Editing IPCC:AR6/WGI/Chapter-2 (section)

==== 2.3.3.1 Ocean Temperature, Heat Content and Thermal Expansion ====

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AR5 assessed that since 1971, global ocean warming was ''virtually certain'' for the upper 700 m and ''likely'' for the 700–2000 m layer. The SROCC reported linear warming trends for the 0–700 m and 700–2000 m layers of the ocean, respectively, of 4.35 ± 0.8 and 2.25 ± 0.64 ZJ yr <sup>–1</sup> over 1970–2017; 6.28 ± 0.48 and 3.86 ± 2.09 ZJ yr <sup>–1</sup> over 1993–2017; and 5.31 ± 0.48 and 4.02 ± 0.97 ZJ yr <sup>–1</sup> over 2005–2017. Both AR5 and SROCC assessed that the ocean below 2000 m had ''likely'' warmed since 1992. The SROCC reported global mean thermosteric sea level (ThSL) rise, associated with thermal expansion of the ocean, with a trend of 0.89 ± 0.05 mm yr <sup>–1</sup> for 1970–2015; 1.36 ± 0.40 mm yr <sup>–1</sup> for 1993–2015; and 1.40 ± 0.40 mm yr <sup>–1</sup> for 2006–2015, and also reported that the rate of ocean warming over 1993–2017 had ''likely'' more than doubled since 1969–1992.

New ocean heat content (OHC) reconstructions derived from paleo proxies ( [[#Bereiter--2018|Bereiter et al., 2018]] ; [[#Baggenstos--2019|Baggenstos et al., 2019]] ; [[#Shackleton--2019|Shackleton et al., 2019]] ; [[#Gebbie--2021|Gebbie, 2021]] ) indicate that the global ocean warmed by 2.57°C ± 0.24°C, at an average rate of about 0.3°C ka <sup>–1</sup> (equivalent to an OHC change rate of 1.3 ZJ yr <sup>–1</sup> ) from the LGM (about 20 ka) to the early Holocene (about 10 ka; Section 9.2.2.1 and Figure 9.9). Over the LDT, ocean warming occurred in two stages, offset by some heat loss during the Antarctic Cold Reversal (14.58–12.75 ka). Only during a short period of rapid warming at the end of the Younger Dryas (12.75–11.55 ka) were rates comparable to those observed since the 1970s ( [[#Bereiter--2018|Bereiter et al., 2018]] ; [[#Shackleton--2019|Shackleton et al., 2019]] ). Ice cores imply a small decrease in the global mean ocean temperature during the early Holocene (&lt;0.4°C) ( [[#Bereiter--2018|Bereiter et al., 2018]] ; [[#Baggenstos--2019|Baggenstos et al., 2019]] ). Sediment cores from the equatorial Pacific and Atlantic Ocean (0–1000 m) indicate a stronger regional cooling (compared to mean ocean temperature) of 1.0°C ± 0.7°C to 1.8°C ± 0.4°C from the early/mid-Holocene to ca.1750 CE ( [[#Rosenthal--2013|Rosenthal et al., 2013]] , 2017; [[#Morley--2014|Morley et al., 2014]] ; [[#Kalansky--2015|Kalansky et al., 2015]] ). Sediment cores from the western equatorial Pacific suggest 0.8°C ± 0.1°C higher temperatures in the upper 700 m of the ocean during 950–1100 CE compared to 1400–1750 CE. These changes are consistent with a global estimate derived from combined surface and subsurface ocean temperature proxy records ( [[#PAGES%202k%20Consortium--2013|PAGES 2k Consortium, 2013]] ; [[#McGregor--2015|McGregor et al., 2015]] ). A combined study of model and observational data further confirmed these results, treating temperature as a passive tracer ( [[#Gebbie--2019|Gebbie and Huybers, 2019]] ) and addressing the role of circulation dynamics ( [[#Scheen--2020|Scheen and Stocker, 2020]] ). Collectively, the proxy records indicate a global OHC decrease of about 400 ± 70 ZJ (about 170 ± 100 ZJ in the Pacific) in the upper 700 m between 950–1100 CE and 1400–1750 CE, and also suggest that the deep Pacific is still adjusting to this cooling ( [[#Rosenthal--2013|Rosenthal et al., 2013]] ), partially offsetting the global increase since 1750 CE ( [[#Gebbie--2019|Gebbie and Huybers, 2019]] ; [[#Gebbie--2021|Gebbie, 2021]] ).

