Editing IPCC:AR6/WGI/Chapter-12 (section)

==== 12.4.2.5 Coastal and Oceanic ====

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'''Relative sea level:''' Around Asia, from 1900–2018, a new tide gauge-based reconstruction finds a regional mean RSL change of 1.33 [0.80 to 1.86] mm yr <sup>–1</sup> in the Indian Ocean–Southern Pacific and 1.68 [1.27 to 2.09] mm yr <sup>–1</sup> in the North-west Pacific ( [[#Frederikse--2020|Frederikse et al., 2020]] ), compared to a GMSL change of around 1.7 mm yr <sup>–1</sup> [[IPCC:Wg1:Chapter:Chapter-2#2.3.3.3|Section 2.3.3.3]] and Table 9.5). For the period 1993–2018, the RSLR rates around Asia, based on satellite altimetry, increased to 3.65 [3.23 to 4.08] mm yr <sup>–1</sup> and 3.53 [2.64 to 4.45] mm yr <sup>–1</sup> respectively ( [[#Frederikse--2020|Frederikse et al., 2020]] ), compared to a GMSL change of 3.25 mm yr <sup>–1</sup> [[IPCC:Wg1:Chapter:Chapter-2#2.3.3.3|Section 2.3.3.3]] and Table 9.5). The rate of RSLR along the coastline of China ranges from –2.3 ± 1.9 to +5.7 ± 0.4 mm yr <sup>–1</sup> during 1980–2016; after removing the vertical land movement, the average rate of sea level rise is 2.9 ± 0.8 mm yr <sup>–1</sup> over 1980–2016 and 3.2 ± 1.1 mm yr <sup>–1</sup> since 1993 ( [[#Qu--2019|Qu et al., 2019]] ). However, the rates of land subsidence reported by [[#Minderhoud--2017|Minderhoud et al. (2017)]] are substantially higher than those reported by [[#Qu--2019|Qu et al. (2019)]] . RSL change in many coastal areas in Asia, especially in EAS, is affected by land subsidence due to sediment compaction under building mass and groundwater extraction ( ''high confidence'' ) ( [[#Erban--2014|Erban et al., 2014]] ; [[#Nicholls--2015|Nicholls, 2015]] ; [[#Minderhoud--2019|Minderhoud et al., 2019]] ; [[#Qu--2019|Qu et al., 2019]] ). During 1991–2016, the Mekong Delta in Vietnam sank on average about 18 cm as a consequence of groundwater withdrawal, and the subsidence related to groundwater extraction has gradually increased with highest sinking rates estimated to be 11 mm yr <sup>–1</sup> in 2015 ( [[#Minderhoud--2017|Minderhoud et al., 2017]] ).

Relative sea level rise is ''very likely'' to continue in the oceans around Asia. Regional mean RSLR projections for the oceans around Asia range from 0.3–0.5 m under SSP1-2.6 to 0.7–0.8 m under SSP5-8.5 for 2081–2100 relative to 1995–2014 (median values), which is within the range of projected GMSL change ( [[IPCC:Wg1:Chapter:Chapter-9#9.6.3.3|Section 9.6.3.3]] ). These RSLR projections may, however, be underestimated due to potential partial representation of land subsidence in their assessment ( [[IPCC:Wg1:Chapter:Chapter-9#9.6.3.2|Section 9.6.3.2]] ).

'''Coastal flood:''' The present-day 1-in-100-year ETWL is between 0.5–8 m around Asia, with values above 2.5 m or above common along the coasts of Central and north-eastern Asia ( [[#Vousdoukas--2018|Vousdoukas et al., 2018]] ; [[#Kirezci--2020|Kirezci et al., 2020]] ). Sea level rise and land subsidence will jointly lead to more flooding in delta areas in Asia ( ''high confidence'' ) ( [[#Takagi--2016|Takagi et al., 2016]] ; J. [[#Wang--2018|]] [[#Wang--2018|Wang et al., 2018]] ).

Extreme total water level magnitude and occurrence frequency are expected to increase throughout the region ( ''high confidence'' ) (Figure 12.4p–r and Figure 12.SM.6). Across the region, the 5–95th percentile range of the 1-in-100-year ETWL is projected to increase (relative to 1980–2014) by 7–44 cm and by 10–52 cm by 2050 under RCP4.5 and RCP8.5 respectively. By 2100, this range is projected to be 11–91 cm and 28–187 cm under RCP4.5 and RCP8.5 respectively ( [[#Vousdoukas--2018|Vousdoukas et al., 2018]] ; [[#Kirezci--2020|Kirezci et al., 2020]] ). Furthermore, the present-day 1-in-100-year ETWL is projected to have median return periods of around 1-in-50-years by 2050 and 1-in-10-years by 2100 under RCP4.5 in most of Asia, except SEA and ARP, in which the present-day 1-in-100-year ETWL is projected occur once per year or more, both by 2050 and 2100 ( [[#Vousdoukas--2018|Vousdoukas et al., 2018]] ). The present-day 1-in-50-year ETWL is projected to occur around three times a year by 2100 with a SLR of 1 m across Asia ( [[#Vitousek--2017|Vitousek et al., 2017]] ). Compound impacts of precipitation change, land subsidence, sea level rise, upstream hydropower development, and local water infrastructure development may lead to larger flood extent and prolonged inundation in the Vietnamese Mekong Delta ( [[#Triet--2020|Triet et al., 2020]] ).

