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

==== 3.5.1.3 Ocean Heat Content Change Attribution ====

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The ocean plays an important role as the Earth’s primary energy store. The AR5 and SROCC assessed that the ocean accounted for more than 90% of the Earth’s energy change since the 1970s ( [[#Rhein--2013|Rhein et al., 2013]] ; [[#Bindoff--2019|Bindoff et al., 2019]] ). These assessments are consistent with recent studies assessed in Section 7.2 and Cross-Chapter Box 9.1, which find that 91% of the observed change in Earth’s total energy from 1971 to 2018 was stored in the ocean ( [[#von%20Schuckmann--2020|von Schuckmann et al., 2020]] ). The AR5 concluded that anthropogenic forcing has ''very likely'' made a substantial contribution to ocean warming above 700 m, whereas below 700 m, limited measurements restricted the assessment of ocean heat content changes in AR5 and prevented a robust comparison between observations and models ( [[#Bindoff--2013|Bindoff et al., 2013]] ).

With the recent increase in ocean sampling by Argo to 2000 m ( [[#Roemmich--2015|Roemmich et al., 2015]] ; [[#Riser--2016|Riser et al., 2016]] ; [[#von%20Schuckmann--2016|von Schuckmann et al., 2016]] ) and the resulting improvements in estimates of ocean heat content ( [[#Abraham--2013|Abraham et al., 2013]] ; [[#Balmaseda--2013|Balmaseda et al., 2013]] ; [[#Durack--2014b|Durack et al., 2014b]] ; [[#Cheng--2017|Cheng et al., 2017]] ; [[#von%20Schuckmann--2020|von Schuckmann et al., 2020]] ), a more quantitative assessment of the global ocean heat content changes that extends into the intermediate ocean (700–2000 m) over the more recent period (from 2005 to the present; [[#Durack--2018|Durack et al., 2018]] ) can be performed. Observed ocean heat content changes are discussed in [[IPCC:Wg1:Chapter:Chapter-2#2.3.3.1|Section 2.3.3.1]] , where it is reported that it is ''virtually certain'' that the global upper ocean (0–700 m) and ''very likely'' that the global intermediate ocean (700–2000 m) warmed substantially from 1971 to the present. Further, ocean layer warming contributions are reported as 61% (0–700 m), 31% (700–2000 m) and 8% (&gt;2000 m) for the 1971 to 2018 period (Table 2.7). CMIP5 model simulations replicate this partitioning fairly well for the industrial-era (1865 to 2017) throughout the upper (0–700 m, 65%), intermediate (700–2000 m, 20%) and deep (&gt;2000 m, 15%) layers ( [[#Gleckler--2016|Gleckler et al., 2016]] ; [[#Durack--2018|Durack et al., 2018]] ). The corresponding warming percentages for the multi-model mean of a subset of CMIP6 simulations over the 1850–2014 period are 58% for the upper, 21% for the intermediate, and 22% for the deep-ocean layers (Figure 3.26). These results are consistent with SROCC which assessed that it is ''virtually certain'' that both the upper and intermediate ocean warmed from 2004 to 2016, with an increased rate of warming since 1993 ( [[#Bindoff--2019|Bindoff et al., 2019]] ). The spatial distribution of these changes for different ocean depths is assessed in Section 9.2.2.1.

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[[File:dcba862d8c947f6b3c983b6ca47cb154 IPCC_AR6_WGI_Figure_3_26.png]]

Figure 3.26 | '''Global ocean heat content in CMIP6 simulations and observations.''' Time series of observed (black) and simulated (red) global ocean heat content anomalies with respect to 1995–2014 for the full ocean depth '''(left-hand panel)''' ; upper layer: 0–700 m '''(top right-hand panel)''' ; intermediate layer: 700–2000 m '''(middle right-hand panel)''' ; and the abyssal ocean: &gt;2000 m '''(bottom right-hand panel)''' . The best estimate observations (black solid line) for the period of 1971–2018, along with ''very likely'' ranges (black shading) are from [[IPCC:Wg1:Chapter:Chapter-2#2.3.3.1|Section 2.3.3.1]] . For the models (1860–2014), ensemble members from 15 CMIP6 models are used to calculate the multi-model mean values (red solid line) after averaging across simulations for each independent model. The ''very likely'' ranges in the simulations are shown in red shading. Simulation drift has been removed from all CMIP6 historical runs using a contemporaneous portion of the linear fit to each corresponding pre-industrial control run ( [[#Gleckler--2012|Gleckler et al., 2012]] ). Units are zettajoules (ZJ; 10 <sup>21</sup> joule). Further details on data sources and processing are available in the chapter data table (Table 3.SM.1).

The multi-model means of both CMIP5 and CMIP6 historical simulations forced with time varying natural and anthropogenic forcing shows robust increases in ocean heat content in the upper (0–700 m) and intermediate (700–2000 m) ocean ( ''high confidence'' ) (Figure 3.26; [[#Cheng--2016|Cheng et al., 2016]] , [[#Cheng--2019|2019]] ; [[#Gleckler--2016|Gleckler et al., 2016]] ; [[#Bilbao--2019|Bilbao et al., 2019]] ; [[#Tokarska--2019|Tokarska et al., 2019]] ). Temporary (&lt;10 years) surface and subsurface cooling during and after large volcanic eruptions is also captured in the upper-ocean, and global mean ocean heat content ( [[#Balmaseda--2013|Balmaseda et al., 2013]] ). The ocean heat content increase is also reflected in the corresponding ocean thermal expansion which is a leading contributor to global mean sea level rise (Sections 3.5.3.2 and 9.2.4, and Box 9.1).

