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

==== 12.4.7.4 Coastal and Oceanic ====

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'''Relative sea level:''' Relative sea level rise (RSLR) continues to be a major threat to small islands and atolls, since it can exacerbate the impacts of other climate hazards on low-lying coastal communities and infrastructures, ecosystems, and freshwater resources ( [[#Nurse--2014|Nurse et al., 2014]] ; [[#Hoegh-Guldberg--2018|Hoegh-Guldberg et al., 2018]] ). In the Indian Ocean–South Pacific region, 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> over 1900–2018 ( [[#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). RSLR rates based on satellite altimetry for the period 1993–2018 in the region increased to 3.65 [3.23 to 4.08] mm yr <sup>–1</sup> ( [[#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).

Relative sea-level rise is ''very likely'' to continue surrounding the oceans in the Small Island States. Around the small islands, regional mean RSLR projections vary widely, from 0.4–0.6 m under SSP1-2.6 to 0.7–1.6 m under SSP5-8.5 for 2081–2100 relative to 1995–2014 (median values), but in general they are situated in areas with RSL changes ranging from the mean projected GMSL change to above-average values ( [[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:''' Relative sea level rise, storm surges and swells contribute to coastal inundation in the small islands, where studies on historical trends in coastal flooding are currently limited. For example, a swell event due to distant extratropical cyclones in December 2008 raised extreme water levels leading to flooding affecting five Pacific island nations: Marshall Islands, Micronesia, Papua New Guinea, Kiribati and Solomon Islands ( [[#Hoeke--2013|Hoeke et al., 2013]] ; [[#Merrifield--2014|Merrifield et al., 2014]] ). Over low-lying atoll islands in the north-west tropical Pacific, potential increases in the frequency and areal extent of coastal flooding, especially at higher SLR scenarios, are expected to have negative consequences for freshwater resources and island habitability ( [[#Storlazzi--2015|Storlazzi et al., 2015]] , 2018). Select tide gauges across the Pacific also indicate increasing trends in the frequency of minor flooding since the 1960s ( [[#Marra--2017|Marra and Kruk, 2017]] ).

As relative sea levels increase, the potential for coastal flooding increases in the small islands ( ''high confidence'' ). Across the Pacific and CAR small islands, the 5–95th percentile range of the 1-in-100-year ETWL is projected to increase (relative to 1980–2014) by 10–35 cm and by 14–41 cm by 2050 under RCP4.5 and RCP8.5, respectively (Figure 12.4q). By 2100, this range is projected to be 27–81 cm and 44–188 cm under RCP4.5 and RCP8.5, respectively (Figure 12.4p,r; [[#Vousdoukas--2018|Vousdoukas et al., 2018]] ; [[#Kirezci--2020|Kirezci et al., 2020]] ). Furthermore, by 2050, the present-day 1-in-100-year ETWL is projected to have median return periods of between 1-in-1-year and 1-in-50-year in both the Pacific and CAR small islands, with some Pacific islands projected to experience the present-day 1-in-100-year ETWL more than once a year ( [[#Vousdoukas--2018|Vousdoukas et al., 2018]] ; [[#Oppenheimer--2019|Oppenheimer et al., 2019]] ). By 2100, the present-day 1-in-50-year ETWL is projected to occur around three times a year by 2100 with an SLR of 1 m at Pacific and CAR small islands ( [[#Vitousek--2017|Vitousek et al., 2017]] ). In the western tropical Pacific, the magnitude and frequency of coastal flooding due to SLR can be modulated by changes in the wave climate ( [[#Shope--2016|Shope et al., 2016]] ).

