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

== Atlas.10 Small Islands ==

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The assessment in this section focuses on changes in average temperature and precipitation for the main Small Islands regions, including the most recent years of observations, updates to observed datasets, the consideration of recent studies using CMIP5 and those using CMIP6 and CORDEX simulations. Assessment of changes in extremes is in [[IPCC:Wg1:Chapter:Chapter-11|Chapter 11]] (Sections 11.3.2, 11.4.2, 11.7.1.5 and, for the Caribbean, Tables 11.13–15) and of changes in climatic impact-drivers in [[IPCC:Wg1:Chapter:Chapter-12|Chapter 12]] (Section, 12.4.7 and Table 12.9).

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=== Atlas.10.1 Key Features of the Regional Climate and Findings From Previous IPCC Assessments ===

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==== Atlas.10.1.1 Key Features of the Regional Climate ====

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Many small islands lie in tropical regions and their climate varies depending on a range of factors with location, extent and topography having major influences. In general, their climate is determined by that of the broader region in which they lie as they have little influence on the regional climate, although steep topography can induce higher rainfall totals locally. Temperature variability tends to be low due to the influence of the surrounding ocean, most marked in the tropics where oceanic temperature ranges are small. However, seasonal rainfall variability can often be significant, both through the annual cycle and also interannually through the influence of many modes of variability (Cross-Chapter Box [[#Atlas.2|Atlas.2]] :, [[IPCC:Wg1:Chapter:Annex-iv|Annex IV]] and [[#Atlas.7.1|Atlas.7.1]] for the Caribbean). Many small islands are exposed to tropical cyclones and the associated hazards of high winds, storm surges and extreme rainfall, and many low-lying islands are exposed to regular flooding from natural high-tide and wave activity. In the Pacific, phases of the El Niño–Southern Oscillation result in periods of warmer or cooler than average temperatures following the upper ocean warming of El Niño events or cooling of La Niña events, and respectively weaker and stronger trade winds. El Niño conditions also lead to drought in Melanesian islands and increased tropical cyclones and storm surges in French Polynesia with La Niña conditions causing drought in Kiribati. Other islands experience increased rainfall during these periods.

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==== Atlas.10.1.2 Findings From Previous IPCC Assessments ====

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The AR5 noted observed temperature increases of 0.1°C–0.2°C per decade in the Pacific Islands and that warming was ''very likely'' to continue across all Small Islands regions ( [[#Christensen--2013|Christensen et al., 2013]] ; [[#IPCC--2013a|IPCC, 2013a]] ). It also reported decreased rainfall over the Caribbean, increases over the Seychelles, streamflow reductions over the Hawaiian Islands and projections of reduced rainfall over the Caribbean and drier rainy season for many of the south-west Pacific Islands ( [[#Christensen--2013|Christensen et al., 2013]] ; [[#IPCC--2013a|IPCC, 2013a]] ; [[#Nurse--2014|Nurse et al., 2014]] ). The remaining findings are derived from the SROCC ( [[#IPCC--2019a|IPCC, 2019a]] ). Ocean warming rates have ''likely'' increased in recent decades with marine heatwaves increasing and ''very likely'' to have become longer-lasting, more intense and extensive as a result of anthropogenic warming. Open ocean oxygen levels have ''very likely'' decreased and oxygen minimum zones have ''likely'' increased in extent. There is ''very high confidence'' that global mean sea level rise has accelerated in recent decades which, combined with increases in tropical cyclone winds and rainfall and increases in extreme waves, has exacerbated extreme sea level events and coastal hazards ( ''high confidence'' ). It is ''virtually certain'' that during the 21st century, the ocean will transition to unprecedented conditions with further warming and acidification ''virtually certain'' , increased upper ocean stratification ''very likely'' and continued oxygen decline ( ''medium confidence'' ). There is ''very'' ''high confidence'' that marine heatwaves and ''medium confidence'' that extreme El Niño and La Niña events will become more frequent. It is ''very likely'' that these changes will be smaller under scenarios with low greenhouse gas emissions. Global mean sea level will continue to rise and there is ''high confidence'' that the consequent increases in extreme levels will result in local sea levels in most locations that historically occurred once per century occurring at least annually by the end of the century under all RCP scenarios ( ''high confidence'' ). In particular, many small islands are projected to experience historical centennial events at least annually by 2050 under RCP2.6 and higher emissions. The proportion of Category 4 and 5 tropical cyclones, and associated precipitation rates and storm surges, along with average tropical cyclone intensity are projected to increase with a 2°C global temperature rise, thereby exacerbating coastal hazards.

