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

===== 5.2.2.2.1 Observed and projected global ocean heat uptake =====

As AR5 concluded, the ocean is warming as a direct result of anthropogenic changes to the radiative properties of the atmosphere and the heat budget of the Earth ( ''very likely'' ) (Bindoff et al., 2013 <sup>[[#fn:r22|22]]</sup> ). Over the past few decades our ocean observing system has measured an increase in ocean temperature (Figure 5.1). This temperature increase corresponds to an uptake of over 90% of the excess heat accumulated in the Earth system over this period (Bindoff et al., 2013 <sup>[[#fn:r23|23]]</sup> ; Rhein et al., 2013 <sup>[[#fn:r24|24]]</sup> ). This heat in the ocean also causes it to expand and has contributed about 43% of the observed global mean SLR from 1970–2015 (Section 4.2.2.3.6).

Since AR5, there have been further improvements in our ability to understand and correct instrumental errors and new estimates also attempt to minimise biases in estimating temperature changes arising from traditional data-void filling strategies (Abraham et al., 2013 <sup>[[#fn:r25|25]]</sup> ; Durack, 2015 <sup>[[#fn:r26|26]]</sup> ; Cheng and Chen, 2017 <sup>[[#fn:r27|27]]</sup> ; Cheng et al., 2017 <sup>[[#fn:r28|28]]</sup> ). New estimates from ocean observations of ocean heat uptake in the top 2000 m between 1993 and 2017 ''very likely'' range from 9.2 ± 2.3 ZJ yr -1 to 12.1 ± 3.1 ZJ yr -1 (Johnson et al., 2018 <sup>[[#fn:r29|29]]</sup> ) <sup>[[#fn:4|4]]</sup> . Three recent independent estimates do a better job of accounting for instrumental biases and the sparseness of historical ocean temperature measurements than the older studies assessed in AR5, and provide larger and more consistent estimates of heat uptake rates for the 0-2000 m layer of 5.8 ± 1.0 ZJ yr -1 (Cheng and Chen, 2017 <sup>[[#fn:r30|30]]</sup> ; Cheng et al., 2017 <sup>[[#fn:r31|31]]</sup> ; Ishii et al., 2017 <sup>[[#fn:r32|32]]</sup> ), 6.0 ± 0.8 ZJ yr -1 (updated from Domingues et al. (2008)) and 6.3 ± 1.8 ZJ yr -1 (Cheng and Chen, 2017 <sup>[[#fn:r33|33]]</sup> ; Cheng et al., 2017 <sup>[[#fn:r34|34]]</sup> ; Ishii et al., 2017 <sup>[[#fn:r35|35]]</sup> ) for the 1971–2010 period assessed by AR5.

Based on these new published methods and revised atlases we update the estimates for ocean heat uptake (Table 5.1, and SM5.1). For all of the periods assessed in Table 5.1, it is ''virtually certain'' that the upper ocean (0–700 m) has warmed. These results are consistent with earlier research into the duration of record needed to detect a significant signal in global ocean heat content (Gleckler et al., 2012 <sup>[[#fn:r36|36]]</sup> ). Critically, the ''high confidence'' and ''high'' ''agreement'' in the ocean temperature data means we can detect discernable rates of increase in ocean heat uptake (Gleckler et al., 2012 <sup>[[#fn:r37|37]]</sup> ; Cheng et al., 2019 <sup>[[#fn:r38|38]]</sup> ). The rate of heat uptake in the upper ocean (0–700 m) is ''very likely'' higher in the 1993–2017 (or 2005–2017) period compared with the 1969–1993 period (see Table 5.1). The deeper layer (700–2000 m) heat uptake rate is ''likely'' to be higher in the 1993–2017 period compared with the 1969–1993 period.

<span id="table-5.1"></span>
<!-- START TABLE -->
'''Table 5.1'''

The assessed rate of increase in ocean heat content in the two depth layers 0–700 m and 700–2000 m and their ''very likely'' ranges. Fluxes in Wm -2 are averaged over the Earth’s entire surface area. The four periods cover earlier and more recent trends; the 2005–2017 period has the most complete interior ocean data coverage and the greatest consistency between estimates, while longer trends are better for distinguishing between forced changes and internal variability. These observationally-estimated rates come from an assessment of the recent research (see SM5.1), while the Coupled Model Intercomparison Project Phase 5 (CMIP5) Earth System Models (ESM) estimates are based on a combined 28-member ensemble of historical, Representative Concentration Pathway (RCP)2.6 and RCP8.5 simulations.

