Editing IPCC:AR6/SROCC/Chapter-5 (section)

===== 5.2.2.3.1 Ocean carbon fluxes and inventories =====

The analyses of the steadily growing number of surface ocean CO 2 observations (now more than 20 million observations, SOCATv6 ( [http://www.socat.info/index.php/2018/06/19/v6-release/ www.socat.info/index.php/2018/06/19/v6-release] ) demonstrate that the net ocean uptake of CO 2 from the atmosphere has increased from around 1.2 ± 0.5 Pg C yr -1 in the early 1980s to 2.0 ± 0.5 Pg C yr -1 in the years 2010–2015 (Rödenbeck et al., 2014; Landschützer et al., 2016). Once new estimates of the outgassing flux stemming from river derived carbon of 0.8 Pg C yr -1 (Resplandy et al. 2018) are accounted for, these new observations imply that the rate of global ocean uptake of anthropogenic CO 2 increased from 2.0 ± 0.5 Pg C yr -1 to 2.8 ± 0.5 Pg C yr -1 between the early 1980s and 2010–2015 (Rödenbeck et al., 2014; Landschützer et al., 2016; Le Quéré et al., 2018). This increase is supported by the current generation of ocean carbon cycle models (Le Quéré et al., 2018), and commensurate with the increase in atmospheric CO 2 .

The continuing efforts to re-measure dissolved inorganic carbon (DIC) along many of the repeat hydrographic lines that were occupied during the 1980s and 1990 (Talley et al., 2016), alongside the preparation of a global quality controlled database of ocean interior observations (Olsen et al., 2016a), have led to progress since AR5 regarding to the oceanic interior storage of anthropogenic CO 2 . Several studies analysed the changes in the amount of anthropogenic CO 2 that have accumulated between different occupations in the different ocean basins (Wanninkhof et al., 2010; Pérez et al., 2013; Woosley et al., 2016; Carter et al., 2017), confirming that the anthropogenic CO 2 taken up from the atmosphere is transported to depth, where most of it is stored. Using a newly developed reconstruction method, Gruber et al. (2019) extended these results to the globe. They find that between 1994 and 2007, across two standard deviations, that the global ocean has accumulated an additional 30-38 Pg C of anthropogenic CO 2 , which is equivalent to an air-sea CO 2 flux of between 2.3–2.9 Pg C yr -1 (coherent with surface ocean CO 2 observations), bringing the total inventory for the year 2007 to 150 ± 20 Pg C. Extrapolating this estimate to the year 2010 gives an inventory of 158 ± 18 Pg C, which is statistically indistinguishable from the ‘best’ estimate provided by Khatiwala et al. (2013) of 155 ± 31 Pg C and more recently also found from a steady-state ocean model (DeVries, 2014) for this reference year. If the inventory-based estimates are adjusted for the loss of natural carbon, a ''very likely'' total increase in storage between 1994 and 2007 of 24–34 Pg C, or around 25% of total emissions, is found (Gruber, 2019).

Thus, there is ''very high confidence'' from surface ocean and ocean interior carbon data that the strength of the ocean sink for anthropogenic carbon has increased in the last two decades in response to the growth of atmospheric CO 2 . Multiple lines of evidence indicate that it is ''very likely'' that the ocean has taken up 20–30% of the global emissions of CO 2 from the burning of fossil fuels, cement production, and land-use change since the mid 1980s. The consistency between independent surface ocean observations and the ocean interior data-based reconstructions supports the assessment of ''very high confidence'' and provides ''robust evidence'' that fraction of emissions taken up by the ocean has not changed in a statistically significant manner in the last few decades and remains consistent with AR5.

Alongside a globally integrated perspective, these new surface ocean observations also reveal a substantial degree of variability at interannual and decadal scales (Rödenbeck et al., 2015; Landschützer et al., 2016; Le Quéré et al., 2018). Most notable are the air-sea CO 2 flux variations in the tropics linked to ENSO variations (Rödenbeck et al., 2015; Landschützer et al., 2016), as well as the strong decadal variations in the high latitudes, especially the Southern Ocean (Landschützer et al., 2015; Munro et al., 2015; Ritter et al., 2017), discussed further in Chapter 3 (Section 3.2.1.2.4). Fluctuations in the Southern Ocean CO 2 flux are important as they impart a substantial imprint also on the global uptake fluxes. For instance, reduced Southern Ocean uptake in the 1990–2000 period coincided with an exceptionally weak global net uptake of only about 0.8 ± 0.5 Pg C yr −1 .

Thus, there is growing evidence from multiple datasets that the ocean carbon sink exhibits decadal variability at regional scales that significantly alter the globally integrated sink ( ''medium confidence'' ).

Detailed analyses of the spatial structure of the change in storage of anthropogenic CO 2 confirm the variable nature of the ocean carbon sink suggested by the surface observations (Pérez et al., 2013), which are most likely a consequence of changes in ocean circulation (DeVries and Weber, 2017). The increase in anthropogenic CO 2 between 1994 and 2007 occurs throughout the upper 1000 m, but with very different penetration depths, reflecting largely differences in the efficiency, with which the anthropogenic CO 2 is transported from the surface to depth (Gruber et al., 2019) (Figure 5.7). This spatial distribution of how the amount of anthropogenic CO 2 has changed between 1994 and 2007 is similar to the distribution of anthropogenic CO 2 reconstructed for 1994 (Sabine et al., 2004), although the imprint of regional variations in ocean circulation and transport are discernible (Gruber, 2019).

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'''Figure 5.7'''

<span id="figure-5.7-vertical-sections-of-the-change-in-anthropogenic-co2-from-1994-to-2007-represented-by-the-zonal-mean-sections-in-each-ocean-basin-organised-around-the-southern-ocean-in-the-centre.-the-upper-500-m-are-expanded.-contour-intervals-of-anthropogenic-co2-are-2-μmol-kg1-gruber-2019."></span>
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'''Figure 5.7 | Vertical sections of the change in anthropogenic CO2 from 1994 to 2007 represented by the zonal mean sections in each ocean basin, organised around the Southern Ocean in the centre. The upper 500 m are expanded. Contour intervals of anthropogenic CO2 are 2 μmol kg–1 (Gruber, 2019).'''
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[[File:61e952057d81b9dbc6f16f4911f31d5c IPCC-SROCC-CH_5_7.jpg]]

Figure 5.7 | Vertical sections of the change in anthropogenic CO2 from 1994 to 2007 represented by the zonal mean sections in each ocean basin, organised around the Southern Ocean in the centre. The upper 500 m are expanded. Contour intervals of anthropogenic CO2 are 2 μmol kg–1 (Gruber, 2019).

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