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=== Atlas.10.2 Assessment and Synthesis of Observations, Trends and Attribution ===

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Significant positive trends in temperature ranging from 0.15°C per decade (over the period 1953–2010) to 0.18°C per decade (over the period 1961–2011) are noted in the tropical western Pacific, where the significant increasing and decreasing trends in warm and cool extremes, respectively, are also spatially homogeneous ( [[#Jones--2013|Jones et al., 2013]] ; [[#Whan--2014|Whan et al., 2014]] ; [[#Wang--2016|Wang et al., 2016]] ). Similarly, much of the Caribbean region showed statistically significant warming (at the 95% level) over the period 1901–2010 (P.D. [[#Jones--2016|Jones et al., 2016]] b). Observation records in the Caribbean region indicate a significant warming trend of 0.19°C per decade and 0.28°C per decade in daily maximum and minimum temperatures, respectively, with statistically significant increases (at the 5% level) in the number of warm days and warm nights during 1961–2010 ( [[#Taylor--2012|]] [[#Taylor--2012|M.A. Taylor et al., 2012]] ; [[#Stephenson--2014|Stephenson et al., 2014]] ; [[#Beharry--2015|Beharry et al., 2015]] ).

A weather station-based annual precipitation trend analysis over 1901–2010 in the Caribbean region indicated some locations with detectable decreasing trends ( [[#Knutson--2018|Knutson and Zeng, 2018]] ), which were attributable in part to anthropogenic forcing. These include southern Cuba, the northern Bahamas, and the Windward Islands, although significant trends were not found over the shorter periods of 1951–2010 and 1981–2010. In the Caribbean islands, a dataset of the Palmer Drought Severity Index (PDSI) from 1950 to 2016 showed a clear drying trend in the region ( [[#Herrera--2017|Herrera and Ault, 2017]] ). The 2013–2016 period showed the most severe drought during the period and was strongly related to anthropogenic warming, which would have increased the severity of the event by 17% and its spatial extent by 7% ( [[#Herrera--2018|Herrera et al., 2018]] ). However, a seasonal analysis of observations grouped into large sub-regions of the Caribbean revealed no significant long-term trends in rainfall over 1901–2012 but significant inter-decadal variability (P.D. [[#Jones--2016|Jones et al., 2016]] b). Declines in summer rainfall (–4.4% per decade) and maximum five-day rainfall (–32.6 mm per decade) over 1960–2005 were reported for Jamaica ( [[#CSGM--2012|CSGM, 2012]] ), and an insignificant decrease in summer precipitation was observed for Cuba for 1960–1995 ( [[#Naranjo-Diaz--1998|Naranjo-Diaz and Centella, 1998]] ). Three of four stations examined for Puerto Rico exhibited declining JJA rainfall over 1955–2009 with the trend statistically significant at the 95% level for Canóvana ( [[#Méndez-Lázaro--2014|Méndez-Lázaro et al., 2014]] ). In the Caribbean, positive regional trends in precipitation and trends in extremes during 1961–2010 were found to be not statistically significant (at the 5% level; [[#Stephenson--2014|Stephenson et al., 2014]] ; [[#Beharry--2015|Beharry et al., 2015]] ). Positive trends in JJA rainfall over Cuba and Jamaica are seen in CRU, whereas they are negative over Cuba for GPCC; over eastern Hispaniola they are positive in CRU and negative in CHIRPS ( [[#Cavazos--2020|Cavazos et al., 2020]] ).

In Hawaii, between 1920 and 2012, over 90% of the islands showed reduced rainfall and streamflow, an increase in the frequency of days with zero flow ( [[#Strauch--2015|Strauch et al., 2015]] ; [[#Frazier--2017|Frazier and Giambelluca, 2017]] ), and robust positive trends in drought frequency and severity ( [[#McGree--2016|McGree et al., 2016]] ). Over the western Pacific, interannual and decadal variabilities also drive long-term trends in rainfall. Recent analysis of station data showed spatial variations in the mostly decreasing but non-significant trends in annual and extreme rainfall over the western Pacific from 1961 to 2011 ( ''low confidence'' ) ( [[#McGree--2014|McGree et al., 2014]] ). Over the southern subtropical Pacific, decreases in annual, JJA, SON and extreme rainfall, and increasing drought frequency in the western region, has been observed since 1951 ( [[#Jovanovic--2013|Jovanovic et al., 2013]] ; [[#McGree--2016|McGree et al., 2016]] , 2019).

Over the western Indian Ocean significant warming trends have been reported for Mauritius (1.2°C during 1951–2016; [[#MESDDBM--2016|MESDDBM, 2016]] ), La Réunion (0.18°C per decade over 1968–2019; [[#Météo-France--2020|Météo-France, 2020]] ) and Maldives ( [[#MEE--2016|MEE, 2016]] ). Both Mauritius and La Réunion have experienced rainfall decreases of 8% during 1951–2016 and 1.2% per decade during 1961–2019 with generally weak, non-significant rainfall trends during 1967–2012.

