Editing IPCC:AR6/WGII/Chapter-13 (section)

=== 13.2.1 Observed Impacts and Projected Risks ===

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==== 13.2.1.1 Risk of Coastal Flooding and Erosion ====

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Almost 50 million Europeans live within 10 m above mean sea level ( [[#Vousdoukas--2020|Vousdoukas et al., 2020]] ; McEvoy et al., 2021). Without further adaptation ( [[#13.2.2|Section 13.2.2]] ), flood risks along Europe’s low-lying coasts and estuaries will increase due to SLR compounded by storm surges, rainfall and river runoff ( ''high confidence'' ) ( [[#Mokrech--2015|Mokrech et al., 2015]] ; [[#Arns--2017|Arns et al., 2017]] ; [[#Sayol--2018|Sayol and Marcos, 2018]] ; [[#Vousdoukas--2018a|Vousdoukas et al., 2018a]] ; [[#Bevacqua--2019|Bevacqua et al., 2019]] ; [[#Couasnon--2020|Couasnon et al., 2020]] ). The population at risk of a 100-year flood event starts to rapidly increase beyond 2040 ( [[#Vousdoukas--2018a|Vousdoukas et al., 2018a]] ) reaching 10 million people under RCP8.5 by 2100, but it stays just below 10 million people under RCP2.6 by 2150 (Figure 13.5; [[#Haasnoot--2021b|Haasnoot et al., 2021b]] ) assuming present population and protection. The number of people at risk is projected to increase and risk to materialise earlier especially in response to increasing population under SSP5 ( [[#Vousdoukas--2018a|Vousdoukas et al., 2018a]] ; [[#Haasnoot--2021b|Haasnoot et al., 2021b]] ). Under high rates of SLR resulting from rapid ice sheet loss from Antarctica, risks may increase by a third by 2150 ( [[#Haasnoot--2021b|Haasnoot et al., 2021b]] ). Expected annual (direct) damages due to coastal flooding are projected to rise from 1.3 billion EUR today to 13–39 billion EUR by 2050 between 2°C and 2.5°C GWL and 93–960 billion EUR by 2100 between 2.5° and 4.4°C GWL, largely depending on socioeconomic developments (Cross-Chapter Box SLR in Chapter 3; [[#Vousdoukas--2018a|Vousdoukas et al., 2018a]] ) ( ''high confidence'' in the sign; ''low confidence'' in the numbers). UNESCO World Heritage sites in the coastal zone are at risk due to SLR, coastal erosion and flooding ( [[#13.8.1.3|Section 13.8.1.3]] ; Cross-Chapter Paper 4; [[#Marzeion--2014|Marzeion and Levermann, 2014]] ; [[#Reimann--2018b|Reimann et al., 2018b]] ) as are coastal landfills and other key infrastructures in Europe (AR6/SROCC; [[#Brand--2018|Brand et al., 2018]] ; [[#Beaven--2020|Beaven et al., 2020]] ).

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[[File:45f5737249250a5357d80d774e41fd9c IPCC_AR6_WGII_Figure_13_005.png]]

'''Figure 13.5 |''' '''Sea level rise (SLR) vulnerability and national planning in Europe:'''

'''(a)''' map of countries in Europe summarising the amount of SLR each country is planning for, at different time horizons (blue bars), and the present population (2020) at risk of a 100-year coastal flood event (orange bars) ( [[#Haasnoot--2021b|Haasnoot et al., 2021b]] ). The amounts of SLR and time horizons reflect national guidance or planning (local or project-based levels may differ) (McEvoy et al., 2021);

'''(b)''' projected population at risk to experience a 1-in-10-year coastal flood event under RCP2.6-SSP1 and RCP8.5-SSP5 assuming present protection and population levels, as well as population change according to, respectively, SSP1 and SSP5, based on Merkens (2016);

'''(c)''' projected population at risk to experience a 1-in-100-year coastal flood event under RCP2.6-SSP1 and RCP8.5-SSP5, assuming the present protection and population levels, as well as population change according to, respectively, SSP1 and SSP5, based on Merkens (2016) (based on [[#Haasnoot--2021b|Haasnoot et al., 2021b]] ).

Observations indicate that soft cliffs and beaches are most affected by erosion in Europe with, for example, 27–40% of Europe’s sandy coast eroding today, without climate change being identified as the main driver so far ( [[#Pranzini--2015|Pranzini et al., 2015]] ; [[#Luijendijk--2018|Luijendijk et al., 2018]] ; [[#Mentaschi--2018|Mentaschi et al., 2018]] ; [[#Oppenheimer--2019|Oppenheimer et al., 2019]] ). SLR will increase coastal erosion of sandy shorelines ( ''high confidence'' ) ( [[#Ranasinghe--2021|Ranasinghe et al., 2021]] ), but there is ''low confidence'' in quantitative values assessment of erosion rates and amounts ( [[#Athanasiou--2019|Athanasiou et al., 2019]] ; [[#Le%20Cozannet--2019|Le Cozannet et al., 2019]] ; [[#Thieblemont--2019|Thieblemont et al., 2019]] ). Without nourishment or other natural or artificial barriers to erosion, sandy shorelines could retreat by about 100 m in Europe at 4°C GWL; limiting warming to 3°C GWL could reduce this value by one-third ( [[#Vousdoukas--2020|Vousdoukas et al., 2020]] ).