<!-- TABLE -->
{| class="wikitable"
|-
|
| colspan="4"| '''Ocean Heat Uptake Rate, ZJ yr''' '''-1'''
| colspan="4"| '''Ocean Heat Uptake as Average Fluxes, W m''' '''-2'''
|-
| '''Period'''
| '''1969–1993'''
| '''1993–2017'''
| '''1970–2017'''
| '''2005–2017'''
| '''1969–1993'''
| '''1993–2017'''
| '''1970–2017'''
| '''2005–2017'''
|-
| colspan="9"| '''Observationally Based Ocean Heat Uptake Estimates:'''
|-
| '''0–700 m'''
| 3.22 ± 1.61
| 6.28 ± 0.48
| 4.35 ± 0.80
| 5.31 ± 0.48
| 0.20 ± 0.1
| 0.39 ± 0.03
| 0.27 ± 0.05
| 0.33 ± 0.03
|-
| '''700–2000 m '''
| 0.97 ± 0.64
| 3.86 ± 2.09
| 2.25 ± 0.64
| 4.02 ± 0.97
| 0.06 ± 0.04
| 0.24 ± 0.13
| 0.14 ± 0.04
| 0.25 ± 0.06
|-
| colspan="9"| '''CMIP5 ESM Ensemble-mean Ocean Heat Uptake with 90% Certainty Range from Ensemble Spread:'''
|-
| '''0–700 m'''
| 3.60 ± 1.92
| 7.37 ± 2.09
| 5.64 ± 1.90
| 7.85 ± 2.71
| 0.22 ± 0.12
| 0.46 ± 0.13
| 0.35 ± 0.12
| 0.49 ± 0.17
|-
| '''700–2000 m '''
| 1.32 ± 1.49
| 2.72 ± 1.41
| 1.99 ± 1.51
| 3.33 ± 1.75
| 0.08 ± 0.09
| 0.17 ± 0.09
| 0.12 ± 0.09
| 0.21 ± 0.11
|}
<!-- END TABLE -->

The direct comparison of the observed changes in ocean heat content and the simulated historical changes is undertaken to detect climate change, to attribute the causes of climate change to the forcings in the system, and to evaluate the performance of ESMs. Attribution studies also reject competing hypotheses to explain the global ocean changes such as natural forcing from solar variability or volcanic eruptions (see Section 1.3) (Bindoff et al., 2013 <sup>[[#fn:r39|39]]</sup> ). Detection and attribution studies have since been used to detect changes in the rate of ocean heat uptake and to attribute these changes to human activity (Gleckler et al., 2016 <sup>[[#fn:r40|40]]</sup> ).

Updated observationally-based estimates of ocean heat uptake are consistent with simulations of equivalent time-periods from an ensemble of CMIP5 ESMs (Table 5.1 and the inset panel in Figure 5.1) ( ''high confidence'' ), once the limitations of the historical ocean observing network and the internally generated variability with a single realisation of the real world are taken into account (see Section 5.2.2.2). Following the CMIP5 protocol, the ESMs are radiatively forced with observationally derived estimates of greenhouse gas concentrations and aerosols, including natural forcing variations from volcanic eruptions and solar forcing, through 2005; after 2006 each of the ESMs uses either the RCP2.6 or RCP8.5 emissions scenarios. The ''very likely'' ranges of the observed trends of heat uptake for the four periods and two layers all fall within the ''very likely'' range of simulated heat uptake from the ESM ensemble (Table 5.1). The difference between observations and average of the simulations in the upper ocean is an overestimate of heat uptake by about 20% and for the deeper layer there an underestimate by a similar amount, but this difference is still well within the ''very likely'' range from the ensemble of simulations. The overall consistency between observationally-based estimates and ESM simulations of the historical period gives greater confidence in the projections; it is ''very likely'' that historical simulations agree with observations of the global ocean heat uptake (Table 5.1).

While the collection of the worlds’ ESMs have been criticised for having an ensemble mean that does not exhibit the observed ‘hiatus’ or ‘slowdown’ of global mean surface temperature increase in the early 21st century (Meehl et al., 2011 <sup>[[#fn:r57|57]]</sup> ; Trenberth et al., 2016 <sup>[[#fn:r58|58]]</sup> ) , it is increasingly clear that this is at least in part due to the redistribution of heat within the climate system from the surface into the interior ocean and between ocean basins. Individual realisations of ESMs do show decades with slow increases in mean surface temperature change comparable to what was observed, even though these cases exhibit continued interior ocean heat uptake, and every ensemble member exhibits surface warming closer to the ensemble-mean over multi-decadal timescales (Meehl et al., 2011 <sup>[[#fn:r59|59]]</sup> ; England et al., 2015 <sup>[[#fn:r60|60]]</sup> ; Knutson et al., 2016 <sup>[[#fn:r61|61]]</sup> )

<span id="figure-5.1"></span>
<!-- START IMG -->
<!-- IMG TITLE -->
'''Figure 5.1'''

