7.3.4.3 Carbon Cycle Feedbacks to Changes in Physical Forcing

A more sluggish ocean circulation and increased density stratification, both expected in a warmer climate, would slow down the vertical transport of carbon, alkalinity and nutrients, and the replenishment of the ocean surface with water that has not yet been in contact with anthropogenic CO₂. This narrowing of the ‘bottleneck’ for anthropogenic CO₂ invasion into the ocean would provide a significant positive feedback to atmospheric greenhouse gas concentrations (Bolin and Eriksson, 1959; see also the carbon cycle climate model simulations by Cox et al., 2000; Friedlingstein et al., 2001, 2006). As long as the vertical transfer rates for marine biogenic particles remain unchanged, in a more sluggish ocean the biological carbon pump will be more efficient (Boyle, 1988; Heinze et al., 1991), thus inducing a negative feedback, which is expected to be smaller than the physical transport feedback (Broecker, 1991; Maier-Reimer et al., 1996; Plattner et al., 2001; see Figure 7.10). However, a modelling study by Bopp et al. (2005) predicts a decrease in vertical particle transfer and hence shallower depths of re-mineralization of particulate organic carbon resulting in a positive CO₂ feedback. Further changes in plankton community structure including the role of N₂-fixing organisms can feed back to the carbon cycle (Sarmiento et al., 2004; Mahaffey et al., 2005). Changes in ocean circulation can affect the regional circulation of shelf and coastal seas, leading either to increased export of nutrients plus carbon from the shallow seas into the open ocean or to increased upwelling of nutrients plus carbon onto the shelf and towards coastal areas (Walsh, 1991; Smith and Hollibaugh, 1993; Chen et al., 2003; Borges, 2005). A reduction in sea ice cover may increase the uptake area for anthropogenic CO₂ and act as a minor negative carbon feedback (ACIA, 2005). The physical ‘bottleneck’ feedback dominates over biological feedbacks induced by circulation change, resulting in an anticipated overall positive feedback to climate change. Both feedbacks depend on details of the future ocean circulation and model projections show a large range.

The solubility of CO₂ gas in seawater and the two dissociation constants of carbonic acid in seawater depend on temperature and salinity (Weiss, 1974; Millero et al., 2002). A 1°C increase in sea surface temperature produces an increase in pCO₂ of 6.9 to 10.2 ppm after 100 to 1,000 years (Heinze et al., 2003; see also Broecker and Peng, 1986; Plattner et al., 2001). Warming may increase the biological uptake rate of nutrients and carbon from surface waters, but the net effect on export and DIC is uncertain. Laws et al. (2000) proposed that export efficiency increases with net photosynthesis at low temperatures, which implies a positive feedback to warming. In addition, DOC may be degraded more quickly at higher temperatures.