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

==== 5.2.2.4 Changing Ocean Oxygen ====

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Ocean oxygen (O 2 ) levels at the surface are controlled by the balance between oxygen production during photosynthesis, temperature-controlled solubility and air-sea exchange. Deeper in the water column, consumption of oxygen during respiration and redistribution by ocean circulation and mixing are dominant processes. In theory, a warmer more stratified ocean would have a reduced oxygen content, due to the combined influence of lowered gas solubility and a greater interior respiration of organic matter due to enhanced physical isolation of subsurface waters. In accord, global changes in ocean oxygen assessed from three different analyses of compiled global oxygen datasets going back to the 1960s agree that there is a net loss of oxygen from the ocean over all depths (see Table 5.2). For the 0–1000 m depth stratum that contains the most data and is common to all three analyses, oxygen is assessed to have declined by a ''very likely'' range of 0.5–3.3% between 1970 and 2010. For the surface ocean (0–100 m) and the thermocline later of 100–600 m the ''very likely'' range of oxygen declines are 0.2–2.1% and 0.7–3.5%, respectively (Table 5.2). Across two studies, global oxygen is assessed to have declined by a ''very likely'' range of 0.3–2.0%, with a similar range of decline for waters deeper than 600 m (Table 5.2). The regions of lowest oxygen, known as OMZs, with oxygen levels lower than 80 μ mol L -1 ), are observed to be expanding by a ''very likely'' range of 3.0–8.3% across the three studies.

Regionally, all studies agree that the north Pacific and Southern Oceans have shown the largest overall oxygen declines (Figure 5.9), but there is some disagreement regarding the magnitude of the oxygen change in the tropical ocean, with some studies suggesting significant declines (Schmidtko et al., 2017) and other reporting more modest reductions (Helm et al., 2011; Ito et al., 2017) and data coverage is still limited for some regions and deeper than 1000 m. Based on the available data, the strongest declines in deep ocean oxygen have occurred in the Equatorial Pacific, North Pacific, Southern Ocean and South Atlantic, with intermediate declines in the Arctic, South Pacific and Equatorial Atlantic, while the north Atlantic has experienced a moderate oxygen increase below 1200 m (Figure 5.9). A particular difference between parallel oxygen analyses concerns the means of integrating and mapping sparse data across the ocean, both horizontally and vertically, with different studies making specific decisions about averaging grids and integration methods. Moreover, data remains sparse for some ocean regions, depths and periods. Taken together, the challenges of data sparsity, regional differences and the relatively large uncertainties on the oxygen changes across different studies, but also recognising that oxygen declines are significantly different to zero, leads to ''medium confidence'' in the observed oxygen decline.

Syntheses of datasets from local time series tend to document stronger trends, with oxygen declines of over 20% at sites in the northeastern Pacific between 1956–2006 (Whitney et al., 2007), the Northwestern Pacific between 1954–2014 (Sasano et al., 2015) and the California Current between 1984–2011 (Bograd et al., 2015). Despite holding the highest inventory of oxygen in the ocean, oxygen levels in Southern Ocean contributed 25% to the global decline between 1970–1992 (Helm et al., 2011) and have fallen by over 150 Tmol per decade from the 1960s to present (Schmidtko et al., 2017). Observations along ocean cruises as part of the CLIVAR programme have also documented broad thermocline oxygen declines in the northern hemisphere oceans, accompanied by well understood oxygen increases in subtropical and southern hemispheres (Talley et al., 2016).

Overall there is ''medium confidence'' that the oxygen content of the upper 1000 m has declined with a ''very likely'' loss of 0.5–3.3% between 1970-2010. OMZ are expanding in volume, by a ''very likely'' range of 3.0–8.3%. There is ''medium confidence'' that the largest regional changes have occurred in the Southern Ocean, equatorial regions, North Pacific and South Atlantic due to ''medium agreement'' among studies.