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==== 13.2.1.2 Risks Related to Inland Water ====

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===== 13.2.1.2.1 Riverine and pluvial flooding =====

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Precipitation has raised river flood hazards in WCE and the UK by 11% per decade from 1960 to 2010 and decreased in EEU and SEU by 23% per decade ( [[#Douville--2021|Douville et al., 2021]] ; [[#Ranasinghe--2021|Ranasinghe et al., 2021]] ). The most recent three decades had the highest number of floods in the past 500 years with increases in summer ( [[#Blöschl--2020|Blöschl et al., 2020]] ). Economic flood damages increased strongly, reflecting increasing exposure of people and assets ( [[#Visser--2014|Visser et al., 2014]] ; [[#Hoegh-Guldberg--2018|Hoegh-Guldberg et al., 2018]] ; [[#Merz--2021|Merz et al., 2021]] ).

Projections indicate a continuation of the observed trends of river flood hazards in WCE ( ''high confidence'' ) of 10% at 2°C GWL and 18% at 4.4°C GWL, and a decrease in NEU and SEU ( ''medium confidence'' ) with, respectively, 5 and 11% in NEU and SEU for a 100-year peak flow, making Europe one of the regions with the largest projected increase in flood risk ( [[#Di%20Sante--2021|Di Sante et al., 2021]] ; [[#Ranasinghe--2021|Ranasinghe et al., 2021]] ). While there is disagreement on the magnitude of economic losses and people affected, there is ''high agreement'' on direction of change, particularly in WCE ( [[#Alfieri--2018|Alfieri et al., 2018]] ). New research increases confidence in AR5 statements that without adaptation measures, increases in extreme rainfall will substantially increase direct flood damages (e.g., [[#Madsen--2014|Madsen et al., 2014]] ; [[#Alfieri--2015a|Alfieri et al., 2015a]] ; [[#Alfieri--2015b|Alfieri et al., 2015b]] ; [[#Blöschl--2017|Blöschl et al., 2017]] ; [[#Dottori--2020|Dottori et al., 2020]] ; [[#Mentaschi--2020|Mentaschi et al., 2020]] ). With low adaptation, damages from river flooding are projected to be three times higher at 1.5°C GWL, four times at 2°C GWL and six times at 3°C GWL ( [[#Alfieri--2018|Alfieri et al., 2018]] ; [[#Dottori--2020|Dottori et al., 2020]] ). At 2°C GWL, the incidence of summer floods is expected to decrease across the whole alpine region, whereas winter and spring floods will increase due to extreme precipitation ( [[#Gobiet--2014|Gobiet et al., 2014]] ) and snowmelt-driven runoff ( [[#Coppola--2018|Coppola et al., 2018]] ).

Pluvial flooding and flash floods due to intense rainfall constitute most flood events in SEU and a substantial risk in other European regions (Cross-Chapter Paper 4; [[#Llasat--2016|Llasat et al., 2016]] ; [[#Rudd--2020|Rudd et al., 2020]] ). The majority (56%) of flood events between 1860 and 2016 were flash floods ( [[#Paprotny--2018a|Paprotny et al., 2018a]] ). These floods had considerable impacts including danger to human lives, for example, causing total economic damage of 1 billion USD in Copenhagen (Denmark) in 2011 ( [[#Wójcik--2013|Wójcik et al., 2013]] ), damage to private households of more than 70 million EUR in Münster (Germany) in 2014 ( [[#Spekkers--2017|Spekkers et al., 2017]] ) and during the 2021 floods in Belgium, Germany and the Netherlands over 200 deaths, damage to thousands of homes and disrupted water and electricity supply ( [[#Kreienkamp--2021|Kreienkamp et al., 2021]] ). The intensity and frequency of heavy rainfall events is projected to increase ( ''high confidence'' ) (Figure 13.3; [[#Ranasinghe--2021|Ranasinghe et al., 2021]] ). Combined with increasing urbanisation, the risk of pluvial flooding is projected to increase ( [[#Westra--2014|Westra et al., 2014]] ; [[#Rosenzweig--2018|Rosenzweig et al., 2018]] ; [[#Papalexiou--2019|Papalexiou and Montanari, 2019]] ). Small catchments, steep river channels and cities are particularly vulnerable due to large areas of impermeable surfaces where water cannot penetrate ( [[#13.6|Section 13.6]] ).

