Editing IPCC:AR6/WGI/Chapter-3 (section)

=== 3.6.2 Ocean Biogeochemical Variables ===

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Since CMIP5, there has been a general increase in ocean horizontal and vertical grid resolution in ocean model components ( [[#Arora--2020|Arora et al., 2020]] ; [[#Séférian--2020|Séférian et al., 2020]] ). The latter of these developments is particularly significant for projections of ocean stressors as it directly affects the representation of stratification. Updates in the representation of ocean biogeochemical processes between CMIP5 and CMIP6 have typically involved an increase in model complexity. Specific developments have been the more widespread inclusion of micronutrients, such as iron, variable stoichiometric ratios, more detailed representation of lower trophic levels including bacteria and the cycling and sinking of organic matter. CMIP6 biogeochemical model performance is generally an improvement on that of the parent CMIP5 generation of models ( [[#Séférian--2020|Séférian et al., 2020]] ). The global representation of present-day air-sea carbon fluxes and surface chlorophyll concentrations show moderate improvements between CMIP5 and CMIP6. Similar improvements are seen in the representation of subsurface oxygen concentrations in most ocean basins, while the representation of surface macronutrient concentrations in CMIP6 is shown to have improved with respect to silicic acid but declined slightly with respect to nitrate. Model representation of the micronutrient iron has not improved substantially since CMIP5, but many more models are capable of representing iron. In addition, a comparison of the carbon concentration and carbon climate feedbacks shows no significant change between CMIP5 and CMIP6 ( [[#Arora--2020|Arora et al., 2020]] ).

Since AR5, research has also focused on the detection and attribution of regional patterns in ocean biogeochemical change relating to interior deoxygenation, air-sea CO <sub>2</sub> flux, and ocean carbon uptake and associated acidification. Characterization of flux variability requires understanding of the suite of physical and biological processes including transport, heat fluxes, interior ventilation, biological production and gas exchange which can have very different controls on seasonal versus interannual time scales in both the North Pacific ( [[#Ayers--2012|Ayers and Lozier, 2012]] ) and North Atlantic ( [[#Breeden--2016|Breeden and McKinley, 2016]] ). In the Southern Ocean, models have difficulty reproducing the observed seasonal cycle and interannual variability, making attribution particularly challenging ( [[#Lovenduski--2016|Lovenduski et al., 2016]] ; [[#Mongwe--2016|Mongwe et al., 2016]] , 2018).

The AR5 concluded that oxygen concentrations have decreased in the open ocean since 1960 and such decreases can be attributed in part to human influence with ''medium confidence'' . The decrease in ocean oxygen content in the upper 1000 m, between 1970 and 2010, is further confirmed in SROCC ( ''medium confidence'' ), with the oxygen minimum zone expanding in volume (see also Section 5.3.3.2). Observed oxygen declines over the last several decades ( [[#Stendardo--2012|Stendardo and Gruber, 2012]] ; [[#Stramma--2012|Stramma et al., 2012]] ; [[#Schmidtko--2017|Schmidtko et al., 2017]] ) match model estimates in the surface ocean ( [[#Oschlies--2017|Oschlies et al., 2017]] ) but are much larger than model derived estimates in the interior ( [[#Bopp--2013|Bopp et al., 2013]] ; [[#Cocco--2013|Cocco et al., 2013]] ). Some of this difference has been interpreted as due to a lack of representation of coastal eutrophication in these models ( [[#Breitburg--2018|Breitburg et al., 2018]] ), but much of it remains unexplained. This disparity is particularly apparent in the eastern Pacific oxygen minimum zone, where some CMIP5 models showed increasing trends whereas observations show a strong decrease ( [[#Cabré--2015|Cabré et al., 2015]] ). However, proxy reconstructions suggest that over the last century the ocean may have in fact undergone increases in oxygen in the most oxygen poor regions ( [[#Deutsch--2014|Deutsch et al., 2014]] ). As discussed in Section 5.3.1, ocean oxygen went through wide oscillations on multi-centennial time scales through the last deglaciation, with abrupt warming resulting in loss of oxygen in subsurface waters of the North Pacific ( [[#Praetorius--2015|Praetorius et al., 2015]] ). The global upper ocean oxygen inventory is negatively correlated with ocean heat content with a regression coefficient comparable to that found in ocean models ( [[#Ito--2017|Ito et al., 2017]] ). Variability and trends in the observed upper ocean oxygen concentration are mainly driven by the apparent oxygen utilization component with small contributions from oxygen solubility, suggesting that changing ocean circulation, mixing, and/or biochemical processes, rather than thermally induced solubility effects may be the main drivers of observed deoxygenation. The spatial distribution of the ocean deoxygenation in the interior of the ocean as well as over coastal areas is further assessed in Section 5.3.

