Editing IPCC:AR6/SR15/Chapter-3 (section)

=== 3.3.9 Sea Level ===

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Sea level varies over a wide range of temporal and spatial scales, which can be divided into three broad categories. These are global mean sea level (GMSL), regional variation about this mean, and the occurrence of sea-level extremes associated with storm surges and tides. GMSL has been rising since the late 19th century from the low rates of change that characterized the previous two millennia (Church et al., 2013) <sup>[[#fn:r282|282]]</sup> . Slowing in the reported rate over the last two decades (Cazenave et al., 2014) <sup>[[#fn:r283|283]]</sup> may be attributable to instrumental drift in the observing satellite system (Watson et al., 2015) <sup>[[#fn:r284|284]]</sup> and increased volcanic activity (Fasullo et al., 2016) <sup>[[#fn:r285|285]]</sup> . Accounting for the former results in rates (1993 to mid-2014) between 2.6 and 2.9 mm yr <sup>–1</sup> (Watson et al., 2015) <sup>[[#fn:r286|286]]</sup> . The relative contributions from thermal expansion, glacier and ice-sheet mass loss, and freshwater storage on land are relatively well understood (Church et al., 2013; Watson et al., 2015) <sup>[[#fn:r287|287]]</sup> and their attribution is dominated by anthropogenic forcing since 1970 (15 ± 55% before 1950, 69 ± 31% after 1970) (Slangen et al., 2016) <sup>[[#fn:r288|288]]</sup> .

There has been a significant advance in the literature since AR5, which has included the development of semi-empirical models (SEMs) into a broader emulation-based approach (Kopp et al., 2014; Mengel et al., 2016; Nauels et al., 2017) <sup>[[#fn:r289|289]]</sup> that is partially based on the results from more detailed, process-based modelling Church et al. (2013) <sup>[[#fn:r290|290]]</sup> assigned ''low confidence'' to SEMs because these models assume that the relation between climate forcing and GMSL is the same in the past (calibration) and future (projection). Probable future changes in the relative contributions of thermal expansion, glaciers and (in particular) ice sheets invalidate this assumption. However, recent emulation-based studies overcame this shortcoming by considering individual GMSL contributors separately, and they are therefore employed in this assessment. In this subsection, the process-based literature of individual contributors to GMSL is considered for scenarios close to 1.5°C and 2°C of global warming before emulation-based approaches are assessed.

A limited number of processes-based studies are relevant to GMSL in 1.5°C and 2°C worlds. Marzeion et al. (2018) <sup>[[#fn:r291|291]]</sup> used a global glacier model with temperature-scaled scenarios based on RCP2.6 to investigate the difference between 1.5°C and 2°C of global warming and found little difference between scenarios in the glacier contribution to GMSL for the year 2100 (54–97 mm relative to present-day levels for 1.5°C and 63–112 mm for 2°C, using a 90% confidence interval). This arises because glacier melt during the remainder of the century is dominated by the response to warming from pre-industrial to present-day levels, which is in turn a reflection of the slow response times of glaciers. Fürst et al. (2015) <sup>[[#fn:r292|292]]</sup> made projections of the Greenland ice sheet’s contribution to GMSL using an ice-flow model forced by the regional climate model Modèle Atmosphérique Régional (MAR; considered by Church et al. (2013) <sup>[[#fn:r293|293]]</sup> to be the ‘most realistic’ such model). They projected an RCP2.6 range of 24–60 mm (1 standard deviation) by the end of the century (relative to the year 2000 and consistent with the assessment of Church et al. (2013) <sup>[[#fn:r294|294]]</sup> ; however, their projections do not allow the difference between 1.5°C and 2°C worlds to be evaluated.

