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

=== 12.4.7 Small Islands ===

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This section covers the climatic impact-drivers affecting small islands around the world (see definition of SIDS in the Glossary; Cross-Chapter Box Atlas.2) with a particular focus on small islands in the Caribbean (CAR) Sea and the Pacific Ocean. Caribbean and Pacific small islands have mostly tropical climates and local conditions are also influenced by diverse topography ranging from low-lying islands and atolls to volcanic and mountainous terrain. Climate variability in these islands is influenced by the trade winds, easterly waves, tropical cyclones (TC), and the migrations of the Inter-tropical Convergence Zone (ITCZ), the North Atlantic Subtropical High, and the South Pacific Convergence Zone (SPCZ), and other modes of climate variability as discussed in Cross-Chapter Box Atlas.2. Furthermore, changes in the ocean temperature and chemistry, and relative sea level have strong impacts on these small islands given their geographical location and dependence on coastal and marine ecosystem services.

The AR5 recognized the heterogeneity in these small islands in terms of physical geography, socio-economic and cultural backgrounds, as well as their vulnerability to the impacts of climate change. Similar to previous reports, these regions have been assessed together in this section, given the similarities in the challenges they face in addressing climate change impacts and risk, which were thought – until AR4 – to be dominated by sea level rise ( [[#Nurse--2014|Nurse et al., 2014]] ; [[#Betzold--2015|Betzold, 2015]] ). Since then there has been a substantial increase in the number and complexity of the literature on the drivers and impacts of climate change on small islands (BOM and CSIRO, 2011, 2014; [[#Nurse--2014|Nurse et al., 2014]] ; [[#Gould--2018|Gould et al., 2018]] ; [[#Keener--2018|Keener et al., 2018]] ). There are also increasing efforts being made to produce higher resolution climate projections for small islands through downscaling methods ( [[#Elison%20Timm--2015|Elison Timm et al., 2015]] ; [[#McLean--2015|McLean et al., 2015]] ; [[#Khalyani--2016|Khalyani et al., 2016]] ; [[#Zhang--2016|]] [[#Zhang--2016|C. Zhang et al., 2016]] ; [[#Stennett-Brown--2017|Stennett-Brown et al., 2017]] ; [[#Bhardwaj--2018|Bhardwaj et al., 2018]] ; [[#Bowden--2021|Bowden et al., 2021]] ).

The AR5 identified the key climate and ocean-related hazards affecting small islands, which occur at different time scales and have diverse impacts on multiple sectors ( [[#Christensen--2013|Christensen et al., 2013]] ; [[#Nurse--2014|Nurse et al., 2014]] ). Recent findings from SR1.5 and SROCC emphasize that the multiple interrelated climate hazards currently faced by low-lying islands and coastal areas will be amplified in the future, especially at higher global warming levels ( [[#Hoegh-Guldberg--2018|Hoegh-Guldberg et al., 2018]] ; [[#IPCC--2019b|IPCC, 2019b]] ).

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==== 12.4.7.1 Heat and Cold ====

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'''Mean air temperature:''' Significant warming trends are clearly evident in the small islands, such as those in the Pacific, CAR, and western Indian Ocean, particularly over the latter half of the 20th century (see Figure Atlas.11; Atlas.10.2; Cross-Chapter Box Atlas.2, Table 1). This observed warming signal in the tropical western Pacific has been attributed to anthropogenic forcing ( [[#Wang--2016|Wang et al., 2016]] ). There is ''high confidence'' of warming over small islands even at 1.5°C GWL (Atlas.10.4 and Figure Atlas.28; [[#Hoegh-Guldberg--2018|Hoegh-Guldberg et al., 2018]] ). Mean temperature is ''very likely'' to increase by 1°C–2°C (2°C–4°C) by 2041–2060 (2081–2100) under RCP8.5 (BOM and CSIRO, 2014) and SSP3-7.0 (Atlas.10.4, Figure 4.19 and Figure Atlas.12; [[#Almazroui--2021|Almazroui et al., 2021]] ).

