Editing IPCC:AR6/SROCC/Chapter-4 (section)

==== 4.2.3.2 Global and Regional Projections of Sea Level Rise ====

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In addition to the model including MICI from DeConto and Pollard (2016) , only a subset of studies ( Levermann et al., 2014; Golledge et al., 2015; Ritz et al., 2015; Bulthuis et al., 2019; Golledge et al., 2019) , and statistical emulation of DeConto and Pollard (2016) by Edwards et al. (2019) provide continental-scale estimates of future Antarctic ice loss, under a range of GHG emissions scenarios. They all provide probabilistic information, but vary considerably, both in their physical approaches and their resulting projections of Antarctica’s future contribution to GMSL. Such variations facilitate the first quantitative uncertainty assessment of the full dynamical contribution of Antarctica, which could not be made by Church et al. (2013) in AR5. The assessment by Church et al. (2013) , based on a single statistical-physical model, reported median values (and ''likely'' ranges) of 0.05 m (-0.04–0.13) and 0.04 m (-0.06–0.12), for RCP4.5 and RCP8.5, respectively, for the total Antarctic contribution in 2081–2100 relative to 1986–2005, and added the following: ‘Based on current understanding, only the collapse of marine-based sectors of the AIS, if initiated, could cause GMSL to rise substantially above the ''likely'' range during the 21st century. This potential additional contribution cannot be precisely quantified but there is ''medium confidence'' that it would not exceed several tenths of a metre of SLR during the 21st century (Church et al., 2013) . Given the above-mentioned publications after AR5, Antarctica’s contribution to sea level change was reassessed and now include the possibility of MISI allowing for a more complete assessment of the ''likely'' range of the projections for three RCP scenarios. Our assessment is based on process-based numerical models of the AIS, driven by diverse climate scenarios. Results are discussed in the context of an expert elicitation study (Bamber et al., 2019 <sup>[[#fn:r546|546]]</sup> ) , probabilistic studies (Perrette et al., 2013 <sup>[[#fn:r547|547]]</sup> ; Slangen et al., 2014a <sup>[[#fn:r548|548]]</sup> ; Grinsted et al., 2015 <sup>[[#fn:r549|549]]</sup> ; Jackson and Jevrejeva, 2016 <sup>[[#fn:r550|550]]</sup> ) and a sensitivity study (Schlegel et al., 2018 <sup>[[#fn:r551|551]]</sup> ) assessing the uncertainty in snow accumulation, ocean-induced melting, ice viscosity, basal friction, bedrock elevation and the effect of ice shelves on ice mass loss in 2100, Figure 4.4.

Ritz et al. (2015) is difficult to contextualise as they only provided estimates for the A1B scenario and not for the RCP scenarios. Despite this limitation their results, which are close to the other studies, are included as if they represent RCP8.5 and as such supports the assessment. The results by DeConto and Pollard (2016) <sup>[[#fn:r552|552]]</sup> indicate significantly higher mass loss even for RCP4.5, potentially related to their high surface melt rates on the ice shelves as contested by Trusel et al. (2015) <sup>[[#fn:r553|553]]</sup> . This early onset of high surface melt rates in DeConto and Pollard (2016) <sup>[[#fn:r554|554]]</sup> leads to extensive hydrofracturing of ice shelves before the end of the 21st century and therefore to rapid ice mass loss. For this reason, their results and probabilistic (e.g., Kopp et al., 2017; Le Bars et al., 2017) and statistical emulation estimates that build on them (Edwards et al., 2019 <sup>[[#fn:r555|555]]</sup> ) , are not used in SROCC sea level projections. Consequently, the process-based studies by Golledge et al. (2015) <sup>[[#fn:r556|556]]</sup> , Ritz et al. (2015) , Levermann et al. (2014) <sup>[[#fn:r558|558]]</sup> , Golledge et al. (2019) <sup>[[#fn:r559|559]]</sup> , and Bulthuis et al. (2019) are used to assess the Antarctic contribution for the different RCP scenarios. The study by Schlegel et al. (2018) does not provide RCP based scenarios, but is considered as an extensive sensitivity estimate providing a high-end estimate based on physical process understanding of the Antarctic contribution.

