Editing IPCC:AR6/WGI/Chapter-3 (section)

=== 3.5.3 Sea Level ===

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In keeping with the scope of this chapter, this section addresses global and basin-scale sea level changes, whereas regional and local sea level changes are assessed in Section 9.6. In AR5, the observed sea level budget was closed by considering all contributing factors including ocean warming, mass contributions from terrestrial storage, glaciers, and the Antarctic and Greenland ice sheets ( [[#Church--2013b|Church et al., 2013b]] ). The SROCC found that the observed global mean sea level (GMSL) rise is consistent within uncertainties with the sum of the estimated observed contributions for 1993–2015 and 2006–2015.

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==== 3.5.3.1 Sea Level Evaluation ====

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The current generation of climate models do not fully resolve many of the components required to close the observed sea level budget, such as glaciers, ice sheets and land water storage (see Section 9.6 and Box 9.1). Consequently, most CMIP-based analyses of sea level change have focused on thermosteric sea level changes (i.e., thermal expansion due to warming) and ocean dynamic sea level change, both of which are simulated in the CMIP5-generation of models. The improved agreement between modelled thermal expansion and observed estimates during the historical period led the SROCC to assess a ''high confidence'' level in the simulated thermal expansion using climate models and ''high confidence'' in their ability to project future thermal expansion.

Since CMIP5 models do not include all necessary components of sea level change, this gap has been bridged by using offline models (for glacier melt and ice-sheet surface mass balance) driven by reanalyses and model output. Some studies have used offline mass inputs to account for dynamic ice-sheet and terrestrial contributions. [[#Slangen--2017|Slangen et al. (2017)]] and [[#Meyssignac--2017|Meyssignac et al. (2017)]] suggested including corrections to several contributions to sea level changes including to the Greenland surface mass balance and glacier contributions, based on differences between CMIP5-driven model results and reanalysis-driven results. This helps close the gap between models and observations for the 20th century globally, as well as providing better agreement with tide gauge observations in terms of interannual and multi-decadal variability at the regional scale.

In CMIP6, ice sheets (see Sections 3.4.3.2 and 9.4) are included for the first time in ISMIP6 ( [[#Nowicki--2016|Nowicki et al., 2016]] ). There is also scope for new insights into terrestrial water contributions from land surface (and sub-surface) modelling in the Land Surface, Snow and Soil moisture Model Intercomparison Project (LS3MIP; [[#van%20den%20Hurk--2016|van den Hurk et al., 2016]] ). In parallel, the GlacierMIP project ( [[#Hock--2019a|Hock et al., 2019a]] ; [[#Marzeion--2020|Marzeion et al., 2020]] ; see Sections 3.4.3.1 and 9.5) is also underway, and has provided more quantitative guidance and a comprehensive assessment of the uncertainties and best estimates of the current and future contributions of glaciers to the sea level budget.

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==== 3.5.3.2 Sea Level Change Attribution ====

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The SROCC concluded with ''high confidence'' that the dominant cause of GMSL rise since 1970 is anthropogenic forcing. Prior to that, AR5 had concluded that it is ''very likely'' that there has been a substantial contribution from anthropogenic forcings to GMSL rise since the 1970s. Since AR5, several studies have identified a human contribution to observed sea level change resulting from a warming climate as manifest in thermosteric sea level change and the contribution from melting glaciers and ice sheets.

For the global mean thermosteric sea level change, [[#Slangen--2014|Slangen et al. (2014)]] showed the importance of anthropogenic forcings (combined greenhouse gas and aerosol forcings) for explaining the magnitude of the observed changes between 1957 and 2005, considering the full depth of the ocean and natural forcings in order to capture the variability (see also Figure 3.29). Over the 1950–2005 period, [[#Marcos--2014|Marcos and Amores (2014)]] found that human influence explains 87% of the 0–700 m global thermosteric sea level rise. Both thermosteric and regional dynamic patterns of sea level change in individual forcing experiments from CMIP5 were considered by [[#Slangen--2015|Slangen et al. (2015)]] who showed that responses to anthropogenic forcings are significantly different from both internal variability and inter-model differences and that although greenhouse gas and anthropogenic aerosol forcings produce opposite GMSL responses, there are differences in the response on regional scales. Based on these studies, we conclude that it is ''very likely'' that anthropogenic forcing was the main driver of the observed global mean thermosteric sea level change since 1970.

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[[File:35bad91d2d74122df9a5fba37539908f IPCC_AR6_WGI_Figure_3_29.png]]

'''Figure 3.29 |''' '''Simulated and observed global mean sea level change due to thermal expansion for CMIP6 models and observations relative to the baseline period 1850–1900.''' Historical simulations are shown in brown, natural only in green, greenhouse gas only in grey, and aerosol only in blue (multi-model means shown as thick lines, and shaded ranges between the 5th and 95th percentile). The best estimate observations (black solid line) for the period of 1971–2018, along with ''very likely'' ranges (black shading) are from [[IPCC:Wg1:Chapter:Chapter-2#2.3.3.1|Section 2.3.3.1]] and are shifted to match the multi-model mean of the historical simulations for the 1995–2014 period. Further details on data sources and processing are available in the chapter data table (Table 3.SM.1).

In an attribution study of the sea-level contributions of glaciers, [[#Marzeion--2014|Marzeion et al. (2014)]] found that between 1991 and 2010, the anthropogenic fraction of global glacier mass loss was 69 ± 24% (see also [[#3.4.3.1|Section 3.4.3.1]] ). [[#Slangen--2016|Slangen et al. (2016)]] considered all quantifiable components of the GMSL budget and showed that anthropogenically forced changes account for 69 ± 31% of the observed sea level rise over the period 1970 to 2005, whereas natural forcings combined with internal variability have a much smaller effect – only contributing 9 ± 18% of the change over the same period. These studies indicate that about 70% of the combined change in glaciers, ice-sheet surface mass balance and thermal expansion since 1970 can be attributed to anthropogenic forcing, and that this percentage has increased over the course of the 20th century. Detection studies on GMSL change in the 20th century ( [[#Becker--2014|Becker et al., 2014]] ; [[#Dangendorf--2015|Dangendorf et al., (2015)]] found that observed total GMSL change in the 20th century was inconsistent with internal variability. [[#Dangendorf--2015|Dangendorf et al. (2015)]] determined that for 1900 to 2011 at least 45% of GMSL change is human-induced. A study that developed a semi-empirical model to link sea-level change to observed GMST change concluded that at least 41% of the 20th century sea-level rise would not have happened in the absence of the century’s increasing GMST and that there was a 95% probability that by 1970 GMSL was higher than that which would have occurred in the absence of increasing GMST ( [[#Kopp--2016|Kopp et al., 2016]] ). [[#Richter--2020|Richter et al. (2020)]] compared modelled sea level change with the satellite altimeter observations from 1993 to 2015; a period short enough that internal variability can dominate the spatial pattern of change. They found that when GMSL is not removed, model simulated zonally averaged sea level trends are consistent with altimeter observations globally as well as in each ocean basin and much larger than might be expected from internal variability. Using spatial correlation, [[#Fasullo--2018|Fasullo and Nerem (2018)]] showed that the satellite altimeter trend pattern is already detectable.

We note that current detection and attribution studies do not yet include all processes that are important for sea-level change (see Section 9.6). However, based on the body of literature available, we conclude that the main driver of the observed GMSL rise since at least 1971 is ''very likely'' anthropogenic forcing. The assessed period starts in 1971 for consistency with observations assessed in Cross-Chapter Box 9.1.

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