'''Extreme heat:''' Observational records indicate warming trends in the temperature extremes since the 1950s in CAR and the Pacific small islands ( ''high confidence'' ) (Sections 11.3.2 and 11.9, and Table 11.13). A detectable anthropogenic increase in summer heat stress has been identified over a number of island regions in CAR, western tropical Pacific, and tropical Indian Ocean, based on wet bulb globe temperature (WBGT) index trends for 1973–2012 ( ''medium confidence'' ) ( [[#Knutson--2016|Knutson and Ploshay, 2016]] ). An increasing trend in the maximum daytime heat index is also noted in CAR during the 1980–2014 period, as well as more extreme heat events since 1991 ( [[#Ramirez-Beltran--2017|Ramirez-Beltran et al., 2017]] ).

Compared with the recent past, it is ''likely'' that the intensity and frequency of hot (cold) temperature extremes will increase (decrease) in the small islands ( [[IPCC:Wg1:Chapter:Chapter-11#11.9|Section 11.9]] and Table 11.13; BOM and CSIRO, 2014). Warm spell conditions will occur up to half the year in CAR at 1.5°C GWL with an additional 70 days at 2°C ( [[#Hoegh-Guldberg--2018|Hoegh-Guldberg et al., 2018]] ; [[#Taylor--2018|Taylor et al., 2018]] ), with livestock temperature–humidity tolerance thresholds increasingly surpassed ( [[#Lallo--2018|Lallo et al., 2018]] ). In CAR, a median increase of more than a month per year where temperatures exceed 35°C is projected by end of the 21st century under SSP5-8.5 (Figure 12.4a–c and Figure 12.SM.1). Heatwaves are projected to increase in CAR by the mid- and end-century under RCP8.5 (Sections 11.3.5 and 11.9, and Table 11.13). Figure 12.4d–f and Figure 12.SM.2 also show an increase of about 30–60 days in which HI exceeds 41°C by 2041–2060 under SSP5-8.5 relative to 1995–2014 in CAR, with an additional increase of about 50–100 days by end of the 21st century for RCP8.5/SSP5-8.5, but this increase remains below 50 days for RCP2.6/SSP1-2.6. The Pacific Islands region is also among those projected to have an increase in WBGT by end-century under RCP8.5, increasing the risk of heat stress in the region ( [[#Newth--2018|Newth and Gunasekera, 2018]] ).

'''It is''' very likely '''that the significant recent warming trends observed in the small islands will continue in the 21st century, which will''' likely '''further increase heat stress in these regions.'''

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==== 12.4.7.2 Wet and Dry ====

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'''Mean precipitation:''' Observational datasets have generally revealed no significant long-term trends in rainfall in the Caribbean over the 20th century when analysed at seasonal and inter-decadal timescales, except for some areas where there is evidence for decreasing trends for the period 1901–2010 but not for the period 1951–2010 (Cross-Chapter Box Atlas.2, Table 1, and Atlas.10.2; [[#Knutson--2018|Knutson and Zeng, 2018]] ). Although there are spatial variations, annual rainfall trends in the western Indian Ocean are mostly decreasing, with generally non-significant trends in the western tropical Pacific since the 1950s ( ''low confidence'' ). Significant drying trends are noted in the southern Pacific subtropics and south-western French Polynesia during the 1951–2015 period ( [[#McGree--2019|McGree et al., 2019]] ), and in some areas of Hawaii during the 1920–2012 period ( ''medium confidence'' ) (Cross-Chapter Box Atlas.2, Table 1, and Atlas.10.2).

Atlas.10.4 projects precipitation reduction over the Caribbean ( ''high confidence'' ) ( [[#Almazroui--2021|Almazroui et al., 2021]] ) and parts of the Atlantic and Indian oceans, particularly in June to August, by end of 21st century under SSP5-8.5. Precipitation is generally projected to increase under SSP5-8.5 and for higher GWLs in the small islands in parts of the western and equatorial Pacific, but there is ''low confidence'' in broad changes given drier conditions projected for the southern subtropical and eastern Pacific Ocean ( ''limited agreement'' given spatial and seasonal variability) (Atlas.10.4 and Figure Atlas.28).

