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

==== 5.2.1.3 Ocean Carbon Fluxes and Storage ====

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Since AR5 and SROCC, major advances in globally coordinated ocean CO <sub>2</sub> observations (Surface Ocean CO <sub>2</sub> Atlas, SOCAT; and Global Ocean Data Analysis Project, GLODAP), the harmonization of ocean and coastal-observation-based products, atmospheric and oceanic inversion models and forced global ocean biogeochemical models (GOBMs) have increased the level of confidence in the assessment of trends and variability of air–sea fluxes and storage of CO <sub>2</sub> in the ocean during the historical period (1960–2018; see also Supplementary Materials 5.SM.1; [[#Ciais--2013|Ciais et al., 2013]] ; [[#Bakker--2016|Bakker et al., 2016]] ; [[#Landschützer--2016|Landschützer et al., 2016]] , 2020; [[#Bindoff--2019|Bindoff et al., 2019]] ; [[#DeVries--2019|DeVries et al., 2019]] ; [[#Gregor--2019|Gregor et al., 2019]] ; [[#Gruber--2019a|Gruber et al., 2019a]] , b; [[#Tohjima--2019|Tohjima et al., 2019]] ; [[#Friedlingstein--2020|Friedlingstein et al., 2020]] ; [[#Hauck--2020|Hauck et al., 2020]] ; [[#Olsen--2020|Olsen et al., 2020]] ). A major advance since SROCC is that, for the first time, all six published observational product fluxes used in this assessment, are made more comparable using a common ocean and sea ice cover area, integration of climatological coastal fluxes scaled to increasing atmospheric CO <sub>2</sub> and an ensemble mean of ocean fluxes calculated from three re-analysis wind products (Supplementary Materials 5.SM.2; [[#Landschützer--2014|Landschützer]] et al., 2014, 2020; [[#Rödenbeck--2014|Rödenbeck et al., 2014]] ; [[#Zeng--2014|Zeng et al., 2014]] ; [[#Denvil-Sommer--2019|Denvil-Sommer et al., 2019]] ; [[#Gregor--2019|Gregor et al., 2019]] ; [[#Iida--2021|Iida et al., 2021]] ). From a process point of view, the ocean uptake of anthropogenic carbon is a two-step set of abiotic processes that involves the exchange of CO <sub>2</sub> , first across the air–sea boundary into the surface mixed layer, followed by its transport into the ocean interior where it is stored for decades to millennia, depending on the depth of storage ( [[#Gruber--2019b|Gruber et al., 2019b]] ). Two definitions of air–sea fluxes of CO <sub>2</sub> are used in this assessment for both observational products and models: S <sub>ocean</sub> is the global mean ocean CO <sub>2</sub> sink and F <sub>net</sub> denotes the net spatially varying CO <sub>2</sub> fluxes ( [[#Hauck--2020|Hauck et al., 2020]] ). Adjustment of the mean global F <sub>net</sub> for the pre-industrial sea-to-air CO <sub>2</sub> flux associated with land-to-ocean carbon flux term makes F <sub>net</sub> comparable to S <sub>ocean</sub> ( [[#Jacobson--2007|Jacobson et al., 2007]] ; [[#Resplandy--2018|Resplandy et al., 2018]] ; [[#Hauck--2020|Hauck et al., 2020]] ).

There are multiple lines of observational and modelling evidence that support with ''high confidence'' the finding that, in the historical period (1960–2018), air–sea fluxes and storage of anthropogenic CO <sub>2</sub> are largely influenced by atmospheric CO <sub>2</sub> concentrations, physical ocean processes and physicochemical carbonate chemistry, which determines the unique properties of CO <sub>2</sub> in seawater ( [[IPCC:Wg1:Chapter:Chapter-9|Chapter 9]] and Cross-Chapter Box 5.3; [[#Wanninkhof--2014|Wanninkhof, 2014]] ; [[#DeVries--2017|DeVries et al., 2017]] ; [[#McKinley--2017|McKinley et al., 2017]] , 2020, [[#Gruber--2019a|Gruber et al., 2019a]] , b; [[#Hauck--2020|Hauck et al., 2020]] ). Here we assess three different approaches (Figures 5.8a,b and 5.9) that together provide ''high'' ''confidence'' that, during the historical period (1960–2018), the ocean carbon sink (S <sub>ocean</sub> ) and its associated ocean carbon storage have grown in response to global anthropogenic CO <sub>2</sub> emissions ( [[#Gruber--2019a|Gruber et al., 2019a]] ; [[#Hauck--2020|Hauck et al., 2020]] ; [[#McKinley--2020|McKinley et al., 2020]] ).