Assessing observed climate change for Small Islands is often constrained by low station density ( [[#Ryu--2014|Ryu and Hayhoe, 2014]] ; P.D. [[#Jones--2016|Jones et al., 2016]] a), digitization requirements or data-sharing limitations (P.D. [[#Jones--2016|Jones et al., 2016]] a). Station data typically have longer temporal coverage relative to satellite products but are limited in spatial coverage ( [[#Cavazos--2020|Cavazos et al., 2020]] ). For Small Island nations, spatial gaps between observations can be very large due to the isolation of the islands ( [[#Wright--2016|Wright et al., 2016]] ). Additionally, over past decades, the number of station observations has declined substantially in Mauritius ( [[#Dhurmea--2019|Dhurmea et al., 2019]] ), Hawai’i ( [[#Bassiouni--2013|Bassiouni and Oki, 2013]] ; [[#Frazier--2017|Frazier and Giambelluca, 2017]] ) and most Pacific Island countries since the 1980s ( [[#Jones--2013|Jones et al., 2013]] ; [[#McGree--2014|McGree et al., 2014]] , 2016). In Fiji, meteorological stations were located on or by the coast and are sparse in the interior ( [[#Kumar--2013|Kumar et al., 2013]] ). Notable topography and land use may result in changes in climatic conditions over small distances ( [[#Foley--2018|Foley, 2018]] ), making the observational density particularly relevant.

Moreover, many stations have little metadata available, including those in Vanuatu, the Solomon Islands and Papua New Guinea ( [[#Whan--2014|Whan et al., 2014]] ). Compared to earlier decades, few metadata are currently being documented in the western Pacific islands ( [[#McGree--2014|McGree et al., 2014]] ), which will challenge the homogenization of long-term observational records. Challenges in the Caribbean include maintaining continuous daily time series with metadata, converting climatological data into digital formats and making them freely available ( [[#Stephenson--2014|Stephenson et al., 2014]] ; [[#Beharry--2015|Beharry et al., 2015]] ; P.D. [[#Jones--2016|Jones et al., 2016]] a). This is also an issue in the Pacific as many data are kept in national (local) databases, with only a fraction having been incorporated into global datasets ( [[#Whan--2014|Whan et al., 2014]] ).

Because of the small number of stations used for interpolation and the complex mountainous topography, gridded product for these small islands should be interpreted with caution ( [[#Frazier--2017|Frazier and Giambelluca, 2017]] ). For the Antilles, the error in estimating CRU2.0 monthly precipitation can stand locally between 20% and 40%. Over the Caribbean, [[#Cavazos--2020|Cavazos et al. (2020)]] found a discrepancy across gridded observational datasets (CRU, CHIRPS and GPCP) in detecting orographicprecipitation, especially during boreal summer, making their use in climate model evaluation challenging ( [[#Herrera--2017|Herrera and Ault, 2017]] ). Furthermore, some reanalysis products such as the 0.7° × 0.7° ERA-Interim reanalysis are not adequate as many of the smaller Caribbean islands are not represented as land (P.D. [[#Jones--2016|Jones et al., 2016]] a).

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=== Atlas.10.3 Assessment of Model Performance ===

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An assessment of model performance for the Caribbean region is contained in [[#Atlas.7.1|Atlas.7.1]] on Central America. In summary, the ability of climate models to simulate the climate over the region has improved in many key respects with the application of increased model resolution and a better representation of the land surface processes of particular importance in these advances ( ''high confidence'' ) ''.'' Regional climate models (RCMs) simulate realistically seasonal surface temperature and precipitation patterns including the bimodal rainfall in the precipitation annual cycle although with some timing biases in some regions ( ''high confidence'' ). The important regional circulation and precipitation features, the Caribbean low-level jet and the midsummer drought (MSD), are well represented over a variety of RCM domains covering the region ( ''high confidence'' ).

Over the tropical Pacific, surface temperature biases in CMIP6 models remain similar to those in CMIP5, although are reduced in the higher-resolution models in the HiResMIP ensemble. CMIP6 models generally represent trends in sea surface temperatures better than CMIP5 (see [[IPCC:Wg1:Chapter:Chapter-9#9.2|Section 9.2.1]] for more details). For precipitation, the persistent tropical Pacific bias of the double ITCZ (erroneous bands of excessive rainfall both sides of the equatorial Pacific) is still present in CMIP6 models although is slightly improved compared to those in CMIP3 and CMIP5 models ( [[IPCC:Wg1:Chapter:Chapter-3#3.3.2.3|Section 3.3.2.3]] ). Application of downscaling techniques (RCMs and stretched-grid GCMs) using resolutions finer than 10 km over the Pacific can capture topographic influences on wind and rainfall to generate realistic simulations of island climates – for example over Fiji and New Caledonia ( [[#Chattopadhyay--2015|Chattopadhyay and Katzfey, 2015]] ; [[#Dutheil--2019|Dutheil et al., 2019]] ). In both cases applying bias adjustment to the sea surface temperatures used as a lower boundary condition for the downscaling models was important to generate realistic simulations.

