This chapter gives a comprehensive overview of the main coastal impacts and risks affecting Mediterranean coasts. Due to their importance and potential impact on the Mediterranean coastal system, the main risks covered are erosion, flooding, water-related risks (e.g. saltwater intrusion and pollution), and biological risks. They are analysed at different time scales associated with drivers of different origin, as described in Chapter 2. The different risks under current conditions and under the effect of climate change are characterised, providing the respective magnitude for the Mediterranean and identifying coastal hotspots. The main impacts induced by analysed drivers and hazards are characterised by considering their potential effects on the economy (related to main economic sectors in the Mediterranean coastal zone such as tourism, agriculture, and fisheries), ecology (impacts on representative habitats and ecosystems such as deltas, wetlands, and seagrass) and on the human system (cultural heritage and human health).

It is important to bear in mind that some of the economic activities potentially affected by analysed hazards, such as coastal tourism, are also drivers of impacts along the Mediterranean coastal system. The current picture for the Mediterranean Basin is a coastal fringe affected by multiple hazards, with significant exposure due to the socio-economic cultural setting of the Mediterranean with high population density and concentration of assets and relatively high vulnerability due to the decrease in natural coastal resilience.

Coastal erosion

The Mediterranean coastline is currently experiencing significant long-term erosion. Based on observed rates, the median projected shoreline retreat by 2100 compared to 2010 is estimated to be 17 m, with very likely (5th, 95th percentiles) values between –32 m and –1 m. This trend presents significant spatial variability, with the western Mediterranean concentrating the largest regional erosion hotspots, which are mainly located in river mouth areas and coastal stretches around harbours and other coastal infrastructure.

This erosion will increase under the effect of climate change, as sea level rise (SLR) will induce widespread shoreline retreat (very high confidence). The estimated median value of SLR-induced shoreline retreat for the Mediterranean with respect to 2010 is –17.5 m and –23 m by 2050 under the IPCC AR5 RCP4.5 and RCP8.5 scenarios, respectively, increasing to –40 m and –65 m respectively by the year 2100. Although shoreline retreat is certain, there is significant uncertainty regarding the computed induced rates due to the models used. Nevertheless, this SLR-induced component must be considered in addition to ongoing shoreline evolution rates.

In the absence of adaptation and protection measures, Mediterranean beaches will continuously erode over the next decades. In urbanised areas, where the coast is limited by physical barriers, this will lead to the progressive narrowing and eventual disappearance of beaches. This constitutes a high risk for intensive sun-and-beach tourism areas due to the expected decrease in beach carrying capacity and the associated economic impacts (very likely).

The progressive narrowing of beaches will reduce the degree of protection provided to existing infrastructure along the coast, with the corresponding increase in the risk of storm-induced damage. Therefore, even despite the absence of any significant increasing trend in storm intensity and frequency, storm-induced damage will increase (very likely) over the next decades along the Mediterranean coast.

Coastal flooding

In the Mediterranean, coastal flooding from coastal storms is mainly caused by waves, due to the relatively low magnitude of surges. The most exposed areas are waterfronts and the more seaward parts of urban developments. The highestrisk areas show significant spatial variability, depending on local storm climate and the extent and dimensions of existing natural and human defences.

In the future, SLR will very likely increase the frequency and magnitude of storm-induced flooding along the Mediterranean coastal zone due to an increase in the total water level at the shoreline, leading to an increase in the existing risk in the absence of adaptation and protection measures. This effect will be more significant in low-lying areas and will be enhanced by the decrease in protection due to beach erosion.

SLR will result in the gradual and permanent inundation of low-lying unprotected areas (high confidence). Within the Mediterranean Basin, deltas and coastal plains are particularly vulnerable, as subsidence can locally and significantly increase relative sea level rise. These areas, often home to the highest natural values in the Mediterranean coastal zone, while being also used for agriculture, are at the greatest risk from relative sea level rise (RSLR).

The Mediterranean Basin is one of the areas in Europe where disastrous flash floods caused by intense precipitation events are more frequent, mainly affecting river mouths and coastal areas. This is due to local climate and topographic conditions, and the existing high population and urban settlements in flood-prone areas. In the future, without adaptation measures, the risk is likely to increase due to more frequent heavy rainfall episodes linked to climate change and the continuous growth of urban areas along the coast, although showing large spatial variability (medium confidence).

