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

==== 9.6.3.1 Global Mean Sea Level Projections Based on the Representative Concentration Pathways ====

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The AR5 ( [[#Church--2013b|Church et al., 2013b]] ) generated GMSL projections for the RCPs by combining information from CMIP5 climate models with glacier and ice-sheet surface mass balance (SMB) models and assessments of projected ice-sheet dynamic and land-water storage contributions ( [[#9.6.3.2|Section 9.6.3.2]] ). The SROCC ( [[#Oppenheimer--2019|Oppenheimer et al., 2019]] ) updated AR5 projections based on a revised assessment of the AIS contribution to GMSL rise. The AR5 and SROCC employ a baseline period of 1986 to 2005, which is updated in this Report to a baseline period of 1995 to 2014 ( [[IPCC:Wg1:Chapter:Chapter-1#1.4.1|Section 1.4.1]] ). Between these two periods, GMSL rose by 3 cm, and this correction is applied to projections from previous reports to allow comparison (Table 9.8). Accounting for this shift, SROCC concludes that, with ''medium confidence,'' GMSL will rise between 0.40 (0.26–0.56, ''likely'' range) m (RCP2.6) and 0.81 (0.58–1.07 m, ''likely'' range) m (RCP8.5) by 2100 relative to 1995–2014. The AR5 and SROCC GMSL projections for the 2007–2018 period have been shown to be consistent with observed trends in GMSL and regional weighted mean tide gauges (J. [[#Wang--2021|]] [[#Wang--2021|Wang et al., 2021]] ).

Since AR5, a number of projections of GMSL rise have been published based on the RCPs ( [[#Kopp--2014|Kopp et al., 2014]] , 2017; [[#Slangen--2014b|Slangen et al., 2014b]] ; Grinsted et al., 2015; [[#Jackson--2016|Jackson et al., 2016]] ; [[#Mengel--2016|Mengel et al., 2016]] ; [[#Bakker--2017|Bakker et al., 2017]] ; [[#Bittermann--2017|Bittermann et al., 2017]] ; [[#Le%20Bars--2017|Le Bars et al., 2017]] ; [[#Nauels--2017|Nauels et al., 2017]] ; [[#Wong--2017|Wong et al., 2017]] ; [[#Goodwin--2018|Goodwin et al., 2018]] ; [[#Nicholls--2018|Nicholls et al., 2018]] ; [[#Le%20Cozannet--2019|Le Cozannet et al., 2019]] ; [[#Palmer--2020|Palmer et al., 2020]] ). See [[#Garner--2018|Garner et al. (2018)]] or a database (Tables 9.SM.5, 9.SM.6). Some studies also produced associated global sets of regional projections ( [[#Kopp--2014|Kopp et al., 2014]] , 2017; [[#Slangen--2014b|Slangen et al., 2014b]] ; [[#Le%20Cozannet--2019|Le Cozannet et al., 2019]] ; [[#Palmer--2020|Palmer et al., 2020]] ). Since SROCC ( [[#Le%20Cozannet--2019|Le Cozannet et al., 2019]] ) focused on the low end of the probability distribution of GMSL rise, [[#Palmer--2020|Palmer et al. (2020)]] extended projections beyond 2100 using a climate model emulator (Cross-Chapter Box 7.1), and [[#Horton--2020|Horton et al. (2020)]] conducted a survey of 106 sea level experts, providing additional context for interpreting sea level rise projections for 2100 and 2300.

As noted by SROCC, the largest differences between projections of GMSL in 2100 are due to the ice-sheet projection method, which generally fall into one of three categories: (i) projections from ice-sheet models that represent processes where there is at least ''medium confidence'' (Sections 9.4.1.2 and 9.4.2.2); (ii) projections from an Antarctic ice-sheet model that incorporates the marine ice cliff instability (MICI; [[#9.4.2.4|Section 9.4.2.4]] ; [[#DeConto--2016|DeConto and Pollard, 2016]] ); or (iii) projections based on SEJ (Sections 9.4.1.3, 9.4.1.4, 9.4.2.5 and 9.4.2.6; [[#Bamber--2013|Bamber and Aspinall, 2013]] ; [[#Bamber--2019|Bamber et al., 2019]] ). ''Low confidence'' is ascribed to projections incorporating MICI because there is ''low confidence'' in the current ability to quantify MICI ( [[#9.4.2.4|Section 9.4.2.4]] ). ''Low confidence'' is also ascribed to projections based on SEJ, because individual experts participating in the SEJ study may have incorporated processes in whose quantification there is ''low confidence'' , and the experts’ reasoning has not been examined in detail. In general, the range of GMSL projections based on ice-sheet models not incorporating MICI overlaps with, but is lower than, projections incorporating MICI or employing SEJ (Figure 9.25).