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===== 13.2.1.2.2 Low Flows and Water Scarcity =====

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The frequency and severity of low flows are projected to increase, making streamflow drought and water scarcity more severe and persistent in SEU and WCE ( ''medium confidence'' ) (Figure 13.3; [[#Ranasinghe--2021|Ranasinghe et al., 2021]] ), but decreases are projected in most of NEU except the southern UK ( [[#Forzieri--2014|Forzieri et al., 2014]] ; [[#Prudhomme--2014|Prudhomme et al., 2014]] ; [[#Schewe--2014|Schewe et al., 2014]] ; [[#Roudier--2016|Roudier et al., 2016]] ; [[#Ranasinghe--2021|Ranasinghe et al., 2021]] ). In EEU, uncertainty about changes in water scarcity pose distinct challenges for adaptation ( [[#Greve--2018|Greve et al., 2018]] ). At 1.5°C GWL, the number of days with water scarcity (water availability as opposed to water demand) and drought will increase slightly in SEU ( [[#Schleussner--2016|Schleussner et al., 2016]] ; [[#Naumann--2018|Naumann et al., 2018]] ), resulting in 18% of the population exposed to at least moderate water scarcity, increasing to 54% at 2°C GWL ( [[#Byers--2018|Byers et al., 2018]] ). Moderate water scarcity is emerging in some parts of WCE ( [[#Bisselink--2018|Bisselink et al., 2018]] ) increasing to16% of the population under 2°C GWL and SSP2 ( [[#Byers--2018|Byers et al., 2018]] ). Under 4°C GWL, areas in WCE experience water scarcity, especially in summer and autumn. Future intensive water use can aggravate the situation, in particular in SEU (Sections 13.5.1, 13.10.3).

Groundwater abstraction rates reach up to 100 million m³ yr –1 across WCE and SEU, and exceed 100 million m³ yr –1 in parts of SEU ( [[#Wada--2016|Wada, 2016]] ). Low recharge rates lead to a depletion of groundwater resources in parts of SEU and WCE ( [[#Doll--2014|Doll et al., 2014]] ; [[#Wada--2016|Wada, 2016]] ; [[#de%20Graaf--2017|de Graaf et al., 2017]] ), increasing the impacts on water scarcity in SEU. Groundwater pumping and declines in groundwater discharge already threaten environmental flow limits in many European catchments, especially in SEU, extending to almost all basins and sub-basins within the next 30–50 years ( [[#de%20Graaf--2019|de Graaf et al., 2019]] ).

The combined effect of increasing water demand and successive dry climatic conditions further exacerbates groundwater depletion and lowers groundwater levels in SEU but also WCE ( [[#Goderniaux--2015|Goderniaux et al., 2015]] ). Declines in groundwater recharge of up to 30% further increase groundwater depletion ( [[#Aeschbach-Hertig--2012|Aeschbach-Hertig and Gleeson, 2012]] ) especially in SEU and semiarid to arid regions ( [[#Moutahir--2017|Moutahir et al., 2017]] ). Even in WCE and NEU, projected increases in groundwater abstraction will impact groundwater discharge, threatening sustaining environmental flows under dry conditions ( [[#de%20Graaf--2019|de Graaf et al., 2019]] ).

The risks for soil moisture drought are projected to increase in WCE and SEU for all climate scenarios ( [[#Grillakis--2019|Grillakis, 2019]] ; [[#Tramblay--2020|Tramblay et al., 2020]] ; [[#Ranasinghe--2021|Ranasinghe et al., 2021]] ). At 3°C GWL compared with 1.5°C GWL, the drought area will increase by 40% and the population under drought by up to 42%, especially affecting SEU, and to a lesser extent in WCE ( [[#Samaniego--2018|Samaniego et al., 2018]] ).

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===== 13.2.1.2.3 Water Temperature and Quality =====

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Water temperatures in rivers and lakes have increased over the past century by ~1–3°C in major European rivers (CBS, 2014; [[#EEA--2017a|EEA, 2017a]] ; [[#Woolway--2017|Woolway et al., 2017]] ). Warming is accelerating for all European river basins ( [[#Wanders--2019|Wanders et al., 2019]] ) increasing by 0.8°C in response to 1.5°C GWL and 1.2°C for 3°C GWL relative to 1971–2000 ( [[#van%20Vliet--2016a|van Vliet et al., 2016a]] ) aggravated by declines in summer river flow.