Each study expresses an uncertainty in the Antarctic contribution to GMSL rise which is, in part, dependent on a common driver, namely regional warming. The uncertainties were therefore interpreted as being dependent and propagate the total uncertainty accordingly. As a result, the total uncertainty exceeds that of the individual studies, which reflects that the individual studies only sample a fraction of the total uncertainty. The uncertainty estimates of Levermann et al. (2014) <sup>[[#fn:r561|561]]</sup> concentrate on the oceanic basal melt rates including a time delay between atmosphere and ocean temperature, but do not consider other sources of uncertainty. Ritz et al. (2015) <sup>[[#fn:r562|562]]</sup> is constrained by observations and provides an asymmetric distribution of the rate of mass loss. The ice sheet simulations by Golledge et al. (2015) and Golledge et al. (2019) only provide two alternative subgrid parameterisations for sub-ice melt, rather than a statistical estimate of the uncertainty. The more sensitive of these two parameterisations which induces more ice loss is challenged by Seroussi and Morlighem (2018) <sup>[[#fn:r565|565]]</sup> . In order to assess a realistic uncertainty for the total Antarctic contribution, it was first assumed that Golledge et al. (2015) <sup>[[#fn:r566|566]]</sup> and Golledge et al. (2019) <sup>[[#fn:r567|567]]</sup> are dependent, because they use similar parameterisations. For each study, a probabilistic distribution is used, assuming a normal distribution with a ''likely'' range bounded by the high and low estimate from those studies. Levermann et al. (2014) <sup>[[#fn:r568|568]]</sup> also provides two alternatives, one with and one without a time delay between oceanic temperatures below the Antarctic ice shelves and global mean atmospheric temperature. As it is unclear which version best matches the updated record of ice loss presented by The IMBIE team, (2018) , results are combined assuming full probabalistic dependence as for the two Golledge studies. Bulthuis et al. (2019) <sup>[[#fn:r572|572]]</sup> uses a simplified ice sheet model to study the uncertainty caused by the atmospheric forcing, ice dynamics, ice and bed rheology, calving and sub-shelf melting. Finally, the studies by Ritz et al. (2015) <sup>[[#fn:r571|571]]</sup> , Bulthuis et al. (2019) and the averages for Golledge and Levermann are combined to identify a best estimate for the Antarctic contribution under RCP8.5. This results in a median contribution of 16 cm in 2100 under RCP8.5. A Monte Carlo technique is used to combine the uncertainties in the aforementioned studies, assuming mutual dependence. The resulting 5–95 percentile range, 2–37 cm in 2100 under RCP8.5, is assessed as the ''likely'' range. This assessment is used in order to reflect ongoing limited understanding of the physics and the fact that the individual studies only reflect part of the total uncertainty. The distribution is slightly skewed to higher values, because of an underlying skewness in the studies of Levermann et al. (2014) <sup>[[#fn:r569|569]]</sup> and Ritz et al. (2015) <sup>[[#fn:r571|571]]</sup> . This skewed distribution is supported by an expert elicitation study (Bamber et al., 2009) . The expert elicitation approach (Bamber et al., 2018) , which applied elicitation to both ice sheets, suggests considerably higher values for total SLR for RCP2.6, RCP4.5 and RCP8.5 than provided in Table 4.3.

As the importance of MISI and MICI is difficult to assess on longer time scales, there remains deep uncertainty for the Antarctic contribution to GMSL after 2100 (Cross-Chapter Box 4 in Chapter 1). Results on these long-time scales are discussed in 4.2.3.5.

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<span id="table-4.3"></span>
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'''Table 4.3:'''
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An overview of different studies estimating the future Antarctic contribution to sea level rise (SLR), listed here are median values. Estimates from Golledge et al. (2015) are based on the average contribution to Global Mean Sea Level (GMSL) over the full 21st century, based on two alternative ensembles using different sub-ice melt schemes. This average is not explicitly reported in the original paper where the individual values of 0.1 and 0.39 m are reported. SMB is the surface mass balance, BMB the basal melt balance, LIG is Last Interglacial, MICI is marine ice cliff instability, GCM is General Circulation Model, PDD is positive-degree day.

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[[File:4bc1e6307f36dfeee2ae0eafcf4efea3 table4.3.png]]

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There is limited evidence for major changes since AR5 in the non-Antarctic components. Recent projections of the glacier contribution are nearly identical to AR5 results used here (see Cross-Chapter Box 6 in Chapter 2). Greenland, thermal expansion and land water storage are also not updated, mainly due to a lack of updated CMIP simulations. Hence, our revised projections replace only the AR5 estimate for Antarctica by a new assessment as outlined in the previous paragraph based on post-AR5 literature and maintaining identical contributions for the non-Antarctic components. As no general dependence between the Antarctic contribution and the non-Antarctic components can be derived from the four studies, independent uncertainties are assumed, which is close to the uncertainty propagation by Church et al. (2013) <sup>[[#fn:r576|576]]</sup> .