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=== Atlas.10.4 Assessment and Synthesis of Projections ===

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Projected median temperature increases for Small Islands from the CMIP5 ensemble range from 1°C (RCP4.5) to 1.5°C (RCP8.5) in the period 2046–2065, and from 1.3°C (RCP4.5) to 2.8°C (RCP8.5) by 2081–2100 relative to 1986–2005 ( [[#Harter--2015|Harter et al., 2015]] ). Spatial variations in the warming trend are projected to increase by the end of the 21st century, with relatively higher increases in the Arctic and sub-Arctic islands, and in the equatorial regions compared with islands in the Southern Ocean ( [[#Harter--2015|Harter et al., 2015]] ). In the western Pacific, temperatures are projected to increase by 2.0°C–4.5°C by the end of the 21st century relative to 1961–1990 ( [[#Wang--2016|Wang et al., 2016]] ). The warming over land in the Lesser Antilles is estimated to be about 1.6°C (3.0°C) by 2071–2100 for the RCP4.5 (RCP8.5) scenario, relative to 1971–2000 ( [[#Cantet--2014|Cantet et al., 2014]] ). Projections from the CMIP6 ensemble support these findings (Figure Atlas.28) and across global warming levels from 1.5°C to 4°C CMIP5 and CMIP6 consistently project lower levels of warming for Small Islands than the global average (Interactive Atlas).

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[[File:a34a6364b13c6719d233de39579e8938 IPCC_AR6_WGI_Atlas_Figure_28.png]]

'''Figure Atlas.28''' '''|''' '''Regional mean changes in annual mean surface air temperature, precipitation and sea level rise relative to the 1995–2014 baseline for the reference regions in the Small Islands (warming since the 1850–1900 pre-industrial baseline is also provided as an offset).''' Maps on the top show global June–July–August (JJA) precipitation changes (%, relative to 1995–2014) projected for 2081–2100 under RCP8.5 (left) and SSP5-8.5 (right) for the CMIP5 and CMIP6 ensembles, respectively. Bar plots in the left panel of each region triplet show the median (dots) and 10th–90th percentile range (bars) across each model ensemble for annual mean temperature changes for four datasets (CMIP5 in intermediate colours; a subset of CMIP5 used to drive CORDEX in light colours; CORDEX overlying the CMIP5 subset with dashed bars; and CMIP6 in solid colours); the first six groups of bars represent the regional warming over two time periods (near-term 2021–2040 and long-term 2081–2100) for three scenarios (SSP1-2.6/RCP2.6, SSP2-4.5/RCP4.5 and SSP5-8.5/RCP8.5), and the remaining bars correspond to four global warming levels (GWLs: 1.5°C, 2°C, 3°C and 4°C). Bar plots in the right panel show the median (dots) and 5th–95th percentile range (bars) sea level rise from the CMIP6 ensemble (see [[IPCC:Wg1:Chapter:Chapter-9|Chapter 9]] for details) for the same time periods and scenarios. The scatter diagrams of temperature against precipitation changes display the median (dots) and 10th–90th percentile ranges for the above four warming levels for December–January–February (DJF; middle panel) and June–July–August (JJA; right panel), respectively; for the CMIP5 subset only the percentile range of temperature is shown, and only for 3°C and 4°C GWLs. Changes are absolute for temperature (in °C) and relative (as %) for precipitation. See [[#Atlas.1.3|Atlas.1.3]] for more details on reference regions ( [[#Iturbide--2020|Iturbide et al., 2020]] ) and [[#Atlas.1.4|Atlas.1.4]] for details on model data selection and processing. The script used to generate this figure is available online ( [[#Iturbide--2021|Iturbide et al., 2021]] ) and similar results can be generated in the Interactive Atlas for flexibly defined seasonal periods. Further details on data sources and processing are available in the chapter data table (Table Atlas.SM.15).

