5.4.2 Carbon

5.4.2.1 Total Change in Dissolved Inorganic Carbon and Air-Sea Carbon Dioxide Flux

Direct observations of oceanic dissolved inorganic carbon (DIC; i.e., the sum of CO₂ plus carbonate and bicarbonate) reflect changes in both the natural carbon cycle and the uptake of anthropogenic CO₂ from the atmosphere. Links between the main modes of climate variability and the marine carbon cycle have been observed on interannual time scales in several regions of the world (see Section 7.3.2.4 for quantitative estimates). In the equatorial Pacific, the reduced upwelling associated with El Niño events decreases the regional outgas of natural CO₂ to the atmosphere (Feely et al., 1999). In the subtropical North Atlantic, reduced mode water formation and reduced deep winter mixing during the positive NAO phase increase the storage of carbon in the intermediate ocean (Bates et al., 2002). These observations show that variability in the content of natural DIC in the ocean has occurred in association with climate variability.

Longer observations exist for the partial pressure of CO₂ (pCO₂) at the surface only. Over more than two decades, the oceanic pCO₂ increase has generally followed the atmospheric CO₂ within the given uncertainty, although regional differences have been observed (Feely et al., 1999; Takahashi et al., 2006). The three stations with the longest time series, all in the northern subtropics, show pCO₂ increases at a rate varying between 1.6 and 1.9 μatm yr^–1 (Figure 5.9), indistinguishable from the atmospheric increase of 1.5 to 1.9 μatm yr^–1. Variability on the order of 20 μatm over periods of five years was observed in the three time series, as well as in other data sets, and has been associated with regional changes in the natural carbon cycle driven by changes in ocean circulation and by climate variability (Gruber et al., 2002; Dore et al., 2003) or with variations in biological activity (Lefèvre et al., 2004).

Figure 5.9. Changes in surface oceanic pCO₂ (left; in μatm) and pH (right) from three time series stations: Blue: European Station for Time-series in the Ocean (ESTOC, 29°N, 15°W; Gonzalez-Dávila et al., 2003); green: Hawaii Ocean Time-Series (HOT, 23°N, 158°W; Dore et al., 2003); red: Bermuda Atlantic Time-series Study (BATS, 31/32°N, 64°W; Bates et al., 2002; Gruber et al., 2002). Values of pCO₂ and pH were calculated from DIC and alkalinity at HOT and BATS; pH was directly measured at ESTOC and pCO₂ was calculated from pH and alkalinity. The mean seasonal cycle was removed from all data. The thick black line is smoothed and does not contain variability less than 0.5 years period.

Direct surface pCO₂ observations have been used to compute a global air-sea CO₂ flux of 1.6 ± 1 GtC yr^–1 for the year 1995 (Takahashi et al., 2002; Section 7.3.2.3.2, Figure 7.8). It is not yet possible to detect large-scale changes in the global air-sea CO₂ flux from direct observations because of the large influence of climate variability. However, estimates from inverse methods of the air-sea CO₂ flux from the spatio-temporal distribution of atmospheric CO₂ suggest that the global air-sea CO₂ flux increased by 0.1 to 0.6 GtC yr^–1 between the 1980s and 1990s, consistent with results from ocean models (Le Quéré et al., 2003).