaCO3) shells. Shallow water environments, primarily coral reefs and carbonate shelves, produce ~0.3 Pg C/year, largely as metastable aragonite and

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Revelle factor

Fig. 3.7. Plot of the change in dissolved inorganic carbon for a 10 ppm change in pCO2 as a function of Revelle factor for surface waters (<60 m) from the GLODAP bottle data-set. Inset shows a map of surface Revelle factor from the same data-set.

high-magnesian calcite. Open-ocean plankton produces an estimated 0.7-1.4 Pg C/year (Milliman, 1993; Lee, 2001), mostly as calcite but also some aragonite. These open-ocean calcifiers include phototrophic coccolitho-phorids and heterotrophic foraminifera as well as pteropods. Using Eq. 3.3, 1 mol of CaCO3 produced releases 1 mol of CO2:

Numerous studies have suggested that the rate of calcification in a wide variety of organisms is reduced when they are exposed to elevated CO2 levels (see summary in Feely et al., 2004). As atmospheric CO2 levels increase, one might expect calcification to decrease, which would lead to a lower natural release of CO2 from the ocean, providing a negative feedback.

The situation, however, is not that straightforward. A decrease in carbonate precipitation in the upper ocean would also lower the RF, increasing the capacity of the ocean to thermodynamically take up CO2 from the atmosphere. A complete shutdown of surface ocean calcification would decrease surface ocean pCO2 by ~20 ppm (Wolf-Gladrow et al., 1999). On the other hand, if these organisms are primary producers, the decrease in organic matter production could result in a positive feedback. Furthermore, a decrease in CaCO3 production would affect the ratio of organic/inorganic carbon delivery to the deep sea. If processes regulating this 'rain' of organic and inorganic carbon to deep-sea sediments are uncoupled, a decrease in CaCO3 production would lead to increased dissolution of CaCO3 in deep-sea sediments, which would raise the ocean pH and its capacity to store CO2 (Archer and Maier-Reimer, 1994). However, if these two processes are coupled and the denser carbonate particles are necessary for transporting the organic matter into the deep ocean quickly (Armstrong et al., 2002), reducing the carbonate production could result in shallower remineraliza-tion of organic carbon, producing a positive feedback (Klaas and Archer, 2002; Ridgwell, 2003), and a diminished role of sediments in the buffering of atmospheric CO2 increases. It is also not clear how elevated CO2 selection against a certain species (e.g. calcifying organisms) will affect the overall ecosystem structure and net CO2 uptake by ocean biology in the future. Clearly, there is a need for more research on these mechanistic controls of the long-term changes in the carbonate system.

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