'''Coastal erosion:''' Over the past 30 years, South, South East and East Asia exhibit the most pronounced delta changes globally due to strong human-induced changes to the fluvial sediment flux ( [[#Nienhuis--2020|Nienhuis et al., 2020]] ). Satellite derived shoreline change estimates over 1984–2015 indicate shoreline retreat rates between 0.5 m yr <sup>–1</sup> and 1 m yr <sup>–1</sup> along the coasts of WCA and ARP, increasing to 3 m yr <sup>–1</sup> in SAS. Over the same period, shoreline progradation has been observed along the coasts of RFE (0.2 m yr <sup>–1</sup> ), SEA (0.1 m yr <sup>–1</sup> ) and EAS (0.5 m yr <sup>–1</sup> ) ( [[#Luijendijk--2018|Luijendijk et al., 2018]] ; [[#Mentaschi--2018|Mentaschi et al., 2018]] ). Meanwhile, there has been a gross coastal area loss of 3,590 km <sup>2</sup> in South Asia, and a loss of 2,350 km <sup>2</sup> in Pacific Asia, over a 30-year period (1984–2015) ( [[#Mentaschi--2018|Mentaschi et al., 2018]] ).

Projections indicate that a majority of sandy coasts in the Asia region will experience shoreline retreat ( ''high confidence'' ) ( [[#Udo--2017|Udo and Takeda, 2017]] ; [[#Ritphring--2018|Ritphring et al., 2018]] ; [[#Vousdoukas--2020b|Vousdoukas et al., 2020b]] ), while parts of the RFE, EAS, SEA and WCA coastline are projected to prograde over the 21st century, if present ambient shoreline change trends continue. Median shoreline change projections (CMIP5), relative to 2010, presented by [[#Vousdoukas--2020b|Vousdoukas et al. (2020b)]] show that, by mid-century, sandy shorelines in Asia will retreat by between 10–50 m, except in SAS where shoreline retreat is projected to exceed 100 m, under both RCP4.5 and RCP8.5. By 2100, and under RCP4.5, shoreline retreats of around 85, 100 and 300 m are projected along the sandy coastlines of SEA and WCA, ARP and SAS respectively (50 m or less in other Asian regions), while under RCP8.5, over the same period, sandy shorelines along all regions with coastlines, except RFE and EAS, are projected to retreat by more than 100 m, with the retreat in SAS reaching 350 m (2100 RCP8.5 projections for RFE and EAS are about 60 m and about 85 m respectively; Figure 12.6).

'''Marine heatwave:''' There have been frequent marine heatwaves (MHW) in the coastal oceans of Asia, connected to the increase between 0.25°C and 1°C in mean SST of the coastal oceans since 1982–1998 ( [[#Oliver--2018|Oliver et al., 2018]] ). There is ''high confidence'' that MHWs will increase around most of Asia. Mean SST is projected to increase by 1°C (2°C) around Asia by 2100, with a hotspot of around 2°C (5°C) along the coastlines of the Sea of Japan and the RFE under RCP4.5 (RCP8.5; Interactive Atlas). Under global warming conditions, MHW intensity and duration are projected to increase in the coastal zones of all sub-regions of Asia, but most notably in SEA and SAS ( [[#Frölicher--2018|Frölicher et al., 2018]] ). Projections for SSP1-2.6 and SSP5-8.5 both show an increase in MHWs around Asia by 2081–2100, relative to 1985–2014 (Box 9.2, Figure 1).

'''In general, there is''' high confidence '''that most coastal/ocean-related climatic impact-drivers in Asia will increase over the 21st century. Relative sea level rise is''' very likely '''to continue around Asia, contributing to increased coastal flooding in low-lying areas''' ( high confidence ''') and shoreline retreat along most sandy coasts''' ( high confidence '''). Marine heatwaves are also expected to increase around the region over the 21st century''' ( high confidence ''').'''

The assessed direction of change in climatic impact-drivers for Asia and associated confidence levels are illustrated in Table 12.4.

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'''Table 12.4''' '''|''' '''Summary of confidence in direction of projected change in climatic impact-drivers in Asia, representing their aggregate characteristic changes for mid-century for scenarios RCP4.5, SSP2-4.5, SRES A1B or above within each AR6 region (defined in Chapter 1), approximately corresponding (for CIDs that are independent of sea level rise) to global warming levels between 2°C and 2.4°C (see [[#12.4|Section 12.4]] for more details of the assessment method).''' The table also includes the assessment of observed or projected time-of-emergence of the CID change signal from the natural interannual variability if found with at least ''medium confidence'' in [[#12.5.2|Section 12.5.2]] .

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