153.8.5 Conclusions

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Based on high agreement between independent estimates using different methods and data sets (e.g., oceanic carbon, oxygen, and transient tracer data), it is very likely that the global ocean inventory of anthropogenic carbon (Cant) increased from 1994 to 2010. The oceanic Cant inventory in 2010 is estimated to be 155 PgC with an uncertainty of ±20%. The annual global oceanic uptake rates calculated from independent data sets (from oceanic Cant inventory changes, from atmospheric O₂/N₂ measurements or from pCO₂ data) and for different time periods agree with each other within their uncertainties and very likely are in the range of 1.0 to 3.2 PgC/yr. (Section 3.8.1, Figures 3.16 and 3.17)

Oceanic uptake of anthropogenic CO₂ results in gradual acidification of the ocean. The pH of surface seawater has decreased by 0.1 since the beginning of the industrial era, corresponding to a 26% increase in hydrogen ion concentration. The observed pH trends range between –0.0014 and –0.0024 /yr in surface waters. In the ocean interior, natural physical and biological processes, as well as uptake of anthropogenic CO₂, can cause changes in pH over decadal and longer time scales (Section 3.8.2, Table 3.2, Box 3.2, Figures 3.18 and 3.19, FAQ 3.3).

High agreement among analyses provides medium confidence that oxygen concentrations have decreased in the open ocean thermocline in many ocean regions since the 1960s. The general decline is consistent with the expectation that warming-induced stratification leads to a decrease in the supply of oxygen to the thermocline from near surface waters, that warmer waters can hold less oxygen, and that changes in wind-driven circulation affect oxygen concentrations. It is likely that the tropical oxygen minimum zones have expanded in recent decades (Section 3.8.3, Figure 3.20).

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