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

==== 9.6.3.3 Sea Level Projections to 2150 Based on Shared Socio-economic Pathway Scenarios ====

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Up to 2050, consistent with AR5 and SROCC, GMSL projections exhibit little scenario dependence ( ''high confidence'' ) (Figure 9.27 and Table 9.9) with ''likely'' ( ''medium confidence'' ) sea level rise between the baseline period (1995–2014) and 2050 of 0.19 (0.16–0.25) m under SSP1-2.6 and 0.23 (0.20–0.30) m under SSP5-8.5. These projections fall centrally within the range of published projections for RCP2.6 and RCP8.5 ( [[#9.6.3.1|Section 9.6.3.1]] ).

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'''Table 9.9''' '''|''' '''Global mean sea level projections for five Shared Socio-economic Pathway (SSP) scenarios, relative to a baseline of 199''' '''5–2''' '''014, in metres.''' Individual contributions are shown for the year 2100. Median values ( ''likely'' ranges) are shown. Average rates for total sea level change are shown in mm yr <sup>–1</sup> . Unshaded cells represent processes in whose projections there is ''medium confidence'' . Shaded cells incorporate a representation of processes in which there is ''low confidence'' ; in particular, the SSP5-8.5 ''low confidence'' column shows the 17th–83rd percentile range from a p-box including SEJ- and MICI-based projections rather than an assessed ''likely'' range. Methods are described in 9.6.3.2.

{| class="wikitable"
|-
|
| '''SSP1-1.9'''

| '''SSP1-2.6'''

| '''SSP2-4.5'''

| '''SSP3-7.0'''

| '''SSP5-8.5'''

| '''SSP5-8.5''' Low Confidence

|-
| '''Thermal expansion'''

| 0.12 (0.09–0.15)

| 0.14 (0.11–0.18)

| 0.20 (0.16–0.24)

| 0.25 (0.21–0.30)

| 0.30 (0.24–0.36)

| 0.30 (0.24–0.36)

|-
| '''Greenland'''

| 0.05 (0.00–0.09)

| 0.06 (0.01–0.10)

| 0.08 (0.04–0.13)

| 0.11 (0.07–0.16)

| 0.13 (0.09–0.18)

| 0.18 (0.09–0.59)

|-
| '''Antarctica'''

| 0.10 (0.03–0.25)

| 0.11 (0.03–0.27)

| 0.11 (0.03–0.29)

| 0.11 (0.03–0.31)

| 0.12 (0.03–0.34)

| 0.19 (0.02–0.56)

|-
| '''Glaciers'''

| 0.08 (0.06–0.10)

| 0.09 (0.07–0.11)

| 0.12 (0.10–0.15)

| 0.16 (0.13–0.18)

| 0.18 (0.15–0.21)

| 0.17 (0.11–0.21)

|-
| '''Land-water Storage'''

| 0.03 (0.01–0.04)

| 0.03 (0.01–0.04)

| 0.03 (0.01–0.04)

| 0.03 (0.02–0.04)

| 0.03 (0.01–0.04)

| 0.03 (0.01–0.04)

|-
|
|-
| '''Total (2030)'''

| 0.09 (0.08–0.12)

| 0.09 (0.08–0.12)

| 0.09 (0.08–0.12)

| 0.10 (0.08–0.12)

| 0.10 (0.09–0.12)

| 0.10 (0.09–0.15)

|-
| '''Total (2050)'''

| 0.18 (0.15–0.23)

| 0.19 (0.16–0.25)

| 0.20 (0.17–0.26)

| 0.22 (0.18–0.27)

| 0.23 (0.20–0.29)

| 0.24 (0.20–0.40)

|-
| '''Total (2090)'''

| 0.35 (0.26–0.49)

| 0.39 (0.30–0.54)

| 0.48 (0.38–0.65)

| 0.56 (0.46–0.74)

| 0.63 (0.52–0.83)

| 0.71 (0.52–1.30)

|-
| '''Total (2100)'''

| 0.38 (0.28–0.55)

| 0.44 (0.32–0.62)

| 0.56 (0.44–0.76)

| 0.68 (0.55–0.90)

| 0.77 (0.63–1.01)

| 0.88 (0.63–1.60)

|-
| '''Total (2150)'''

| 0.57 (0.37–0.86)

| 0.68 (0.46–0.99)

| 0.92 (0.66–1.33)

| 1.19 (0.89–1.65)

| 1.32 (0.98–1.88)

| 1.98 (0.98–4.82)

|-
|
|-
| '''Rate (204''' '''0–2060)'''

| 4.1 (2.8–6.0)

| 4.8 (3.5–6.8)

| 5.8 (4.4–8.0)

| 6.4 (5.0–8.7)

| 7.2 (5.6–9.7)

| 7.9 (5.6–16.1)

|-
| '''Rate (2080–2100)'''

| 4.2 (2.4–6.6)

| 5.2 (3.2–8.0)

| 7.7 (5.2–11.6)

| 10.4 (7.4–14.8)

| 12.1 (8.6–17.6)

| 15.8 (8.6–30.1)

