Editing IPCC:AR6/WGI/TS (section)

== Box TS.4 | Sea Level ==

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'''Global mean sea level (GMSL) increased by 0.20 [0.15 to 0.25] m over the period 1901 to 2018, with a rate of rise that has accelerated since the 1960s to 3.7 [3.2 to 4.2] mm yr <sup>–1</sup> for the period 2006–2018 <sup></sup> (''high confidence''). Human activities were ''very likely'' the main driver of observed GMSL rise since 1971, and new observational evidence leads to an assessed sea level rise over the period 1901 to 2018 that is consistent with the sum of individual components contributing to sea level rise, including expansion due to ocean warming and melting of glaciers and ice sheets (''high confidence''). It is ''virtually certain'' that GMSL will continue to rise over the 21st century in response to continued warming of the climate system (Box TS.4, Figure 1). Sea level responds to greenhouse gas (GHG) emissions more slowly than global surface temperature, leading to weaker scenario dependence over the 21st century than for global surface temperature (''high confidence''). This slow response also leads to long-term committed sea level rise, associated with ongoing ocean heat uptake and the slow adjustment of the ice sheets, that will continue over the centuries and millennia following cessation of emissions (''high confidence'') (Box TS.9). By 2100, GMSL is projected to rise by 0.28–0.55 m (''likely'' range) under SSP1-1.9 and 0.63–1.01 m (''likely'' range) under SSP5-8.5 relative to the 1995–2014 average (''medium confidence''). Under the higher CO <sub>2</sub> emissions scenarios, there is deep uncertainty in sea level projections for 2100 and beyond associated with the ice-sheet responses to warming. In a low-likelihood, high-impact storyline and a high CO <sub>2</sub> emissions scenario, ice-sheet processes characterized by deep uncertainty could drive GMSL rise up to about 5 m by 2150. Given the long-term commitment, uncertainty in the timing of reaching different GMSL rise levels is an important consideration for adaptation planning. Links to chapters 2.3, 3.4, 3.5, 9.6, Box 9.4, Cross-Chapter Box 9.1, Table 9.5'''

GMSL change is driven by warming or cooling of the ocean (and the associated expansion/contraction) and changes in the amount of ice and water stored on land. Paleo-evidence shows that GMSL has been about 70 m higher and 130 m lower than present within the past 55 million years and was ''likely'' 5 to 10 m higher during the Last Interglacial (Box TS.2, Figure 1). Sea level observations show that GMSL rose by 0.20 [0.15 to 0.25] m over the period 1901–2018 at an average rate of 1.7 [1.3 to 2.2] mm yr <sup>–1</sup>. New analyses and paleo-evidence since AR5 show this rate is ''very likely'' faster than during any century over at least the last three millennia (''high confidence''). Since AR5, there is strengthened evidence for an increase in the rate of GMSL rise since the mid-20th century, with an average rate of 2.3 [1.6–3.1] mm yr <sup>–1</sup> over the period 1971–2018 increasing to 3.7 [3.2–4.2] mm yr <sup>–1</sup> for the period 2006–2018 (''high confidence''). Links to chapters 2.3.3, 9.6.1, 9.6.2

[[File:716d64a7d5c79cf79e3eb0d85c55b6b9 IPCC_AR6_WGI_TS_Box_4_Figure_1.png]]

'''Box TS.4, Figure 1 |'''

'''Global mean sea level (GMSL) change on different time scales and under different scenarios.''' ''The intent of this figure is to (i) show the century-scale GMSL projections in the context of the 20th century observations, (ii) illustrate ‘deep uncertainty’ in projections by considering the timing of GMSL rise milestones, and (iii) show the long-term commitment associated with different warming levels, including the paleo evidence to support this.'' '''(a)''' GMSL change from 1900 to 2150, observed (1900–2018) and projected under the SSP scenarios (2000–2150), relative to a 1995–2014 baseline. Solid lines show median projections. Shaded regions show ''likely'' ranges for SSP1-2.6 and SSP3-7.0. Dotted and dashed lines show respectively the 83rd and 95th percentile ''low confidence'' projections for SSP5-8.5. Bars at right show ''likely'' ranges for SSP1-1.9, SSP1-2.6, SSP2-4.5, SSP3-7.0 and SSP5-8.5 in 2150. Lightly shaded thick/thin bars show 17th–83rd/5th–95th percentile ''low-confidence'' ranges in 2150 for SSP1-2.6 and SSP5-8.5, based upon projection methods incorporating structured expert judgement and marine ice cliff instability. ''Low confidence'' range for SSP5-8.5 in 2150 extends to 4.8/5.4 m at the 83rd/95th percentile. '''(b)''' GMSL change on 100- (blue), 2000- (green) and 10,000-year (magenta) time scales as a function of global surface temperature, relative to 1850–1900. For 100-year projections, GMSL is projected for the year 2100, relative to a 1995–2014 baseline, and temperature anomalies are average values over 2081–2100. For longer-term commitments, warming is indexed by peak warming above 1850–1900 reached after cessation of emissions. Shaded regions show paleo-constraints on global surface temperature and GMSL for the Last Interglacial and mid-Pliocene Warm Period. Lightly shaded thick/thin blue bars show 17th–83rd/5th–95th percentile ''low confidence'' ranges for SSP1-2.6 and SSP5-8.5 in 2100, plotted at 2°C and 5°C. '''(c)''' Timing of exceedance of GMSL thresholds of 0.5, 1.0, 1.5 and 2.0 m, under different SSPs. Lightly shaded thick/thin bars show 17th–83rd/5th–95th percentile ''low-confidence'' ranges for SSP1-2.6 and SSP5-8.5. Links to chapters 4.3.2, 9.6.1, 9.6.2, 9.6.3, Box 9.4