(Ground)water extractions or drainage have caused saltwater intrusions ( [[#Rasmussen--2013|Rasmussen et al., 2013]] ; [[#Ketabchi--2016|Ketabchi et al., 2016]] ). During summer, seawater will also penetrate estuaries further upstream in response to reduced river flow and SLR, resulting in more frequent closure of water inlets in the downstream part of the rivers in a period when water is most needed ( ''high agreement, low evidence'' ) (e.g., [[#Haasnoot--2020b|Haasnoot et al., 2020b]] ).

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'''Box 13.1 | Venice and Its Lagoon'''
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Venice and its lagoon are a UNESCO World Heritage Site. This socio-ecological system is the result of millennia of interactions between people and the natural environment. It is exposed to climatic and non-climatic hazards: more frequent floods, warming, pollution, invasive species, reduction of salt marshes, hydrodynamic and bathymetric changes, and waves generated by cruise ships and boat traffic.

The elevation of the average city pedestrian level and of its inner historic area are, respectively, 105 and 55 cm above the present relative mean sea level (RMSL). Consequently, even small surges and compound events cause floods when they coincide with high tide ( [[#Lionello--2021a|Lionello et al., 2021a]] ). During the 20th century, RMSL rose at about 2.5 mm yr –1 due to SLR and land subsidence ( [[#Zanchettin--2021|Zanchettin et al., 2021]] ). The frequency of floods affecting the city has increased from once per decade in the first half of the 20th century to 40 times per decade in the period 2010–2019 (Figure Box 13.1.1a).

In 1973, the Italian government established a legal framework for safeguarding Venice and its lagoon. Construction of the flood protection system started in 2003 and was used for the first time in October 2020 ( [[#Lionello--2021b|Lionello et al., 2021b]] ). This system of mobile barriers (MoSE) closes the lagoon inlets to avoid floods when needed, while under normal conditions they lay on the seabed, thus allowing ship traffic and the exchange between the lagoon and the sea ( [[#Molinaroli--2019|Molinaroli et al., 2019]] ). To prevent flooding of the central monument area, additional measures have been proposed including inlets, expansion of salt marshes and pumping seawater into deep brackish aquifers to raise the city’s level ( [[#Umgiesser--1999|Umgiesser, 1999]] ; [[#Umgiesser--2004|Umgiesser, 2004]] ; [[#Teatini--2011|Teatini et al., 2011]] ).

Without adaptation, potential economic damages between 7 and 17 billion EUR have been estimated for the next 50 years ( [[#Caporin--2016|Caporin and Fontini, 2016]] ). Additionally, the ecosystem is vulnerable to warming ( [[#Solidoro--2010|Solidoro et al., 2010]] ) and SLR (Day Jr et al., 1999; [[#Marani--2007|Marani et al., 2007]] ). The duration of the closure of the lagoon inlets is expected to increase from 2 to 3 weeks yr –1 for RMSL rises of 30 cm, to 2 months yr –1 for 50 cm and 6 months yr –1 for 75 cm (Figure Box 13.1.1b; [[#Umgiesser--2020|Umgiesser, 2020]] ; [[#Lionello--2021b|Lionello et al., 2021b]] ), resulting in disconnection from the sea for most of the time for RMSL rise exceeding 75 cm. Frequent closures of the inlets would prevent ship traffic and in/outflow of water. For Venice, adaptation pathways considering the full range of plausible RMSL (Figure Box 13.1.1c) levels are not available, indicating a long-term adaptation gap. As planning and implementation of adaptation of this extent can take several decades ( [[#Haasnoot--2020b|Haasnoot et al., 2020b]] ; Cross-Chapter Box SLR in Chapter 3), this increases the risk that the city will not be prepared in case of rapid SLR.

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'''Figure Box 13.1.1 |''' '''Venice sea level rise (SLR) and coastal flooding: (a)''' ''evolution of relative and mean sea level in Venice and decadal frequency of floods above the safeguard level in the city centre (Frederikse et al.'' '','' 2020; [[#Lionello--2021a|Lionello et al., 2021a]] ; [[#Lionello--2021b|Lionello et al., 2021b]] ; [[#Zanchettin--2021|Zanchettin et al., 2021]] ); '''(b)''' projected relative SLR at the Venetian coast ( [[#Fox-Kemper--2021|Fox-Kemper et al., 2021]] ); “very likely ” corresponds to 5–95th percentile range, “likely ” to 17–83rd percentile range; '''(c)''' timing when critical relative sea level thresholds will be reached depending on scenarios and confidence level ( [[#Lionello--2012|Lionello, 2012]] ; [[#Umgiesser--2020|Umgiesser, 2020]] ; [[#Lionello--2021a|Lionello et al., 2021a]] ), the upper limit of the medium confidence range under SSP5–8.5 represents a low-likelihood, high-impact storyline, low confidence processes include ice sheet instability; '''(d)''' Landsat view of Venice and its lagoon with the three inlets connecting it to the Adriatic Sea.

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