The CMIP5 ensemble median projected precipitation decreases of up to –16% over the Caribbean, parts of the Atlantic and Indian oceans, and the southern subtropical and eastern Pacific Ocean, and increases of up to 10% over parts of the western Pacific and Southern oceans, and up to 55% in the equatorial Pacific Islands under RCP6.0 in the period 2081–2100 relative to 1986–2005 ( [[#Harter--2015|Harter et al., 2015]] ). A projected decrease in annual precipitation is also noted over the Lesser Antilles under the RCP4.5 and RCP8.5 scenarios ( [[#Cantet--2014|Cantet et al., 2014]] ). Seasonal rainfall is projected to decrease in most areas in Hawaii, except for the climatically wet windward side of the mountains, which would increase the wet to dry gradient over the area ( [[#Timm--2015|Timm et al., 2015]] ). The average precipitation changes in Hawaii are estimated to be about –11% to –28% under RCP4.5 during the wet season, and about –4% to –28% under RCP4.5 during the dry season in the period 2041–2071 relative to 1975–2005, with larger changes under RCP8.5 ( [[#Timm--2015|Timm et al., 2015]] ). There are still uncertainties in the projected changes, which have been attributed to factors including insufficient model skill in representing topography in the small islands, and high variability in climate drivers. However, the broad-scale pattern of projected wetter conditions in the western and equatorial Pacific, and the north Indian and Southern oceans, and of drier conditions over the Caribbean, and in parts of the Atlantic, Indian and southern subtropical and eastern Pacific oceans are further strengthened in the CMIP6 ensemble (Figure Atlas.28), which are thus ''likely'' regional responses as the climate continues to warm.

The negative trend in future summer rainfall in the Caribbean and Central America is projected to be strongest during midsummer (June–August) based on studies using GCMs ( [[#Rauscher--2008|Rauscher et al., 2008]] ; [[#Karmalkar--2013|Karmalkar et al., 2013]] ; [[#Karmacharya--2017a|Karmacharya et al., 2017a]] ; [[#Taylor--2018|Taylor et al., 2018]] ). The future summer drying over the Caribbean is associated with a projected future strengthening of the Caribbean low-level jet ( [[#Taylor--2013a|Taylor et al., 2013a]] ). [[#Rauscher--2008|Rauscher et al. (2008)]] hypothesized that the simulated 21st-century drying over Central America represents an early onset and intensification of the MSD. The westward expansion and intensification of the NASH associated with the MSD occurs earlier with stronger low-level easterlies. [[#Rauscher--2008|Rauscher et al. (2008)]] further suggested that the eastern Pacific ITCZ is also located further southward and that there are some indications that these changes could be forced by ENSO-like warming of the tropical eastern Pacific and increased land-ocean heating contrasts over the North American continent. Other studies also suggest a future intensification of the NASH due to changes in land-sea temperature contrast resulting from increased greenhouse-gas concentrations (W. [[#Li--2012|]] [[#Li--2012|Li et al., 2012]] ).

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=== Atlas.10.5 Summary ===

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It is ''very likely'' that all Small Island regions have warmed with significant trends recorded from at least the 1960s in all territories or nations. Trends include increases of 0.15°C–0.18°C per decade in the tropical western Pacific (1953–2011), significant warming over the Caribbean (1901–2010) with trends of 0.19°C (0.28°C) per decade in daily maximum (minimum temperatures) (1961–2010) and in La Réunion of 0.18°C per decade (1968–2019). There are fewer significant trends in precipitation in these regions though several locations in the Caribbean have detectable decreasing trends ( ''high confidence'' ), in part attributable to anthropogenic forcing ( ''limited evidence'' ). Also, it is ''likely'' that drying has occurred since the mid-20th century in some parts of the western Indian Ocean, and in the Pacific poleward of 20° latitude in both the northern and southern hemispheres.

It is ''very likely'' that Small Island regions will continue to warm in the coming decades at a level slightly lower than the global mean. Small Island regions in the western and Equatorial Pacific, north Indian and Southern oceans are ''likely'' to be wetter in the future; and those in the Caribbean, parts of the Atlantic and west Indian oceans, and the southern subtropical and eastern Pacific Ocean drier.

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'''Cross-Chapter Box Atlas.2 | Climate information relevant to water resources in Small Islands'''

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'''Coordinators:''' Tannecia Stephenson (Jamaica), Faye Abigail Cruz (The Philippines)

'''Contributors:''' Donovan Campbell (Jamaica), Subimal Ghosh (India), Rafiq Hamdi (Belgium), Mark Hemer (Australia), Richard G. Jones (United Kingdom), James Kossin (United States of America), Simon McGree (Australia/Fiji), Blair Trewin (Australia), Sergio M. Vicente-Serrano (Spain)

Constructing regional climate information for Small Islands involves synthesis from multiple sources. This cross-chapter box presents information relevant to water resources, drawing on several chapters in AR6 and [[#Atlas.10|Atlas.10]] . It introduces the context and current evidence base followed by an assessment of trends and projections in rainfall, temperature and sea levels across Small Islands and it highlights key findings.

