Editing IPCC:AR6/WGI/Chapter-2 (section)

==== 2.3.3.3 Sea Level ====

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The AR5 concluded based on proxy and instrumental data that the rate of global mean sea level (GMSL) rise since the mid-19th century was larger than the mean rate during the previous two millennia ( ''high confidence'' ). The SROCC reported with ''high confidence'' that GMSL increases were 1.5 [1.1 to 1.9] mm yr <sup>–1</sup> for 1902–2010 (with an acceleration rate between –0.002 and +0.019 mm yr <sup>–2</sup> ), 2.1 [1.8 to 2.3] mm yr <sup>–1</sup> for 1970–2015, 3.2 [2.8 to 3.5] mm yr <sup>–1</sup> for 1993–2015 and 3.6 [3.1 to 4.1] mm yr <sup>–1</sup> for 2006–2015. AR5 reported that GMSL during the LIG was, over several thousand years, between 5 and 10 m higher than 1985–2004 ( ''medium confidence'' ) whereas SROCC concluded it was ''virtually certain'' that GMSL exceeded current levels ( ''high confidence'' ), and reached a peak that was ''likely'' 6–9 m higher than today, but did not exceed 10 m ( ''medium confidence'' ). The AR5 concluded with ''high confidence'' that there were two intra-LIG GMSL peaks and that the millennial-scale rate during these periods exceeded 2 mm yr <sup>–1</sup> . The AR5 had ''high confidence'' that GMSL during the MPWP did not exceed 20 m above present. Based on new understanding, SROCC placed the upper bound at 25 m but with ''low confidence'' .

The Earth was largely ice free during the EECO ( [[#Cramer--2011|Cramer et al., 2011]] ; [[#Miller--2020|Miller et al., 2020]] , Section 9.6.2), and complete loss of current land ice reservoirs would raise GMSL by 65.6 ± 1.8 m ( [[#Morlighem--2017|Morlighem et al., 2017]] , 2020; [[#Farinotti--2019|Farinotti et al., 2019]] ). Given that GMSL change must be due to some combination of transient land ice growth and changes in terrestrial water storage, additional global mean thermosteric sea-level increase of 7 ± 2 m ( [[#Fischer--2018|Fischer et al., 2018]] ) implies a peak EECO GMSL of 70–76 m ( ''low confidence'' ). Changes in ocean basin size driven by plate tectonics contributed a comparable amount to global mean geocentric sea level in the Eocene, but are definitionally excluded from GMSL assessment ( [[#Wright--2020|Wright et al., 2020]] ).

For the MPWP, several studies of coastal features have provided additional quantitative sea-level estimates of: 5.6–19.2 m from Spain ( [[#Dumitru--2019|Dumitru et al., 2019]] ), approximately 14 m from South Africa ( [[#Hearty--2020|Hearty et al., 2020]] ), 15 m from the United States ( [[#Moucha--2017|Moucha and Ruetenik, 2017]] ), and 25 m from New Zealand ( [[#Grant--2019|Grant et al., 2019]] ). Thus, consistent with SROCC, GMSL during the MPWP was higher than present by 5–25 m ( ''medium confidence'' ).

Reconstructions of GMSL from marine oxygen isotopes in foraminifera shells show variations of more than 100 m over intervals of 10–100 kyr during glacial-interglacial cycles of the Quaternary ( [[#Shackleton--1987|Shackleton, 1987]] ; [[#McManus--1999|McManus et al., 1999]] ; [[#Waelbroeck--2002|Waelbroeck et al., 2002]] ; [[#Miller--2020|Miller et al., 2020]] ). Correction for past temperatures and a calibration for ice-volume changes implies uncertainty estimates of ± 10–13 m (1 SD) ( [[#Grant--2014|Grant et al., 2014]] ; [[#Shakun--2015|Shakun et al., 2015]] ; [[#Spratt--2016|Spratt and Lisiecki, 2016]] ). A recent marine oxygen-isotope-based GMSL reconstruction ( [[#Spratt--2016|Spratt and Lisiecki, 2016]] ) agrees with previous reconstructions, while focusing on the past 800 kyr (Figure 2.28). It shows that GMSL during the Holocene was among the highest over this entire interval, and was surpassed only during the LIG (Marine Isotope Stage (MIS 5e)) and MIS 11 ( ''medium confidence'' ); however, relatively brief (about 2 kyr) highstands during other interglacial periods might be obscured by dating limitations.

