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

=== 9.6.1 Global and Regional Sea Level Change in the Instrumental Era ===

<div id="h2-21-siblings" class="h2-siblings"></div>

<div id="9.6.1.1" class="h3-container"></div>

<span id="global-mean-sea-level-change-budget-in-the-pre-satellite-era"></span>
==== 9.6.1.1 Global Mean Sea Level Change Budget in the Pre-satellite Era ====

<div id="h3-38-siblings" class="h3-siblings"></div>

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.

<div id="9.6.1.2" class="h3-container"></div>

<span id="global-mean-sea-level-change-budget-in-the-satellite-era"></span>
==== 9.6.1.2 Global Mean Sea Level Change Budget in the Satellite Era ====

<div id="h3-39-siblings" class="h3-siblings"></div>

The SROCC ( [[#Oppenheimer--2019|Oppenheimer et al., 2019]] ) concluded that GMSL increased at a rate of 3.16 [2.79 to 3.53, ''very likely'' range] mm yr <sup>–1</sup> in the period 1993–2015 (the satellite altimetry era), and a rate of 3.58 [3.10 to 4.06, ''very likely'' range] mm yr <sup>–1</sup> in the period 2006–2015 – the Gravity Recovery and Climate Experiment (GRACE)/Argo data era ( ''high confidence'' ). An updated assessment for the periods 1993–2018 and 2006–2018 yields values of 3.25 [2.88 to 3.61] and 3.69 [3.21 to 4.17] mm yr <sup>–1</sup> ( ''high confidence'' ) (Table 9.5), with the slightly larger central estimates consistent with the observed acceleration in GMSL rise since the late 1960s ( [[#Dangendorf--2019|Dangendorf et al., 2019]] ), given the longer assessment periods. Based on the GMSL assessed time series presented in [[IPCC:Wg1:Chapter:Chapter-2#2.3.3.3|Section 2.3.3.3]] , GMSL acceleration is estimated as 0.075 [0.066 to 0.080] mm yr <sup>–2</sup> for 1971–2018 and 0.094 [0.082–0.115] mm yr <sup>–2</sup> for 1993–2018 ( ''high confidence'' ). For the common period of 1993–2010, the assessed rate of GMSL rise based on tide gauge reconstructions (3.19 [1.18 to 5.20] mm yr <sup>–1</sup> ) is consistent with the assessment based on satellite altimetry (2.77 [2.26 to 3.28] mm yr <sup>–1</sup> ), within the estimated uncertainties.

Since SROCC, two new estimates of the LWS contribution have been published ( [[#9.6.1.1|Section 9.6.1.1]] ; [[#Cáceres--2020|Cáceres et al., 2020]] ; [[#Frederikse--2020b|Frederikse et al., 2020b]] ). For the early 21st century (the periods 1993–2018 and 2006–2018) both publications find a positive LWS contribution (Table 9.5), based on the most recent GRACE-derived estimates. This contrasts with the negative LWS contribution presented for the same periods in SROCC based on World Climate Research Programme (WCRP) Global Sea Level Budget Group (2018), and reinforces the ''low confidence'' assessment of the LWS contribution.

For both periods in the satellite era – that is, 1993–2018 and 2006–2018 – the sum of contributions is consistent with the total observed GMSL change ( ''high confidence'' ) (Table 9.5). However, the latter period, which is characterized by improved data quality and coverage associated with satellite and Argo observations, shows much closer agreement in the central estimates. The marginal sea level budget closure for the period 1993–2018 may indicate underestimated uncertainty, which may be structural as well as parametric. The sea level budget assessments across the various periods in Table 9.5 demonstrate that the acceleration in GMSL rise ( [[IPCC:Wg1:Chapter:Chapter-2#2.3.3.3|Section 2.3.3.3]] ) since the late 1960s is mostly the result of increased ice-sheet mass loss. However, all contributions to GMSL rise show their largest rate during 2006–2018, with the ice sheets accounting for 27% of the total change during this period. Because of the increased ice-sheet mass loss, the total loss of land ice (glaciers and ice sheets) was the largest contributor to GMSL rise over the period 2006–2018 ( ''high confidence'' ).