The Mediterranean coast is one of the areas in Europe at highest risk to compound flooding due to co-occurrence of heavy rainfall and high-water levels. The expected evolution of these events under climate change shows large spatial variability in their occurrence without a clear trend regarding their intensity (medium confidence).

SLR will increase the extreme total water level at the shoreline and the associated flood risk. However, except for certain localised areas like the northern Adriatic, the Mediterranean stands out among European coastlines for experiencing relatively low extreme total water levels under SLR (high confidence). The Mediterranean region is particularly vulnerable to coastal flooding, due to the widespread presence of unprotected buildings and activities along the Mediterranean coastline, and the relatively small values of maximum coastal water levels. In the absence of coastal protection or adaptation, climate change will very likely be the main driver of future increase in coastal flood losses.

Tsunamis and meteotsunamis

Due to the high seismicity of the Mediterranean Basin, the short travel times of tsunami waves to the coast from source areas and the high population density and concentration of assets along the coast, tsunamis are a significant threat for Mediterranean coastal zones despite their low frequency, with the eastern Mediterranean being the most affected area.

Meteotsunamis occur regularly on the Mediterranean coast and show large spatial variability, with their highest intensities in bays and inlets where resonance is favoured. As a result, the greatest damage and, consequently, the areas of highest risk, are concentrated in local hotspots where existing coastal infrastructure and developments are not adapted to accommodate significant changes in sea level. Despite this, hazard assessments are only available for a few areas, while risk assessments are lacking.

Freshwater resource scarcity

Without appropriate adaptation and protection strategies, the quantity and quality of freshwater resources in coastal areas will decline, reducing the water available for future uses (very likely). Maintaining the socio-economic activities related to the significant urban, agricultural and/or industrial development in Mediterranean coastal areas, which require supplying significant freshwater demands, will be a challenging issue which will be exacerbated due to seawater intrusion in coastal aquifers. In the future, associated risks will be amplified due to the expected reduction in aquifer recharge, sea level rise, the increase in water demands and the frequency and severity of droughts.

The use of unconventional water resources generated by desalination will reduce the risk of water scarcity and their socio-economic implications, but it may increase the risks of environmental impacts, especially on coastal ecosystems (e.g. adverse impact due to brine water discharge) and will increase associated CO2 emissions.

Coastal pollution risks

Water pollution along the Mediterranean coast is mainly generated by land-based point and diffuse sources (80%) due to the existing high urbanisation of the coastal zone, with ship-induced and air pollution contributing the remaining part. This puts the coastal zone at risk of impacts on ecological systems and human health due to diverse pollutants generated either on land and discharged into the coastal waters or accidental oil spills.

Along the Mediterranean coast there are numerous sites that present a high risk associated with eutrophication of coastal waters due to nutrient inputs from land. Coastal eutrophication is of medium or important significance in 13 Mediterranean countries. This has adverse impacts on coastal ecosystems and may also have significant local socio-economic impacts due to its impact on aquaculture, fishing and coastal tourism.

In the future, it is expected that the risk along the Mediterranean coasts will increase following the expected increase in their occurrence and the increasing pressure on the coastal zone (high confidence). The presence of areas with high concentrations of plastics along the Mediterranean coast has high adverse impacts on marine biodiversity and human health due to the ingestion and accumulation by marine fish.

Synergistic interactions between climate change impacts and emerging pollutants in the coastal environment will become more frequent (medium confidence) due to multiple stressors from both natural and anthropogenic sources. Accumulated pollution from various sources in coastal and bathing waters endangers coastal ecosystems and human health. The magnitude of anthropogenic impacts has been higher in coastal waters compared to offshore waters due to the increasing anthropogenic pressure (e.g. overfishing, land-based pollution) on coastal zones and climate change as prevailing pressures altogether (medium confidence).

Biological risks

The Mediterranean coastal zone is subjected to a high risk associated with invasive non-indigenous species that produce different ecological and socio-economic impacts through their interaction with native species, which significantly affect native biodiversity. In addition, there is strong evidence that most of the services provided by Mediterranean marine ecosystems are affected by invasive non-indigenous species, those related to food provision being the most impacted ones (high confidence).

The massive abundance of jellyfish is a threat to anthropogenic coastal activities and human health due to their competitiveness and predatory impacts on marine biodiversity and their multiple direct and indirect consequences. Jellyfish blooms have potential adverse impacts on the marine ecosystem with consequent health risks for marine organisms and humans because they represent vectors of potential bacterial pathogens affecting fish aquaculture, in particular.