|}

Beyond 2050, the scenarios increasingly diverge. Between the baseline period (1995–2014) and 2100, processes in whose projection there is ''medium confidence'' drive ''likely'' GMSL rise of 0.44 (0.32–0.62) m and 0.77 (0.63–1.01) m under SSP1-2.6 and SSP5-8.5, respectively (Tables 9.8, 9.9). While derived using substantially updated methods, these projections are broadly consistent with SROCC, which projected ''likely'' GMSL rise of 0.41 (0.26–0.56) m and 0.81 (0.58–1.07) m under RCP2.6 and RCP8.5, respectively, over this period. They are modestly higher than those of AR5, which projected ''likely'' GMSL rise of 0.41 (0.25–0.58) m under RCP2.6 and 0.71 (0.49–0.95) m under RCP8.5 (Figure 9.25, Table 9.8). They are also broadly consistent with projections produced by driving AR5 methods with CMIP6 temperature and thermal expansion projections, which leads to 0.44 (0.27–0.61) m under SSP1-2.6 and 0.73 (0.49–1.02) m under SSP5-8.5 ( [[#Hermans--2021|Hermans et al., 2021]] ). The SSP1-2.6 and SSP5-8.5 projections are consistent with the ranges of published projections for RCP2.6 and RCP8.5 that do not incorporate MICI or SEJ ( [[#9.6.3.1|Section 9.6.3.1]] ).

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[[File:5985eebb8bd48aff38effb27264ccbea IPCC_AR6_WGI_Figure_9_27.png]]

'''Figure 9.27''' '''|''' '''Projected global mean sea level rise under different Shared Socio-economic Pathway (SSP) scenarios.''' ''Likely'' global mean sea level (GMSL) change for SSP scenarios resulting from processes in whose projection there is ''medium confidence'' . Projections and ''likely'' ranges at 2150 are shown on right. Lightly shaded ranges and thinner lightly shaded ranges on the right show the 17th–83rd and 5th–95th percentile ranges for projections including ''low confidence'' processes for SSP1-2.6 and SSP5-8.5 only, derived from a p-box including structured expert judgement and marine ice-cliff instability projections. Black lines show historical GMSL change, and thick solid and dash-dotted black lines show the mean and ''likely'' range extrapolating the 1993–2018 satellite altimeter trend and acceleration. Further details on data sources and processing are available in the chapter data table (Table 9.SM.9).

The ''likely'' GMSL projections for SSP3-7.0 and SSP5-8.5 are consistent with a continuation of the GMSL satellite-observed rate ( ''very likely'' 3.25 [2.88–3.61] mm yr <sup>–1</sup> ) and acceleration ( ''very likely'' 0.094 [0.082–0.115] mm yr <sup>–2</sup> ) of GMSL rise over 1993–2018 (Table 9.5 and [[IPCC:Wg1:Chapter:Chapter-2#2.3.3.3|Section 2.3.3.3]] ), which would imply a ''likely'' GMSL rise of 0.24 m (0.23–0.25 m) by 2050 and 0.73 m (0.69–0.77 m) by 2100. This extrapolation would also imply a ''likely'' rate of GMSL rise of 7.5 (7.4–7.6) mm yr <sup>–1</sup> over 2040–2060 and 11.2 (10.6–11.8) mm yr <sup>–1</sup> over 2080–2100. Over the satellite period, the observed acceleration has been driven primarily by ice-sheet contributions ( [[#9.6.1.2|Section 9.6.1.2]] and Table 9.5); in the median projections for SSP3-7.0 and SSP5-8.5, these accelerations are projected to continue at a slightly lower level, while the GMSL acceleration is augmented by an acceleration of thermal expansion and glacier loss associated with rising global temperature. Overall, these extrapolations imply that, under SSP1-1.9, SSP1-2.6, and SSP2-4.5, the GMSL acceleration is projected to decrease from its current level.

While ice-sheet processes in whose projection there is ''low confidence'' have little influence up to 2100 on projections under SSP1-1.9 and SSP1-2.6 (Table 9.9), this is not the case under higher emissions scenarios, where they could lead to GMSL rise well above the ''likely'' range. In particular, under SSP5-8.5, ''low-confidence'' processes could lead to a total GMSL rise of 0.6–1.6 m over this time period (17th–83rd percentile range of p-box, including SEJ- and MICI-based projections), with 5th–95th percentile projections extending to 0.5–2.3 m ( ''low confidence'' ). The assessed ''low confidence'' range is slightly narrower than, but broadly consistent with, the full 0.4–2.4 m range of published 5th–95th percentile projections for RCP8.5 since AR5 ( [[#9.6.3.1|Section 9.6.3.1]] ) – including those based on SEJ or incorporating MICI – and highlights the ''deep uncertainty'' in GMSL rise under the highest emissions scenarios (Box 9.4). The assessment of the potential contribution of processes in which there is ''low confidence'' to GMSL rise by 2100 is broadly consistent with the AR5’s assessment ( [[#Church--2013b|Church et al., 2013b]] ), which concluded that collapse of marine-based sectors of the AIS could cause several tenths of a metre of GMSL rise above the ''likely'' range.