Cross-Chapter Box [[#Atlas.2|Atlas.2]]

'''Regional context'''

Small Islands are predominantly located in the Pacific, Atlantic and Indian oceans, and in the Caribbean ( [[#Nurse--2014|Nurse et al., 2014]] ; [[#Shultz--2019|Shultz et al., 2019]] ). They are characterized by their small physical size, being surrounded by large ocean expanses, vulnerability to natural disasters and extreme events, and relative isolation ( [[IPCC:Wg1:Chapter:Chapter-12#12.4.7|Section 12.4.7]] , [[#Atlas.10|Atlas.10]] and Glossary; [[#Nurse--2014|Nurse et al., 2014]] ). These and nearby larger islands (e.g., Madagascar and Cuba) are often water-scarce with low water volumes due to increasing demand (from population growth and tourism), aging and poorly designed infrastructure ( [[#Burns--2002|Burns, 2002]] ), and decreasing supply (from pollution, changes in precipitation patterns, drought, saltwater intrusion, regional sea level rise, inadequate water governance ( [[#Belmar--2016|Belmar et al., 2016]] ; [[#Mycoo--2018|Mycoo, 2018]] ) and competing and conflicting uses ( [[IPCC:Wg1:Chapter:Chapter-8#8.1.1.1|Section 8.1.1.1]] ; [[#Cashman--2014|Cashman, 2014]] ; [[#Gheuens--2019|Gheuens et al., 2019]] ). In the Caribbean, groundwater is the main freshwater source and depends strongly on rainfall variability ( [[#Post--2018|Post et al., 2018]] ), while rain, ground or surface water are the primary sources for the Pacific Islands depending on island type (volcanic or atoll), size and quality of groundwater reserves ( [[#Burns--2002|Burns, 2002]] ). Groundwater pumping and increasing sea levels also affect water availability by increasing the salinity of the aquifer (e.g., [[#Bailey--2015|Bailey et al., 2015]] , 2016), thus reinforcing negative drought effects from reduced rainfall and increased evaporative demand from higher temperatures. For example, in 54% of the Marshall Islands, groundwater is highly vulnerable to droughts ( [[#Barkey--2017|Barkey and Bailey, 2017]] ).

'''The climate of Small Islands and findings from previous IPCC assessments'''

Intra-seasonal to interannual rainfall in the Caribbean and in the Indian and Pacific oceans is influenced by the trade winds, the passage of tropical cyclones (TCs), Madden–Julian Oscillation (MJO), easterly waves, migrations of the Inter-tropical Convergence Zone (ITCZ) and the North Atlantic Subtropical High (NASH) for the Caribbean; the South Pacific Convergence Zone (SPCZ) and western North Pacific summer monsoon for the Pacific; and the South Asian monsoons for the Indian Ocean. The relevant dominant modes of climate variability ( [[IPCC:Wg1:Chapter:Chapter-8#8.3.2.9|Section 8.3.2.9]] and Annex IV) are El Niño–Southern Oscillation (ENSO) and the Indian Ocean Dipole (IOD) which have been associated with extreme events in the islands (Annex IV; [[#Stephenson--2014|Stephenson et al., 2014]] ; [[#Kruk--2015|Kruk et al., 2015]] ; [[#Frazier--2018|Frazier et al., 2018]] ). The modes of climate variability are modulated by Pacific Decadal Variability (PDV), Inter-decadal Pacific Oscillation (IPO) and Atlantic Multi-decadal Variability (AMV). These modes show no sustained trend since the late 19th century ( ''high confidence'' ) ( [[IPCC:Wg1:Chapter:Chapter-2#2.4|Section 2.4]] ).

The AR5 WGI reports observed temperature increases of 0.1°C–0.2°C per decade in the Pacific Islands with these trends ''very likely'' to continue under high emissions, and projects a drier rainy season for many islands in the south-west Pacific ( [[#Christensen--2013|Christensen et al., 2013]] ). The AR5 WGII reports rainfall reductions over the Caribbean, increases over the Seychelles, streamflow reductions over the Hawaiian Islands and saltwater intrusion into groundwater reserves in the Pacific Islands resulting from storm surges and high tides ( [[#Nurse--2014|Nurse et al., 2014]] ). The SROCC ( [[#IPCC--2019a|IPCC, 2019a]] ) finds ''very high confidence'' that global mean sea level rise has accelerated in recent decades which has exacerbated extreme sea level events and flooding ( ''high confidence'' ). It will continue to rise with consequent increases in extreme levels so that the historical one-in-a-century extreme local sea level will become an annual event by the end of the century under all RCP scenarios ( ''high confidence'' ). In particular, many Small Islands are projected to experience historical centennial events at least annually by 2050 under RCP2.6, RCP4.5 and RCP8.5 emissions. The proportion of Category 4 and 5 TCs and associated precipitation rates along with their average intensity are projected to increase with a 2°C global temperature rise which will further increase the magnitude of resultant storm surges and flooding. The SROCC Cross-Chapter Box on Low-lying Islands and Coasts ( [[#Magnan--2019|Magnan et al., 2019]] ) focused on sea level rise and oceanic changes and their impacts, therefore the assessment presented here on climate changes relevant to water resources, including precipitation and temperature, is complementary.

