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

==== 9.6.3.4 Sea Level Projections up to 2100 Based on Global Warming Levels ====

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Global warming levels represent a new dimension of integration in the AR6 cycle ( [[IPCC:Wg1:Chapter:Chapter-1#1.6.2|Section 1.6.2]] , Cross-Chapter Box 11.1). The SR1.5 ( [[#Hoegh-Guldberg--2018|Hoegh-Guldberg et al., 2018]] ) concluded that, based on an assessment of GMSL projections published for 1.5°C and 2.0°C scenarios, there is ''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 17–84% confidence interval projections (0.00–0.24 m based on 5–95% confidence interval projections) with a central value of around 0.1 m. The SR1.5 did not attempt to standardize the definition of warming-level scenarios, or to examine additional warming levels. No new integrated GMSL projections for 1.5°C or 2.0°C scenarios have been published since SR1.5.

Most of the contributors to GMSL are more closely tied to time-integrated GSAT than instantaneous GSAT ( [[#Hermans--2021|Hermans et al., 2021]] ), which means that sea level projections by warming level can only be interpreted if the warming levels are linked to a specific time frame. Here, the warming level projections are defined based on the 2081–2100 GSAT anomaly (Supplementary Material 9.SM.4.7). Different pathways in GSAT can be followed to reach a certain temperature level, which affects the temporal evolution of the different contributors to sea level change. For instance, there will be different ice-sheet and glacier responses to a fast increase to a peak warming of 2°C in 2050, followed by a plateau or a decrease, compared to a gradual increase to the same level of warming in 2100. The sea level projections presented might include different pathways to the same warming level in 2100, which is reflected in the uncertainty ranges, and should therefore be interpreted as illustrative of sea level scenarios under a certain warming level.

Projections of ''likely'' 21st-century GMSL rise along climate trajectories leading to different increases in GSAT between 1850–1900 and 2081–2100 are shown in Table 9.10, along with the SSPs for which the temperature-level projections are most closely aligned. For example, considering only processes in which there is ''medium confidence'' , from the baseline period (1995–2014) up to 2100, GMSL in a 2°C scenario is ''likely'' to rise by 0.40–0.69, which is intermediate between the projections for SSP1-2.6 and SSP2-4.5. GMSL in a 4°C scenario is ''likely'' to rise by 0.58–0.92 m, similar to the projection for SSP3-7.0. Consistent with the discussion in [[#9.6.3.3|Section 9.6.3.3]] , there is ''deep uncertainty'' in the projections for temperature levels above 3°C, and alternative approaches to projecting ice-sheet changes may yield substantially different projections in 4°C and 5°C futures. For example, employing SEJ ice-sheet projections ( [[#Bamber--2019|Bamber et al., 2019]] ) instead of the projections for ''medium confidence'' processes only leads to a 17th–83rd percentile rise between the baseline period (1995–2014) and 2100 of 0.7–1.6 m, rather than 0.7–1.1 m in a 5°C scenario.

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'''Table 9.10''' '''|''' '''Global mean sea level (GMSL) projections and commitments for exceedance of five global warming levels, defined by sorting GSAT change in 208''' '''1–2''' '''100 with respect to 185''' '''0–1''' '''900.''' Median values and ( ''likely'' ) ranges are in metres relative to a 1995–2014 baseline. Rates are 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 projections based on structured expert judgement (SEJ) and marine ice cliff instability (MICI) rather than an assessed ''likely'' range. Methods are described in 9.6.3.2.

{| class="wikitable"
|-
|
| '''1.5''' ° '''C'''

| '''2.0''' ° '''C'''

| '''3.0''' ° '''C'''

| '''4.0''' ° '''C'''

| '''5.0''' ° '''C'''

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

|-
| '''Closest SSPs'''

| SSP1-2.6

| SSP1-2.6/SSP2-4.5

| SSP2-4.5/SSP3-7.0

| SSP3-7.0

| SSP5-8.5

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

| 0.18 (0.16–0.24) m

| 0.20 (0.17–0.26) m

| 0.21 (0.18–0.27) m

| 0.22 (0.19–0.28) m

| 0.25 (0.22–0.31) m

| 0.24 (0.20–0.40) m

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

| 0.44 (0.34–0.59) m

| 0.51 (0.40–0.69) m

| 0.61 (0.50–0.81) m

| 0.70 (0.58–0.92) m

| 0.81 (0.69–1.05) m

| 0.88 (0.63–1.60) m

|-
| '''Rate (2040–2060)'''

| 4.1 (2.9–5.7) mm yr <sup>–1</sup>

| 5.0 (3.7–7.0) mm yr <sup>–1</sup>

| 6.0 (4.6–8.1) mm yr <sup>–1</sup>

| 6.4 (5.0–8.6) mm yr <sup>–1</sup>

| 7.2 (5.7–9.8) mm yr <sup>–1</sup>

| 7.9 (5.6–16.1) mm yr <sup>–1</sup>

|-
| '''Rate (2080–2100)'''

| 4.3 (2.6–6.4) mm yr <sup>–1</sup>

| 5.5 (3.4–8.4) mm yr <sup>–1</sup>

| 7.8 (5.3-–11.6) mm yr <sup>–1</sup>

| 9.9 (7.1–14.3) mm yr <sup>–1</sup>

| 11.7 (8.5–17.0) mm yr <sup>–1</sup>

| 15.8 (8.6–30.1) mm yr <sup>–1</sup>

|-
|
|-
| '''2000-yr commitment'''

| 2 to 3 m

| 2 to 6 m

| 4 to 10 m

| 12 to 16 m

| 19 to 22 m

|
|-
| '''10,000-yr commitment'''

| 6 to 7 m

| 8 to 13 m

| 10 to 24 m

| 19 to 33 m

| 28 to 37 m

|
|}

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