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[[File:afaeaca739981232e0771a71f50accc9 IPCC_AR6_WGI_Figure_2_28.png]]

'''Figure 2.2''' '''8 |''' '''Changes in global mean sea level. (a)''' Reconstruction of sea-level from ice core oxygen isotope analysis for the last 800 kyr. For target paleo periods (CCB2.1) and MIS11 the estimates based upon a broader range of sources are given as box whiskers. Note the much broader axis range (200 m) than for later panels (tenths of metres). '''(b)''' Reconstructions for the last 2500 years based upon a range of proxy sources with direct instrumental records superposed since the late 19th century. '''(c)''' Tide-gauge and, more latterly, altimeter-based estimates since 1850. The consensus estimate used in various calculations in Chapters 7 and 9 is shown in black. '''(d)''' The most recent period of record from tide-gauge and altimeter-based records. Further details on data sources and processing are available in the chapter data table (Table 2.SM.1).

Few sites globally have well-preserved MIS 11 sea-level indicators ( [[#Dutton--2015|Dutton et al., 2015]] ). As reported in AR5, [[#Raymo--2012|Raymo and Mitrovica (2012)]] used glacial isostatic adjustment models to correct the elevation of MIS 11 sea-level proxies from Bermuda and Bahamas to estimate a peak MIS 11 GMSL between 6 and 13 m above present-day. This agrees with the elevation of 13 ± 2 m for the MIS 11 subtidal–intertidal transition in South Africa ( [[#Roberts--2012|Roberts et al., 2012]] ). A revised glacial isostatic adjustment at this location resulted in a peak GMSL estimate of 8–11.5 m ( [[#Chen--2014|Chen et al., 2014]] ). In light of these data, and the review by [[#Dutton--2015|Dutton et al. (2015)]] , the AR5 estimate of 6–13 m for MIS 11 remains the best available ( ''medium confidence'' ).

Recent studies have highlighted uncertainties in estimates of GMSL during the LIG, including the extent of GMSL variability ( [[#Capron--2019|Capron et al., 2019]] ). Vertical land motions ( [[#Austermann--2017|Austermann et al., 2017]] ) are starting to be considered quantitatively (e.g., [[#Stephenson--2019|Stephenson et al., 2019]] ), but are still bounded by large uncertainties. The distribution and thickness of pre-LIG ice sheets ( [[#Dendy--2017|Dendy et al., 2017]] ; [[#Rohling--2017|Rohling et al., 2017]] ) and isostasy driven by sediment loading since the LIG ( [[#Pico--2020|Pico, 2020]] ) add further uncertainty. In light of these recent studies and previous assessments, there is ''medium confidence'' that peak GMSL during the LIG was ''likely'' between 5 and 10 m higher than modern. Relative sea-level estimates from some sites (e.g., Bahamas and Seychelles) report ephemeral, metre-scale fluctuations ( [[#Vyverberg--2018|Vyverberg et al., 2018]] ). Different generations of LIG reef growth at other sites (e.g., Yucatan Peninsula, Western Australia) suggest the occurrence of sudden accelerations in GMSL change ( [[#Blanchon--2009|Blanchon et al., 2009]] ; [[#O’Leary--2013|O’Leary et al., 2013]] ). However, other sites (e.g., South Australia, Mediterranean), indicate that LIG sea level was substantially stable (T.-Y. [[#Pan--2018|]] [[#Pan--2018|Pan et al., 2018]] ; [[#Polyak--2018|Polyak et al., 2018]] ). In addition, there are uncertainties in the interpretation of local relative sea level from some GMSL reconstructions ( [[#Barlow--2018|Barlow et al., 2018]] ). Therefore, ''low confidence'' is assigned to any GMSL rate of change estimated within the LIG.