'''River flood:''' There is ''limited evidence'' on observed changes in river flooding in the small islands. Long-term records in Hawaii indicate no clear trends in peak flow, except for the significant decrease in peak streamflow in Hawaii Island over the period 1967–2016 ( [[#Bassiouni--2013|Bassiouni and Oki, 2013]] ; [[#Clilverd--2019|Clilverd et al., 2019]] ). Similarly, there is no significant trend in the frequency and height (after adjusting for average sea level rise) of river flood in Fiji over the period 1892–2013 ( [[#McAneney--2017|McAneney et al., 2017]] ). There is ''low confidence'' on the direction of future change of river flooding in the small islands due to the limited literature. In Oahu, Hawaii, extreme peak flow events with high return periods are projected to increase by end of the 21st century under RCP8.5, but there is also high uncertainty in these projections ( [[#Leta--2018|Leta et al., 2018]] ).

'''Heavy precipitation and pluvial flood:''' Heavy precipitation days in CAR have increased in magnitude, and have been more frequent in the northern part during the latter part of the 20th century ( ''low confidence'' ) ( [[IPCC:Wg1:Chapter:Chapter-11#11.4.2|Section 11.4.2]] and Table 11.14). The direction of change in extreme precipitation varies across the Pacific and depends on the season ( ''low confidence'' ) ( [[IPCC:Wg1:Chapter:Chapter-11#11.4.2|Section 11.4.2]] and Cross-Chapter Box Atlas.2, Table 1). Although pluvial flooding events have been observed in some islands, there is ''limited evidence'' for an assessment on past changes in pluvial flooding, unlike in other regions. There is ''low confidence'' in the projected change in magnitude of very heavy precipitation days in CAR across different GWLs (Table 11.14). On the other hand, there is ''high confidence'' in the increase in frequency and intensity of extreme rainfall events (i.e., 1-in-20-year rainfall events) in the western tropical Pacific in the 21st century, even for RCP2.6 scenario, based on model agreement and mechanistic understanding but ''low confidence'' in the magnitude of change in extreme rainfall due to model bias (BOM and CSIRO, 2014).

'''Landslide:''' Heavy rainfall, such as from tropical cyclones, can trigger landslides over steep terrain in the small islands ( [[#Bessette-Kirton--2019|Bessette-Kirton et al., 2019]] ). There is ''limited evidence'' to determine long-term trends in rainfall-induced landslides in the small islands ( [[#Kirschbaum--2015|Kirschbaum et al., 2015]] ; [[#Sepúlveda--2015|Sepúlveda and Petley, 2015]] ; [[#Froude--2018|Froude and Petley, 2018]] ; [[#Bessette-Kirton--2019|Bessette-Kirton et al., 2019]] ). There is ''low confidence'' in future changes in landslides in the small islands. The direction of change may depend on future changes in precipitation, tropical cyclones, climate modes (e.g., El Niño–Southern Oscillation, ENSO), as well as human disturbance, but more data and understanding of the complexity of these relationships are needed, especially in these vulnerable areas ( [[#Sepúlveda--2015|Sepúlveda and Petley, 2015]] ; [[#Gariano--2016|Gariano and Guzzetti, 2016]] ; [[#Froude--2018|Froude and Petley, 2018]] ).

'''Aridity:''' Current estimates identify many small islands as being under water stress and thus particularly sensitive to variations in rainfall and groundwater, population growth and demand, and land-use change, among others (Cross-Chapter Box Atlas.2; [[#Holding--2016|Holding et al., 2016]] ). From 1950 to 2016, a heterogeneous but prevalent drying trend is found in CAR ( ''low confidence'' ), where drought variability is modulated by the tropical Pacific and North Atlantic oceans (Table 11.15 and Cross-Chapter Box Atlas.2, Table 1; [[#Herrera--2017|Herrera and Ault, 2017]] ). In the future, increased aridity and decreased freshwater availability are projected in many small islands due to higher evapotranspiration in a warmer climate that partially offsets increases or exacerbates reductions in precipitation ( [[#Karnauskas--2016|Karnauskas et al., 2016]] , 2018b; [[#Hoegh-Guldberg--2018|Hoegh-Guldberg et al., 2018]] ). Increased aridity is projected for the majority of the small islands, such as in CAR, southern Pacific and western Indian Ocean, by 2041–2059 relative to 1981–1999 under RCP8.5 or at 1.5°C and 2°C GWLs, which will further intensify by 2081–2099 ( ''medium confidence'' ) ( [[#Karnauskas--2016|Karnauskas et al., 2016]] , 2018b). Groundwater recharge is projected to increase in Maui, Hawaii except on the leeward side of the island, which underscores the importance of topography and elevation on freshwater availability in different island microclimates ( [[#Brewington--2019|Brewington et al., 2019]] ; [[#Mair--2019|Mair et al., 2019]] ).

