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

==== 9.6.1.2 Global Mean Sea Level Change Budget in the Satellite Era ====

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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'' ).

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'''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.

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