Executive Summary

This chapter provides a comprehensive overview of the main natural and socio-economic drivers affecting the Mediterranean coasts. These drivers are of different origins and nature, including atmospheric, marine, terrestrial, biological, pollution-related, and socio-economic factors. They contribute to phenomena such as coastal flooding, changes in ecosystem services, utilisation and exploitation of marine and coastal resources, degradation of natural and built infrastructure, among others, which impact the lives and livelihoods of the large and densely populated coastal areas and the proximate urban areas that depend heavily on marine and coastal resources. Some drivers are linked to climate change (exacerbated by human activities), while others are partially or entirely of anthropogenic origin (e.g. air and water pollution, tourism, urbanisation, socio-economic development). The situation can become complex as these drivers occur in temporal sequence, jointly, or in synergy. This chapter introduces the drivers, while their impacts are be considered in subsequent chapters.

Introduction

A driver is any natural or human-induced factor that directly or indirectly causes a change in a system (IPCC 2021: Annex VII: Glossary). Most drivers, especially those related to climate change, pollution, or human activities, have been presented and discussed in IPCC 2021 and MAR1 (MedECC 2020). This Chapter is mainly grounded on them, but with some updating and additional items. Drivers may operate singularly, or in conjunction, and may generate negative feedback loops, where drivers can be either the cause or consequence of changes. The aim is to summarise the key drivers that govern the coastal climate, the sea level and the coastal ecosystem of the Mediterranean and are a necessary prerequisite to understanding what is explained in the next Chapters. This Chapter considers a comprehensive set of drivers relevant for coastal communities, with special attention to projections and their potential synergisms with other natural or anthropic drivers. The spatial and temporal combination of concurrent drivers and/or meteorological conditions may amplify each other and lead to even greater secondary impacts with unprecedented social, ecological and economic consequences (Bevacqua et al. 2021; Xoplaki et al. 2023).

The identification of drivers helps to identify critical issues, predict changes and hazards, and assess risks (Chapter 3). On this basis, it will be possible to adopt measures to reduce potential damage to ecosystems and human systems, or to adapt them to climate change (Chapter 4), as well as to plan sustainable development pathways (Chapter 5). Therefore, the presentation of the drivers, their long-term trends, and related future scenarios, has been organised to produce a comprehensive overview.

Key messages on Climate and geological drivers

Coastal air is warming. At the beginning of the 2020s, the near-surface air temperature of the Mediterranean region is +1.5°C warmer than in the 1850–1900 preindustrial period (high confidence). On the Mediterranean coasts, referring to the 1850–1900 period, there is high confidence that the projected increase of in air temperature is will be +1.6°C to +2.7°C on forduring the the mid-term and +1.6°C to +3°C on forduring the long- term, for the SSP1-2.6 low emission scenario (very likely) and values up to +2.3°C to +3.6°C on for the mid-medium term, and +4.2°C to +6.8°C on for the long- long term, for under the SSP5-8.5 very high emission scenario (very likely). {2.2.1}

Coastal waters are warming. Since the preindustrial period, the surface temperature of the Mediterranean water is has been rising with a long-term positive trend of about +0.86°C per century. This trend is not constant but is characterised by a multidecadal periodicity (~70 years) superimposed to on it (high confidence). {2.2.1}. Since the 1980s, satellite data has shown that the warming rate of the sea surface is spatially inhomogeneous, ranging between +0.29°C and +0.44°C per decade, and is stronger in the eastern Mediterranean. In addition, over the last two decades, the mean frequency of marine heatwaves (MHW) has increased by +40%, and the duration by +15% (high confidence). {2.2.5}

Significant warming is expected in the surface waters of the Mediterranean Sea (virtually certain). Compared to the end of the 20th century, the annual mean basin sea surface temperature is expected to increase by +0.6°C to +1.3°C before the mid-21st century and by +2.7°C to +3.8°C at the end of the 21st century period for the pessimistic RCP8.5 scenario, and by +1.1°C to +2.1°C for the medium RCP4.5 scenario (high confidence). {2.2.5}

Sea level is rising. Sea level changes have long been documented with instrumental and non-instrumental data. The pre-instrumental period is known from proxy data, tide gauges started in 1871, and satellite altimetry started in 1992. The rise rate increases over time, and the longest reconstructed series, for example Venice's seven century long record, shows an exponential trend (observation evidence). {2.2.7}

The Mediterranean Sea level is projected to rise further during the coming decades and centuries (high confidence), likely reaching +0.28 m to +0.55 m for shared socioeconomic pathways (SSP1-1.9) and +0.63 m to +1.01 m for SSP5-8.5 in 2100 (relative to 1995–2014) (medium confidence). The process is irreversible at the scale of centuries to millennia (high confidence). {2.2.7}