'''Hydrological drought:''' There is ''low confidence'' of widespread changes to hydrological drought in CAR or Pacific small islands in recent decades, although an increasing number of studies document local changes. Records in Hawaii indicate downward trends in low streamflow and base flow from 1913 to 2008 ( [[#Bassiouni--2013|Bassiouni and Oki, 2013]] ). Decadal variability of Hawaiian streamflow coincides with rainfall fluctuations associated with the Pacific Decadal Variability although significant average declines in surface and baseflow runoff of about 8% and 11% per decade, respectively, have been noted during the 1987–2016 period ( [[#Clilverd--2019|Clilverd et al., 2019]] ).

There is ''low confidence'' in hydrological drought change projections, given low signal-to-noise ratios and the challenge in representing island scales in global analyses. [[#Prudhomme--2014|Prudhomme et al. (2014)]] recognized CAR as one of the regions with the highest increase in regional deficit index (RDI; a measure of the fraction of area in hydrological drought conditions) by the end of the 21st century under RCP8.5. Daily streamflow and extreme low flows in two watersheds in Oahu, Hawaii are projected to decline by mid- and end of the 21st century under RCP4.5 and RCP8.5, which would result in more frequent hydrological droughts in this area ( [[#Leta--2018|Leta et al., 2018]] ).

'''Agricultural and ecological drought:''' Recent trends toward more frequent and severe droughts have been noted in the small islands but only with ''low confidence'' in broad trend patterns, given high spatial variability including heightened drought on the leeward side of islands (e.g., [[#Frazier--2017|Frazier and Giambelluca, 2017]] ; [[#Herrera--2017|Herrera and Ault, 2017]] ; [[#McGree--2019|McGree et al., 2019]] ; see Table 11.15, Cross-Chapter Box Atlas.2, Table 1). Agricultural and ecological droughts are projected to increase in frequency, duration, magnitude, and extent in small islands, such as in CAR ( ''medium confidence'' ) and parts of the Pacific ( ''low confidence'' ), particularly where future declines in precipitation are compounded by higher evapotranspiration, under increasing levels of warming ( [[#Naumann--2018|Naumann et al., 2018]] ; [[#Taylor--2018|Taylor et al., 2018]] ; [[#Vichot-Llano--2021|Vichot-Llano et al., 2021]] ). Relative to the period 1985–2014, decreases in annual surface and total column soil moisture become more robust in more areas in CAR by 2071–2100 under SSP3-7.0 and SSP5-8.5 scenarios (B.I. [[#Cook--2020|]] [[#Cook--2020|Cook et al., 2020]] ), but reliably representing drought features in small island domains with global simulations is challenging (see also [[IPCC:Wg1:Chapter:Chapter-11#11.9|Section 11.9]] ).

'''Fire weather:''' There is ''limited evidence'' on trends in wildfire in CAR and the Pacific. Records of wildfire in Hawaii from 2005 to 2011 indicate a peak in area burned during the hot and dry summer months, but [[#Trauernicht--2015|Trauernicht et al. (2015)]] note the difficulty in establishing the link between past climate and wildfire trends due to human activities and vegetation changes. Availability of literature limits assessment on future fire weather in the small islands. Drying and warming trends tend to increase fire probability aside from the climate impact on fuel loading, for example, grassland fires in Hawaii ( [[#Trauernicht--2019|Trauernicht, 2019]] ), and wildfires in Puerto Rico ( [[#Van%20Beusekom--2018|Van Beusekom et al., 2018]] ).

'''Observed and projected rainfall trends vary spatially across the small islands. Higher evapotranspiration under a warming climate are projected to partially offset future increases or amplify future reductions in rainfall, resulting in drier conditions and increased water stress in the small islands''' ( medium confidence ''').'''