'''Observations and attribution of changes'''

Cross-Chapter Box [[#Atlas.2|Atlas.2]] : presents an overview of observed sub-regional trends relevant to water resources in some Small Islands and island regions largely from 1951. Some general observed climate trends include higher magnitude and frequency of temperatures including warm extremes ( ''high confidence'' ) ( [[IPCC:Wg1:Chapter:Chapter-12#12.4.7.1|Section 12.4.7.1]] , Table 11.13 and [[#Atlas.10.2|Atlas.10.2]] ), declines in high-intensity rainfall events ( ''low'' to ''medium confidenc'' e) (Table 11.14), regional sea level rises with strong storm surges and waves resulting in increased coastal flood intensity ( ''high confidence'' ) ( [[IPCC:Wg1:Chapter:Chapter-12#12.4.7.4|Section 12.4.7.4]] and [[#Atlas.10.2|Atlas.10.2]] ), and increased intensity and intensification rates of tropical cyclones at global scale ( ''medium confidence'' ) (Sections 11.7.1.2 and 12.4.7.3) and ocean acidification ( ''virtually certain'' ) (Chapters 2, 6 and 9, and [[#Atlas.3.2|Atlas.3.2]] ).

No significant long-term trends are observed for annual Caribbean rainfall over the 20th century ( ''low confidence'' ) ( [[#Atlas.10.2|Atlas.10.2]] ). Over the western Pacific, generally decreasing but non-significant trends are noted in annual total rainfall from 1961 to 2011 ( ''low confidence'' ) ( [[#Atlas.10.2|Atlas.10.2]] ). June–July–August (JJA) rainfall over the Caribbean shows some drying tendencies that may be linked to the combined effect of warm ENSO events and a positive NAO phase ( [[#Giannini--2000|Giannini et al., 2000]] ; [[#Méndez-Lázaro--2014|Méndez-Lázaro et al., 2014]] ; [[#Fernandes--2015|Fernandes et al., 2015]] ), or to warm ENSO events and a positive PDV ( [[#Maldonado--2016|Maldonado et al., 2016]] ). However, the work of [[#Herrera--2018|Herrera et al. (2018)]] suggests that anthropogenic influences may also be possible, although mechanisms proposed to date have not decoupled the influence of anthropogenic trends from natural decadal variability ( [[#Vecchi--2006|Vecchi et al., 2006]] ; [[#Vecchi--2007|Vecchi and Soden, 2007]] ; [[#DiNezio--2009|DiNezio et al., 2009]] ).

Cross-Chapter Box [[#Atlas.2|Atlas.2]]

'''Cross-Chapter Box [[#Atlas.2|Atlas.2]] , Table'''

'''1 |'''

'''Summary of observed trends for Small Island regions.''' SLR = sea level rise; TC = tropical cyclone; SPCZ = South Pacific Convergence Zone.
[[File:4d6bb8f17fb5465e4d97036fab9ce07b IPCC_AR6_WGI_Atlas_CCB_Atlas_2_Table_1_1.jpg]] [[File:847b4717e77c22a0397dce54f775beea IPCC_AR6_WGI_Atlas_CCB_Atlas_2_Table_1_2.jpg]]
Cross-Chapter Box [[#Atlas.2|Atlas.2]]

Southern Hemisphere subtropical Pacific June–November drying has been associated with intensification of the subtropical ridge and associated declines in baroclinicity ( [[#Whan--2014|Whan et al., 2014]] ). Austral summer drying in the south-west French Polynesia sub-region has been linked with increased greenhouse gas and ozone changes ( [[#Fyfe--2012|Fyfe et al., 2012]] ). The Southern Hemisphere jet stream has ''likely'' shifted polewards ( [[IPCC:Wg1:Chapter:Chapter-2#2.3.1.4.3|Section 2.3.1.4.3]] ) which is attributed largely to a trend in the Southern Annular Mode ( [[IPCC:Wg1:Chapter:Chapter-3#3.7.2|Section 3.7.2]] ).

