Longterm accumulation of anthropogenic CO2

Recognizing the need to constrain the oceanic uptake, transport and storage of anthropogenic CO2 during the anthropo-cene as well as to provide a baseline for future estimates of oceanic CO2 uptake, two international ocean research programmes, the World Ocean Circulation Experiment (WOCE) and the Joint Global Ocean Flux Study (JGOFS), jointly conducted a comprehensive survey of inorganic carbon distributions in the global ocean in the 1990s (Wallace, 2001). After completion of the US field programme in 1998, a 5-year effort was started to compile and rigorously quality control the US and international data-

sets including a few pre-WOCE data-sets in regions that had limited data (Key et al., 2004). The final data-set, with 9618 hydro-graphic stations collected on 95 cruises, provides the most accurate and comprehensive view of the global ocean inorganic carbon distribution available (see http://cdiac. esd.ornl.gov/oceans/glodap/Glodap_home. htm). By combining these data with a back calculation technique (Gruber et al., 1996) for isolating the anthropogenic component of the measured DIC, Sabine et al. (2004b) estimated that 118 ± 19 Pg C has accumulated in the ocean between 1800 and 1994. This inventory accounts for 48% of the fossil fuel and cement manufacturing CO2 emissions to the atmosphere over this time frame.

A map of the anthropogenic CO2 ocean column inventory (Fig. 3.5) shows that CO2 is not evenly distributed in space. More than 23% of the inventory can be found in the North Atlantic, a region covering ~15% of the global ocean. By contrast, the region south of 50°S represents approximately the same ocean area but has only ~9% of

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Fig. 3.5. Global map of anthropogenic CO2 column inventory in mol/m2. (From Sabine et al., 2004b.)

the global inventory (Sabine et al., 2004b). Despite the relatively slow equilibration rate for CO2 in sea water (~1 year versus weeks for oxygen), uptake at the surface does not fully explain the spatial differences in storage. The primary reason for these differences is due to the slow mixing time in the ocean interior and the fact that waters move into the deep ocean only in a few locations. The highest inventories are found in locations where mode and intermediate waters move anthropogenic CO2 into the ocean interior (e.g. the northern North Atlantic or in the southern hemisphere associated with the subtropical convergence zone at 40°S-50°S; Fig. 3.5).

One exception to the observation of higher inventories associated with water mass formation regions is the fact that no large inventories are associated with the formation of bottom waters around Antarctica. There are several possible reasons:

1. The anthropogenic signal has not been properly identified because of poor data coverage in these regions.

2. Low vertical stratification results in substantial mixing, invalidating a basic assump-

tion of the technique used to estimate the anthropogenic CO2 concentrations.

3. Newly formed bottom waters mix with old anthropogenic CO2 free waters, diluting the signal below the limit of detection (~5 mmol/kg).

4. Short residence times of the waters at the surface and ice cover do not allow the CO2 to equilibrate, resulting in incomplete uptake.

5. Carbon chemistry (i.e. high Revelle factor) makes the Southern Ocean very inefficient at taking up CO2.

In reality, it is likely to be a combination of all these factors that has limited our ability to detect substantial anthropogenic CO2 concentrations in the bottom waters around Antarctica.

Figure 3.6 shows sections of anthropogenic CO2 in the Atlantic, Pacific and Indian oceans. One feature that clearly stands out in these examples is that most of the deep ocean has still not been exposed to elevated CO2 levels. Nearly 50% of all the anthropogenic CO2 is stored in the upper 10% of the global ocean (depths less than 400 m) and detectable concentrations of anthropogenic CO2 average only as deep as 1000 m.

The global ocean is far from being saturated with CO2. This further illustrates that the primary rate-limiting step for oceanic carbon uptake is not the exchange across the sea-air interface, but the rate at which that carbon is transported into the ocean int erior. Model studies suggest that the ocean ultimately will absorb 70-85% of the CO2 released from human activity, but given the slow mixing time of the ocean, this will take millennia to accomplish (Le Quere and Metzl, 2004).


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Fig. 3.6. Representative sections of anthropogenic CO2 (mmol/kg) from the (a) Atlantic, (b) Pacific and (c) Indian oceans. Insets show maps of the station locations used to generate the sections.

Fig. 3.6. Representative sections of anthropogenic CO2 (mmol/kg) from the (a) Atlantic, (b) Pacific and (c) Indian oceans. Insets show maps of the station locations used to generate the sections.

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