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

==== 12.4.5.5 Coastal and Oceanic ====

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'''Relative sea level:''' Around Europe, over 1900–2018, a new tide gauge-based reconstruction finds a regional mean RSL change of 1.08 [0.79 to 1.38] mm yr <sup>–1</sup> in the subpolar North Atlantic ( [[#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, the RSLR rates around Europe, based on satellite altimetry, increased to 2.17 [1.66 to 2.66] 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 ''extremely likely'' to continue in the oceans around Europe. Regional mean RSLR projections for the oceans around Europe range from 0.4–0.5 m under SSP1-2.6 to 0.7–0.8 m under SSP5-8.5 for 2081–2100 relative to 1995–2014 (median values), which means that there are locally large deviations from 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]] ). The signal is strongest for the North Sea and Atlantic coasts, followed by the Black Sea. The Baltic Sea, on the contrary, shows the lowest increase due to land uplift ( [[#Vousdoukas--2017|Vousdoukas et al., 2017]] ). The model agreement is higher for the Mediterranean and in line with previous findings by [[#Gualdi--2013|Gualdi et al. (2013)]] .

'''Coastal flood:''' The present-day 1-in-100-year ETWL is between 0.5 and 1.5 m in the MED basin and 2.5 and 5.0 m in the western Atlantic European coasts, around the UK and along the North Sea coast, and lower at 1.5–2.5 m along the Baltic Sea coast ( [[#Kirezci--2020|Kirezci et al., 2020]] ). Similar values are reported by [[#Vousdoukas--2018|Vousdoukas et al. (2018)]] .

There is ''high confidence'' that extreme total water level (ETWL) magnitude and occurrence frequency will increase throughout Europe (see Figure 12.4p–r), except in the northern Baltic Sea. Across the region, the 5–95th percentile range of the 1-in-100-year ETWL is projected to increase (relative to 1980–2014) by 4–40 cm and by 6–47 cm by 2050 under RCP4.5 and RCP8.5, respectively. By 2100, this range is projected to be 6–88 cm and 25–186 cm under RCP4.5 and RCP8.5, respectively (Figure 12.SM.6; [[#Vousdoukas--2018|Vousdoukas et al., 2018]] ; [[#Kirezci--2020|Kirezci et al., 2020]] ). Mass addition across the Gibraltar Strait may play a role, although the extent of this contribution is currently unclear ( [[#Lionello--2017|Lionello et al., 2017]] ). Furthermore, under RCP4.5, the present day 1-in-100-year ETWL is projected to have median return periods of between 1-in-5 and 1-in-20 years by 2050 and occur at least once per year by 2100 in the Mediterranean and Black Sea, while in the rest of Europe it is mostly projected to have median return periods of between 1-in-20-years and 1-in-50-years by 2050 and between 1-in-5-years and 1-in-20-years by 2100 ( [[#Vousdoukas--2018|Vousdoukas et al., 2018]] ). Under RCP8.5, occurrence of the present day 1-in-100-year ETWL is projected to increase further to median return periods of 1-in-1-year to 1-in-5-years by 2050 and occur more than once per year by 2100 in the Mediterranean and Black Sea, while in the rest of Europe it is mostly projected to have median return periods between 1-in-5-years and more than once per year by 2100.

'''Coastal erosion:''' Satellite-derived shoreline change estimates over 1984–2015 indicate shoreline retreat rates of around 0.5 m yr <sup>–1</sup> along the sandy coasts of WCE and MED, around 4 m yr <sup>–1</sup> in EEU (Caspian Sea region) and more or less stable shorelines in NEU ( [[#Luijendijk--2018|Luijendijk et al., 2018]] ; [[#Mentaschi--2018|Mentaschi et al., 2018]] ). [[#Mentaschi--2018|Mentaschi et al. (2018)]] report a coastal area loss of 270 km <sup>2</sup> over a 30-year period (1984–2015) along the Atlantic coastlines of Europe.