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===== 5.2.1.3.1 Ocean carbon fluxes and storage: Global multi-decadal trends =====

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In the first assessment approach, the mean global multi-decadal (1960–2019) trends in the ocean sink (S <sub>ocean</sub> ) for CO <sub>2</sub> show a high degree of coherence across the nine GOBMs and six ''p'' CO <sub>2</sub> -based observational product reconstructions (1987–2018) which, despite a temporary slowdown (or ‘hiatus’) in the 1990s, is also quasi-linear over that period (Figure 5.8a; [[#Gregor--2019|Gregor et al., 2019]] ; [[#Hauck--2020|Hauck et al., 2020]] ). This coherence between the GOBMs and observations-based reconstructions (1987–2018; r <sup>2</sup> =0.85) provides ''high confidence'' that the ocean sink (S <sub>ocean</sub> in [[#5.2.1.5|Section 5.2.1.5]] ) evaluated from GOBMs (1960–2019) grew quasi-linearly from 1.0 ± 0.3 PgC yr <sup>–1</sup> to 2.5 ± 0.6 PgC yr <sup>–1</sup> between the decades 1960–1969 and 2010–2019 in response to global CO <sub>2</sub> emissions (Figure 5.8a; Table 5.1; [[#Friedlingstein--2020|Friedlingstein et al., 2020]] ; [[#Hauck--2020|Hauck et al., 2020]] ). The cumulative ocean CO <sub>2</sub> uptake (105 ± 20 PgC) is 23% of total anthropogenic CO <sub>2</sub> emissions (450 ± 50 PgC) for the same period ( [[#Friedlingstein--2020|Friedlingstein et al., 2020]] ). Notwithstanding the ''high confidence'' in the magnitude of the annual to decadal trends for S <sub>ocean,</sub> this assessment is moderated to ''medium'' c ''onfidence'' by the ''low confidence'' in the currently inadequately constrained uncertainties in the pre-industrial land-to-ocean carbon flux, the uncertain magnitude of winter outgassing from the Southern Ocean, and the uncertain effect of the ocean surface cool-skin, the effect of data sparsity, differences between wind products and the uncertain contribution from the changing land–ocean continuum on global and regional fluxes ( [[#Jacobson--2007|Jacobson et al., 2007]] ; [[#Resplandy--2018|Resplandy et al., 2018]] ; [[#Roobaert--2018|Roobaert et al., 2018]] ; [[#Bushinsky--2019|Bushinsky et al., 2019]] ; [[#Hauck--2020|Hauck et al., 2020]] ; [[#Watson--2020|Watson et al., 2020]] ; [[#Gloege--2021|Gloege et al., 2021]] ). However, both GOBMs and ''p'' CO <sub>2</sub> -based observational products independently reveal a slowdown or ‘hiatus’ of the ocean sink in the 1990s, which provides a valuable constraint for model verification and leads to greater confidence in the model outputs (Figure 5.8a; [[#Landschützer--2016|Landschützer et al., 2016]] ; [[#Gregor--2018|Gregor et al., 2018]] ; [[#DeVries--2019|DeVries et al., 2019]] ; [[#Hauck--2020|Hauck et al., 2020]] ). A number of studies point to the role of the Southern Ocean in the global ‘1990s hiatus’ in air–sea CO <sub>2</sub> fluxes, but provide different process-based explanations linking ocean temperature, mixing and meridional overturning circulation (MOC) responses to variability in large-scale climate systems, wind stress and volcanic activity, as well as the sensitivity of the air–sea CO <sub>2</sub> flux to small changes in the atmospheric forcing from anthropogenic CO <sub>2</sub> ( [[#Landschützer--2016|Landschützer et al., 2016]] ; [[#DeVries--2017|DeVries et al., 2017]] ; [[#Bronselaer--2018|Bronselaer et al., 2018]] ; [[#Gregor--2018|Gregor et al., 2018]] ; [[#Gruber--2019a|Gruber et al., 2019a]] ; [[#Keppler--2019|Keppler and Landschützer, 2019]] ; [[#McKinley--2020|McKinley et al., 2020]] ; [[#Nevison--2020|Nevison et al., 2020]] ). Data sparsity in the Southern Ocean could also be a factor amplifying the global decadal perturbation of the 1990s ( [[#Gloege--2021|Gloege et al., 2021]] ). Therefore, while there is ''high confidence'' in the 1990s hiatus of the global ocean sink for anthropogenic CO <sub>2,</sub> and that the Southern Ocean makes an observable contribution to it, there is still ''low confidence'' in the attribution for the processes behind the 1990s hiatus ( [[#5.2.1.3.2|Section 5.2.1.3.2]] ). Observed increases in the amplitude of the seasonal cycle of ocean ''p'' CO <sub>2</sub> and reductions in the mean global buffering capacity provide ''high confidence'' that the growing CO <sub>2</sub> sink is also beginning to drive observable large-scale changes in ocean carbonate chemistry ( [[#Jiang--2019|Jiang et al., 2019]] ). However, there is ''medium confidence'' that these changes which, depending on the emissions scenario, could drive future ocean feedbacks, are still too small to emerge from the historical multi-decadal observed growth rate of S <sub>ocean</sub> (Sections 5.1.2; 5.3.2 and 5.4.2, and Figure 5.8a; SROCC ( [[#5.2.2.3.2|Section 5.2.2.3.2]] ; [[#Bates--2014|Bates et al., 2014]] ; [[#Sutton--2016|Sutton et al., 2016]] ; [[#Fassbender--2017|Fassbender et al., 2017]] ; [[#Landschützer--2018|Landschützer et al., 2018]] ; [[#Jiang--2019|Jiang et al., 2019]] ). A recent model-based study suggests that re-emergence of previously stored anthropogenic CO <sub>2</sub> is changing the buffering capacity of the mixed layer and reducing the ocean sink for anthropogenic CO <sub>2</sub> during the historical period ( [[#Rodgers--2020|Rodgers et al., 2020]] ). This trend is not reflected in observations-based products (Figure 5.8a), so we attribute a ''low confidence'' .

