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

==== 9.6.1.1 Global Mean Sea Level Change Budget in the Pre-satellite Era ====

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The SROCC ( [[#Oppenheimer--2019|Oppenheimer et al., 2019]] ) discussed the development and application of new statistical methodologies for reconstructing global mean sea level (GMSL) from tide gauge data over the 20th century (Box 9.1). Based on an ensemble of tide gauge reconstructions, SROCC assessed an average rate of GMSL rise of 1.38 [0.81 to 1.95, ''very likely'' range] mm yr <sup>–1</sup> for the period 1901–1990. Since SROCC, two new GMSL reconstructions have been published ( [[#Dangendorf--2019|Dangendorf et al., 2019]] ; [[#Frederikse--2020b|Frederikse et al., 2020b]] ) and are included in an updated ensemble estimate of GMSL change ( [[IPCC:Wg1:Chapter:Chapter-2#2.3.3.3|Section 2.3.3.3]] ; [[#Palmer--2021|Palmer et al., 2021]] ). Based on these updated data and methods, the GMSL change over the (pre-satellite) period 1901–1990 is assessed to be 0.12 [0.07 to 0.17, ''very likely'' range] m with an average rate of 1.35 [0.78 to 1.92, ''very likely'' range] mm yr <sup>–1</sup> ( ''high confidence'' ) (Table 9.5; [[IPCC:Wg1:Chapter:Chapter-2#2.3.3.3|Section 2.3.3.3]] ) in agreement with SROCC assessment. Both this assessment and SROCC have substantially larger uncertainties than the AR5 assessment, which was based on a single tide gauge reconstruction and did not account for structural uncertainty (see [[#Palmer--2021|Palmer et al., 2021]] for a discussion).

The SROCC found that four of the five available tide gauge reconstructions that extend back to at least 1902 showed a robust acceleration ( ''high confidence'' ) of GMSL rise over the 20th century, with estimates for the period 1902–2010 (–0.002 to +0.019 mm yr <sup>–2</sup> ) that were consistent with AR5. New tide gauge reconstructions published since SROCC ( [[#Dangendorf--2019|Dangendorf et al., 2019]] ; [[#Frederikse--2020b|Frederikse et al., 2020b]] ) support this assessment and suggest that increased ocean heat uptake related to changes in Southern Hemisphere winds and increased mass loss from Greenland are the primary physical mechanisms for the acceleration ( [[IPCC:Wg1:Chapter:Chapter-2#2.3.3.3|Section 2.3.3.3]] ). Therefore, the SROCC assessment on the acceleration of GMSL rise over the 20th century is maintained.

The evaluation of the sea level budget presented here, and in [[#9.6.1.2|Section 9.6.1.2]] , draws on assessments of the individual components (Sections 2.3.3.1 and 9.2.4.1 for global-mean thermosteric and Sections 9.5.1.1, 9.4.1.1 and 9.4.2.1 for ice mass loss contributions to GMSL change from glaciers and ice sheets). Following SROCC approach, the mass loss from ice sheet peripheral glaciers is included in the ice-sheet contributions to GMSL change (glacier mass loss from regions 5 and 19 of the Randolph Glacier Inventory 6.0 ( [[#RGI%20Consortium--2017|RGI Consortium, 2017]] ) are added to ice-sheet mass loss where applicable, with uncertainties added in quadrature). The total change in GMSL for each component, and their sum, is summarized in Table 9.5 (uncertainties added in quadrature). For consistency across the report, and to simplify the treatment of uncertainties, all budget calculations are based on the difference between the first and last year in each period ( [[#Palmer--2021|Palmer et al., 2021]] ), rather than a linear fit to the underlying time series as used in SROCC and AR5.

The sea level budget in SROCC included the anthropogenic contribution of land-water storage (LWS; Box 9.1) change from a single estimate ( [[#Wada--2016|Wada, 2016]] ). Since SROCC, two studies have combined estimates of natural LWS change with anthropogenic LWS changes from reservoir impoundment and groundwater depletion ( [[#Cáceres--2020|Cáceres et al., 2020]] ; [[#Frederikse--2020b|Frederikse et al., 2020b]] ). For [[#Cáceres--2020|Cáceres et al. (2020)]] , zero change is assumed for the period 1901–1948, since their LWS change estimates are not available before 1948. Given the large year-to-year changes associated with hydrological variability, the assessed changes in LWS (Table 9.5) are based on linear trends for each period, following [[#Palmer--2021|Palmer et al. (2021)]] . Structural uncertainty is estimated from the standard deviation of the trends across the two studies, and parametric uncertainty is estimated based on the Monte Carlo simulations of [[#Frederikse--2020b|Frederikse et al. (2020b)]] . These two sources of uncertainty are combined in quadrature, and the assessed central estimate is taken as the average of the ensemble mean trends. Compared to SROCC-assessed LWS trend of -0.12 mm yr <sup>–1</sup> for the period 1901–1990, the updated assessment leads to a more negative trend of –0.16 [–0.35 to 0.04] mm yr <sup>–1</sup> , although the two are consistent within the estimated uncertainties. Previous studies and SROCC have highlighted the large uncertainty in estimates of LWS change over the 20th century ( [[#Gregory--2013|Gregory et al., 2013]] ), and therefore SROCC assessment of ''low confidence'' in the estimated LWS contribution to GMSL change is maintained.

Since SROCC, a new ocean heat content reconstruction ( [[IPCC:Wg1:Chapter:Chapter-2#2.3.3.1|Section 2.3.3.1]] ; [[#Zanna--2019|Zanna et al., 2019]] ) has allowed global thermosteric sea level change to be estimated over the 20th century. As a result, the sea level budget for the 20th century can now be assessed for the first time. For the periods 1901–1990 and 1901–2018, the assessed ''very likely'' range for the sum of components is found to be consistent with the assessed ''very likely'' range of observed GMSL change ( ''medium confidence'' ), in agreement with Frederikse et al. (2020b; Table 9.5). This represents a major step forward in the understanding of observed GMSL change over the 20th century, which is dominated by glacier (52%) and Greenland Ice Sheet mass loss (29%) and the effect of ocean thermal expansion (32%), with a negative contribution from the LWS change (–14%). While the combined mass loss for Greenland and glaciers is consistent with SROCC, updates in the underlying datasets lead to differences in partitioning of the mass loss.

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