Editing IPCC:AR6/WGI/Chapter-9 (section)

==== 9.2.4.1 Global Mean Thermosteric Sea Level Change ====

<div id="h3-13-siblings" class="h3-siblings"></div>

Changes in globally averaged ocean heat content (OHC) cause global mean thermosteric sea level (GMTSL) change (Box 9.1). The observed increased OHC for 1971–2018 of 325 to 546 ZJ ( ''very likely'' range) ( [[IPCC:Wg1:Chapter:Chapter-7#7.2|Section 7.2]] , Box 7.2) has led to a GMTSL rise of 0.03 to 0.06 m out of a total global mean sea level (GMSL) of 0.07 to 0.15 m ( ''very likely'' range) ( [[IPCC:Wg1:Chapter:Chapter-2#2.3.3.3|Section 2.3.3.3]] , Tables 2.7 and 9.5, and Cross-Chapter Box 9.1).

Projections of GMTSL rise in AR5 ( [[#Church--2013b|Church et al., 2013b]] ) and SROCC ( [[#Oppenheimer--2019|Oppenheimer et al., 2019]] ) were derived from the CMIP5 ensemble, after removing drift estimated based on pre-industrial control simulations. Differences between removing a linear and a quadratic drift are small ( [[#Hobbs--2016a|Hobbs et al., 2016a]] ; [[#Hermans--2021|Hermans et al., 2021]] ). These prior assessments filled in projections for models that did not provide GMTSL rise for all scenarios, by calculating the heat content of the climate system from global surface air temperature and net radiative flux, then converting this to GMTSL rise using each model’s diagnosed expansion efficiency coefficient. In AR5, the associated uncertainties were derived by assuming a normal distribution, with the 5th–95th percentile CMIP5 ensemble range taken as the ''likely'' range (±1 standard deviation).

In this Report, global surface air temperature projections are not derived directly from the CMIP6 ensemble (Box 4.1). Therefore, to produce projections of OHC and GMTSL rise consistent with the Report’s assessment of equilibrium climate sensitivity and transient climate response ( [[IPCC:Wg1:Chapter:Chapter-7#7.5.2.2|Section 7.5.2.2]] ), this chapter employs a two-layer energy budget emulator (Supplementary Materials 7.SM.2, 9.SM.4.3). Since AR5, climate model emulators have been increasingly used to predict GMTSL (Cross-Chapter Box 7.1; [[#Kostov--2014|Kostov et al., 2014]] ; [[#Palmer--2018|Palmer et al., 2018]] , 2020; [[#Nauels--2019|Nauels et al., 2019]] ). The expansion efficiency coefficient that relates GMTSL and OHC for the two-layer emulator has a mean and standard deviation of 0.113 ± 0.013 m YJ <sup>–1</sup> (Supplementary Material 9.SM.4.3). This approach yields a ''likely'' thermosteric contribution between 1995–2014 and 2100 that represents a minimal change from AR5 and SROCC (Table 9.8). The two-layer emulator GMTSL projected median and 17th–83rd percentile, or ''likely'' , range is 0.12 (0.09 to 0.15) m for SSP1-1.9, 0.14 (0.11 to 0.18) m for SSP1-2.6, 0.20 (0.16 to 0.24) m for SSP2-4.5, 0.25 (0.21 to 0.30) m for SSP3-7.0, and 0.30 (0.24 to 0.36) m for SSP5-8.5 by 2100 ( [[#9.6.3.2|Section 9.6.3.2]] and Tables 9.1, 9.8 and 9.9). The two-layer model heat content increases slightly faster than that of the total depth CMIP6 ensemble, which is related to its role in the assessed energy balance (Section 7.SM.2), but with a similar ensemble spread (Table 9.1). Projecting the ''likely'' factor by which 1995–2014 to 2081–2100 OHC change exceeds change over 1971 to 2018 in CMIP6 yields 3 to 5 for SSP1-2.6, 4 to 6 for SSP2-4.5, 5 to 7 for SSP3-7.0, and 5 to 8 for SSP5-8.5. The two-layer model ''likely'' equivalents are 2 to 3 for SSP1-2.6, 3 to 4 for SSP2-4.5, 4 to 5 for SSP3-7.0, and 4 to 6 for SSP5-8.5.

For reconstructions, the expansion efficiency coefficient is required for the conversion between ocean temperature and steric sea level over a specific time scale. Combining the assessed sea level and energy data over 1995 to 2014 (drawn from the analysis in Cross-Chapter Box 9.1) results in a coefficient of 0.1210 ± 0.0014 m YJ <sup>–1</sup> , or 0.6607 ± 0.0076 m °C <sup>–1</sup> in terms of mean ocean temperature. The two-layer emulator assessment used in AR6 results in 0.113 ± 0.013 m YJ <sup>–1</sup> , or 0.617 ± 0.071 m °C <sup>–1</sup> (Appendices 7.SM.2, 9.SM.4). Both of these estimates are in line with an independent estimate of 0.70 m/°C ( [[#Hieronymus--2019|Hieronymus, 2019]] ) and other estimates, for example, 0.116 ± 0.011 m YJ <sup>–1</sup> ( [[#Kuhlbrodt--2012|Kuhlbrodt and Gregory, 2012]] ), but are significantly larger than the temperature to sea level conversion used in AR5 (0.42 m °C <sup>–1</sup> based on SST and the estimated range from [[#Levermann--2013|Levermann et al., 2013]] ). The expansion coefficient is not fixed across models, nor in time, as it varies depending on which water masses are storing the added heat, and the commitment time scale ( [[#Hallberg--2013|Hallberg et al., 2013]] ). For paleoclimate, a scaling for sea surface temperature (0.6 m °C <sup>–1</sup> ) or global surface air temperature (GSAT; see Cross-Chapter Box 2.3) can be estimated, but mean ocean temperature is in phase with steric sea level change, while sea surface temperatures are not (Figure 9.9; [[#Shakun--2012|Shakun et al., 2012]] ; [[#Tierney--2020|Tierney et al., 2020]] ). Thus, while conversions between OHC, mean ocean temperature and GMTSL across applications are within uncertainty ranges ( ''medium confidence'' ) (Table 9.1), little consistency is found when correlating these variables to SST or GSAT, which may vary independently.

