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

==== 12.4.4.5 Coastal and Oceanic ====

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'''Relative sea level:''' Around Central and South America, over 1900–2018, a new tide gauge-based reconstruction finds a regional mean relative sea level (RSL) change of 2.07 [1.36 to 2.77] mm yr <sup>–1</sup> in the South Atlantic, 2.49 [1.89 to 3.06] mm yr <sup>–1</sup> in the subtropical North Atlantic and 1.20 [0.76 to 1.62] mm yr <sup>–1</sup> in the East Pacific ( [[#Frederikse--2020|Frederikse et al., 2020]] ), compared to a global mean sea level (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, these RSLR rates, based on satellite altimetry, increased to 3.45 [3.04 to 3.86)] mm yr <sup>–1</sup> , 4.04 [2.77 to 5.24] mm yr <sup>–1</sup> and 2.35 [0.70 to 4.06] 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).

Relative sea level rise is ''extremely likely'' to continue in the oceans around Central and South America. Regional mean RSLR projections for the oceans around Central and South America range from 0.3–0.5 m under SSP1-2.6 to 0.5–0.9 m under SSP5-8.5 for 2081–2100 relative to 1995–2014 (median values), which is around the 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 ranges from 0.5 to 2.5 m around most of Central and South America, except in SSA and SWS, where 1-in-100-year ETWLs can be as large as 5 to 6 m ( [[#Vousdoukas--2018|Vousdoukas et al., 2018]] ). ETWL 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 8–34 cm and by 10–43 cm by 2050 under RCP4.5 and RCP8.5 respectively ( [[#Vousdoukas--2018|Vousdoukas et al., 2018]] ; [[#Kirezci--2020|Kirezci et al., 2020]] ). By 2100, this range is projected to be 21–93 cm and 34–190 cm under RCP4.5 and RCP8.5 respectively ( [[#Vousdoukas--2018|Vousdoukas et al., 2018]] ; [[#Kirezci--2020|Kirezci et al., 2020]] ). Furthermore, under RCP4.5, the present-day 1-in-100-year ETWL is projected to have median return periods of 1-in-10-years to 1-in-50-years by 2050 and 1-in-1-year by 2100 in SES, SSA and SWS. In other regions of Central and South America, the present-day 1-in-100-year ETWL is projected to occur once per year or more by both 2050 and 2100 under RCP4.5. 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 ( [[#Vitousek--2017|Vitousek et al., 2017]] ).

'''Coastal erosion:''' According to satellite data, shoreline retreat rates of around 1 m yr <sup>–1</sup> have been observed along the sandy coasts of SCA, SES and SSA over the period 1984–2015, while shoreline progradation rates of around 0.25 m yr <sup>–1</sup> has been observed in NWS and NSA. The sandy shorelines in NES and SWS have remained more or less stable over the same period ( [[#Luijendijk--2018|Luijendijk et al., 2018]] ; [[#Mentaschi--2018|Mentaschi et al., 2018]] ). Using satellite observations, [[#Mentaschi--2018|Mentaschi et al. (2018)]] report a coastal area loss of 250 km <sup>2</sup> over a 30-year period (1984–2015) along the Pacific coast of South America, and of 780 km <sup>2</sup> along the Atlantic coastlines.

Projections indicate that a majority of sandy coasts in the region will experience shoreline retreat throughout the 21st century ( ''high confidence'' ). Median shoreline change projections (CMIP5) for the mid-century period show that, relative to 2010, sandy shorelines will retreat by between 30 and 75 m in SCA, NES, SES and SSA under both RCP4.5 and RCP8.5, while the projected mid-century retreats are less than 30 m in NSA, NWS and SWS for both RCPs ( [[#Vousdoukas--2020b|Vousdoukas et al., 2020b]] ). Parts of the coastline in these latter three regions are projected to prograde over the 21st century, if present ambient shoreline change trends continue ( [[#Vousdoukas--2020b|Vousdoukas et al., 2020b]] ). By 2100, median retreats of more than 100 m are projected in SCA, NES, SES and SSA under both RCPs, while retreats between 50–100 m are projected for NSA, NWS and SWS under both RCPs (Figure 12.8). Notably, the projected shoreline retreats in SCA and SES approach 150 m by 2100 under RCP4.5 and 200 m under RCP8.5. The total length of sandy coasts in Central and South America that is projected to retreat by more than a median of 100 m by 2100 under RCP4.5 and RCP8.5 is about 12,000 and 15,000 km respectively, an increase of approximately 30%.

'''Marine heatwave:''' The mean sea surface temperature (SST) of the Atlantic Ocean and the Caribbean around Central and South America increased from 0.25°C to 1°C over the period 1982–1998 ( [[#Oliver--2018|Oliver et al., 2018]] ). This mean ocean surface warming is connected to longer and more frequent marine heatwaves (MHW) in the region ( [[#Oliver--2018|Oliver et al., 2018]] ). Over the period 1982–2016, the coastlines experienced on average more than 1.0 MHW per year, with the Pacific coast of Northern Central America and the coast of SES (Atlantic) experiencing on average 2.5–3 MHWs per year. The average duration was between 10 and 15 days, with the notable exception of the equatorial Pacific coastline, which experiences MHWs with &gt;30 days average duration related to ENSO conditions. In the south-western Atlantic shelf (32–38°S), more than half of the days with MHWs have occurred since 2014, and the most intense event (1.7°C above previous maximum) took place in the austral summer of 2017 ( [[#Manta--2018|Manta et al., 2018]] ). Changes over the 20th century, derived from MHW proxies, show an increase in frequency between 0.5 and 2 MHW per decade in the South Atlantic, the Caribbean and the Pacific coast of Northern Central America, an increase in intensity per event in the South Atlantic, and a decrease along the equatorial Pacific coastline. The total number of MHW days per year increases around Central and South America, with the exception of the equatorial Pacific coastline ( [[#Oliver--2018|Oliver et al., 2018]] ).

There is ''high confidence'' that MHWs will increase around Central and South America. Mean SST is projected to increase by 1°C (2°C) by 2100, with a hotspot of about 2°C (4°C) along the coast of South-Eastern South America and North-Western South America under RCP4.5 (RCP8.5; Interactive Atlas). More frequent MHWs are projected around the region for GWLs of 1.5°C, 2°C and 3.5°C relative to the modelled reference value for 1861–1880 ( [[#Frölicher--2018|Frölicher et al., 2018]] ). Projections for SSP1-2.6 and SSP5-8.5 both show an increase in MHWs around Central and South America by 2081–2100, relative to 1985–2014 (Box 9.2, Figure 1).

'''In summary, relative sea level rise is''' extremely likely '''to continue in the oceans around Central and South America, 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 Central and South America and associated confidence levels are illustrated in Table 12.6. No assessable literature could be found for sand and dust storm, lake and sea ice, heavy snowfall and ice storms, hail and snow avalanches, although these phenomena may be relevant in parts of the region.

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'''Table 12.6''' '''|''' '''Summary of confidence in direction of projected change in climatic impact-drivers in Central and South America, 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 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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