Time series for the different RCP scenarios are shown in Figure 4.9 indicating a divergence in median and upper ''likely'' range for RCP8.5 during the second half of the century between this report and the AR5 projections (Church et al., 2013 <sup>[[#fn:r577|577]]</sup> ) . The value of the Antarctic contribution in 2081–2100 under RCP8.5 is the individual component with the largest uncertainty. As a consequence, the uncertainty in the GMSL projections is slightly increased compared to Church et al. (2013) . Nevertheless, results can also be considered to be consistent with Church et al. (2013) <sup>[[#fn:r578|578]]</sup> . In AR5, the potential additional contribution by ice dynamics, was estimated to be not more than several tenths of a metre but excluded from projections; here this value was assessed to be 16 cm (5–95 percentile; 2–37 cm) and include it in the projections. As the projections build on the CMIP5 work presented in AR5, and also given the limited exploration of uncertainty in estimates from each individual study, the results of the 5–95 percentile are interpreted to represent the ''likely'' range, that is, the 17–83 percentile, as assessed by Church et al. (2013) and as assessed in AR5 for other CMIP5-derived results.

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<span id="table-4.4"></span>
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'''Table 4.4'''
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Median values and likely ranges for projections of global mean sea level (GMSL) rise in metres in 2081–2100 relative to 1986–2005 for three scenarios. In addition, values of GMSL rise are given for 2046-2065 and 2100, and the rate of GMSL rise is given for 2100. Values between parentheses reflect the likely range. SMB is surface mass balance, DYN is dynamical contribution, LWS is land water storage. Total AR5 minus Antarctica AR5 is the GMSL rise contribution in Church et al. (2013) without the Antarctic contribution of Church et al. (2013). The newly derived Antarctic contribution is added to this to arrive at the GMSL rise.

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[[File:aa63ba81f162ddaf5b939f462585d229 table4.4.png]]

Notes:

\*The uncertainty in this value is calculated as in Church et al. (2013).

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<span id="figure-4.9"></span>
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'''Figure 4.9'''

<span id="figure-4.9-time-series-of-global-mean-sea-level-gmsl-for-representative-concentration-pathway-rcp2.6-rcp4.5-and-rcp8.5-as-used-in-this-report-and-for-reference-the-ipcc-5th-assessment-report-ar5-results-church-et-al.-2013.-results-are-based-on-ar5-results-for-all-components-except-the-antarctic-contribution.-results-for-the-antarctic"></span>
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'''Figure 4.9 | Time series of Global Mean Sea Level (GMSL) for Representative Concentration Pathway (RCP)2.6, RCP4.5 and RCP8.5 as used in this report and, for reference the IPCC 5th Assessment Report (AR5) results (Church et al., 2013). Results are based on AR5 results for all components except the Antarctic contribution. Results for the Antarctic […]'''
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[[File:77840ce1305f31bf8c3a27055b8fff52 IPCC-SROCC-CH_4_9-3000x895.jpg]]

Figure 4.9 | Time series of Global Mean Sea Level (GMSL) for Representative Concentration Pathway (RCP)2.6, RCP4.5 and RCP8.5 as used in this report and, for reference the IPCC 5th Assessment Report (AR5) results (Church et al., 2013). Results are based on AR5 results for all components except the Antarctic contribution. Results for the Antarctic contribution in 2081–2100 are provided in Table 4.4. The shaded region is considered to be the likely range.

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Projections as presented in Table 4.4 are used to calculate the regional RSL projections as outlined in AR5 by including gravitational and rotational patterns as shown in Figure 4.10 and subsequently used in 4.2.3.4 to calculate ESL projections. Including the updated results in terms of magnitude and uncertainty for the Antarctic component also changes the regional patterns in sea level projections. Results of the regional patterns in Figure 4.10 show an increased SLR with respect to the results presented in AR5 nearly everywhere for RCP8.5 because of the increased Antarctic contribution.

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<span id="figure-4.10"></span>
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'''Figure 4.10'''

<span id="figure-4.10-regional-sea-level-change-for-rcp2.6-rcp4.5-and-rcp8.5-in-metres-as-used-in-this-report-for-extreme-sea-level-esl-events.-results-are-median-values-based-on-the-values-in-table-4.4-for-antarctica-including-gia-and-the-gravitational-and-rotational-effects-and-results-by-church-et-al.-2013-for-glaciers"></span>
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'''Figure 4.10 | Regional sea level change for RCP2.6, RCP4.5 and RCP8.5 in metres as used in this report for extreme sea level (ESL) events. Results are median values based on the values in Table 4.4 for Antarctica including GIA and the gravitational and rotational effects, and results by Church et al. (2013) for glaciers, […]'''
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[[File:551a1ce25a7fff5251607a55b7ae6dc3 IPCC-SROCC-CH_4_10-3000x2591.jpg]]

Figure 4.10 | Regional sea level change for RCP2.6, RCP4.5 and RCP8.5 in metres as used in this report for extreme sea level (ESL) events. Results are median values based on the values in Table 4.4 for Antarctica including GIA and the gravitational and rotational effects, and results by Church et al. (2013) for glaciers, land water storage (LWS) and Greenland. The left column is for the time slice 2046–2065 and the right column for 2081–2100.

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