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[[File:b43a260d7811a80998c166fc4239ff8e IPCC_AR6_WGI_Figure_5_8.png]]

'''Figure 5.8 |''' '''Multi-decadal trends for the ocean sink of CO''' <sub>2</sub> '''.''' '''(a)''' The multi-decadal (1960–2019) trends in the annual ocean sink (S <sub>ocean</sub> ) reconstructed from nine Global Ocean Biogeochemical Models (GOBM) forced with atmospheric re-analysis products ( [[#Hauck--2020|Hauck et al., 2020]] ), six observationally based gap-filling products that reconstructed spatial and temporal variability in the ocean CO <sub>2</sub> flux from sparse observations of surface ocean ''p'' CO <sub>2</sub> (Supplementary Materials 5.SM.2). The trends in S <sub>ocean</sub> were calculated from the mean annual GOBM outputs, and the observational products were used to provide confidence in the GOBM assessments (r <sup>2</sup> =0.85). Thick lines represent the multi-model mean. Observationally based products have been corrected for pre-industrial river carbon fluxes (0.62 PgC yr <sup>–1</sup> ) based on the average of estimates from [[#Jacobson--2007|Jacobson et al. (2007)]] and [[#Resplandy--2018|Resplandy et al. (2018)]] . '''(b)''' Mean decadal constraints and their confidence intervals for global ocean sink (S <sub>ocean</sub> ) of anthropogenic CO <sub>2</sub> using multiple independent or quasi-independent lines of evidence or methods for the period 1990–2019 (see Supplementary Materials Tables 5.SM.1 and 5.SM.2 for magnitudes, uncertainties and published sources). Further details on data sources and processing are available in the chapter data table (Table 5.SM.6).