New geological proxies and glacial isostatic adjustment (GIA) modelling studies confirm that, at the LGM, GMSL was 125–134 m below present ( [[#Lambeck--2014|Lambeck et al., 2014]] ; [[#Yokoyama--2018|Yokoyama et al., 2018]] ). During the LDT, GMSL rose from approximately –120 m to –50 m, implying an average rate of about 10 mm yr <sup>–1</sup> ( [[#Lambeck--2014|Lambeck et al., 2014]] ). The fastest rise occurred during Meltwater Pulse 1A, at about 14.6–14.3 ka ( [[#Deschamps--2012|Deschamps et al., 2012]] ; [[#Sanborn--2017|Sanborn et al., 2017]] ), when GMSL rose by between 8 m and 15 m ( ''medium confidence'' ) (J. [[#Liu--2016|]] [[#Liu--2016|Liu et al., 2016]] ) at an average rate of 24–44 mm yr <sup>–1</sup> .

Recent GIA modelling studies tuned to both near- and far-field relative sea level (RSL) data yield MH GMSL estimates of –3.8 to –1.0 m ( [[#Lambeck--2014|Lambeck et al., 2014]] ; [[#Peltier--2015|Peltier et al., 2015]] ; [[#Bradley--2016|Bradley et al., 2016]] ; [[#Roy--2017|Roy and Peltier, 2017]] ). Estimates from relatively stable locations where the effects of GIA are small and relatively insensitive to parameters defining Earth rheology, and where RSL is expected to approximate GMSL to within about 1 m (e.g., [[#Milne--2008|Milne and Mitrovica, 2008]] ), suggest that RSL was between about –6 to +1.5 m at around 6 ka at multiple locations ( [[#Camoin--1997|Camoin et al., 1997]] ; [[#Braithwaite--2000|Braithwaite et al., 2000]] ; [[#Frank--2006|Frank et al., 2006]] ; [[#Montaggioni--2008|Montaggioni and Faure, 2008]] ; [[#Vacchi--2016|Vacchi et al., 2016]] ; [[#Khan--2017|Khan et al., 2017]] ; [[#Hibbert--2018|Hibbert et al., 2018]] ). The assessment of GMSL change at 6 kyr is challenging considering the proportionately large GIA effect ( [[#Kopp--2016|Kopp et al., 2016]] ), insufficient resolution of marine geochemical proxies ( δ <sup>18</sup> O, Mg/Ca) and uncertainties in the contribution of the Antarctic Ice Sheet during the MH ( [[#2.3.2.4|Section 2.3.2.4]] ). The possibility that GMSL was at least somewhat higher than present cannot be excluded.

For the last 3 kyr, GMSL has been estimated from global databases of sea-level proxies, including numerous densely-sampled high-resolution salt-marsh records with decimetre scale vertical resolution and sub-centennial temporal resolution ( [[#Kopp--2016|Kopp et al., 2016]] ; [[#Kemp--2018|Kemp et al., 2018]] ). Over the last about 1.5 kyr, the most prominent century-scale GMSL trends include average maximum rates of lowering and rising of –0.7 ± 0.5 mm yr <sup>–1</sup> (2 SD) over 1020–1120 CE, and 0.3 ± 0.5 (2 SD) over 1460–1560, respectively. Between 1000 and 1750 CE, GMSL is estimated to have been within the range of about –0.11 to +0.09 m relative to 1900 ( [[#Kemp--2018|Kemp et al., 2018]] ). This was followed by a sustained increase of GMSL that began between 1820 and 1860 and has continued to the present day. New analyses demonstrate that it is ''very likely'' that GMSL rise over the 20th century was faster than over any preceding century in at least 3 kyr ( [[#Kopp--2016|Kopp et al., 2016]] ; [[#Kemp--2018|Kemp et al., 2018]] ) (Figure 2.28).