Land subsidence increases coastal submersion. Relative sea level is determined by the sum of the mean sea level and vertical land movements (i.e. negative subsidence and positive uplift). Relative sea level rise increases especially in areas affected by significant land subsidence. The situation across the European coasts has been documented by studies and especially satellite data (Copernicus Sentinel) since 2016, and the most affected areas are the coastal region of the Adriatic Sea and the Po Delta in Italy, Thessaloniki in Greece, and some small islands (high confidence). The non-European coasts on the eastern and southern Mediterranean are less documented, except for Mejerda near Tunis, and the eastern Nile Delta in Egypt (high confidence). Land subsidence is mainly determined by geological factors, but it may be increased by human activities, such as extraction of water, gases, or building load. In certain areas, subsidence may reach values of the order of −10 mm yr-1 (observation evidence). {2.2.8}

For the combined effect of sea level rise and subsidence, the risk of coastal floods will increase in low-lying areas that constitute 37% of the Mediterranean coastline (high confidence). {2.2.4}

Saltwater intrusion in rivers, estuaries, and coastal aquifers will likely increase, affecting groundwater resources, river discharges, the use of coastal areas, and the most extensive wetlands that are found in relation to the major Mediterranean rivers (high confidence). {2.2.4}

Storm surges and coastal floods. In the cold season, the penetration of Atlantic fronts, or low-pressure areas developing over the Mediterranean, may generate storm surges and exceptionally deep coastal floods, high wind waves and other phenomena such as flash floods that are potentially dangerous to people, the environment, and the whole coastal area (high confidence). {2.2.4}. In the warm season, increasing aridity or intense precipitation combined with punctual occasional high intensity precipitation events will likely constitute the main challenges (medium confidence). {2.2.2}

Water salinity and acidity are related to water temperature. Not only temperature, but water salinity and acidity will also be affected, with likely impacts on the terrestrial and marine environment. Acidification is projected to continue (virtually certain) with a pH decrease of up to –0.46 unit in a high emission scenario (medium confidence). {2.2.5}
Future reduced precipitation, associated with increased evaporation will lead to a decline in runoff in the Mediterranean region and fresh water supply. Droughts are projected to become more severe, more frequent and longer under moderate emission scenarios, and strongly enhanced under severe emission scenarios (high confidence) {2.2.6}

Key messages on biological drivers

With over a thousand non-indigenous species, the Mediterranean, which is a major invasion hotspot (virtually certain), is the most heavily invaded marine region in the world. Non-indigenous species outcompete indigenous species, causing regional biodiversity shifts and altering ecosystem functions and services (high confidence). The Suez Canal has provided the most important entrance for non-native species in the Mediterranean. At present, other pathways such as shipping vectors and the aquarium trade are responsible for a considerably higher number of the non-indigenous species that have been introduced. {2.3.1}

The Mediterranean is warming faster than other seas, becoming increasingly suitable to be colonised and invaded by organisms of tropical origin. The effect of global warming is therefore contributing to species colonisation through the Strait of Gibraltar, but also to the dispersal of these and truly non-indigenous species within the Mediterranean. Moreover, species are changing their life-history traits and patterns due to warming, which can lead to a loss of competitive abilities to cope with the effects of biological drivers, especially those caused by biological invasions. {2.3.2}

Recent studies show an increase in the frequency of jellyfish blooms in the Mediterranean Sea (medium confidence). There is some evidence that this is occurring due to eutrophication and other human-induced stressors, such as global warming (medium confidence). {2.3.3}

Pollution drivers

There is robust evidence that the high fluxes of nutrients transported by air, surface water and groundwater to Mediterranean coasts are related to agricultural practices and urban and industrial uses. Nutrient fluxes are expected to decrease in the north due to the implementation of environmental regulations, but nutrient increases are expected in the south as a result of urban development and agricultural intensification (high confidence). Submarine groundwater discharge inputs, which lag a few decades behind agricultural inputs, can contribute to sustained nutrient increases in the coming years and compromise water quality (medium confidence). The overall projected changes in land-derived nutrients will contribute to widening the current nutrient imbalance in coastal ecosystems, increasing the availability of N relative to P and ultimately exacerbating eutrophication problems (high agreement). {2.4.1}

Concentrations of certain persistent organic pollutants (POPs), such as polychlorinated biphenyl (PCBs) and dichlorodiphenyltrichloroethane (DDT), will very likely continue to decline in the Mediterranean coasts due to regulations (medium confidence). Concentrations of emerging pollutants, such as pharmaceuticals and personal care products, will not show a downward trend due to emerging industries and socioeconomic change (medium confidence). {2.4}.

The amount of plastic pollution along Mediterranean coasts has remained steady for the past two decades (medium confidence). Annual plastic leakage into the Mediterranean coastal area is likely to reach 500,000 tonnes by 2040 if annual plastic production continues to grow at a rate of 4% and waste management is not radically improved. In the scenario of 1% annual growth in plastic production and improved waste management, the leakage is likely to decrease by 2040 (medium confidence). {2.4.4}
Given the high concentrations of plastics, trace elements and emerging pollutants in the Mediterranean Sea, their co-occurrence with seawater warming, acidification, and deoxygenation is likely to rise along the Mediterranean shores (high confidence). {2.2.5; 2.4}