The coastal zone of the Mediterranean Sea is affected by multiple drivers of change: climate, pollution, biologic and socio-economic processes. This report describes their evolution, their impacts on ecosystems and people, the risks that are posed and solutions to reduce them, together with pathways for sustainable development.

The Mediterranean coastal region is characterised by rapid, spatially diverse and geographically unbalanced socioeconomic development, mainly related to demographic trends, human settlement patterns and on-going wars and armed conflicts in different countries. The total coastal population of the Mediterranean is expected to grow faster than the inland population, thus leading to increased exposure of people and assets to coastal hazards. The northern Mediterranean may experience coastal population decline under some scenarios, while the highest increases in coastal population are expected in the Mediterranean Middle Eastern and Maghreb countries.

Climate change is affecting both the terrestrial and marine components of the Mediterranean coastal zone. Projections show an increase in surface air temperatures, frequency and intensity of hot extremes, sea level, evapotranspiration and decreased precipitation, depending on the level of future greenhouse gas emissions. Climate change is expected to pose serious risks for ecosystems and important economic sectors such as summer beach tourism, agriculture, aquaculture and fisheries.
The Mediterranean coastlines have experienced an accelerating relative sea level rise, which is expected to continue during the coming decades and centuries. Rising sea levels will exacerbate the risks of coastal floods, permanent inundation of some areas, and coastal erosion, with consequences for ecosystems and efficiency of present defences. Coastal structures, such as airports, transport networks, ports, and cultural heritage sites will be at risk. Both protection against coastal flooding and management of coastal erosion generally do not adequately consider future sea level rise, with risks of limited future efficiency. Climate change and growing urbanisation will further increase the risk posed by flash floods in some coastal areas.

Risks of water scarcity in the coastal areas of the Mediterranean are caused by the overall drying trend affecting the region, salinisation of coastal aquifers, increasing demand associated with population growth, irrigation, tourist use, industry and the energy sector. Risks of water scarcity are expected to increase in the future. Adaptation to decreasing water availability is taking place in Mediterranean coastal areas, with needs that vary significantly across sub-regions, depending on the population dynamics, the hydrogeological context and water management practices. These adaptation options consist in increasing water supply, improving water quality, supporting measures and governance, and to a lesser extent, reducing water demand.

In the Mediterranean Sea, observed mass mortalities in coastal waters have been partially attributed to marine heat waves and are expected to increase in the future. Mediterranean coastal wetlands have significantly declined since the beginning of the 20th century and further reduction is expected in the future. The efficiency of the conservation measures in coastal ecosystems strongly depends on the success of climate change mitigation and an increasing number of hard limits will be reached for every increment of global warming. The Mediterranean is also becoming increasingly colonised by non-indigenous tropical species and changes in the distribution and population of species have been observed. However, solutions have been rarely attempted.

Mediterranean coastal areas are polluted by micro- and macro-plastics, metals, persistent organic pollutants and emerging pollutants, with nutrient inputs from land, producing eutrophication in several coastal areas with negative impacts on ecological systems, human health and economic sectors (aquaculture, fishing, and coastal tourism). Pollution originates from numerous human activities, which are mainly land-based, such as industry, agriculture, urbanisation, and tourism. Future pollution levels along the Mediterranean coasts are expected to exhibit varying trends across regions and pollutants, depending on regulations, dependency, production, treatment, and socioeconomic changes. Pollution control at its source is generally more efficient than treating it at the endpoints. Actions to tackle pollution at a Mediterranean basin-scale have not been implemented yet, and both technical and decision-making challenges remain.
The engagement of scientists with policymakers, stakeholders, and citizens is a key factor to removing barriers (including lack of understanding and trust) and can be particularly beneficial during the planning process. Turning stakeholders into partners significantly increases the potential of successfully implementing solutions and adaptation measures.

In the Mediterranean coastal zone, present actions towards solutions to environmental problems, adaptation to climate change and its mitigation are insufficient to attain the UN Sustainable Development Goals (SDGs) ensuring the well-being of people and the sustainability of resources. Without transformative actions across all sectors, systems, and scales, climate change risks will be exacerbated and the sustainable development goals will not be met. Social-economic and gender-based inequalities, lack of access to basic services will act as further barriers to the implementation of sustainable development pathways.

