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

===== 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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