<span id="figure-5.1-time-series-of-globally-integrated-upper-2000-m-ocean-heat-content-changes-in-zj-relative-to-the-20002010-period-average-as-inferred-from-observations-magenta-and-as-simulated-for-historical-tan-representative-concentration-pathway-rcp2.6-blue-and-rcp8.5-red-forcing-by-a-25-member-ensemble-of-coupled-model-intercomparison-project-phase-5-cmip5"></span>
<!-- IMG CAPTION -->
'''Figure 5.1 | Time series of globally integrated upper 2000 m ocean heat content changes in ZJ, relative to the 2000–2010 period average, as inferred from observations (magenta) and as simulated for historical (tan), Representative Concentration Pathway (RCP)2.6 (blue) and RCP8.5 (red) forcing by a 25-member ensemble of Coupled Model Intercomparison Project Phase 5 (CMIP5) […]'''
<!-- IMG FILE -->
[[File:e2518a5a96f60c4991d9797d8d3fea43 IPCC-SROCC-CH_5_1.jpg]]

Figure 5.1 | Time series of globally integrated upper 2000 m ocean heat content changes in ZJ, relative to the 2000–2010 period average, as inferred from observations (magenta) and as simulated for historical (tan), Representative Concentration Pathway (RCP)2.6 (blue) and RCP8.5 (red) forcing by a 25-member ensemble of Coupled Model Intercomparison Project Phase 5 (CMIP5) Earth System Models (ESMs) (Cheng et al. 2019 <sup>[[#fn:r41|41]]</sup> ). The shaded magenta in the outer panel is the very likely range determined by combining data from 4 long-term estimates (Palmer et al. 2007 <sup>[[#fn:r42|42]]</sup> ; Levitus et al. 2012 <sup>[[#fn:r43|43]]</sup> ; Lyman and Johnson, 2014 <sup>[[#fn:r44|44]]</sup> ; Cheng and Chen, 2017 <sup>[[#fn:r45|45]]</sup> ; Cheng et al. 2017 <sup>[[#fn:r46|46]]</sup> ; Ishii et al. 2017 <sup>[[#fn:r47|47]]</sup> ) processed as in Johnson et al. (2018) <sup>[[#fn:r48|48]]</sup> . The tan, blue and red lines are the ESM ensemble means, while shading shows each ensemble’s 5th to 95th percentile range. In the inset subpanel, the four different shaded magenta areas are the reported very likely range of heat content changes as inferred from observations by four independent groups (Magenta shading; Palmer et al. 2007 <sup>[[#fn:r49|49]]</sup> ; Lyman and Johnson, 2014 <sup>[[#fn:r50|50]]</sup> ; Cheng and Chen, 2017 <sup>[[#fn:r51|51]]</sup> ; Cheng et al. 2017 <sup>[[#fn:r52|52]]</sup> ; Ishii et al. 2017) <sup>[[#fn:r53|53]]</sup> processed as in Johnson et al. (2018) <sup>[[#fn:r54|54]]</sup> . In the inset subpanel the RCP2.6 and RCP8.5 projections after 2005 are combined into a single ensemble with the historical simulations.

<!-- END IMG -->
<span id="figure-5.2"></span>
<!-- START IMG -->
<!-- IMG TITLE -->
'''Figure 5.2'''

<span id="figure-5.2-heat-uptake-by-the-top-700-m-of-the-ocean-as-determined-by-differences-between-the-averages-over-two-5--or-20-year-intervals-converted-to-a-heat-flux-into-the-ocean-w-m2-either-from-observationally-based-analyses-or-a-38-member-ensemble-of-coupled-model-intercomparison-project-phase-5-cmip5-earth-system-models"></span>
<!-- IMG CAPTION -->
'''Figure 5.2 | Heat uptake by the top 700 m of the ocean, as determined by differences between the averages over two 5- or 20-year intervals converted to a heat flux into the ocean (W m–2), either from observationally-based analyses or a 38-member ensemble of Coupled Model Intercomparison Project Phase 5 (CMIP5) Earth System Models […]'''
<!-- IMG FILE -->
[[File:b7d41eb2445844a63fceb78ad98cdcb4 IPCC-SROCC-CH_5_2-1.jpg]]