The role of ocean warming alone in driving the oxygen changes can be appraised using solubility estimates, which vary between around 15–50% for the upper 1000 m oxygen trend between studies (Helm et al., 2011; Ito et al., 2017; Schmidtko et al., 2017). The role of other processes, linked to changing ocean ventilation and respiration are challenging to appraise directly, but tend to reinforce the impacts from warming and are probably predominant overall (Oschlies et al., 2018). Indeed, that the observed oxygen decline is negatively correlated with ocean heat content changes (Ito et al., 2017) reflects the overriding role of changing ocean ventilation and associated processes (see also Section 5.2.2). That the ratio of the associated oxygen to heat changes is larger than would be expected from thermal processes alone also highlights the role played by other processes (Oschlies et al., 2018). Local oxygen trends have emphasised the role of changes to ocean physics in western Northern Pacific (Whitney et al., 2013); Sasano et al. (2015), the southern California Current region (Goericke et al., 2015), and the Santa Barbara Basin (Goericke et al., 2015). In regions of high mesoscale activity, such as the tropical north Atlantic, low oxygen eddies can have a significant impact on oxygen dynamics (Karstensen et al., 2015; Grundle et al., 2017). Oxygen fluctuations in the deep ocean have been linked to changes in large scale ocean circulation (Watanabe et al., 2003; Stendardo and Gruber, 2012) and at the global scale, the observed oxygen decline is negatively correlated with ocean heat content changes (Ito et al., 2017). Changes to respiration rates, either due to temperature enhancement or in the amount/quality of organic material can also be important and the enhanced respiratory demand associated with an intensified monsoon has been invoked as a driver of the expansion of the Arabian Sea OMZ (Lachkar et al., 2018).

Ocean oxygen changes are also affected by climate variability on interannual and decadal timescales, especially for the tropical ocean OMZs (Deutsch et al., 2011). ENSO variability in particular affects the thermocline structure, which then alongside changes in circulation modulates oxygen solubility and respiratory demand in this region (Ito and Deutsch, 2013; Eddebbar et al., 2017). These drivers may then be combined with modifications to overturning and ventilation of OMZs by lateral jets and equatorial current intensity (Duteil et al., 2014). Centennial scale studies based on isotope proxies for low oxygen regions have demonstrated fluctuations in OMZ extent linked to decadal changes in tropical trade winds that affects interior ocean respiratory oxygen demand, which implies that it will be difficult to attribute recent changes in the Pacific OMZ to anthropogenic forcing alone (Deutsch et al., 2015). Parallel work based on oxygen observations (Llanillo et al., 2013), as well as modelling (Duteil et al., 2018) supports the importance of decadal scale variability in the eastern tropical Pacific OMZ. There is some evidence for the potential of a modulating impact on tropical Pacific oxygen at interannual timescales from atmospheric deposition of nitrogen and iron (Ito et al., 2016; Yang and Gruber, 2016).

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<span id="table-5.2"></span>
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'''Table 5.2'''

Observed oxygen changes for the period 1970–2010 for 6 different layers within the ocean. The changes are shown as percentage change of global averages. The layers are depths 0–100, 100–600, 0–1000, and 600–bottom are in metres. The oxygen minimum zone (OMZ) is defined as the ocean volume change that is less than 80 μ mol L -1 . The estimates and confidence intervals are based published papers (Schmidtko et al. 2018, Ito et al. 2017 and Helm et al. 2011). The assessed change is the average of the available estimates and the 90% Confidence Interval (CI) combines the confidence as their standard deviation with two degrees of freedom.