Mass mortality events (MMEs) are attributed to the increase in frequency and intensity of marine heat waves (MHWs) and pathogen infections. The frequency and intensity of MMEs will likely increase in the future in parallel with rising MHWs (high confidence).

The frequency of the mucilage phenomenon has been reported to have increased significantly over the last decades. Mucilage adversely affects seawater and makes it unsuitable for bathing due to the adherence of this mucus-like substance on bathers’ skin. Marine mucilage settled on the benthos in the form of large aggregates, coats the sediment, causing hypoxic and/or anoxic conditions resulting in the suffocation of benthic organisms. The mucilage phenomenon can cause serious economic damage to tourism and fisheries (high confidence).

Impacts on the economic system

Coastal tourism along the Mediterranean is likely to be affected by climate change due to a decrease in climate comfort during the summer season and its increase in spring and autumn (medium confidence). In addition to this, sun-and-beach tourism will be negatively affected by the decrease in beach carrying capacity due to SLR-induced beach erosion. This will result in the risk of a substantial decrease in revenue for coastal communities and consequently declines in GDPs of countries in the region (medium confidence).

Without effective adaptation, agriculture production in Mediterranean coastal zones will be negatively affected by climate change due to the expected decline in water resources, soil degradation, and increase in salinity. This can directly affect food security (medium confidence).

There is a high agreement that Mediterranean fisheries are overexploited, and the majority of the commercial stocks are declining. In addition, the risk of traditional fish stocks shrinking in quantity and economic value is further increased by pollution and widespread emergence of non-indigenous species. This represents a financial and technical challenge for many artisanal fishermen, who are the main operators in coastal fisheries (high confidence).

Climate change exacerbates challenges related to water and energy security through increasing temperatures, as well as decreasing precipitation and enhanced droughts (high confidence).

The risk for coastal infrastructure along the Mediterranean due to climate change in general, and SLR in particular, is related to the decrease in their functionality, mainly associated with increased coastal flooding and overtopping. For Mediterranean ports, the expected change in risk level by 2100 will be small under low-medium SLR scenarios, but it will significantly increase, becoming high or very high under the RCP8.5 scenario (medium confidence). Although an increase in risk associated with decreased functionality of coastal protection infrastructure due to SLR is expected at basin scale (high confidence), its significance will depend on its specific local configurations.

Impacts on the human system

A large part of the existing UNESCO cultural World Heritage Sites (WHS) in the low elevation coastal zone of the Mediterranean are currently at risk to erosion and coastal flooding (medium confidence), with an expected increase in flood and erosion risk that will reach 50% and 13% higher values respectively by 2100 under a high-end SLR scenario (unlikely). Additionally, the built heritage is likely to be affected by climate change through slow cumulative deterioration processes. As a result, the risk of decohesion and fracturing in porous building materials is expected to increase.

The occurrence of natural disasters and environmental degradation linked to pollution have multiple direct and indirect impacts on the health and well-being of coastal populations along the Mediterranean Basin (high confidence). In the absence of adaptation, their impacts are expected to increase in the near future due to the expected increase in hazardous conditions due to climate change and rising coastal population (medium confidence).

Impacts on the natural system

Mediterranean coastal wetlands have significantly declined during the 20th century due to a combination of erosion, extreme events, salt-water intrusion, and mainly human-induced pressures such as the expansion of irrigated agriculture and urban development. They are projected to be significantly affected by future changes in precipitation (high agreement, medium evidence), although with high spatial variability. SLR-induced hazards are expected to result in the loss of coastal wetlands (high agreement, robust evidence), with locally significant impacts in areas where rigid inland boundaries limit their potential for horizontal migration.

SLR-induced erosion along the Mediterranean coast will induce a decline in ecosystem services provided by coastal habitats, as these areas degrade and, eventually, disappear with ongoing erosion (high confidence). For the northern Mediterranean coast, a decline in approximately 5% in services by 2100 relative to current conditions under RCP8.5 has been estimated (medium confidence), presenting a high spatial variability; the eastern Mediterranean is expected to experience the largest declines.

Changes in sediment supply, industrial development, and urban processes will enhance the vulnerability of natural coastal ecosystems (e.g. sandy coastal beaches, saltmarshes, coastal lagoons) to sea level rise. In addition, coastal systems are experiencing compound threats from ocean warming, sea level rise, eutrophication, and the expansion of low-oxygen zones as a result of climate change. The risk to these ecosystems is projected to become very high by the end of the century.