<div id="_idContainer062" class="Basic-Text-Frame"></div>

'''Table''' '''9.5 |''' '''Observed contributions to global mean sea level (GMSL) change for five different periods.''' Values are expressed as the total change (Δ) in the annual mean or year mid-point value over each period (mm) along with the equivalent rate (mm yr <sup>–1</sup> ). The ''very likely'' ranges appear in brackets based on the various section assessments as indicated. Uncertainties for the sum of contributions are added in quadrature, assuming independence. Percentages are based on central estimate contributions compared to the central estimate of the sum of contributions.

{| class="wikitable"
|-
| '''Observed contribution to GMSL change'''

|
| '''1901–1990'''

{9.6.1.1}

| '''1971–2018'''

{CCBox 9.1}

| '''1993–2''' '''018'''

{9.6.1.2}

| '''2006–2018'''

{9.6.1.2}

| '''1901–2018'''

{9.6.1.1}

|-
| rowspan="2"| Thermal expansion

( [[IPCC:Wg1:Chapter:Chapter-2#2.3.3.1|Section 2.3.3.1]] ; Table 2.7)

| Δ (mm)

| 31.6

[14.7 to 48.5]

(31.9%)

| 47.5

[34.3 to 60.7]

(50.4%)

| 32.7

[23.8 to 41.6]

(45.9%)

| 16.7

[8.9 to 24.6]

(38.6%)

| 63.2

[47.0 to 79.4]

(38.4%)

|-
| mm yr <sup>–1</sup>

| 0.36

[0.17 to 0.54]

| 1.01

[0.73 to 1.29]

| 1.31

[0.95 to 1.66]

| 1.39

[0.74 to 2.05]

| 0.54

[0.40 to 0.68]

|-
| rowspan="2"| Glaciers (excluding peripheral glaciers)

(Sections 2.3.2.3, 9.5.1.1)

| Δ (mm)

| 51.8

[30.4 to 73.2]

(52.3%)

| 20.9

[10.0 to 31.7]

(22.2%)

| 13.8

[10.0 to 17.6]

(19.4%)

| 7.5

[6.8 to 8.2]

(17.3%)

| 67.2

[41.8 to 92.6]

(40.8%)

|-
| mm yr <sup>–1</sup>

| 0.58

[0.34 to 0.82]

| 0.44

[0.21 to 0.67]

| 0.55

[0.40 to 0.70]

| 0.62

[0.57 to 0.68]

| 0.57

[0.36 to 0.79]

|-
| rowspan="2"| Greenland Ice Sheet (including peripheral glaciers)

(Sections 2.3.2.4.1, 9.4.1.1)

| Δ (mm)

| 29.0

[16.3 to 41.7]

(29.3%)

| 11.9

[7.7 to 16.1]

(12.6%)

| 10.8

[8.9 to 12.7]

(15.2%)

| 7.5

[6.2 to 8.9]

(17.3%)

| 40.4

[27.2 to 53.5]

(24.5%)

|-
| mm yr <sup>–1</sup>

| 0.33

[0.18 to 0.47]

| 0.25

[0.16 to 0.34]

| 0.43

[0.36 to 0.51]

| 0.63

[0.51 to 0.74]

| 0.35

[0.23 to 0.46]

|-
| rowspan="2"| Antarctic Ice Sheet (including peripheral glaciers)

(Sections 2.3.2.4.2, 9.4.2.1)

| Δ (mm)

| 0.4

[–8.8 to 9.6]

(0.4%)

| 6.7

[–4.0 to 17.3]

(7.1%)

| 6.1

[4.0 to 8.3]

(8.6%)

| 4.4

[2.9 to 6.0]

(10.2%)

| 6.7

[–4.0 to 17.4]

