Changes in ocean uptake rate

By combining the Sabine et al. (2004b) estimate of the anthropogenic CO2 that has accumulated in the ocean between 1800 and 1994 with a synthesis of the average uptake estimate for the last 20 years (Sabine et al., 2004a), we can evaluate potential changes in the decadal-scale uptake rate of anthropogenic CO2 by the ocean. Table 3.4 shows the change in carbon inventories during the first 180 years of the anthropocene versus inventory changes over the last 20 years. These estimates suggest that the oceanic uptake of net CO2 emissions decreased from ~44% during the first period to ~36% over the last two decades. Although this difference is not statistically significant, there is a suggestion that the oceanic uptake efficiency is decreasing with time.

Several countries have initiated programmes to evaluate decadal-scale changes in oceanic CO2 uptake. For example, Ocean Station Papa in the north-eastern Pacific Ocean has been sampled for oceanic carbon on a semi-regular basis for the last 40 years (Signorini et al., 2001). Within the USA, the Hawaii Ocean Time-series (HOT) programme and Bermuda Atlantic Time-series Study (BATS) have been measuring carbon concentrations in the water column for more than 15 years. These projects have focused most of their attention on seasonal to interannual variability, but are beginning to have records long enough to see longer-term variability in CO2 uptake (e.g. Bates, 2001; Gruber et al., 2002; Dore et al., 2003; Keeling et al., 2004). Additional sites

Table 3.4. The global carbon budget (Pg C).

1800-1979

1 980-1 999

Atmospheric increase

+116 ± 4

+65 ± 1

Emissions (fossil fuel

and cement)

+ 156 ± 20

+117 ± 5

Ocean inventory

-90 ± 19

-3 7 ± 8

Net terrestrial

+50 ± 28

-15 ± 9

Notes: Positive values represent atmospheric increase (or ocean/land sources); negative numbers represent atmospheric decrease (sinks).

Notes: Positive values represent atmospheric increase (or ocean/land sources); negative numbers represent atmospheric decrease (sinks).

are also being examined by European and Asian countries.

Changes in the carbon concentrations along hydrographic sections sampled several years apart can also provide useful information on decadal-scale CO2 uptake. At least seven countries have agreed to coordinate hydrographic survey cruises to monitor the decadal-scale changes in ocean carbon inventory. For example, the US CLIVAR/ CO2 Repeat Hydrography Program has outlined 19 cruises that will reoccupy sections that were last sampled in the 1990s. To date, six lines have been run. Preliminary results have suggested interesting basin-to-basin differences in the inferred uptake rates on these lines (e.g. 0.7 mol/m2/year in the North Atlantic versus 1.1 mol/m2/year in the North Pacific; see Feely et al., 2005; Wanninkhof et al., 2005). These changes in oceanic uptake may reflect changes in ocean circulation and/ or the enactment of feedback mechanisms in the ocean that can serve to either enhance or reduce the uptake of anthropogenic CO2 in the ocean.

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