Figure 5.2 | Heat uptake by the top 700 m of the ocean, as determined by differences between the averages over two 5- or 20-year intervals converted to a heat flux into the ocean (W m–2), either from observationally-based analyses or a 38-member ensemble of Coupled Model Intercomparison Project Phase 5 (CMIP5) Earth System Models (ESMs). (a) Change between (1971–1990) and (1998–2017) as inferred from observations (Good et al. 2013 <sup>[[#fn:r55|55]]</sup> ); (b) The ensemble mean change in CMIP5 ESMs for the same time periods as in (a); (c) Projected ensemble mean change in CMIP5 ESMs between (1986–2005) and (2081–2100) for the RCP8.5 forcing scenario. In panels (b) and (c), stippling indicates regions where the ensemble mean change is not significantly different from 0 at the 95% confidence level based on the models’ temporal variability. (d) Change between (2004–2008) and (2013–2017) as inferred from observations by the SODA 3.4.2 reanalysis product (Carton et al. 2018 <sup>[[#fn:r56|56]]</sup> ); (e) and (f) Estimates of change in heat uptake as in (d) but from two individual realisations of the CCSM ESM (Table SM5.2). These two realisations are identical apart from their initial conditions, which leads to different timing in their internal modes of variability; they were selected from the full CMIP5 ensemble as examples where one is reminiscent of the recent observed changes while the other has regional changes that have dissimilar timing.

<!-- END IMG -->
<div id="section-5-2-2-2changing-temperature-salinity-circulation-block-3"></div>

The ocean will continue to take up heat in the coming decades for all plausible scenarios. As depicted in Figure 5.1, the ensemble of CMIP5 ESMs used by Cheng et al. (2019) project that under RCP2.6, the top 2000 m of the ocean will take up 935 ZJ of heat between 2015 and 2100 (with a ''very likely'' range of 650–1340 ZJ based on the 5th and 95th percentiles of the 25 ESMs used here that have available data from the historical, scenario and control runs for RCP2.6). Under RCP8.5 this ensemble projects heat uptake of 2180 ZJ (with a ''very likely'' range of 1710–2790 ZJ, based on 35 ESMs) between 2015 and 2100. By 2100 the ocean is ''very likely'' to warm by 2 to 4 times as much for low emissions (RCP2.6) and 5 to 7 times as much for the high emissions scenario (RCP8.5) compared with the observed changes since 1970. With the RCP8.5 scenario, the ocean is ''very likely'' to take up about twice as much heat as RCP2.6 (Figure. 5.1). Even under RCP2.6 the ocean will continue to warm for several centuries to come (Collins et al., 2013 <sup>[[#fn:r52|52]]</sup> ). It is ''virtually certain'' that the ocean will continue to take up heat throughout the 21st century, and the rate of uptake will depend upon on the emissions scenario we collectively choose to follow.

<div id="section-5-2-2-2changing-temperature-salinity-circulation-block-4"></div>

<span id="figure-5.3"></span>
<!-- START IMG -->
<!-- IMG TITLE -->
'''Figure 5.3'''

<span id="figure-5.3-side-view-basin-averaged-zonal-mean-trends-change-per-century-in-water-mass-properties-in-the-top-2000-m-by-basin-a-as-inferred-from-observations-average-of-20132017-minus-average-of-20052009-and-b-coupled-model-intercomparison-project-phase-5-cmip5-model-projections-with-representative-concentration-pathway-rcp8.5-forcing-average-of-20812100-minus-average-of"></span>
<!-- IMG CAPTION -->
'''Figure 5.3 | Side-view basin-averaged zonal-mean trends (change per century) in water-mass properties in the top 2000 m by basin (a) as inferred from observations (average of 2013–2017 minus average of 2005–2009) and (b) Coupled Model Intercomparison Project Phase 5 (CMIP5) model projections with Representative Concentration Pathway (RCP)8.5 forcing (average of 2081–2100 minus average of […]'''
<!-- IMG FILE -->
[[File:e6f8060870293eb7dd25c79ffa06efb5 IPCC-SROCC-CH_5_3.jpg]]

Figure 5.3 | Side-view basin-averaged zonal-mean trends (change per century) in water-mass properties in the top 2000 m by basin (a) as inferred from observations (average of 2013–2017 minus average of 2005–2009) and (b) Coupled Model Intercomparison Project Phase 5 (CMIP5) model projections with Representative Concentration Pathway (RCP)8.5 forcing (average of 2081–2100 minus average of 1981–2000) trends in water-mass changes forcing. Subpanels within each group: top-to-bottom (Atlantic, combined Pacific and Indian, Global); left-to-right (Temperature, in situ Density, Salinity). Shaded areas show where the projected changes are not statistically significant at the 95% level. This figure uses the same observationally-derived reanalysis datasets and ensemble of Earth System Models (ESMs) as in Figure 5.2c and 5.2d. Solid lines show present contours of these fields; the notable structure in the northern hemisphere of the global-zonal mean contours of density and salinity are due to the relatively salty Mediterranean and fresh Black seas.

<!-- END IMG -->
<div id="section-5-2-2-2changing-temperature-salinity-circulation-block-5"></div>

<span id="structure-of-anthropogenic-climate-changes-in-the-ocean"></span>