'''Coastal erosion:''' Recent studies have indicated variable and dynamic changes in shorelines of reef islands ( ''medium confidence'' ), including both erosion and accretion, which suggest factors other than SLR affecting shoreline changes, such as in the central and western Pacific within the past 50-to-60-year timeframe ( [[#Webb--2010|Webb and Kench, 2010]] ; [[#Le%20Cozannet--2014|Le Cozannet et al., 2014]] ; [[#Ford--2015|Ford and Kench, 2015]] ; [[#Duvat--2017|Duvat and Pillet, 2017]] ). For example, islands on atolls in the central and western Pacific have not substantially eroded or reduced in size in the past decades while sea level has been rising, but their position and morphology have changed due to anthropogenic factors (e.g., seawalls, reclamation) and climate–ocean processes ( [[#Biribo--2013|Biribo and Woodroffe, 2013]] ; [[#McLean--2015|McLean and Kench, 2015]] ). Analysis of aerial and satellite imagery revealed severe shoreline retreat in six islands and the disappearance of five vegetated reef islands in Solomon Islands in the western Pacific between 1947 and 2014, which may be due to the interaction between SLR and waves ( [[#Albert--2016|Albert et al., 2016]] ). In French Polynesia, changes in shoreline and island area have been observed since the 1960s, partly due to the effect of TCs on sediment changes and human activities ( [[#Duvat--2017|Duvat and Pillet, 2017]] ; [[#Duvat--2017|Duvat et al., 2017]] ). Coastal erosion has also been noted over the small, low-lying, sandy islands, such as in French Polynesia and Solomon Islands, among others ( [[#Luijendijk--2018|Luijendijk et al., 2018]] ; [[#Mentaschi--2018|Mentaschi et al., 2018]] ). Average shoreline retreat rates between 1 and 2 m yr <sup>–1</sup> are estimated for the islands in the equatorial Pacific and in CAR, while a retreat rate of 0.5 m yr <sup>–1</sup> is estimated for islands in the South Pacific, based on satellite observations from 1984–2016 ( [[#Luijendijk--2018|Luijendijk et al., 2018]] ; [[#Mentaschi--2018|Mentaschi et al., 2018]] ). There was also a loss of 610 km <sup>2</sup> compared with a gain of 520 km <sup>2</sup> in coastal area in Oceania during the 1984–2015 period ( [[#Mentaschi--2018|Mentaschi et al., 2018]] ).

Projections indicate that shoreline retreat will occur over most of the small islands in the Pacific and CAR throughout the 21st century with spatial variability ( ''high confidence'' ). Median shoreline change projections (CMIP5) relative to 2010, presented by [[#Vousdoukas--2020b|Vousdoukas et al. (2020b)]] , show that, by mid-century, shorelines in the islands in the equatorial Pacific and South Pacific will retreat by around 40 m, under both RCP4.5 and RCP8.5. In CAR islands, sandy shorelines are projected to retreat by about 80 m by mid-century under both RCPs. By 2100, more than 100 m of median shoreline retreat is projected for all small islands under both RCPs; notably in CAR where retreats approaching 200 m (relative to 2010) are projected under both RCPs. The total length of sandy coasts in CAR and Pacific small islands that is projected to retreat by more than a median of 100 m by 2100 under RCP4.5 and RCP8.5 is about 1100 km and 1200 km respectively, an increase of approximately 14%.

'''Marine heatwave:''' Ocean temperatures from satellite observations noted a moderate increase of 1–4 annual marine heat wave (MHW) events between 1982–1988 and 2000–2016 over some areas in the Indian Ocean, subtropical parts of the North and South Atlantic, and central and western parts of the North and South Pacific, but a decrease in frequency (two annual events) over the eastern Pacific Ocean (Box 9.2; [[#Oliver--2018|Oliver et al., 2018]] ). The intensity of MHWs has also increased between 0.2°C and 0.5°C over the equatorial portions of the North Atlantic and the South Pacific. Over the eastern tropical Pacific, the decrease in intensity and duration of MHW is between 0.5°C and 1.0°C and between 30 and 75 days, respectively (Box 9.2; [[#Oliver--2018|Oliver et al., 2018]] ). There is ''high confidence'' that MHWs will increase around all small island nations. Marine heatwaves are projected to be more intense and prolonged where the largest changes are noted in the equatorial region with maximum annual intensities up to 1.2°C (1.8°C) and annual mean duration reaching 100 days (200 days) at 1.5°C (2.0°C) warming levels ( [[#Frölicher--2018|Frölicher et al., 2018]] ). Projections for SSP1-2.6 and SSP5-8.5 both show an increase in MHWs around all small island nations by 2081–2100, relative to 1985–2014 (Box 9.2, Figure 1).

'''In summary, relative sea level rise is''' very likely '''in the oceans around small islands, and along with storm surges and waves will exacerbate coastal inundation in small islands. Shoreline retreat is projected along sandy coasts of most small islands''' ( high confidence '''). There is''' high confidence '''that MHWs will increase around all small island nations.'''

The assessed direction of change in climatic impact-drivers for CAR and Pacific small islands and associated confidence levels are illustrated in Table 12.9. Cold, snow, and ice-related climatic impact-drivers, and sand and dust storms are not broadly relevant in the small islands that were assessed.

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'''Table 12.9''' '''|''' '''Summary of confidence in direction of projected change in climatic impact-drivers in the small islands, 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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