For the instrumental era, since AR5 and SROCC, new and updated OHC and ThSL observation-based analyses ( [[#Johnson--2020|Johnson et al., 2020]] ; [[#von%20Schuckmann--2020|von Schuckmann et al., 2020]] ) enhance an existing large ensemble of direct and indirect OHC estimates (Figure 2.26), although some rely to varying degrees upon information from ocean-climate models. Direct estimates benefit from improved: bias adjustments (e.g., [[#Cheng--2018|Cheng et al., 2018]] ; [[#Leahy--2018|Leahy et al., 2018]] ; [[#Palmer--2018|Palmer et al., 2018]] ; [[#Ribeiro--2018|Ribeiro et al., 2018]] ; [[#Wang--2018|]] [[#Wang--2018|B. Wang et al., 2018]] ; [[#Bagnell--2020|Bagnell and DeVries, 2020]] ; [[#Gouretski--2020|Gouretski and Cheng, 2020]] ); interpolation methods ( [[#Kuusela--2018|Kuusela and Stein, 2018]] ; [[#Su--2020|Su et al., 2020]] ); and characterization of sources of uncertainty (e.g., [[#Good--2017|Good, 2017]] ; [[#Wunsch--2018|Wunsch, 2018]] ; [[#Allison--2019|Allison et al., 2019]] ; [[#Garry--2019|Garry et al., 2019]] ; [[#Meyssignac--2019|Meyssignac et al., 2019]] ; [[#Palmer--2021|Palmer et al., 2021]] ), including those originating from forced and intrinsic ocean variability ( [[#Penduff--2018|Penduff et al., 2018]] ). After 2006 direct OHC estimates for the upper 2000 m layer benefit from the near-global ARGO array with its superior coverage over 60°S–60°N ( [[#Roemmich--2019|Roemmich et al., 2019]] ). Indirect estimates include OHC and ThSL series inferred from satellite altimetry and gravimetry since 2003 ( [[#Meyssignac--2019|Meyssignac et al., 2019]] ), the passive uptake of OHC (ThSL) at centennial timescales inferred from observed SST anomalies, and time-invariant circulation processes from an ocean state estimation (e.g., [[#Zanna--2019|Zanna et al., 2019]] ). [[#Resplandy--2019|Resplandy et al. (2019)]] estimate the rate of global OHC uptake over 1991–2016 from changes in atmospheric composition and physical relationships based on CMIP5 model simulations. The uncertainties are broader than from direct estimates but the estimate is qualitatively consistent.

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[[File:b5c47168906cc523362848773535b3bc IPCC_AR6_WGI_Figure_2_26.png]]

'''Figure 2.2''' '''6 |''' '''Changes in ocean heat content (OHC).''' Changes are shown over '''(a)''' full depth of the ocean from 1871–2019 from a selection of indirect and direct measurement methods. The series from Table 2.7 is shown in solid black in both (a) and (b) (see Table 2.7 caption for details). '''(b)''' as (a) but for 0–2000 m depths only and reflecting the broad range of available estimates over this period. For further details see chapter data table (Table 2.SM.1).

Collectively, the new and updated analyses strengthen AR5 and SROCC findings of a sustained increase in global OHC (Figure 2.26 and Table 2.7) and associated ThSL rise. Larger warming rates are observed in the upper 700 m compared to deeper layers, with more areas exhibiting significant warming than significant cooling ( [[#Johnson--2020|Johnson and Lyman, 2020]] ). There is an improved consistency among available estimates of OHC rates in the upper 2000 m since 2006. [[#Cheng--2020|Cheng et al. (2020)]] , [[#von%20Schuckmann--2020|von Schuckmann et al. (2020)]] and [[#Johnson--2020|Johnson et al. (2020)]] have further confirmed that the central estimates of rates of OHC change in the upper 2000 m depths have increased after 1993 and particularly since 2010 ( [[IPCC:Wg1:Chapter:Chapter-3#3.5.1.3|Section 3.5.1.3]] and Figures 2.26 and 3.26), although uncertainties are large (Table 2.7). Ocean reanalyses support findings of continued upper ocean warming ( [[#Balmaseda--2013|Balmaseda et al., 2013]] ; [[#von%20Schuckmann--2018|von Schuckmann et al., 2018]] ; [[#Meyssignac--2019|Meyssignac et al., 2019]] ), albeit with higher spread than solely observational estimates, particularly in the poorly sampled deep ocean below 2000 m ( [[#Storto--2017|Storto et al., 2017]] ; [[#Palmer--2018|Palmer et al., 2018]] ).

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'''Table 2.7 |''' '''Rates of global ocean heat content (OHC) and global mean thermosteric sea level (ThSL) change for four depth integrations over different periods.''' For the period up to 1971, the assessment for all depth layers is based on [[#Zanna--2019|Zanna et al. (2019)]] . From 1971 onwards, consistent with AR5, Domingues et al. (2008, updated) is the central estimate for 0–700 m along with uncertainty from a five-member ensemble ( [[#Domingues--2008|Domingues et al., 2008]] , updated; [[#Levitus--2012|Levitus et al., 2012]] ; [[#Good--2013|Good et al., 2013]] ; [[#Cheng--2017|Cheng et al., 2017]] ; [[#Ishii--2017|Ishii et al., 2017]] ), following the approach of [[#Palmer--2021|Palmer et al. (2021)]] . Similarly, [[#Ishii--2017|Ishii et al. (2017)]] is the central estimate for 700–2000 m with uncertainty based on a 3-member ensemble ( [[#Levitus--2012|Levitus et al., 2012]] ; [[#Cheng--2017|Cheng et al., 2017]] ; [[#Ishii--2017|Ishii et al., 2017]] ). For depths below 2000 m, both central estimate and uncertainty are from Purkey and Johnson (2010, updated). In cases when OHC estimates do not have a ThSL counterpart (e.g., [[#Good--2013|Good et al., 2013]] ; [[#Cheng--2017|Cheng et al., 2017]] ), OHC was converted into ThSL using the average linear regression coefficients for 0–700 m and 700–2000 m from all available ensemble members. For consistency with the energy and sea-level budgets presented in Chapters 7 and 9, reported rates are based on the difference between the first and last annual mean value in each period ( [[#Palmer--2021|Palmer et al., 2021]] , Box 7.2, Cross-Chapter Box 9.1). N/A indicates not applicable. Further details on data sources and processing are available in the chapter data table (Table 2.SM.1).