The second assessment approach makes use of six independent methods to constrain the mean decadal ocean sink over the period 1990–2019 (Figure 5.8b). This provides a multi-decadal advance on the 1990–1999 decadal constraint from ( [[#Denman--2007|Denman et al., 2007]] ) that has been widely used as a model constraint for GOBMs used for the global carbon budget ( [[#Hauck--2020|Hauck et al., 2020]] ). The ''medium confidence'' attributed by this assessment of the global multi-decadal trend (Figure 5.8a) is further supported by the broad agreement in magnitude and trend of the decadal mean ocean CO <sub>2</sub> uptake with assessments that also include additional observations-based, independent methods such as ocean CO <sub>2</sub> inversion and atmospheric CO <sub>2</sub> and O <sub>2</sub> /N <sub>2</sub> measurements (Figure 5.8b; Supplementary Materials Tables 5.SM.1 and 5.SM.2).

Here we provide a third comparative assessment approach depicting the spatial coherence of ocean air–sea fluxes and storage rates of CO <sub>2</sub> as well as a quantitative assessment of both fluxes for the same period (1994–2007; Figure 5.9). Observation-based ''p'' CO <sub>2</sub> flux products show that emissions of natural CO <sub>2</sub> occur mostly in the tropics and high-latitude Southern Ocean, and that the uptake and storage of anthropogenic CO <sub>2</sub> occurs predominantly in the mid-latitudes (Chapter 9, Figure 5.9 and Cross-Chapter Box 5.3). Strong ocean CO <sub>2</sub> sink regions are those in the mid-latitudes associated with the cooling of poleward flowing subtropical surface waters as well as equatorward flowing sub-polar surface waters, both of which contribute to the formation of Mode, Intermediate and Deep water masses that transport anthropogenic CO <sub>2</sub> into the ocean interior on time scales of decades to centuries in both hemispheres ( [[IPCC:Wg1:Chapter:Chapter-9#9.2.2.3|Section 9.2.2.3]] and Figure 5.9; [[#DeVries--2014|DeVries, 2014]] ; [[#Gruber--2019b|Gruber et al., 2019b]] ; [[#Wu--2019|Wu et al., 2019]] ). The mean decadal scale magnitude and uncertainties of S <sub>ocean</sub> from net air sea fluxes (F <sub>net</sub> ) were calculated from an ensemble of six observational-based product reconstructions (Figure 5.9a) and the storage rates in the ocean interior derived from multiple ocean interior CO <sub>2</sub> datasets ( [[#Gruber--2019b|Gruber et al., 2019b]] ; Figure 5.9b). The cumulative CO <sub>2</sub> stored in the ocean interior from 1800 to 2007 has been estimated at 140 ±18 PgC ( [[#Gruber--2019b|Gruber et al., 2019b]] ). As reported in SROCC ( [[#5.2.2.3.1|Section 5.2.2.3.1]] ; [[#IPCC--2019b|IPCC, 2019b]] ), the net ocean CO <sub>2</sub> storage between 1994–2007 was 29 ± 4 PgC, which corresponds to a mean storage of 26 ± 5% of anthropogenic CO <sub>2</sub> emissions for that period ( [[#Gruber--2019b|Gruber et al., 2019b]] ). The resulting net annual storage rate of anthropogenic CO <sub>2</sub> , equivalent to S <sub>ocean</sub> for the period mid-1994 to mid-2007 is 2.2 ± 0.3 PgC yr <sup>–1</sup> , which is in very close agreement with the top-down air–sea flux estimate of S <sub>ocean</sub> of 2.1 ± 0.5 PgC yr <sup>–1</sup> from GOBMs and 1.9 ± 0.3PgC yr <sup>–1</sup> from ''p'' CO <sub>2</sub> -based observational products with the steady river carbon flux correction of 0.62 PgC yr <sup>–1</sup> for the same time period ( [[#Gruber--2019b|Gruber et al., 2019b]] ; [[#Hauck--2020|Hauck et al., 2020]] ). This close agreement between these independent ocean CO <sub>2</sub> sink estimates derived from air–sea fluxes and storage rates in the ocean interior support the ''medium confidence'' assessment that the ocean anthropogenic carbon storage rates continue to be determined by the ocean sink (S <sub>ocean</sub> ) in response to growing CO <sub>2</sub> emissions (Figure 5.9; [[#McKinley--2020|McKinley et al., 2020]] ).