As one of the most commonly observed surface parameters, the partial pressure of CO <sub>2</sub> has been the topic of considerable detection and attribution work. In North Atlantic subtropical and equatorial biomes, warming has been shown to be a significant and persistent contributor to the observed increase in the partial pressure of CO <sub>2</sub> since the mid‐2000s with long‐term warming leading to a reduction in ocean carbon uptake ( [[#Fay--2013|Fay and McKinley, 2013]] ), and with both the partial pressure of CO <sub>2</sub> and associated carbon uptake demonstrating strong predictability as a function of interannual to decadal climate state (H. [[#Li--2016|]] [[#Li--2016|Li et al., 2016]] ; [[#Li--2018|Li and Ilyina, 2018]] ). In the Southern Ocean however, detection and attribution of surface trends in the partial pressure of CO <sub>2</sub> has proven more elusive and dependent on methodology, with some studies suggesting that Southern Ocean carbon uptake slowed from about 1990 to 2006 and subsequently strengthened from 2007 to 2010 ( [[#Lovenduski--2008|Lovenduski et al., 2008]] ; [[#Fay--2014|Fay et al., 2014]] ; [[#Ritter--2017|Ritter et al., 2017]] ). Other studies have suggested that poor representation of the seasonal cycle in the Southern Ocean may confound the models’ ability to represent changes in the partial pressure of CO <sub>2</sub> in the Southern Ocean ( [[#Nevison--2016|Nevison et al., 2016]] ; [[#Mongwe--2018|Mongwe et al., 2018]] ).

Section 5.2.1.3 assesses that both observational reconstructions based on the partial pressure of CO <sub>2</sub> and ocean biogeochemical models show a quasi-linear increase in the ocean sink of anthropogenic CO <sub>2</sub> from 1.0 ± 0.3 PgC yr <sup>–1</sup> to 2.5 ± 0.6 PgC yr <sup>–1</sup> between 1960–1969 and 2010–2019 in response to global CO <sub>2</sub> emissions ( ''high confidence'' ). During the 1990s, the global net flux of CO <sub>2</sub> into the ocean is estimated to have weakened to 0.8 ± 0.5 PgC yr <sup>–1</sup> while in 2000 and thereafter, it is estimated to have strengthened considerably to rates of 2.0 ± 0.5 PgC yr <sup>–1</sup> , associated with changes in SST, the surface concentration of dissolved inorganic carbon and alkalinity, and decadal variations in atmospheric forcing ( [[#Landschützer--2016|Landschützer et al., 2016]] , see also Section 5.2).

Ocean acidification is one of the most detectible metrics of environmental change and was well covered in AR5, in which it was assessed that the uptake of anthropogenic CO <sub>2</sub> had ''very likely'' resulted in acidification of surface waters ( [[#Bindoff--2013|Bindoff et al., 2013]] ). Since then, observations and simulations of multi-decadal trends in surface carbon chemistry have increased in robustness. The evidence on ocean pH decline had further strengthened in SROCC with good agreement found between CMIP5 models and observations and an assessment that the ocean was continuing to acidify in response to ongoing carbon uptake ( [[#Bindoff--2019|Bindoff et al., 2019]] ). An observed decrease in global surface open ocean pH is assessed in [[IPCC:Wg1:Chapter:Chapter-2#2.3.3.5|Section 2.3.3.5]] to be ''virtually certain'' to have occurred with a rate of 0.003–0.026 per decade for the past 40 years. The ocean acidification has occurred not only in the surface layer but also in the interior of the ocean (Sections 2.3.3.5 and 5.3.3). Rates have been observed to be between −0.015 and −0.020 per decade in mode and intermediate waters of the North Atlantic through the combined effect of increased anthropogenic and remineralized carbon ( [[#Ríos--2015|Ríos et al., 2015]] ) and acidification has been observed down to 3000 m in the deep water formation regions ( [[#Perez--2018|Perez et al., 2018]] ). There has also been considerable improvement in detection and attribution of anthropogenic CO <sub>2</sub> versus eutrophication-based acidification in coastal waters ( [[#Wallace--2014|Wallace et al., 2014]] ).

The increased evidence in recent studies supports an assessment that it is ''virtually certain'' that the uptake of anthropogenic CO <sub>2</sub> was the main driver of the observed acidification of the global surface open ocean. The observed increase in acidification over the North Atlantic subtropical and equatorial regions since 2000 is ''likely'' associated in part with an increase in ocean temperature, a response which corresponds to the expected weakening of the ocean carbon sink with warming. Due to strong internal variability, systematic changes in carbon uptake in response to climate warming have not been observed in most other ocean basins at present. We further assess, consistent with AR5 and SROCC, that deoxygenation in the upper ocean is due in part to anthropogenic forcing, with ''medium'' confidence. There is ''high confidence'' that Earth system models simulate a realistic time evolution of the global mean ocean carbon sink.

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