The Antarctic ice sheet can contribute both positively, through increases in outflow (solid ice lost directly to the ocean), and negatively, through increases in snowfall (owing to the increased moisture-bearing capacity of a warmer atmosphere), to future GMSL rise. Frieler et al. (2015) <sup>[[#fn:r295|295]]</sup> suggested a range of 3.5–8.7% °C <sup>–1</sup> for this effect, which is consistent with AR5. Observations from the Amundsen Sea sector of Antarctica suggest an increase in outflow (Mouginot et al., 2014) <sup>[[#fn:r296|296]]</sup> over recent decades associated with grounding line retreat (Rignot et al., 2014) <sup>[[#fn:r297|297]]</sup> and the influx of relatively warm Circumpolar Deepwater (Jacobs et al., 2011) <sup>[[#fn:r298|298]]</sup> . Literature on the attribution of these changes to anthropogenic forcing is still in its infancy (Goddard et al., 2017; Turner et al., 2017a) <sup>[[#fn:r299|299]]</sup> . RCP2.6-based projections of Antarctic outflow (Levermann et al., 2014; Golledge et al., 2015; DeConto and Pollard, 2016 <sup>[[#fn:r300|300]]</sup> , who include snowfall changes) are consistent with the AR5 assessment of Church et al. (2013) <sup>[[#fn:r301|301]]</sup> for end-of-century GMSL for RCP2.6, and do not support substantial additional GMSL rise by Marine Ice Sheet Instability or associated instabilities (see Section 3.6). While agreement is relatively good, concerns about the numerical fidelity of these models still exist, and this may affect the quality of their projections (Drouet et al., 2013; Durand and Pattyn, 2015) <sup>[[#fn:r302|302]]</sup> . An assessment of Antarctic contributions beyond the end of the century, in particular related to the Marine Ice Sheet Instability, can be found in Section 3.6.

While some literature on process-based projections of GMSL for the period up to 2100 is available, it is insufficient for distinguishing between emissions scenarios associated with 1.5°C and 2°C warmer worlds. This literature is, however, consistent with the assessment by Church et al. (2013) <sup>[[#fn:r303|303]]</sup> of a ''likely'' range of 0.28–0.61 m in 2100 (relative to 1986–2005), suggesting that the AR5 assessment is still appropriate. Recent emulation-based studies show convergence towards this AR5 assessment (Table 3.1) and offer the advantage of allowing a comparison between 1.5°C and 2°C warmer worlds. Table 3.1 features a compilation of recent emulation-based and SEM studies.

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'''Table 3.1:'''

<span id="compilation-of-recent-projections-for-sea-level-at-2100-in-cm-for-representative-concentration-pathway-rcp2.6-and-1.5c-and-2c-scenarios.-upper-and-lower-limits-are-shown-for-the-17-84-and-5-95-confidence-intervals-quoted-in-the-original-papers."></span>
'''Compilation of recent projections for sea level at 2100 (in cm) for Representative Concentration Pathway (RCP)2.6, and 1.5°C and 2°C scenarios. Upper and lower limits are shown for the 17-84% and 5-95% confidence intervals quoted in the original papers.'''

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{| class="wikitable"
|-
! rowspan="2"| Study
! rowspan="2"| Baseline
! colspan="2"| RCP2.6
! colspan="2"| 1.5°C
! colspan="2"| 2°C
|-
! 67%
! 90%
! 67%
! 90%
! 67%
! 90%
|-
| AR5
| 1986–2005
| 28–61
|
|-
| Kopp et al. (2014)  <sup>[[#fn:r304|304]]</sup>
| 2000
| 37–65
| 29–82
|
|-
| Jevrejeva et al. (2016) <sup>[[#fn:r305|305]]</sup>
| 1986–2005
|
| 29–58
|
|-
| Kopp et al. (2016) <sup>[[#fn:r306|306]]</sup>
| 2000
| 28–51
| 24–61
|
|-
| Mengel et al. (2016) <sup>[[#fn:r307|307]]</sup>
| 1986–2005
| 28–56
|
|-
| Nauels et al. (2017) <sup>[[#fn:r308|308]]</sup>
| 1986–2005
| 35–56
|
|-
| Goodwin et al. (2017) <sup>[[#fn:r309|309]]</sup>
| 1986–2005
|
| 31–59<br />
45–70<br />
45–72
|
|-
| Schaeffer et al. (2012) <sup>[[#fn:r310|310]]</sup>
| 2000
|
| 52–96
|
| 54–99
|
| 56–105
|-
| Schleussner et al. (2016b) <sup>[[#fn:r311|311]]</sup>
| 2000
|
| 26–53
|
| 36–65
|
|-
| Bittermann et al. (2017) <sup>[[#fn:r312|312]]</sup>
| 2000
|
| 29–46
|
| 39–61
|-
| Jackson et al. (2018) <sup>[[#fn:r313|313]]</sup>
| 1986–2005
|
| 30–58<br />
40–77
| 20–67<br />
28–93
| 35–64<br />
47–93
| 24–74<br />
32–117
|-
| Sanderson et al. (2017) <sup>[[#fn:r314|314]]</sup>
|
| 50–80
|
| 60–90
|-
| Nicholls et al. (2018) <sup>[[#fn:r315|315]]</sup>
| 1986–2005
|
| 24–54
|
| 31–65
|-
| Rasmussen et al. (2018) <sup>[[#fn:r316|316]]</sup>
| 2000
|
| 35–64
| 28–82
| 39–76
| 28–96
|-
| Goodwin et al. (2018) <sup>[[#fn:r317|317]]</sup>
| 1986–2005
|
| 26–62
|
| 30–69
|}
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There is little consensus between the reported ranges of GMSL rise (Table 3.1). Projections vary in the range 0.26–0.77 m and 0.35–0.93 m for 1.5°C and 2°C respectively for the 17–84% confidence interval (0.20–0.99 m and 0.24–1.17 m for the 5–95% confidence interval). There is, however, ''medium agreement'' that GMSL in 2100 would be 0.04–0.16 m higher in a 2°C warmer world compared to a 1.5°C warmer world based on the 17–84% confidence interval (0.00–0.24 m based on 5–95% confidence interval) with a value of around 0.1m. There is ''medium confidence'' in this assessment because of issues associated with projections of the Antarctic contribution to GMSL that are employed in emulation-based studies (see above) and the issues previously identified with SEMs (Church et al., 2013) <sup>[[#fn:r318|318]]</sup> .