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'''Figure 5.9 |''' '''Comparative regional characteristics of the mean decadal (1994–2007) sea-air CO''' <sub>2</sub> '''flux (Fnet) and ocean storage of anthropogenic CO''' <sub>2</sub> '''. (a)''' Regional source–sink characteristics for contemporary ocean air – sea CO <sub>2</sub> fluxes (F <sub>net</sub> ) derived from the ensemble of six observation-based products using Surface Ocean CO <sub>2</sub> ( [[IPCC:Wg1:Chapter:Atlas|Atlas]] (SOCAT)v6 observational dataset ( [[#Landschützer--2014|Landschützer et al., 2014]] ; [[#Rödenbeck--2014|Rödenbeck et al., 2014]] ; [[#Zeng--2014|Zeng et al., 2014]] ; [[#Bakker--2016|Bakker et al., 2016]] ; [[#Denvil-Sommer--2019|Denvil-Sommer et al., 2019]] ; [[#Gregor--2019|Gregor et al., 2019]] ; [[#Iida--2021|Iida et al., 2021]] ). Warm colours depict outgassing fluxes and black contours characterize the super-biomes defined from [[#Fay--2014|Fay and McKinley (2014)]] and adjusted by [[#Gregor--2019|Gregor et al. (2019)]] also used to calculate the variability in regional flux anomalies (Supplementary Materials Figure 5.SM.1); '''(b)''' The regional characteristics of the storage fluxes of CO <sub>2</sub> in the ocean interior for the same period ( [[#Gruber--2019b|Gruber et al., 2019b]] ). The dots reflect ocean areas where the 1-sigma standard deviation of Fnet from the six observational-based product reconstructions is larger than the magnitude of the mean. This reflects source–sink transition areas where the mean Fnet is small and more strongly influenced by spatial and temporal variability across the products. Further details on data sources and processing are available in the chapter data table (Table 5.SM.6).

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===== 5.2.1.3.2 Ocean carbon fluxes and storage: Regional and global variability =====

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The intent of this assessment is to show how global variability can be regionally forced ( [[#Gregor--2019|Gregor et al., 2019]] ; [[#Landschützer--2019|Landschützer et al., 2019]] ; [[#Hauck--2020|Hauck et al., 2020]] ). Since AR5 and SROCC, advances in global ocean CO <sub>2</sub> flux products, GOBMs and atmospheric inversion models have strengthened confidence in the assessment of how ocean regions influence mean global variability and trends of ocean CO <sub>2</sub> air–sea fluxes (F <sub>net</sub> ; see Supplementary Materials Figure 5.SM.1; [[#Ciais--2013|Ciais et al., 2013]] ; [[#Landschützer--2014|Landschützer et al., 2014]] , 2015; [[#Rödenbeck--2014|Rödenbeck et al., 2014]] ; [[#McKinley--2017|McKinley et al., 2017]] ; [[#Bindoff--2019|Bindoff et al., 2019]] ; [[#Gregor--2019|Gregor et al., 2019]] ; [[#Friedlingstein--2020|Friedlingstein et al., 2020]] ; [[#Hauck--2020|Hauck et al., 2020]] ). The coherence in the regional variability of the anomalies in F <sub>net</sub> from three independent lines of evidence support with ''high confidence'' that the non-steady state global interannual-decadal variability of F <sub>net</sub> has clear regional influences ( [[#Gregor--2019|Gregor et al., 2019]] ; [[#Landschützer--2019|Landschützer et al., 2019]] ). The tropical oceans contribute the most to the global mean interannual variability (Supplementary Materials Figure 5.SM.1d). The high latitude oceans, particularly the Southern Ocean, contribute the most to the global-scale decadal variability (Supplementary Materials Figure 5.SM5.1b,c; ( [[#Landschützer--2016|Landschützer et al., 2016]] , 2019; [[#Gregor--2019|Gregor et al., 2019]] ; [[#Gruber--2019a|Gruber et al., 2019a]] ; [[#Hauck--2020|Hauck et al., 2020]] ). The influence of the Southern Ocean on the global mean decadal variability and the 1990s hiatus is supported by the highest regional–global correlation coefficients (Supplementary Materials Figures 5.SM.1a,c). In contrast, the equatorial oceans’ influence on global mean F <sub>net</sub> has a low correlation because, notwithstanding the coherence in interannual variability, it does not show the same global mean trend of strengthening sink in response to growing global emissions (Supplementary Materials Figure 5.SM.1d; [[#Gregor--2019|Gregor et al., 2019]] ). All regions, except the equatorial ocean, contribute to varying extents to the multi-decadal trend of growth in the global ocean sink (Supplementary Materials Figure 5.SM.1). Data sparseness in the high latitudes and the relatively short length of the observational records leads to ''low confidence'' in the attribution of the processes that link regional–global variability to climate ( [[#Landschützer--2019|Landschützer et al., 2019]] ; [[#Gloege--2021|Gloege et al., 2021]] ).