Since SROCC, two new tide gauge reconstructions of 20th century GMSL change have been published, although both rely upon CMIP models to varying degrees (Figure 2.28). [[#Frederikse--2020|Frederikse et al. (2020)]] used a ‘virtual station’ method and a probabilistic framework to estimate GMSL change and its uncertainties since 1900. [[#Dangendorf--2019|Dangendorf et al. (2019)]] combined a Kalman Smoother ( [[#Hay--2015|Hay et al., 2015]] ) with Reduced Space Optimal Interpolation ( [[#Church--2011|Church and White, 2011]] ; [[#Ray--2011|Ray and Douglas, 2011]] ) in an effort to better represent both the long-term GMSL change while preserving information on sea-level variability. In addition, new ensemble-based methods for quantifying GMSL change have been presented that account for both structural and parametric uncertainty ( [[#Palmer--2021|Palmer et al., 2021]] ). Altimeter time series of GMSL change (Figure 2.28) have been extended to 2019/2020 but bias adjustments ( [[#Watson--2015|Watson et al., 2015]] ; [[#Beckley--2017|Beckley et al., 2017]] ; [[#Dieng--2017|Dieng et al., 2017]] ; [[#Ablain--2019|Ablain et al., 2019]] ; [[#Legeais--2020|Legeais et al., 2020]] ) did not change since SROCC.

Based on the ensemble approach of [[#Palmer--2021|Palmer et al. (2021)]] and an updated [[#WCRP%20Global%20Sea%20Level%20Budget%20Group--2018|WCRP Global Sea Level Budget Group (2018)]] assessment (Figure 2.28) GMSL rose at a rate of 1.32 [0.58 to 2.06] mm yr <sup>–1</sup> for the period 1901–1971, increasing to 1.87 [0.82 to 2.92] mm yr <sup>–1</sup> between 1971 and 2006, and further increasing to 3.69 [3.21 to 4.17] mm yr <sup>–1</sup> for 2006–2018 ( ''high confidence'' ). The average rate for 1901–2018 was 1.73 [1.28 to 2.17] mm yr <sup>–1</sup> with a total rise of 0.20 [0.15 to 0.25] m (Table 9.5). The acceleration rate ''very likely'' is 0.094 [0.082 to 0.115] mm yr <sup>–2</sup> for 1993–2018 ( [[#WCRP%20Global%20Sea%20Level%20Budget%20Group--2018|WCRP Global Sea Level Budget Group, 2018]] , updated), consistent with other estimates ( [[#Watson--2015|Watson et al., 2015]] ; X. [[#Chen--2017|]] [[#Chen--2017|Chen et al., 2017]] ; [[#Nerem--2018|Nerem et al., 2018]] ; [[#WCRP%20Global%20Sea%20Level%20Budget%20Group--2018|WCRP Global Sea Level Budget Group, 2018]] ; [[#Ablain--2019|Ablain et al., 2019]] ; [[#Legeais--2020|Legeais et al., 2020]] ). For the period 1902–2010 the updated tide gauge reconstructions published since SROCC also show a robust acceleration over the 20th century and the ensemble estimate of [[#Palmer--2021|Palmer et al. (2021)]] gives a value of 0.0053 [0.0042 to 0.0073] mm yr <sup>–2</sup> , based on an unweighted quadratic fit.

In summary, GMSL is rising, and the rate of GMSL rise since the 20th century is faster than over any preceding century in at least the last three millennia ( ''high confidence'' ). Since 1901, GMSL has risen by 0.20 [0.15 to 0.25] m at an accelerating rate. Further back in time, there is ''medium confidence'' that GMSL was within –3.5 to +0.5 m ( ''very likely'' range) of present during the MH, 5–10 m higher ( ''likely range'' ) during the LIG, and 5–25 m higher ( ''very likely'' range) during the MPWP.

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