Translating projections of GMSL to the scale of coastlines and islands requires two further steps. The first step accounts for regional changes associated with changing water and ice loads (such as Earth’s gravitational field and rotation, and vertical land movement), as well as spatial differences in ocean heat uptake and circulation. The second step maps regional sea level to changes in the return periods of particular flood events to account for effects not included in global climate models, such as tides, storm surges, and wave setup and runup. Kopp et al. (2014) <sup>[[#fn:r319|319]]</sup> presented a framework to do this and gave an example application for nine sites located in the US, Japan, northern Europe and Chile. Of these sites, seven (all except those in northern Europe) were found to experience at least a quadrupling in the number of years in the 21st century with 1-in-100-year floods under RCP2.6 compared to under no future sea level rise. Rasmussen et al. (2018) <sup>[[#fn:r320|320]]</sup> used this approach to investigate the difference between 1.5°C and 2°C warmer worlds up to 2200. They found that the reduction in the frequency of 1-in-100-year floods in a 1.5°C compared to a 2°C warmer world would be greatest in the eastern USA and Europe, with ESL event frequency amplification being reduced by about a half and with smaller reductions for small island developing states (SIDS). This last result contrasts with the finding of Vitousek et al. (2017) <sup>[[#fn:r321|321]]</sup> that regions with low variability in extreme water levels (such as SIDS in the tropics) are particularly sensitive to GMSL rise, such that a doubling of frequency may be expected for even small (0.1–0.2 m) rises. Schleussner et al. (2011) <sup>[[#fn:r322|322]]</sup> emulated the AMOC based on a subset of CMIP-class climate models. When forced using global temperatures appropriate for the CP3-PD scenario (1°C of warming in 2100 relative to 2000 or about 2°C of warming relative to pre-industrial) the emulation suggests an 11% median reduction in AMOC strength at 2100 (relative to 2000) with an associated 0.04 m dynamic sea level rise along the New York City coastline.

In summary, there is ''medium confidence'' that GMSL rise will be about 0.1 m (within a 0.00–0.20 m range based on 17–84% confidence-interval projections) less by the end of the 21st century in a 1.5°C compared to a 2°C warmer world. Projections for 1.5°C and 2°C global warming cover the ranges 0.2–0.8 m and 0.3–1.00 m relative to 1986–2005, respectively ( ''medium confidence'' ). Sea level rise beyond 2100 is discussed in Section 3.6; however, recent literature strongly supports the assessment by Church et al. (2013) <sup>[[#fn:r323|323]]</sup> that sea level rise will continue well beyond 2100 ( ''high confidence'' ).

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