Regional decadal-scale anomalies in the variability of ocean CO <sub>2</sub> storage have also emerged, probably associated with changes in the MOC, which may influence the global variability in F <sub>net</sub> (Chapter 9; [[#DeVries--2017|DeVries et al., 2017]] ). In the interior of the Indian and Pacific sectors of the Southern Ocean, and the North Atlantic, the increase in the CO <sub>2</sub> inventory from 1994 to 2007 was about 20% smaller than expected from the atmospheric CO <sub>2</sub> increase during the same period and the anthropogenic CO <sub>2</sub> inventory in 1994 (Sabine eta al., 2004; [[#Gruber--2019a|Gruber et al., 2019a]] ). There is ''medium confidence'' that the ocean CO <sub>2</sub> inventory strengthened again in the decade 2005–2015 ( [[#DeVries--2017|DeVries et al., 2017]] ). In the North Atlantic, a low rate of anthropogenic CO <sub>2</sub> storage at 1.9 ± 0.4 PgC per decade during the time period of 1989–2003 increased to 4.4 ± 0.9 PgC per decade during 2003–2014. This is associated with changing ventilation patterns driven by the North Atlantic Oscillation ( [[#Woosley--2016|Woosley et al., 2016]] ). In the Pacific sector of the Southern Ocean, the rate of anthropogenic CO <sub>2</sub> storage also increased from 8.8 ± 1.1 (1 σ ) PgC per decade during 1995–2005 to 11.7 ± 1.1 PgC per decade during 2005–2015 ( [[#Carter--2019|Carter et al., 2019]] ). However, in the Subantarctic Mode Water of the Atlantic sector of the Southern Ocean, the storage rate of the anthropogenic CO <sub>2</sub> was rather lower after 2005 than before ( [[IPCC:Wg1:Chapter:Chapter-9#9.2.3.2|Section 9.2.3.2]] ; [[#Tanhua--2017|Tanhua et al., 2017]] ; [[#Bindoff--2019|Bindoff et al., 2019]] ). These changes have been predominantly ascribed to the impact of changes in the MOC on the transport of anthropogenic CO <sub>2</sub> into the ocean interior due to regional climate variability, in addition to the increase in the atmospheric CO <sub>2</sub> concentration ( [[IPCC:Wg1:Chapter:Chapter-9#9.2.3.1|Section 9.2.3.1]] ; [[#Wanninkhof--2010|Wanninkhof et al., 2010]] ; [[#Pérez--2013|Pérez et al., 2013]] ; [[#DeVries--2017|DeVries et al., 2017]] , 2019; [[#Gruber--2019b|Gruber et al., 2019b]] ; [[#McKinley--2020|McKinley et al., 2020]] ). However,the low frequency of carbon observations in the interior of the vast ocean leads to ''medium confidence'' in the assessment of temporal variability in the rate of regional ocean CO <sub>2</sub> storage and its controlling mechanisms.

In summary, multiple lines of observational and modelling evidence provide ''high confidence'' in the finding that the ocean sink for anthropogenic CO <sub>2</sub> has increased quasi-linearly over the past 60 years in response to growing global emissions of anthropogenic CO <sub>2,</sub> with a mean fraction of 23% of total emissions. The ''high confidence'' assessment is moderated to ''medium confidence'' due to a number of ocean CO <sub>2</sub> flux terms yet to be adequately constrained. Observed changes in the variability of ocean ''p'' CO <sub>2</sub> and observed reductions in the mean global buffering capacity provide ''high confidence'' that the growing CO <sub>2</sub> sink is also beginning to drive observable large-scale changes in ocean carbonate chemistry. However, there is ''medium confidence'' that these changes which, depending on the emissions scenario, could drive future ocean feedbacks, are still too small to emerge from the historical multi-decadal observed growth rate of S <sub>ocean</sub> .

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