Adopting actions consistent with sustainable development pathways requires the proper identification of vulnerabilities related to human activities and climate change impacts, and assessment of options to reduce risks to the affected communities and ecosystems. A mix of legal, policy and economic instruments, and behavioural nudges are available at local, national, and regional level to promote effective and resilient development pathways in the Mediterranean coastal zone.

A.1 This Special Report identifies and assesses environmental and climate change hazards in the coastal zone of the Mediterranean Basin, the related risks, adaptation options and solutions. It further assesses and provides information on actions to meet the United Nations Sustainable Development Goals (SDGs), such as combating climate change, increasing food security, ensuring water resources, accessing affordable and sustainable energy resources, managing natural resources, creating opportunities for social inclusion, and economic prosperity. Adaptation plans are presented by placing the social and cultural values in context of the region and its local traditions, considering the need to protect communities and biodiversity, minimise impacts on the natural environment, and addressing ethical considerations important for socially-oriented adaptation policies.

A.1.1 Policies to manage coastal risks and adaptation strategies in the Mediterranean coastal zone are important to the whole region, as a third of the Mediterranean population lives close to the sea and depends on infrastructure and economic activities in its immediate vicinity.

A.1.2 The coastal zone can be defined using objective and subjective criteria, often with a high level of uncertainty or fuzziness. Depending on the technical, economic or legal implications, the definition and extent of the coastal zone may vary significantly in the literature. This report does not aim to propose a general definition, instead it adopts a loose criterion that the coastal zone consists of areas geographically connected to the coastline, including land areas directly impacted by marine processes and sea areas directly impacted by terrestrial processes.

A.1.3 The Mediterranean coastal zone is often narrow and over-pressured and requires a specific risk assessment tailored to its characteristics to inform adaptation pathways and support decisions towards risk reduction and sustainability in coastal governance, policies and social perception.

A.2 This Special Report, as with other MedECC assessments, and international and national assessment processes, is based on the available, relevant and traceable evidence in the published scientific literature, including different lines of evidence (observational products, model-based findings and other types of data and analyses).

A.2.1 This report applies the calibrated terms that were adopted transversally by the Intergovernmental Panel on Climate Change (IPCC) since the 5th Assessment Report in order to communicate the robustness and certainty of assessment findings either qualitatively or quantitatively. The calibrated terms quantify confidence and likelihood. The terms are attributed to the assessment outcome by the author team following an evaluation of the available evidence. The designation of confidence and likelihood are agreed upon through a consensus-building discussion of the evidence, reflecting all expert views that are expressed.

A.2.2 A common set of key dimensions is used in this report on the basis of information that is available in the scientific literature, including well-defined time frames, baselines for past changes and conditions, a subset of representative scenarios of future changes, and well-known frameworks, such as the Sustainable Development Goals (SDGs).

A.2.3 In the SPM, Shared Socioeconomic Pathways (SSP) are cited as defined in IPCC AR6 based on future greenhouse gases (GHG) emissions, labelled after the SSP narrative and associated radiative forcing values in the year 2100 (1.9, 2.6, 4.5, 7.0, and 8.5 W m-2). SSP1-1.9 - very low GHG emissions and SSP1-2.6 - low GHG emissions (CO2 emissions reduce to net zero in the 2050s), , SSP2-4.5 - intermediate GHG emissions (CO2 emissions remain around current levels until 2050, then falling but not reaching net zero by 2100), SSP3-7.0 - high GHG emissions and SSP5-8.5 - very high GHG emissions (CO2 emissions roughly double from current levels by 2100 and 2050, respectively).

A.2.4 In the SPM, Representative Concentration Pathways (RCP) defined in IPCC AR5 are cited. RCPs are greenhouse gas concentration (not emissions) trajectories labelled by the associated radiative forcing values in the year 2100 (2.6, 4.5, 6, and 8.5 W m-2, respectively and corresponding to one stringent mitigation scenario (RCP2.6), two intermediate scenarios (RCP4.5 and RCP6.0) and one scenario with very high GHG emissions (RCP8.5).