Short-lived climate forcers (Sections 6.3 and 6.6.3) are associated with a sea level commitment, due to an OHC and mean ocean temperature response that lasts substantially longer than their atmospheric forcing and SST response, although not as long as the sea level commitment associated with CO <sub>2</sub> emissions (Sections 9.2.1.1 and 4.4.4). For example, [[#Zickfeld--2017|Zickfeld et al. (2017)]] find that about 70% of the thermosteric sea level rise associated with methane forcing would persist 100 years after the elimination of methane emissions, and 40% would persist for more than 500 years.

In summary, consistent relationships between OHC ( [[#9.2.2.1|Section 9.2.2.1]] ), mean ocean temperature and GMTSL are found using two-layer emulators, CMIP6 models, and modern and paleo observations to provide ''medium confidence'' in the 0.113 ± 0.013 m YJ <sup>–1</sup> , or 0.617 ± 0.071 m °C <sup>–1</sup> ''likely'' ranges of assessed conversion values. It is possible to estimate relationships between SST or GSAT change and GMTSL rise, but conversions are not generally applicable and depend on time scale and application.

<div id="_idContainer031" class="Basic-Text-Frame"></div>

'''Table 9.1''' '''|''' '''Projected contributions to median and 1''' '''7–8''' '''3% (parentheses) and''' '''5–9''' '''5% [square brackets] ranges of thermosteric sea level from AR5 ( [[#Church--2013b|Church et al., 2013b]] ), CMIP6 ( [[#Jevrejeva--2020|Jevrejeva et al., 2020]] ; [[#Hermans--2021|Hermans et al., 2021]] ) and the two-layer energy balance model (described in Sections 7.SM.2, 9.SM.4 and Box 4.1) averaged over 208''' '''1–2''' '''100, with respect to a baseline of 199''' '''5–2''' '''014.''' Note that AR5 and SROCC interpret 5–95% range as the ''likely'' range, while in this table square brackets are used for consistency.

{| class="wikitable"
|-
| '''Study'''

| '''RCP2.6/SSP1-2.6'''

| '''RCP4.5/SSP2-4.5'''

| '''RCP8.5/SSP5-8.5'''

|-
| '''IPCC AR5 and SROCC GMTSL'''

'''( [[#Church--2013b|Church et al., 2013b]] ; [[#Oppenheimer--2019|Oppenheimer et al., 2019]]''' )

| 0.13 [0.09 to 0.17] m

| 0.18 [0.13 to 0.22] m

| 0.26 [0.20 to 0.32] m

|-
| '''CMIP6 5–95% GMTSL'''

'''( [[#Hermans--2021|Hermans et al., 2021]] )'''

| 0.14 [0.08 to 0.17] m

| 0.18 [0.11 to 0.23] m

| 0.26 [0.17 to 0.33] m

|-
| '''CMIP6 5–95% GMTSL'''

'''( [[#Jevrejeva--2020|Jevrejeva et al., 2020]] )'''

| –

| 0.19 [0.13 to 0.24] m

| 0.27 [0.19 to 0.35] m

|-
| '''Assessed GMTSL based on two-layer model 17–83% and 5–95% (Sections''' '''7.SM.2''' ''', 9.SM.4)'''

| 0.13 (0.11 to 0.16) [0.09 to 0.19] m

| 0.17 (0.14 to 0.21)

[0.12 to 0.25] m

| 0.25 (0.20 to 0.30)

[0.18 to 0.35] m

|-
| '''Total OHC 17–83% and 5–95% from assessed two-layer model (Sections''' '''7.SM.2''' ''', 9.SM.4)'''

| 1.18 (0.99 to 1.42)

[0.86 to 1.65] YJ

| 1.56 (1.33 to 1.86)

[1.19 to 2.12] YJ

| 2.23 (1.92 to 2.64)

[1.71 to 3.00] YJ

|-
| '''0–2000 m OHC 17–83% and 5–95% from CMIP6 (Figure 9.6)'''

| 1.06 (0.80 to 1.31)

[0.66 to 1.64] YJ

| 1.35 (1.08 to 1.67)

[0.90 to 1.84] YJ

| 1.89 (1.60 to 2.29)

[1.28 to 2.58] YJ

|}

<div id="9.2.4.2" class="h3-container"></div>

<span id="ocean-dynamic-sea-level-change"></span>