{| class="wikitable"
|-
| rowspan="2"| Depth

| rowspan="2"| Period

| rowspan="2"| OHC Rate (ZJ y <sup>–1</sup> )

| rowspan="2"| ThSL Rate (mm yr <sup>–1</sup> )

| colspan="2"| Relative Full Ocean Depth Contribution

|-
| OHC

| ThSL

|-
| rowspan="5"| '''0–700 m'''

| 1901–1990

| 2.50

[1.16 to 3.85]

| 0.31

[0.16 to 0.45]

| 81%

| 86%

|-
| 1901–2018

| 3.11

[2.18 to 4.04]

| 0.40

[0.30 to 0.50]

| 66%

| 74%

|-
| 1971–2018

| 5.14

[3.46 to 6.82]

| 0.71

[0.51 to 0.90]

| 61%

| 70%

|-
| 1993–2018

| 6.06

[4.56 to 7.55]

| 0.89

[0.69 to 1.10]

| 58%

| 68%

|-
| 2006–2018

| 6.28

[4.06 to 8.50]

| 0.91

[0.51 to 1.31]

| 54%

| 65%

|-
| rowspan="5"| '''700–2000 m'''

| 1901–1990

| 0.50

[–0.59 to 1.60]

| 0.04

[–0.07 to 0.16]

| 16%

| 11%

|-
| 1901–2018

| 1.26

[0.43 to 2.09]

| 0.11

[0.02 to 0.19]

| 27%

| 20%

|-
| 1971–2018

| 2.62

[2.04 to 3.20]

| 0.23

[0.16 to 0.31]

| 31%

| 23%

|-
| 1993–2018

| 3.31

[2.40 to 4.22]

| 0.30

[0.19 to 0.41]

| 32%

| 23%

|-
| 2006–2018

| 4.14

[2.41 to 5.86]

| 0.36

[0.15 to 0.58]

| 36%

| 26%

|-
| rowspan="5"| '''&gt;2000 m'''

| 1901–1990

| 0.07

[0.02 to 0.12]

| 0.01

[0.00 to 0.01]

| 2%

| 3%

|-
| 1901–2018

| 0.32

[0.18 to 0.46]

| 0.03

[0.02 to 0.05]

| 7%

| 6%

|-
| 1971–2018

| 0.66

[0.33 to 0.99]

| 0.07

[0.03 to 0.10]

| 8%

| 7%

|-
| 1993–2018

| 1.15

[0.58 to 1.72]

| 0.12

[0.06 to 0.18]

| 11%

| 9%

|-
| 2006–2018

| 1.15

[0.58 to 1.72]

| 0.12

[0.06 to 0.18]

| 10%

| 9%

|-
| rowspan="5"| '''Full-depth'''

| 1901–1990

| 3.08

[1.36 to 4.79]

| 0.36

[0.17 to 0.54]

| N/A

| N/A

|-
| 1901–2018

| 4.68

[3.45 to 5.92]

| 0.54

[0.40 to 0.68]

| N/A

| N/A

|-
| 1971–2018

| 8.42

[6.08 to 10.77]

| 1.01

[0.73 to 1.29]

| N/A

| N/A

|-
| 1993–2018

| 10.52

[7.76 to 13.28]

| 1.31

[0.95 to 1.66]

| N/A

| N/A

|-
| 2006–2018

| 11.57

[7.20 to 15.94]

| 1.39

[0.74 to 2.05]

| N/A

| N/A

|}

In summary, current multi-decadal to centennial rates of OHC gain are greater than at any point since the last deglaciation ( ''medium confidence'' ). At multi-centennial timescales, changes in OHC based upon proxy indicators demonstrate a tight link with surface temperature changes during the last deglaciation ( ''high confidence'' ), as well as during the Holocene and CE ( ''low confidence'' ). It is ''likely'' the global ocean has warmed since 1871, consistent with the observed increase in sea surface temperature. It is ''virtually certain'' that OHC increased between 1971 and 2018 in the upper 700 m and ''very likely'' in the 700–2000 m layer, with ''high confidence'' since 2006. It is ''likely'' the OHC below 2000 m has increased since 1992. Confidence in the assessment of multi-decadal OHC increase is further strengthened by consistent closure of both global sea level and energy budgets (Section 7.2.2.2, Box 7.2, Cross-Chapter Box 9.1).

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