(4.1%)

|-
| mm yr <sup>–1</sup>

| 0.00

[–0.10 to 0.11]

| 0.14

[–0.09 to 0.37]

| 0.25

[0.16 to 0.33]

| 0.37

[0.24 to 0.50]

| 0.06

[–0.03 to 0.15]

|-
| rowspan="2"| Land-water storage <sup>a</sup>

( [[#9.6.1.1|Section 9.6.1.1]] )

| Δ (mm)

| –13.8

[–31.4 to 3.8]

(-13.9%)

| 7.3

[–2.4 to 16.9]

(7.7%)

| 7.8

[3.3 to 12.2]

(10.9%)

| 7.2

[3.8 to 10.6]

(16.6%)

| –12.9

[–45.8 to 20.0]

(–7.8%)

|-
| mm yr <sup>–1</sup>

| –0.15

[–0.35 to 0.04]

| 0.15

[–0.05 to 0.36]

| 0.31

[0.13 to 0.49]

| 0.60

[0.32 to 0.88]

| –0.11

[–0.39 to 0.17]

|-
|
|-
| rowspan="2"| '''Sum of observed contributions'''

| Δ (mm)

| '''99.0'''

[63.0 to 135.1]

| '''94.2'''

[71.5 to 117.0]

| '''71.2'''

[60.2 to 82.3]

| '''43.4'''

[34.5 to 52.2]

| '''164.6'''

[116.9 to 212.4]

|-
| mm yr <sup>–1</sup>

| '''1.11'''

[0.71 to 1.52]

| '''2.00'''

[1.52 to 2.49]

| '''2.85'''

[2.41 to 3.29]

| '''3.61'''

[2.88 to 4.35]

| '''1.41'''

[1.00 to 1.82]

|-
| rowspan="2"| '''Observed GMSL change'''

( [[IPCC:Wg1:Chapter:Chapter-2#2.3.3.3|Section 2.3.3.3]] )

| Δ (mm)

| '''120.1''' <sup>T</sup>

[69.3 to 170.8]

| '''109.6''' <sup>T&amp;A</sup>

[72.8 to 146.4]

| '''81.2''' <sup>A</sup>

[72.1 to 90.2]

| '''44.3''' <sup>A</sup>

[38.6 to 50.0]

| '''201.9''' <sup>T&amp;A</sup>

[150.3 to 253.5]

|-
| mm yr <sup>–1</sup>

| '''1.35''' <sup>T</sup>

[0.78 to 1.92]

| '''2.33''' <sup>T&amp;A</sup>

[1.55 to 3.12]

| '''3.25''' <sup>A</sup>

[2.88 to 3.61]

| '''3.69''' <sup>A</sup>

[3.21 to 4.17]

| '''1.73''' <sup>T&amp;A</sup>

[1.28 to 2.17]

|}

<sup>T, A</sup> and <sup>T&amp;A</sup> indicate assessments based on tide gauge reconstructions (T), satellite altimetry (A), or a combination of both (T&amp;A). The assessment uses tide gauge reconstructions before 1993 and satellite altimetry after 1993.

<sup>a</sup> For the periods 1971–2018, 1993–2018, 2006–2018 and 1901–2018 the [[#Cáceres--2020|Cáceres et al. (2020)]] linear trends are based on the period up to 2016.

<div id="9.6.1.3" class="h3-container"></div>

<span id="regional-sea-level-change-in-the-satellite-era"></span>
==== 9.6.1.3 Regional Sea Level Change in the Satellite Era ====

<div id="h3-40-siblings" class="h3-siblings"></div>

Regional sea level changes are resolved by both tide gauge and satellite altimetry observations ( [[#Hamlington--2020a|Hamlington et al., 2020a]] ). Altimeters have the advantage of quasi-global coverage but are limited to a period (1993–present) in which the forced trend response is just emerging on regional scales ( [[#9.6.1.4|Section 9.6.1.4]] ). An analysis of the local altimetry error budget to estimate 90% confidence intervals on regional sea level trends and accelerations reports that 98% of the ocean surface has experienced significant sea level rise over the satellite era ( [[#Prandi--2021|Prandi et al., 2021]] ). The same study finds that sea level accelerations display a less uniform pattern, with an east–west dipole in the Pacific, a north–south dipole in the Southern Ocean and in the North Atlantic, and 85% of the ocean surface experiencing significant sea level acceleration or deceleration, above instrumental and post-processing noise. Longer records are available from tide gauges, albeit with variable coverage by basin. Regional departures from GMSL rise are primarily driven by ocean transport divergences that result from wind stress anomalies and spatial variability in atmospheric heat and freshwater fluxes ( [[#9.2.4|Section 9.2.4]] ).