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==== 12.4.7.3 Wind ====

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'''Mean wind speed:''' Scarcity of observations limits assessment of long-term changes in winds over the small islands in the Pacific and CAR. Records indicate that average daily wind speeds have slowly declined in Hawaii, but have remained constant across western and southern Pacific sites since the mid-20th century ( [[#Marra--2017|Marra and Kruk, 2017]] ). Recent studies of reanalyses and hindcast simulations indicate an intensification of the Pacific trade winds during the 1992–2011 period, which contributed to the ocean cooling in the tropical central and eastern Pacific ( [[#England--2014|England et al., 2014]] ; [[#Takahashi--2016|Takahashi and Watanabe, 2016]] ). Projections estimate up to 0.4 m s <sup>–1</sup> (8%) increase in annual winds in CAR under RCP8.5, which is associated with changes in the extension of the North Atlantic Subtropical High that enhances the Caribbean low-level jet during the wet season, and stronger local easterlies due to enhanced land–ocean temperature differences in the dry season ( [[#Costoya--2019|Costoya et al., 2019]] ) ( ''low confidence'' ).

'''Tropical cyclone:''' Tropical cyclones have devastating impacts on the small islands due to intense winds, storm surge and rainfall, although the associated rainfall can also be beneficial for freshwater resources. It is ''likely'' that tropical cyclone intensity and intensification rates at a global scale have increased in the past 40 years but it is not clear if regional-scale changes are basin-wide or due to shifts in tropical cyclone tracks ( [[IPCC:Wg1:Chapter:Chapter-11#11.7.1.2|Section 11.7.1.2]] ). Other, less data-sensitive tropical cyclone features, such as the poleward migration of where tropical cyclones reach peak intensity in the western North Pacific since the 1940s ( '''medium confidence''' ) and the slowdown in tropical cyclone translational speed over contiguous USA since 1900 ( '''medium confidence''' ), can affect rainfall and flooding over small islands in CAR and the Pacific ( [[IPCC:Wg1:Chapter:Chapter-11#11.7.1.2|Section 11.7.1.2]] ).

Projections of global changes in tropical cyclones indicate more frequent Category 4–5 storms ( ''high confidence'' ) and increased rain rates ( ''high confidence'' ) ( [[#Knutson--2020|Knutson et al., 2020]] ), with relative sea level rise exacerbating storm surge potential, but with large regional differences (see [[IPCC:Wg1:Chapter:Chapter-11#11.7.1.5|Section 11.7.1.5]] ). By the late 21st century, tropical cyclones are projected to be less frequent in the basins of the western and eastern North Pacific, Bay of Bengal, Caribbean Sea and in the Southern Hemisphere, but will be more frequent in the subtropical central Pacific ( [[#Murakami--2014|Murakami et al., 2014]] ; [[#Yoshida--2017|Yoshida et al., 2017]] ; [[#Bell--2019|Bell et al., 2019]] ; [[#Knutson--2020|Knutson et al., 2020]] ). Over CAR, tropical cyclone intensity is expected to increase by the end of the century under RCP8.5 due to higher sea surface temperatures but can be inhibited by increases in vertical wind shear in the region ( ''medium confidence'' ) ( [[#Kossin--2017|Kossin, 2017]] ; [[#Ting--2019|Ting et al., 2019]] ). The poleward movement of the area in which tropical cyclones reach peak intensity in the western North Pacific is ''likely'' to continue, which affects the tropical cyclone frequency over the small islands in the area ( [[IPCC:Wg1:Chapter:Chapter-11#11.7.1.5|Section 11.7.1.5]] ; [[#Kossin--2016|Kossin et al., 2016]] ). Projections also indicate an increase (decrease) in the tropical cyclone frequency during El Niño (La Niña) events in the Pacific at the end of the 21st century ( [[#Chand--2017|Chand et al., 2017]] ). RCP8.5 2080–2099 projections indicate a 2% increase in the number of tropical cyclones in the north-central Pacific relative to 1980–1999, with tracks shifting northward towards Hawaii (N. [[#Li--2018|]] [[#Li--2018|]] [[#Li--2018|]] [[#Li--2018|]] [[#Li--2018|Li et al., 2018]] ). Given projected reductions to the overall number of tropical cyclones but increases in storm intensity, total rainfall and storm surge potential, we assess ''medium confidence'' of overall changes to tropical cyclones affecting the Caribbean and Pacific small islands.

'''Projections indicate that small islands will generally face fewer but more intense tropical cyclones''' ( medium confidence ''') although there is substantial variability across small island regions given projected regional shifts in storm tracks.'''

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==== 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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