5.4.2.2 Anthropogenic Carbon Change

The recent uptake of anthropogenic carbon in the ocean is well constrained by observations to a decadal mean of 2.2 ± 0.4 GtC yr^–1 for the 1990s (see Section 7.3.2, Table 7.1). The uptake of anthropogenic carbon over longer time scales can be estimated from oceanic measurements. Changes in DIC between two time periods reflect the anthropogenic carbon uptake plus the changes in DIC concentration due to changes in water masses and biological activity. To estimate the contribution of anthropogenic carbon alone, several corrections must be applied. From observed DIC changes between surveys in the 1970s and the 1990s, an increase in anthropogenic carbon has been inferred down to depths of 1,100 m in the North Pacific (Peng et al., 2003; Sabine et al., 2004a), 200 to 1,200 m in the Indian Ocean (Peng et al., 1998; Sabine et al., 1999) and 1,900 m in the Southern Ocean (McNeil et al., 2003).

An indirect method was used to estimate anthropogenic carbon from observations made at a single time period based on well-known processes that control the distribution of natural DIC in the ocean. The method corrects the observed DIC concentration for organic matter decomposition and dissolution of carbonate minerals, and removes an estimate of the DIC concentration of the water when it was last in contact with the atmosphere (Gruber et al., 1996). With this method, a global DIC increase of 118 ± 19 GtC between pre-industrial times (roughly 1750) and 1994 has been estimated, using 9,618 profiles from the 1990s (Sabine et al., 2004b; see Figure 5.10). The uncertainty of ±19 GtC in this estimate is based on uncertainties in the anthropogenic DIC estimates and mapping errors, which have characteristics of random error, and on an estimate of potential biases, which are not necessarily centred on the mean value. Potential biases of up to 7% in the technique have been identified, mostly caused by assumptions about the time evolution of CO₂, the age or the identification of water masses (Matsumoto and Gruber, 2005), and the recent changes in surface warming and stratification (Keeling, 2005). Potential biases from assumptions of constant carbon and nutrient uptake ratios for biological activity have not been assessed. While the magnitude and direction of all potential biases are not yet clear, the given uncertainty of ±16% appears realistic compared to the biases already identified.

Figure 5.10. Column inventory of anthropogenic carbon (mol m^–2) as of 1994 from Sabine et al. (2004b). Anthropogenic carbon is estimated indirectly by correcting the measured DIC for the contributions of organic matter decomposition and dissolution of carbonate minerals, and taking into account the DIC concentration the water had in the pre-industrial ocean when it was last in contact with the atmosphere. The global inventory of anthropogenic carbon taken up by the ocean between 1750 and 1994 is estimated to be 118 ± 19 GtC.

Because of the limited rate of vertical transport in the ocean, more than half of the anthropogenic carbon can still be found in the upper 400 m, and it is undetectable in most of the deep ocean (Figure 5.11). The vertical penetration of anthropogenic carbon is consistent with the DIC changes observed between two cruises (Peng et al., 1998, 2003). Anthropogenic carbon has penetrated deeper in the North Atlantic and subantarctic Southern Ocean compared to other basins, due to a combination of: i) high surface alkalinity (in the Atlantic) which favours the uptake of CO₂, and ii) more active vertical exchanges caused by intense winter mixing and by the formation of deep waters (Sabine et al., 2004b). The deeper penetration of anthropogenic carbon in these regions is consistent with similar features observed in the oceanic distribution of chlorofluorocarbons (CFCs) of atmospheric origin (Willey et al., 2004), confirming that it takes decades to many centuries to transport carbon from the surface into the thermocline and the deep ocean. Deeper penetration in the North Atlantic and subantarctic Southern Ocean is also observed in the changes in heat content shown in Figure 5.3. The large storage of anthropogenic carbon observed in the subtropical gyres is caused by the lateral transport of carbon from the region of mode water formation towards the lower latitudes (Figure 5.10).

Figure 5.11. Mean concentration of anthropogenic carbon as of 1994 in μmol kg^–1 from Sabine et al. (2004b) averaged over (a) the Pacific and Indian Oceans and (b) the Atlantic Ocean. The calculation of anthropogenic carbon is described in the caption of Figure 5.10 and in the text (Section 5.4).

The fraction of the net CO₂ emissions taken up by the ocean (the uptake fraction) was possibly lower during 1980 to 2005 (37% ± 7%) compared to 1750 to 1994 (42% ± 7%); however the uncertainty in the estimates is larger than the difference between the estimates (Table 5.1). The net CO₂ emissions include all emissions that have an influence on the atmospheric CO₂ concentration (i.e., emissions from fossil fuel burning, cement production, land use change and the terrestrial biosphere response). It is equivalent to the sum of the atmospheric and oceanic CO₂ increase. Because the atmospheric CO₂ is well constrained by observations, the uncertainty in the net CO₂ emissions is nearly equal to the uncertainty in the oceanic CO₂ increase. The decrease in oceanic uptake fraction would be consistent with the understanding that the ocean CO₂ sink is limited by the transport rate of anthropogenic carbon from the surface to the deep ocean, and also with the nonlinearity in carbon chemistry that reduces the CO₂ uptake capacity of water as its CO₂ concentration increases (Sarmiento et al., 1995).