The air–sea flux of CO₂ is computed from the observed difference in the partial pressure of CO₂ (pCO₂) across the air–water interface (∆pCO₂ = pCO₂sw - pCO₂air), the solubility of CO₂ in seawater, and the gas transfer velocity (Wanninkhof et al., 2009). However, the limited geographic and temporal coverage of the ∆pCO 2 measurement as well as uncertainties in wind forcing and transfer velocity parameterizations mean that uncertainties in global and regional fluxes calculated from measurements of ∆pCO₂ can be as larges as ±50% (Wanninkhof et al., 2013). Using ∆pCO₂ data in combination with the riverine input Gruber et al. (2009) estimated a global uptake rate of 1.9 [1.2 to 2.5] PgC/yr for the time period 1995–2000 and Takahashi et al. (2009) found 2.0 [1.0 to 3.0] PgC/yr normalized to the year 2000. Uncertainties in fluxes calculated from ∆pCO₂ are too large to detect trends in global ocean carbon uptake.
Trends in surface ocean pCO₂ are calculated from ocean time series stations and repeat hydrographic sections in the North Atlantic and North Pacific (Table 3.2). At all locations and for all time periods shown, pCO₂ in both the atmosphere and ocean has increased, while pH and [CO3 2–] have decreased. At some sites, oceanic surface pCO₂ increased faster than the atmospheric trend, implying a decreasing uptake of atmospheric CO₂ at those locations. The oceanic pCO₂ trend can differ from that in the atmosphere owing to changes in the intensity of biological production and changes in physical conditions, for instance between El Niño and La Niña (Keeling et al., 2004; Midorikawa et al., 2005; Yoshikawa-Inoue and Ishii, 2005; Takahashi et al., 2006, 2009; Schuster and Watson, 2007; Ishii et al., 2009; McKinley et al., 2011; Bates, 2012; Lenton et al., 2012).
Although local variations of ∆pCO₂ with time have little effect on the atmospheric CO₂ growth rate in the short term, they provide important information on the dynamics of the ocean carbon cycle and the potential for longer-term climate feedbacks. For example, El Niño and La Niña can drive large changes in the efflux of CO₂ in the Pacific. Differences in ∆pCO₂ can exceed 100 µatm in the eastern and central equatorial Pacific between El Niño and La Niña; an increase in ∆pCO₂ observed between 1998 and 2004 was attributed to wind and circulation changes associated with the Pacific Decadal Oscillation (Feely et al., 2006). CO₂ uptake in the North Atlantic decreased by 0.24 [0.19–0.29] PgC/yr between 1994 and 2003 (Schuster and Watson, 2007) and has partially recovered since then (Watson et al., 2009). Linear trends for the North Atlantic from 1995 to 2009 reveal an increased uptake (Schuster et al., 2013). Uptake of CO₂ in the Subtropical Mode Water (STMW) of the North Atlantic was enhanced during the 1990s, a predominantly positive phase of the NAO, and much reduced in the 2000s when the NAO phase was neutral or negative (Bates, 2012). Observations in the Indian and Pacific sectors of the Southern Ocean were interpreted as evidence for reduced winter-time CO2 uptake as a result of increased winds, increased upwelling and outgassing of natural CO₂ (Metzl, 2009; Lenton et al., 2012).