B.2 Mediterranean coastlines have experienced relative sea level rise, which is the sum of mean sea level rise and vertical land motion, with an accelerated rate during the last three decades (1993–2A018). {2.2.7, 2.2.8}

B.2.1 Mean sea level in the Mediterranean shows an approximate trend of ~1.4 mm yr–1 during the 20th century (high confidence), and has accelerated to 2.8 ± 0.1 mm yr–1 in the last three decades (1993–2018) (high confidence). The interannual and decadal variability that is superimposed to this trend can temporarily mask it. {2.2.7}

B.2.2 Vertical land motion along the Mediterranean coasts generally ranges from 0 to –10 mm yr–1, with isolated positive values. This widespread subsidence is mainly determined by geological factors such as tectonic subsidence and natural sediment compaction but is increased by human activities such as the withdrawal of underground fluids (water, oil and gas, drainage of organic soils) that contribute significantly to relative sea level rise in some areas (high confidence). {2.2.8}

B.2.3 Coastal flooding in the Mediterranean due to storm surges and wind waves threatens the flood-prone areas in the waterfronts (river mouths and deltas) and low-lying coastal plains in many Mediterranean countries. Relative sea level rise has already increased the frequency of floods of the Venice city centre, Italy (high confidence). {2.2.4}

B.3.1 Coastal water pollution originates mainly from land-based sources, followed by air and ship-originated sources. Pollution sources include domestic effluents, agricultural runoff, road transport, maritime transport, mine tailings, manufacturing and extractive industries. {2.4}

B.3.2 The Mediterranean Sea is one of the most heavily plastic polluted areas across the globe and floating plastics accumulate along its coasts as a result of human activities and marine circulation (high confidence). Plastics account for up to 82% of observed litter, 95–100% of total floating marine litter and more than 50% of seabed marine litter in the Mediterranean Sea. About two thirds of all the plastic debris from land-based sources (rivers, urban and industrial areas, and intensive agricultural areas) is retained along the coasts, where its level has remained steady for the past two decades, with several hotspots of plastic fluxes (medium confidence). {2.4.4}

B.3.3 Human activities have led to increased concentrations of potentially toxic metals with hotspots of lead, mercury and cadmium located on the northern, central and south-eastern shores of the Mediterranean Basin (high confidence). Manufacturing of refined petroleum products (southern Mediterranean, Balkans and Türkiye), tanning and dressing of leather, and manufacturing of cement (Balkans and Türkiye) and energy production (Mediterranean EU countries) contribute to the release of heavy metals in coastal waters impacting marine ecosystems. Mercury concentrations exceed European Union regulatory thresholds in many Mediterranean top-predatory fish. Methylated mercury concentrations in western Mediterranean waters are twice as high as in the eastern Mediterranean (high confidence) and are biomagnified in marine food webs (medium confidence). In general, the release of toxic metals is decreasing for the European Union countries, but opposite trends are reported in some areas (high confidence). {2.4.2}

B.3.4 Pollution sources such as domestic effluents, runoff from agricultural practices and urban runoff introduce emerging pollutants and persistent organic pollutants into the coastal zone, with higher concentrations in the northern than the southern shores. Pollution from polycyclic aromatic hydrocarbons (PAHs) and polychlorinated biphenyls (PCBs) has been detected along some stretches of the Mediterranean coastline with the highest levels observed around river mouths, and harbour and industrial areas (medium confidence). Shipping is one of the main sources of oil pollution in Mediterranean coastal areas, with about 90% of tanker spills occurring near the coastlines and particularly affecting the eastern coasts (medium confidence). {2.4.3}

B.3.5 Nutrient flows of nitrogen and phosphate have decreased in most of the northern Mediterranean over the last two decades, following the implementation of best agricultural management practices and technological advances in wastewater treatment plants. However, nutrient pollution has increased in the southern and eastern Mediterranean in parallel with agricultural intensification and urban and industrial development (high confidence). {2.4.1}

B.4.1 Non-indigenous species are accidentally introduced into Mediterranean coastal waters, estuaries or coastal lagoons, by aquaculture facilities, aquarium species trade, boat ballast waters and biofouling on vessels. Most non-indigenous subtropical coastal fish species enter the Mediterranean from the Red Sea. Warming of the Mediterranean waters is creating increasingly suitable conditions for non-indigenous thermophilic species, which are expanding their distribution ranges (high confidence). {2.3.2, 2.3.3}

B.4.2 The frequency of jellyfish blooms has increased in the Mediterranean Sea with some evidence that they benefit from eutrophication, warming of sea water and other human induced stressors (medium confidence). {2.3.4}