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

==== 12.4.1.5 Coastal and Oceanic ====

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'''Relative sea level:''' Around Africa, from 1900 to 2018, a new tide gauge-based reconstruction finds a regional mean RSL change of 2.07 [1.36 to 2.77] mm yr <sup>–1</sup> in the South Atlantic and 1.33 [0.80 to 1.86] mm yr <sup>–1</sup> in the Indian Ocean ( [[#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). 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> and 3.65 [3.23 to 4.08] 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 ''virtually certain'' to continue in the oceans around Africa. Regional mean RSLR projections for the oceans around Africa range from 0.4–0.5 m under SSP1-2.6 to 0.8–0.9 m under SSP5-8.5 for 2081–2100 relative to 1995–2014 (median values), which is within the range of 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 extreme total water level is between 0.1 and 1.2 m around Africa, with values around 1 m or above along the south-west, south-east and central east coasts ( [[#Vousdoukas--2018|Vousdoukas et al., 2018]] ).

Extreme total water level (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 continent, the 5th–95th percentile range of the 1-in-100-year ETWL is projected to increase (relative to 1980–2014) by 7–36 cm and by 14–42 cm by 2050 under RCP4.5 and RCP8.5 respectively. By 2100, this range is projected to be 28–86 cm and 43–190 cm under RCP4.5 and RCP8.5 respectively ( [[#Vousdoukas--2018|Vousdoukas et al., 2018]] ; [[#Kirezci--2020|Kirezci et al., 2020]] ). In terms of ETWL occurrence frequencies, the present-day 1-in-100-year ETWL is projected to have median return periods of around 1-in-10-years to 1-in-20-years by 2050 and 1-in-1-year to 1-in-5-years by 2100 in southern and North Africa and occur more than once per year by 2050 and 2100 in most of East and West Africa under RCP4.5 ( [[#Vousdoukas--2018|Vousdoukas et al., 2018]] ). 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 in Africa ( [[#Vitousek--2017|Vitousek et al., 2017]] ).

'''Coastal erosion:''' Shoreline retreat rates up to 1 m yr <sup>–1</sup> have been observed around the continent during 1984–2015, except in ESAF, which has experienced a shoreline progradation rate of 0.1 m/r over the same period ( [[#Luijendijk--2018|Luijendijk et al., 2018]] ; [[#Mentaschi--2018|Mentaschi et al., 2018]] ). [[#Mentaschi--2018|Mentaschi et al. (2018)]] report a coastal area losses of 160 km <sup>2</sup> and 460 km <sup>2</sup> over a 30-year period (1984–2015) along the Atlantic and Indian Ocean coasts of the continent. At the more regional level, in Ghana along the Gulf of Guinea about 79% of the shoreline was found to be retreating while 21% was found to be stable or prograding over the period 1974–1996 ( [[#Addo--2016|Addo and Addo, 2016]] ).

Projections indicate that a vast majority of sandy coasts in the region will experience shoreline retreat throughout the 21st century ( ''high confidence'' ), while parts of the ESAF and western MDG coastline are projected to prograde over the 21st century, if present ambient trends continue. Median shoreline change projections (CMIP5), relative to 2010, presented by [[#Vousdoukas--2020b|Vousdoukas et al. (2020b)]] show that, under RCP4.5, shorelines in Africa will retreat by between 30 m (SAH, NEAF, WSAF, ESAF, MDG) and 55 m (WAF, CAF), by mid-century. By the same period but under RCP8.5, the median shoreline retreat is projected to be between 35 m (SAH, NEAF, WSAF, ESAF) and 65 m (WAF, CAF). By 2100, more than 100 m of median retreat is projected in WAF, CAF and SEAF under RCP4.5, while under RCP8.5, more than 100 m of shoreline retreat is projected in all regions except NEAF and WSAF. Under RCP8.5 especially, the projected retreat by 2100 is greater than 150 m in WAF and CAF. The total length of sandy coasts in Africa that is projected to retreat by more than a median of 100 m by 2100 under RCP4.5 and RCP8.5 is about 13,000 km and 17,000 km respectively, an increase of approximately 33%.

'''Marine heatwave (MHW):''' From 1982 to 2016, the coastal oceans of Africa have experienced on average 2–3 MHWs per year, with the coastal oceans around the southern half of the continent experiencing on average 2.5–3 MHWs per year. The average duration was between 5 and 15 days ( [[#Oliver--2018|Oliver et al., 2018]] ). Changes over the 20th century, derived from MHW proxies, show an increase in frequency between 0.5 and 2 MHWs per decade over the region, especially off the Horn of Africa; an increase in intensity per event around Southern Africa; and an increase in MHW duration along the North African coastlines ( [[#Oliver--2018|Oliver et al., 2018]] ).

There is ''high confidence'' that MHWs will increase around Africa. Mean SST, a common proxy for MHWs, is projected to increase by 1°C (2°C) around Africa by 2100, with a hotspot of around 2°C (5°C) along the coastlines of South Africa under RCP4.5 (RCP8.5; Interactive Atlas). Under global warming conditions, MHW intensity and duration will increase in the coastal zones of all sub-regions of Africa ( [[#Frölicher--2018|Frölicher et al., 2018]] ). Projections for SSP1-2.6 and SSP5-8.5 both show an increase in MHWs around Australasia by 2081–2100, relative to 1985–2014 (Box 9.2, Figure 1).

'''In general, there is''' high confidence '''that most coastal- and ocean-related hazards in Africa will increase over the 21st century. Relative sea level rise is''' virtually certain '''to continue around Africa, 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 CIDs for Africa and associated confidence levels are illustrated in Table 12.3. No relevant literature could be found for permafrost and hail, although these phenomena may be relevant in parts of the continent.

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'''Table 12.3''' '''|''' '''Summary of confidence in direction of projected change in climatic impact-drivers in Africa, 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]] .

[[File:cd304a9910c93ec3527dd226ee411456 IPCC_AR6_WGI_Chapter12_Table_12_3.jpg]]

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