<!-- TABLE -->
{| class="wikitable"
|-
|
| '''Schmidtko'''
|
| '''Ito'''
|
| '''Helm'''
|
| '''Assessed Change'''
|
|-
| '''Layer'''
| '''Period'''
| '''Change'''
| '''90 CI'''
| '''Change'''
| '''90 CI'''
| '''Change'''
| '''90 CI'''
| '''Change'''
| '''90 CI'''
|-
| '''0–100'''
| 1970–2010
| –0.38%
| ±1.06%
| –1.65%
| ±0.63%
| –1.30%
| ±0.54%
| –1.11%
| ±0.95%
|-
| '''100–600'''
| 1970–2010
| –1.06%
| ±1.36%
| –3.17%
| ±1.34%
| –2.04%
| ±0.60%
| –2.09%
| ±1.42%
|-
| '''0–1000'''
| 1970–2010
| –1.35%
| ±1.38%
| –2.70%
| ±1.30%
| –1.74%
| ±0.54%
| –1.93%
| ±1.39%
|-
| '''600–bottom'''
| 1970–2010
| –1.51%
| ±0.62%
| n.a.
| –0.81%
| ±0.57%
| –1.16%
| ±0.84%
|-
|
|-
| '''OMZ'''
| 1970–2010
| 6.33%
| ±2.52%
| 6.10%
| 1.2%
| 4.49%
| ±2.25%
| 5.64%
| ±2.66%
|-
|
|-
| '''Global'''
| 1970–2010
| –1.43%
| ±0.70%
| n.a.
| –0.87%
| ±0.53%
| –1.15%
| ±0.88%
|}
<!-- END TABLE -->

At the global scale, there is ''high confidence'' that the impact of a warmer ocean on oxygen levels is reinforced by other processes associated with ocean physics and biogeochemistry, which cause the majority of the observed oxygen decline. For the tropical Pacific OMZ, there is ''medium confidence'' arising from ''medium agreement'' from ''medium evidence'' that low frequency decadal changes in ocean physics have controlled past fluctuations in OMZ extent.

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<span id="figure-5.9"></span>
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'''Figure 5.9'''

<span id="figure-5.9-absolute-change-in-dissolved-oxygen-umol-kg1-per-decade-between-water-depths-of-a-0-and-1200-m-and-b-1200-m-and-the-sea-floor-over-the-period-19602010.-lines-indicate-boundaries-of-omzs-with-less-than-80-μ-mol-kg1-oxygen-anywhere-within-the-water-column-dasheddotted-less-than-40"></span>
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'''Figure 5.9 | Absolute change in dissolved oxygen (umol kg–1 per decade) between water depths of (a) 0 and 1200 m, and (b) 1200 m and the sea floor over the period 1960–2010. Lines indicate boundaries of OMZs with less than 80 μ mol kg–1 oxygen anywhere within the water column (dashed/dotted), less than 40 […]'''
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[[File:b75db0c50b18807042b37db95dc0403a IPCC-SROCC-CH_5_9.jpg]]

Figure 5.9 | Absolute change in dissolved oxygen (umol kg–1 per decade) between water depths of (a) 0 and 1200 m, and (b) 1200 m and the sea floor over the period 1960–2010. Lines indicate boundaries of OMZs with less than 80 μ mol kg–1 oxygen anywhere within the water column (dashed/dotted), less than 40 μ mol kg–1 (dashed) and less than 20 μ mol kg–1 (solid). Redrawn from Oschlies et al. (2018).

