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

=== 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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