These assessments are constrained by limited availability of observational datasets and of scientific studies. Assessment of observed climate change for Small Islands is often constrained by low station density ( [[#Ryu--2014|Ryu and Hayhoe, 2014]] ; P.D. [[#Jones--2016|Jones et al., 2016]] a), short periods of record, digitization requirements or data-sharing limitations (P.D. [[#Jones--2016|Jones et al., 2016]] a), availability of metadata ( [[#McGree--2014|McGree et al., 2014]] ; [[#Stephenson--2014|Stephenson et al., 2014]] ; P.D. [[#Jones--2016|Jones et al., 2016]] b), challenges in some gridded product representations of variability, for example, for complex topography ( [[#Frazier--2017|Frazier and Giambelluca, 2017]] ), and challenges characterizing the impact of vertical land motion on sea level rise ( [[#Atlas.10.2|Atlas.10.2]] ; [[#Wöppelmann--2016|Wöppelmann and Marcos, 2016]] ).

'''Information on future climate changes'''
Small Islands will ''very likely'' continue to warm this century, though at a rate less than the global average (Figure Atlas.28), with consequent increased frequency of warm extremes for the Caribbean and western Pacific islands, and heatwave events for the Caribbean ( ''high confidence'' ) (Table 11.13). Annual and JJA rainfall declines are ''likely'' for some Indian and southern Pacific ocean regions with drying over southern French Polynesia (attributed partially to greenhouse gas increases) and farther east clearly evident in CMIP5 and CMIP6 projections ( ''high confidence'' ) (Figure Atlas.28). See also Section [[#Atlas.10.4|Atlas.10.4]] .

Rainfall is ''very likely'' to decline over the Caribbean, in the annual mean and especially in JJA, with a stronger and more coherent signal in CMIP6 compared to CMIP5 (Figure Atlas.28 and Interactive Atlas) and reductions of 20–30% by the end of the century under high future emissions (SSP5-8.5). This JJA drying has been linked to a future strengthening of the Caribbean low level jet (CLLJ) ( [[#Taylor--2013a|Taylor et al., 2013a]] ), a westward expansion and intensification of the NASH, stronger low-level easterlies over the region, a southwardly-placed eastern Pacific ITCZ ( [[#Rauscher--2008|Rauscher et al., 2008]] ), and changing dynamics due to increased greenhouse gas concentrations ( ''very high confidence'' ) (W. [[#Li--2012|]] [[#Li--2012|Li et al., 2012]] ). Projections from 15 GCM and two RCM experiments for 2080–2089 relative to 1970–1989 were for a generally drier Caribbean and a robust summer drying ( [[#Karmalkar--2013|Karmalkar et al., 2013]] ). More recent downscaling studies (e.g., [[#Taylor--2018|Taylor et al., 2018]] ; [[#Vichot-Llano--2021a|Vichot-Llano et al., 2021a]] ) also project a drier Caribbean and longer dry spells ( [[#Van%20Meerbeeck--2020|Van Meerbeeck, 2020]] ).

Sea level rise is ''very likely'' to continue in all Small Island regions (Sections 9.6.3.3 and 12.4.7.4, and Figure Atlas.28) and its effects will be compounded by TC surge events. In general, the most intense TCs are ''likely'' to intensify and produce more flood rains with warming, however detailed effects of climate change on TCs will vary by region ( [[IPCC:Wg1:Chapter:Chapter-11#11.7.1|Section 11.7.1]] ; [[#Knutson--2019|Knutson et al., 2019]] ). [[#Bailey--2016|Bailey et al. (2016)]] projected a 20% decline in groundwater availability by 2050 in coral atoll islands of the Federated States of Micronesia and stressed that under higher sea level rises the decrease could be higher than 50% due to marine water intrusion into aquifers, as well as drought events.

'''Summary of information distilled from multiple lines of evidence'''

It is ''very likely'' that most Small Islands have warmed over the period of instrumental records. The clearest precipitation trend is a ''likely'' decrease in JJA rainfall over the Caribbean since 1950. There is ''limited evidence'' and ''low agreement'' for the cause of the observed drying trend, whether it is mainly caused by decadal-scale internal variability or anthropogenic forcing, but it is ''likely'' that it will continue over coming decades. It is ''likely'' that drying has occurred since the mid-20th century in some parts of the Pacific poleward of 20° latitude in both the Northern Hemisphere and the Southern Hemisphere and that these changes will continue over coming decades. Rainfall trends in most other Pacific Ocean and Indian Ocean Small Islands are mixed and largely non-significant. It is ''very likely'' that sea levels will continue to rise in all Small Island regions, and this will result in increased coastal flooding with the potential to increase saltwater intrusion into aquifers in Small Islands.

Whilst this assessment demonstrates that the climate of Small Islands has and will continue to change in diverse ways, constructing climate information for Small Islands is challenging. This is due to observational issues, incomplete understanding of some modes of variability and their representation by climate models and the lack of availability of large ensembles of regional climate model simulations and limited studies to decouple internal variability and anthropogenic influences.

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