The SROCC ( [[#Oppenheimer--2019|Oppenheimer et al., 2019]] ) noted the occurrence of large multiannual sea level variations in the Pacific, associated with the Pacific Decadal Oscillation (PDO) in particular, and involving the El Niño Southern Oscillation (ENSO), North Pacific Gyre Oscillation (NPGO) and Indian Ocean Dipole (IOD; Annex IV; [[#Royston--2018|Royston et al., 2018]] ; [[#Hamlington--2020b|Hamlington et al., 2020b]] ). There was intensified sea level rise during the 1990s and 2000s, with 10-year trends exceeding 20 mm yr <sup>–1</sup> in the western tropical Pacific Ocean, while sea level trends were negative on the North American west coast. During the 2010s, the situation reversed, with western Pacific sea level falling at more than 10 mm yr <sup>–1</sup> ( [[#Hamlington--2020b|Hamlington et al., 2020b]] ). For the Atlantic Ocean, SROCC described regional sea level variability as being driven primarily by wind and heat flux variations associated with the North Atlantic Oscillation (NAO) and heat transport changes associated with Atlantic Meridional Overturning Circulation (AMOC) variability ''.'' During periods of subpolar North Atlantic warming, winds along the European coast are predominantly from the south and may communicate steric anomalies onto the continental shelf, driving regional sea level rise, with the reverse during periods of cooling ( [[#Chafik--2019|Chafik et al., 2019]] ). High rates of sea level rise in the North Indian Ocean are accompanied by a weakening summer South Asian monsoon circulation ( [[#Swapna--2017|Swapna et al., 2017]] ).

The Arctic ocean is typically excluded from global sea level studies, owing to the uncertainties associated with resolving sea level in ice-covered regions, strong variations in gravitational, rotational, and deformational (GRD) effects, and uncertain glacial isostatic adjustment (GIA) estimates (Box 9.1). Spanning 1991–2018, a ''very likely'' sea level rise of 1.16–1.81 mm yr <sup>–1</sup> is observed ( [[#Rose--2019|Rose et al., 2019]] ). Since SROCC, the forced response in regional sea level varies in time with the relative influence of different forcing agents ( [[#Fasullo--2020|Fasullo et al., 2020]] ).

The SROCC estimated regional sea level changes from combinations of the various contributions to sea level change from CMIP5 climate model outputs, allowing comparison with satellite altimeter and tide gauge observations. Closure of the regional sea level budget is complicated by the fact that regional sea level variability is larger than GMSL variability. Also, there are more processes that need to be considered, such as vertical land movement and ocean dynamical changes (Box 9.1). A number of observation-based studies have focused on specific areas, such as the Mediterranean ( [[#García--2006|García et al., 2006]] ), the South China Sea ( [[#Feng--2012|Feng et al., 2012]] ), the east coast of the USA ( [[#Frederikse--2017|Frederikse et al., 2017]] ; [[#Piecuch--2018|Piecuch et al., 2018]] ), the North Atlantic basin ( [[#Kleinherenbrink--2016|Kleinherenbrink et al., 2016]] ) and the north-western European continental shelf seas ( [[#Frederikse--2016|Frederikse et al., 2016]] ). Studies using tide gauge data and observation-based estimates of the contributions find that, while local agreement is not yet possible, the observational sea level budget can be closed on a basin scale ( [[#Slangen--2014b|Slangen et al., 2014b]] ; [[#Frederikse--2016|Frederikse et al., 2016]] , 2018, 2020b). A budget analysis for the GRACE era found that the budget closes in some, but not all, coastal regions: substantial parts of the sea level change signal in the North Atlantic could not be explained by steric or barystatic changes ( [[#Rietbroek--2016|Rietbroek et al., 2016]] ). This is in agreement with other work comparing climate model estimates to 20th-century tide gauge observations ( [[#Meyssignac--2017|Meyssignac et al., 2017]] ), where the majority of local spatial variability is determined by the ocean dynamic component. Vertical land movement is another major cause of local spatial variability in sea level change and, for instance, relevant for oceanic islands ( [[#Forbes--2013|Forbes et al., 2013]] ; [[#Martínez-Asensio--2019|Martínez-Asensio et al., 2019]] ). In summary, the regional sea level budget, using either observations or models, can currently only be closed on basin scales ( ''medium confidence'' ), with large uncertainties remaining on smaller scales ''.''