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[[File:98d08c28f372fd0e31a76d38b0573521 IPCC_AR6_WGI_Figure_9_25.png]]

'''Figure 9.25''' '''|''' '''Literature global mean sea level (GMSL) projections (m) for 2050 (left) and 2100 (right) since 199''' '''5–2''' '''014, for RCP8.5/SSP5-8.5 (top set), RCP4.5/SSP2-4.5 (middle set), and RCP2.6/SSP1-2.6 (bottom set).''' Projections are standardized to account for minor differences in time periods. Thick bars span from the 17th–83rd percentile projections, and thin bars span the 5th–95th percentile projections. The different assessments of ice-sheet contributions are indicated by ‘MED’ (ice-sheet projections include only processes in whose quantification there is ''medium confidence'' ), ‘MICI’ (ice-sheet projections which incorporate marine ice cliff instability), and ‘SEJ’ (structured expert judgement) to assess the central range of the ice-sheet projection distributions. ‘Survey’ indicates the results of a 2020 survey of sea level experts on global mean sea level (GMSL) rise from all sources ( [[#Horton--2020|Horton et al., 2020]] ). Projection categories incorporating processes in which there is ''low confidence'' (MICI and SEJ) are lightly shaded. Dispersion among the different projections represents ''deep uncertainty'' , which arises as a result of ''low agreement'' regarding appropriate conceptual models describing ice-sheet behaviour and ''low agreement'' regarding probability distributions used to represent key uncertainties. Individual studies are shown in Tables 9.SM.5 and 9.SM.6. Further details on data sources and processing are available in the chapter data table (Table 9.SM.9).

There is ''high'' ''agreement'' across published GMSL projections for 2050, and there is little sensitivity to emissions scenario (Figure 9.25, left panel). Up to 2050, projections are broadly consistent with extrapolation of the observed acceleration of GMSL rise (Sections 2.3.3.3, 9.6.1.1 and 9.6.1.2). Considering only projections incorporating ice-sheet processes in whose quantification there is at least ''medium confidence'' , the GMSL projections for 2050, across all emissions scenarios, fall between 0.1 and 0.4 m (5th–95th percentile range). Projections incorporating MICI or SEJ do not extend this range under RCP2.6 or RCP4.5 but do extend the upper part of the range to 0.6 m under RCP8.5. On the basis of these studies, we therefore have ''high confidence'' that GMSL in 2050 will be between 0.1 and 0.4 m higher than in 1995–2014 under low- and moderate-emissions scenarios, and between 0.1 and 0.6 m under high-emissions scenarios.

Conversely, there is ''low agreement'' across published GMSL projections for 2100, particularly for higher-emissions scenarios, as well as a higher degree of sensitivity to the choice of emissions scenario (Figure 9.25, right panel). Considering only projections representing processes in whose quantification there is at least ''medium'' ''confidence'' , the GMSL projections for 2100 fall between 0.2 and 1.0 m (5th–95th percentile range) under RCP2.6 and RCP4.5, and between 0.3 and 1.6 m under RCP8.5. Considering also projections incorporating MICI or SEJ ( ''low confidence'' ), the projections for 2100 fall between 0.2 and 1.0 m (5th–95th percentile range) under RCP2.6, 0.2, and 1.6 m under RCP4.5, and 0.4 and 2.4 m under RCP8.5. In summary, RCP-based projections published since AR5 show ''high agreement'' for 2050, but exhibit broad ranges and ''low agreement'' for 2100, particularly under RCP8.5.

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