For the period 1971–2014, the rate of ocean heat uptake for the global ocean in the CMIP6 models is about 6.43 [2.08–8.66] ZJ yr <sup>–1</sup> , with the upper, intermediate and deeper layers respectively accounting for 68%, 16% and 16% of the full depth global heat uptake (Figure 3.26). Overall, the simulated ocean heat content changes are consistent with the updated and improved observational analyses, within the ''very likely'' uncertainty range defined for each (see also ( [[IPCC:Wg1:Chapter:Chapter-2#2.3.3.1|Section 2.3.3.1]] , Table 2.7; [[#Domingues--2008|Domingues et al., 2008]] ; [[#Purkey--2010|Purkey and Johnson, 2010]] ; [[#Levitus--2012|Levitus et al., 2012]] ; [[#Good--2013|Good et al., 2013]] ; [[#Cheng--2017|Cheng et al., 2017]] ; [[#Ishii--2017|Ishii et al., 2017]] ; [[#Zanna--2019|Zanna et al., 2019]] ) as well as with the ocean components of total Earth heating assessed in Section 7.2.2.2, Table 7.1. Nevertheless, large uncertainties remain, particularly in the deeper layers due to the poor temporal and spatial sampling coverage, particularly in the Atlantic, Southern and Indian Oceans ( [[#Garry--2019|Garry et al., 2019]] ). The ''very likely'' ranges of the simulated trends for the full ocean depth and below 2000 m fall within the ''very likely'' range of observed uptake during the last two decades. In the intermediate layer, the multi-model ensemble mean mostly stays above the observed 5th–95th percentile range before the year 2000, and below that range after 2000. For the upper ocean, some individual model realizations show a reduced ocean heat content increase during the 1970s and 1980s, which is then compensated by a greater warming than the observations from the early 1990s. These discrepancies have been linked with a temporary increase in the Southern Ocean deep water formation rate, as well as with the models’ strong aerosol cooling effects and high equilibrium climate sensitivity (see also Section 7.5.6 and Box 7.2; [[#Andrews--2019|Andrews et al., 2019]] , [[#Andrews--2020|2020]] ; [[#Golaz--2019|Golaz et al., 2019]] ; [[#Dunne--2020|Dunne et al., 2020]] ; [[#Winton--2020|Winton et al., 2020]] ).

Nevertheless, simulations show that the rate of ocean heat uptake has doubled in the past few decades, when contrasted to the rate over the complete 20th century (Figure 3.26), with over a third of the accumulated heat stored below 700 m ( [[#Cheng--2016|Cheng et al., 2016]] , [[#Cheng--2019|2019]] ; [[#Gleckler--2016|Gleckler et al., 2016]] ; [[#Durack--2018|Durack et al., 2018]] ). The Southern Ocean shows the strongest ocean heat uptake that penetrates to deeper layers (Section 9.2.3.2), whereas ocean heat content increases in the Pacific and Indian Oceans largely occur in the upper layers ( [[#Bilbao--2019|Bilbao et al., 2019]] ).

Since AR5, the attribution of ocean heat content increases to anthropogenic forcing has been further supported by more detection and attribution studies. These studies have shown that contributions from natural forcing alone cannot explain the observed changes in ocean heat content in either the upper or intermediate ocean layers, and a response to anthropogenic forcing is clearly detectable in ocean heat content ( [[#Gleckler--2016|Gleckler et al., 2016]] ; [[#Bilbao--2019|Bilbao et al., 2019]] ; [[#Tokarska--2019|Tokarska et al., 2019]] ). Moreover, a response to greenhouse gas forcing is detectable independently of the response to other anthropogenic forcings ( [[#Bilbao--2019|Bilbao et al., 2019]] ; [[#Tokarska--2019|Tokarska et al., 2019]] ), which has offset part of the greenhouse gas induced warming. Further evidence is provided by the agreement between observed and simulated changes in global thermal expansion associated with the ocean heat content increase when both natural and anthropogenic forcings are included in the simulations ( [[#3.5.3.2|Section 3.5.3.2]] ), though internal variability plays a larger role in driving basin-scale thermosteric sea level trends ( [[#Bilbao--2015|Bilbao et al., 2015]] ). Over the Southern Ocean, warming is detectable over the late 20th century and is largely attributable to greenhouse gases ( [[#Swart--2018|Swart et al., 2018]] ; [[#Hobbs--2021|Hobbs et al., 2021]] ), while other anthropogenic forcings such as ozone depletion have been shown to mitigate the warming in some of the CMIP5 simulations ( [[#Swart--2018|Swart et al., 2018]] ; [[#Hobbs--2021|Hobbs et al., 2021]] ). The use of the mean temperature above a fixed isotherm rather than fixed depth further strengthens a robust detection of the anthropogenic response in the upper ocean ( [[#Weller--2016|Weller et al., 2016]] ), and better accounting for internal variability in the upper ocean ( [[#Rathore--2020|Rathore et al., 2020]] ), helps explain reported hemispheric asymmetry in ocean heat content change ( [[#Durack--2014b|Durack et al., 2014b]] ).

In summary, there is strong evidence for an improved understanding of the observed global ocean heat content increase. It is ''extremely likely'' that human influence was the main driver of the ocean heat content increase observed since the 1970s, which extends into the deeper ocean ( ''very high confidence'' ). Updated observations, like model simulations, show that warming extends throughout the entire water column ( ''high confidence'' ).

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