<div id="9.6.1.4" class="h3-container"></div>

<span id="attribution-and-time-of-emergence-of-regional-sea-level-change"></span>

==== 9.6.1.4 Attribution and Time of Emergence of Regional Sea Level Change ====

<div id="h3-41-siblings" class="h3-siblings"></div>

The SROCC ( [[#Oppenheimer--2019|Oppenheimer et al., 2019]] ) attributed anthropogenic forcing to be the dominant cause of GMSL rise since 1970 (see also [[IPCC:Wg1:Chapter:Chapter-3#3.5.3.2|Section 3.5.3.2]] ), but detection and attribution (Cross-Working Group Box: Attribution in Chapter 1) of 20th century externally forced regional sea level changes is more challenging, as regional variability is larger ( [[#9.6.1.3|Section 9.6.1.3]] ), and therefore the signal-to-noise ratio is smaller ( [[#Richter--2014|Richter and Marzeion, 2014]] ; [[#Monselesan--2015|Monselesan et al., 2015]] ; [[#Palanisamy--2015|Palanisamy et al., 2015]] ). Whereas SROCC assessed with ''high confidence'' that GMSL rise is attributable to anthropogenic greenhouse gas emissions, they assessed with ''medium confidence'' that the regional anomalies in ocean basins are a combination of the response to anthropogenic greenhouse gas emissions and internal variability.

The simulated ocean dynamic and thermosteric response to external forcings during 1861–2005 is only larger than simulated internal variability in the Southern Ocean and North Pacific on a 1° grid ( [[#Slangen--2015|Slangen et al., 2015]] ). However, on spatial scales exceeding 2000 km, a detectable signal is revealed in the last 45 years in 63% of the global ocean area ( [[#Richter--2017|Richter et al., 2017]] ). The thermosteric change in the upper 700 m in the period 1970–2005 shows similar observed and simulated forced geographical patterns, and anthropogenic forcing accounts for part (North Atlantic, 65%) or all (tropical Pacific, Southern Ocean) of the observed regional mean ( [[#Marcos--2014|Marcos and Amores, 2014]] ). The influences of greenhouse gases and anthropogenic aerosols can be partially distinguished by considering geographical or vertical ocean temperature variations ( [[#Slangen--2015|Slangen et al., 2015]] ; [[#Bilbao--2019|Bilbao et al., 2019]] ; [[#Fasullo--2020|Fasullo et al., 2020]] ). Zonal-mean forced ocean dynamic sea level change alone is not detectable but, using spatial correlation, the global geographical pattern during the altimeter period is detectable in sea level trends (Fasullo and Nerem, 2018). This patternmay already or will soon be detectable in individual years, based on an analysis of CMIP5 climate model simulations ( [[#Bilbao--2015|Bilbao et al., 2015]] ). Anthropogenic forcing, dominated by greenhouse gases, has strengthened the meridional sea level gradient in the Southern Ocean since the 1960s ( [[#Slangen--2015|Slangen et al., 2015]] ; [[#Bilbao--2019|Bilbao et al., 2019]] ; [[#Fasullo--2020|Fasullo et al., 2020]] ). New evidence finds that observed zonal-mean total sea level trends during 1993–2018 in all basins are inconsistent with unforced variability alone, but are consistent with the modelled response to external forcing ( [[#Richter--2020|Richter et al., 2020]] ).

