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

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

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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.

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

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'''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).

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[[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.

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