GMSL will continue to rise throughout the 21st century (Box TS.4, Figure 1a). Considering only those processes in whose projections we have at least ''medium confidence'' , relative to the period 1995–2014, GMSL is projected to rise between 0.18 m (0.15–0.23 m, ''likely range'' ; SSP1-1.9) and 0.23 m (0.20–0.30 m, ''likely range'' ; SSP5-8.5) by 2050. By 2100, the projected rise is between 0.38 m (0.28–0.55 m, ''likely range'' ; SSP1-1.9) and 0.77 m (0.63–1.01 m, ''likely range'' ; SSP5-8.5) Links to chapters Table 9.9 . The methods, models and scenarios used for sea level projections in the AR6 are updated from those employed by SROCC, with contributions informed by the latest model projections described in the ocean and cryosphere Sections (Sections TS.2.4 and TS.2.5). Despite these differences, the sea level projections are broadly consistent with those of SROCC. Links to chapters 4.3.2, 9.6.3

Importantly, ''likely'' range projections do not include those ice-sheet-related processes whose quantification is highly uncertain or that are characterized by deep uncertainty. Higher amounts of GMSL rise before 2100 could be caused by earlier-than-projected disintegration of marine ice shelves, the abrupt, widespread onset of marine ice sheet instability (MISI) and marine ice cliff instability (MICI) around Antarctica, and faster-than-projected changes in the surface mass balance and dynamical ice loss from Greenland (Box TS.4, Figure 1). In a low-likelihood, high-impact storyline and a high CO <sub>2</sub> emissions scenario, such processes could in combination contribute more than one additional meter of sea level rise by 2100 (Box TS.3). Links to chapters 4.3.2, 9.6.3, Box 9.4

Beyond 2100, GMSL will continue to rise for centuries to millennia due to continuing deep ocean heat uptake and mass loss from ice sheets, and will remain elevated for thousands of years (''high confidence''). By 2150, considering only those processes in whose projections we have at least ''medium confidence'' and assuming no acceleration in ice-mass flux after 2100, GMSL is projected to rise between 0.6 m (0.4–0.9 m, ''likely'' range, SSP1-1.9) and 1.3 m (1.0–1.9 m, ''likely'' range) (SSP5-8.5), relative to the period 1995–2014 based on the SSP scenario extensions. Under high CO <sub>2</sub> emissions, processes in which there is ''low confidence'' , such as MICI, could drive GMSL rise up to about 5 m by 2150 (Box TS.4, Figure 1a). By 2300, GMSL will rise 0.3–3.1 m under low CO <sub>2</sub> emissions (SSP1-2.6) (''low confidence''). Under high CO <sub>2</sub> emissions (SSP5-8.5), projected GMSL rise is between 1.7 and 6.8 m by 2300 in the absence of MICI and by up to 16 m considering MICI (''low confidence''). Over 2000 years, there is ''medium agreemen'' t and ''limited evidence'' that committed GMSL rise is projected to be about 2–3 m with 1.5°C peak warming, 2–6 m with 2°C of peak warming, 4–10 m with 3°C of peak warming, 12–16 m with 4°C of peak warming, and 19–22 m with 5°C of peak warming. Links to chapters 9.6.3

Looking at uncertainty in time provides an alternative perspective on uncertainty in future sea level rise (Box TS.4, Figure 1c). For example, 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. Given the long-term commitment, uncertainty in the timing of reaching different levels of GMSL rise is an important consideration for adaptation planning. Links to chapters 9.6.3

At regional scales, additional processes come into play that modify the local sea level change relative to GMSL, including vertical land motion, ocean circulation and density changes, and gravitational, rotational, and deformational effects arising from the redistribution of water and ice mass between land and the ocean. These processes give rise to a spatial pattern that tends to increase sea level rise at the low latitudes and reduce sea level rise at high latitudes. However, over the 21st century, the majority of coastal locations have a median projected regional sea level rise within ±20% of the projected GMSL change (''medium confidence''). Further details on regional sea level change and extremes are provided in Section TS.4. Links to chapters 9.6.3
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