A region that has been studied intensely in the context of sea level detection and attribution is the tropical Pacific. Observed sea level trends in the tropical Pacific show a PDO-like (Annex IV) east–west dipole (with a greater rate of rise in the west, see [[#9.6.1.3|Section 9.6.1.3]] ). This dipole does not occur in CMIP5 simulations with the magnitude and duration that was observed in the 1990s and 2000s, neither in response to historical forcing, nor as internal variability after removing the variability associated with the PDO ( [[#Bilbao--2015|Bilbao et al., 2015]] ). [[#Hamlington--2014|Hamlington et al. (2014)]] did obtain a residual trend pattern for 1993–2010 in the tropical Pacific that may link to anthropogenic warming of the tropical Indian Ocean. Allowing for PDO and ENSO variations, ( [[#Royston--2018|Royston et al., 2018]] ) describe patches of the Pacific Ocean where the sea level trend for 1993–2015 is distinguishable from temporally correlated noise. The acceleration in eastern Pacific sea level rise is largely accounted for by variations resembling PDO and ENSO ( [[#Hamlington--2020a|Hamlington et al., 2020a]] ).

In the future, the anthropogenic signal in regional sea level change from ocean density and dynamics is projected to emerge first in regions with relatively small internal variability, such as the tropical Atlantic Ocean and the tropical Indian Ocean ( [[#Jordà--2014|Jordà, 2014]] ; Lyuet al., 2014; [[#Richter--2014|Richter and Marzeion, 2014]] ; [[#Bilbao--2015|Bilbao et al., 2015]] ). The signal is projected to emerge over 50% of the ocean area by the 2040s ( [[#Lyu--2014|Lyu et al., 2014]] ), but in regions where variability is large and projected changes are small, such as the Southern Ocean, the signal will not emerge before late in the century. Adding the projected sea level change from land ice mass loss and groundwater extraction strengthens and modifies the forced signal, leading to times of emergence 10 to 20 years earlier in most parts of the ocean, except in regions close to sources of mass loss, with emergence over 50% of the ocean area by 2020, and nearly everywhere by 2100 ( ''medium confidence'' ) ( [[#Lyu--2014|Lyu et al., 2014]] ; [[#Richter--2017|Richter et al., 2017]] ).

In summary, detection of forced regional changes for some ocean areas in recent decades is possible ( ''medium confidence'' ), but attribution of regional sea level change to forcings over longer periods (20th century) and for all ocean basins is not yet possible.

<div id="cross-chapter-box-9.1" class="h2-container box-container"></div>

'''Cross-Chapter Box 9.1 | Global Energy Inventory and Sea Level Budget'''

<div id="h2-20-siblings" class="h2-siblings"></div>

'''Coordinators:''' Matthew D. Palmer (United Kingdom), Aimée B.A. Slangen (The Netherlands)

'''Contributors:''' Guðfinna Aðalgeirsdóttir (Iceland), Fábio Boeira Dias (Finland/Brazil), Catia M. Domingues (Australia, United Kingdom/Brazil), Gerhard Krinner (France/Germany, France), Johannes Quaas (Germany), Lucas Ruiz (Argentina)

Increased atmospheric greenhouse gas emissions since the 19th century have led to a net positive radiative forcing of Earth’s climate (Sections [[IPCC:Wg1:Chapter:Chapter-2#2.2|2.2]] and [[IPCC:Wg1:Chapter:Chapter-7#7.3|7.3]] ) and a corresponding accumulation of energy in the Earth system. Quantification of this energy gain is essential to our understanding of observed climate change, and for estimates of climate sensitivity ( [[IPCC:Wg1:Chapter:Chapter-7#7.5|Section 7.5]] ). The global energy inventory is closely linked to our understanding of observed global sea level change, through the energy associated with loss of land-based ice and the effect of thermal expansion associated with ocean warming (Box 9.1, Sections 2.3.3.1 and 9.6.1; Table 9.5).