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Future changes in oxygen can be appraised from ESMs that account for the combined effects of ocean physics and biogeochemistry. Globally, these models project that it is ''very likely'' oxygen will decline by 3.2–3.7% or 1.6–2.0% (both across 90% confidence limits) for RCP8.5 or RCP2.6, respectively, relative to 2000 (Bopp et al., 2013). Focussing on the 100–600 m depth stratum, O 2 changes by –4 to –3.1% for the RCP8.5 or by –0.5–0.1% for the RCP2.6 scenario (relative to 2006–2015, Figure 5.8d). It should be noted that ESMs appear to be underestimating the rate of oxygen change from available datasets from the historical period (Oschlies et al., 2018) . Increased tropical ocean stratification reduces interior ocean oxygen by diminishing pathways of ventilation in the subtropical gyres and by inhibiting turbulent mixing with the oxygen-rich surface ocean (see Section 5.2.2.2.4). This relatively robust global modelled trend (Figure 5.8d) however masks important uncertainties in the projection of regional trends (Figure 5.8e), particularly in the tropical ocean OMZs (Bopp et al., 2013; Cocco et al., 2013; Cabré et al., 2015). The uncertainty in the trends in tropical ocean OMZs arises due to the fact that oxygen depletion due to warming induced reductions in oxygen saturation are opposed by oxygen enrichment due to reduced oxygen consumption during respiration in response to predicted declines in marine export production, as well as biases due to model resolution in the tropics and the length of the model spin up (Bopp et al., 2017). The 80 μ mol L -1 threshold that may be used to define the volume of the oxygen minimum is projected to grow by a ''very likely'' range of 7.0 ± 5.6% by 2100 during the RCP8.5 scenario or show virtually no change during the RCP2.6 scenario, relative to a 1850–1900 reference period (Figure 5.10). At the seafloor, between 200–3000 m depth strata, the north Pacific, north Atlantic, Arctic and Southern Oceans may see oxygen declines by 0.3–3.7% by 2100 (relative to 2005), with abyssal ocean changes being lower and more localised around regions in the north Atlantic and Southern Ocean (Sweetman et al., 2017), but will be modulated by any future changes in overturning strength. There is ''high confidence'' that the largest changes in deep sea systems will occur after 2100 (Battaglia and Joos, 2018).

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<span id="figure-5.10"></span>
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'''Figure 5.10'''

<span id="figure-5.10-the-evolution-of-the-volume-of-the-100600-m-layer-of-the-ocean-with-oxygen-concentrations-less-than-80-mmol-l1-for-the-rcp8.5-red-line-and-the-rcp2.6-blue-line-normalised-to-the-volume-in-18501900.-dashed-lines-indicated-the-very-likely-range-90-confidence-intervals-across-the-cmip5-models-cnrm-cm5"></span>
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'''Figure 5.10 | The evolution of the volume of the 100–600 m layer of the ocean with oxygen concentrations less than 80 mmol L–1 for the RCP8.5 (red line) and the RCP2.6 (blue line), normalised to the volume in 1850–1900. Dashed lines indicated the very likely range (90% confidence intervals) across the CMIP5 models (CNRM-CM5, […]'''
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[[File:d0b3d4255f66d71da9ffd86d3c2fc212 IPCC-SROCC-CH_5_10.jpg]]

Figure 5.10 | The evolution of the volume of the 100–600 m layer of the ocean with oxygen concentrations less than 80 mmol L–1 for the RCP8.5 (red line) and the RCP2.6 (blue line), normalised to the volume in 1850–1900. Dashed lines indicated the very likely range (90% confidence intervals) across the CMIP5 models (CNRM-CM5, GFDL-ESM2M, GFDL-ESM2G, IPSL-CM5A-LR, IPSL-CM5A-MR, MPI-ESM-LR, MPI-ESM-MR and the NCAR-CESM1 models). Models are corrected for drift in O2 using their control simulations.

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Simulations extended to 2300 suggest that by 2150 the trend of declining tropical ocean oxygen (both in terms of concentrations and volume of low oxygen waters) may reverse itself, mainly due to the effect of strong declines in primary production and organic matter fluxes to the ocean interior (Fu et al., 2018) or due to enhanced Antarctic ventilation (Yamamoto et al., 2015), but with ''low confidence'' due to ''limited evidence'' . At the global scale, 10,000 year intermediate complexity model simulations find that overall ocean oxygen loss shows near linear relationships to equilibrium temperature, itself linearly related to cumulative emissions, and any climate mitigation scenario will reduce peak oxygen loss by 4.4% per degree Celsius of avoided warming (Battaglia and Joos, 2018).

In summary, the total oxygen content of the ocean is ''very likely'' to decline by 3.2–3.7% by 2100, relative to 2000, for RCP8.5 or by between 1.6–2.0% for RCP2.6 with ''medium confidence'' . There is ''medium confidence'' that sea floor changes will be more localised in the north Atlantic and Southern Oceans by 2100, but ''high confidence'' that the largest deep sea floor changes in oxygen will occur after 2100.

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