Projections indicate that sandy coasts throughout the continent (except those bordering the northern Baltic Sea) will experience shoreline retreat through the 21st century ( ''high confidence'' ). Median shoreline change projections (CMIP5) relative to 2010, show that, by mid-century, shorelines will retreat by between 25 m and 60 m along sandy coasts in WCE and MED under both RCP4.5 and RCP8.5 ( [[#Athanasiou--2020|Athanasiou et al., 2020]] ; [[#Vousdoukas--2020b|Vousdoukas et al., 2020b]] ). Mid-century median projections for NEU indicate virtually no shoreline retreat under RCP4.5, but a retreat of around 40 m under RCP8.5. By 2100, median shoreline retreats of around 50 m are projected in NEU and MED under RCP4.5, increasing to around 80 m under RCP8.5. End-century median projections for WCE are far higher at 100 m (RCP4.5) and 160 m (RCP8.5). The total length of sandy coasts in Europe 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 km and 18,000 km respectively, an increase of approximately 54% ( [[#Vousdoukas--2020b|Vousdoukas et al., 2020b]] ).

Local assessments of both long term shoreline retreat and episodic coastal erosion are given by [[#Li--2014b|Li et al. (2014b)]] , [[#Toimil--2017|Toimil et al. (2017)]] , [[#Bon%20de%20Sousa--2018|Bon de Sousa et al. (2018)]] and [[#Le%20Cozannet--2019|Le Cozannet et al. (2019)]] . In terms of episodic coastal erosion, 31–88% of all Aegean beaches are projected to experience complete erosion, with a RCP4.5 sea level rise of 0.5 m and a surge of 0.6 m, but with substantial uncertainty ( [[#Monioudi--2017|Monioudi et al., 2017]] ).

'''Marine heatwave:''' The mean SST of the Atlantic Ocean and the Mediterranean has increased between 0.25°C and 1°C since 1982–1998. This mean ocean surface warming is correlated to longer and more frequent marine heatwaves in the region ( [[#Oliver--2018|Oliver et al., 2018]] ). Over the period 1982–2016, the coastlines of Europe experienced on average more than 2.0 MHW yr <sup>–1</sup> , with the eastern Mediterranean and Scandinavia experiencing 2.5–3 MHWs yr <sup>–1</sup> . The average duration was between 10 and 15 days. Changes over the 20th century, derived from MHW proxies, show an increase in frequency of between 1.0 and 2.0 MHWs per decade in Europe, although the trend is not statistically significant; with an increase in intensity per event in the North Atlantic and the Mediterranean, and a decrease in the Atlantic off the British Isles. The total number of MHW days per decade has increased in the Mediterranean ( [[#Oliver--2018|Oliver et al., 2018]] ).

Mean SST is projected to increase by 1°C–3°C around Europe by 2100, with a hotspot of around 4°C–5°C along the Arctic coastline of Europe under RCP4.5 and RCP8.5 scenarios (see Interactive Atlas), leading to a continued increase in MHW frequency, magnitude and duration ( [[#Oliver--2018|Oliver et al., 2018]] ; [[#MedECC--2020|MedECC, 2020]] ). Projections for SSP1-2.6 and SSP5-8.5 both show an increase in MHWs around Europe by 2081–2100, relative to 1985–2014 (Box 9.2, Figure 1). [[#Darmaraki--2019|Darmaraki et al. (2019)]] project that, by the end of the 21st century and under RCP8.5, there will be one MHW occurring every year in the northern Mediterranean sea, and that these MHWs would be three months longer, four times more intense, and 42 times more severe than present day MHWs in the region. [[#Frölicher--2018|Frölicher et al. (2018)]] show that, in Europe, the change in the probability for the number of days of MHWs exceeding the 99th percentile of the pre-industrial level is 4%, 15% and 30% for global warming levels of 1°C, 2°C and 3.5°C, respectively. MHW increase in the Mediterranean will impact on many species that live in shallow waters and have reduced motility, with consequences for related economic activities ( [[#Galli--2017|Galli et al., 2017]] ).

'''In general, there is''' high confidence '''that most coastal/ocean-related climatic impact-drivers in Europe will increase over the 21st century for all scenarios and time horizons. Relative sea level rise is''' extremely likely '''to continue around Europe (except in the northern Baltic Sea), contributing to increased coastal flooding in low-lying areas 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 ''').'''

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