<div id="_idContainer002"></div>

[[File:847e2fce28810a2743bd4ce868abd493 IPCC_AR6_WGI_CCBox_9_1_Figure_1.png]]

'''Cross-Chapter 9.1,'''

'''Figure 1 |'''

'''Global Energy Inventory and Sea Level Budget. (a)''' Observed changes in the global energy inventory for 1971–2018 (shaded time series) with component contributions as indicated in the figure legend. Earth System Heating for the whole period and associated uncertainty is indicated to the right of the plot (red bar = central estimate; shading = ''very likely'' range); '''(b)''' Observed changes in components of global mean sea level for 1971–2018 (shaded time series) as indicated in the figure legend. Observed global mean sea level change from tide gauge reconstructions (1971–1993) and satellite altimeter measurements (1993–2018) is shown for comparison (dashed line) as a three-year running mean to reduce sampling noise. Closure of the global sea level budget for the whole period is indicated to the right of the plot (red bar = component sum central estimate; red shading = ''very likely'' range; black bar = total sea level central estimate; grey shading = ''very likely'' range). Full details of the datasets and methods used are available in Annex I. Further details on energy and sea level components are reported in Table 7.1 and Table 9.5.

The Earth system gained substantial energy over the period 1971–2018 ( ''high confidence'' ), with an assessed ''very likely'' range of 325–546 ZJ or 0.43–0.72 W m <sup>–2</sup> expressed per unit area of the Earth’s surface (Cross-Chapter Box 9.1, Figure 1a; [[IPCC:Wg1:Chapter:Chapter-7#7.2|Section 7.2]] , Box 7.2). Ocean warming dominates the energy inventory change ( ''high confidence'' ), accounting for 91% of the observed energy increase for the period 1971–2018, with upper-ocean warming (0–700 m) accounting for 56% ( [[IPCC:Wg1:Chapter:Chapter-7#7.2|Section 7.2]] ). Much smaller amounts went into melting of ice (3%) and heating of the land (5%) and atmosphere (1%). Overall, the percentage contributions are similar to those reported in IPCC’s Fifth Assessment Report (AR5) for the period 1971–2010 ( [[#Rhein--2013|Rhein et al., 2013]] ).

The observed global mean sea level (GMSL) budget is assessed through comparison of the sum of individual components of GMSL change with independent observations of total GMSL change from tide gauge and satellite altimeter observations (Cross-Chapter Box 9.1, Figure 1b; Sections 2.3.3 and 9.6.1 and Table 9.5). The assessed sum of the observed components indicates that GMSL ''very likely'' increased by 72 mm to 117 mm over the period 1971–2018 (Table 9.5), with the largest contributions from ocean thermal expansion (50%) and melting of ice sheets and glaciers (42%). The assessed total GMSL change ( [[IPCC:Wg1:Chapter:Chapter-2#2.3.3|Section 2.3.3]] ) for the period 1971–2018 has a ''very likely'' range of 73–146 mm and, as a result, the sea level budget is closed for this period (Cross-Chapter Box 9.1, Figure 1b; [[#9.6.1|Section 9.6.1]] , Table 9.5).

The sea level budget closure demonstrates improved quantification of the processes of observed GMSL change for this period relative to previous IPCC assessments ( [[#Church--2013b|Church et al., 2013b]] ; [[#Oppenheimer--2019|Oppenheimer et al., 2019]] ). A related assessment presented in [[IPCC:Wg1:Chapter:Chapter-7|Chapter 7]] demonstrates closure of the global energy budget ( ''high confidence'' ) (Box 7.2) and strengthens the confidence in scientific understanding of both of these key aspects of climate change.

<div id="9.6.2" class="h2-container"></div>

<span id="paleo-context-of-global-and-regional-sea-level-change"></span>