While prior assessment reports, starting with the First Assessment Report ( [[#Warrick--1990|Warrick et al., 1990]] ), have focused on projecting GMSL up to the year 2100, time has progressed, and the year 2100 is now within the time frame of some long-term infrastructure decisions. For this reason, projections up to the year 2150 are also highlighted (Table 9.9). Over this time period, assuming no acceleration in ice-sheet mass fluxes after 2100, processes in which there is ''medium confidence'' lead to GMSL rise of 0.5–1.0 m under SSP1-2.6 and 1.0–1.9 m under SSP5-8.5. Processes in which there is ''low confidence'' could drive GMSL rise under SSP5-8.5 to 1.0–4.8 m (17th–83rd percentile) or even 0.9–5.4 m (5th–95th percentile).

Median projected RSL changes are shown in Figure 9.28, with driving factors highlighted in Figure 9.26. Approximately 60% (SSP1-1.9) to 70% (SSP5-8.5) of the global coastline has a projected median 21st century regional RSL rise within ±20% of the global mean increase ( ''medium confidence'' ). Consistent with AR5, loss of land ice mass will be an important contributor to spatial patterns in RSL change ( ''high confidence'' ), with ocean dynamic sea level being particularly important as a dipolar contributor in the north-west Atlantic, a positive contributor in the Arctic Ocean, and a negative contributor in the Southern Ocean south of the Antarctic Circumpolar Current ( ''medium confidence'' ) [[#9.2.4.2|Section 9.2.4.2]] ). As today, VLM will remain a major driver of RSL change ( ''high confidence'' ). Uncertainty in RSL projections is greatest in tectonically active areas in which VLM varies over short distances (e.g., Alaska) and in areas potentially subject to large ocean dynamic sea level change (e.g., the north-western Atlantic) ( ''high confidence'' ).

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[[File:24f4f2d5f1718be2009d41874e62fcbd IPCC_AR6_WGI_Figure_9_28.png]]

'''Figure 9.28''' '''|''' '''Regional sea level change at 2100 for different scenarios (with respect to 199''' '''5–2''' '''014).''' Median regional relative sea level change from 1995–2014 up to 2100 for: '''(a)''' SSP1-1.9; '''(b)''' SSP1-2.6; '''(c)''' SSP2-4.5; '''(d)''' SSP3-7.0; '''(e)''' SSP5-8.5; and '''(f)''' width of the likely range for SSP3-7.0. The high uncertainty in projections around Alaska and the Aleutian Islands arises from the tectonic contribution to vertical land motion, which varies greatly over short distances in this region. Further details on data sources and processing are available in the chapter data table (Table 9.SM.9).

An alternative perspective on uncertainty in future sea level rise is provided by looking at uncertainty in time rather than elevation; that is, looking at the range of dates when specific thresholds of sea level rise are projected to be crossed (Figure 9.29). Considering only ''medium confidence'' processes, GMSL rise is ''likely'' to exceed 0.5 m between about 2080 and 2170 under SSP1-2.6 and between about 2070 and 2090 under SSP5-8.5. It is ''likely'' to exceed 1.0 m between about 2150 and some point after 2300 under SSP1-2.6, and between about 2100 and 2150 under SSP5-8.5. It is ''unlikely'' to exceed 2.0 m until after 2300 under SSP1-2.6, while it is ''likely'' to do so between about 2160 and 2300 under SSP5-8.5. However, processes in whose projections there is ''low confidence'' could lead to substantially earlier exceedances under higher emissions scenarios: under SSP5-8.5, 1.0 m could be exceeded by about 2080 and 2.0 m could be exceeded by about 2110 (17th percentile of p-box, incorporating projections based on SEJ and MICI), with 5th percentile projections as early as about 2070 for 1.0 m and 2090 for 2.0 m.

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[[File:aac96d66d6c3036c972bc39d071d4a58 IPCC_AR6_WGI_Figure_9_29.png]]

'''Figure''' '''9.29 |''' '''Timing of when global mean sea level (GMSL) thresholds of 0.5, 1.0, 1.5 and 2.0 m are exceeded, based on four different ice-sheet projection methods informing post-2100 projections.''' Methods are labelled based on their treatment of ice sheets. ‘No acceleration’ assumes constant rates of mass change after 2100. ‘Assessed ice sheet’ models post-2100 ice-sheet losses using a parametric fit (Supplementary Material 9.SM.4) extending to 2300 based on a multi-model assessment of contributions under RCP2.6 and RCP8.5 at 2300. Structured expert judgement (SEJ) employs ice-sheet projections from [[#Bamber--2019|Bamber et al. (2019)]] . Marine ice-cliff instability (MICI) combines the parametric fit (Supplementary Material 9.SM3.4) for Greenland with Antarctic projections based on [[#DeConto--2021|DeConto et al. (2021)]] . Circles, thick bars and thin bars represent the 50th, 17th–83rd and 5th–95th percentiles of the exceedance timing for the indicated projection method. Further details on data sources and processing are available in the chapter data table (Table 9.SM.9).

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