Tsunamis

According to Imamura (1937: 123), the term tunami or tsunami is a combination of the Japanese word tu (meaning a port) and nami (a long wave), hence long wave in a harbour. He goes on to say that the meaning might also be defined as a seismic sea-wave since most tsunamis are produced by a sudden dip-slip motion along faults during major earthquakes. Other submarine or coastal phenomena, however, such as volcanic eruptions, landslides, and gas escapes, are also known to cause tsunamis. According to Van Dorn (1968), ‘tsunami’ is the Japanese name for the gravity wave system formed in the sea following any large-scale, short-duration disturbance of the free surface. Tsunamis fall under the general classification of long waves. The length of the waves is of the order of several tens or hundreds of kilometres and tsunamis usually consist of a series of waves that approach the coast with periods ranging from 5 to 90 minutes (Murty 1977). Some commonly used terms that describe tsunami wave propagation and inundation are illustrated in Figure 17.2. Because of the active lithospheric plate convergence, the Mediterranean area is geodynamically characterized by significant volcanism and high seismicity as discussed in Chapters 15 and 16 respectively. Furthermore, coastal and submarine landslides are quite frequent and this is partly in response to the steep terrain of much of the basin (Papadopoulos et al. 2007a). Tsunamis are among the most remarkable phenomena associated with earthquakes, volcanic eruptions, and landslides in the Mediterranean basin. Until recently, however, it was widely believed that tsunamis either did not occur in the Mediterranean Sea, or they were so rare that they did not pose a threat to coastal communities. Catastrophic tsunamis are more frequent on Pacific Ocean coasts where both local and transoceanic tsunamis have been documented (Soloviev 1970). In contrast, large tsunami recurrence in the Mediterranean is of the order of several decades and the memory of tsunamis is short-lived. Most people are only aware of the extreme Late Bronge Age tsunami that has been linked to the powerful eruption of Thera volcano in the south Aegean Sea (Marinatos 1939; Chapter 15).

Weathering, Soils, and Slope Processes

Hillslopes are the dominant landform features of the Earth’s surface. They make up the interface between the atmosphere and Earth systems, providing a substrate that supports life and thus the basis for human activities within the Mediterranean. Their location at this interface means that hillslopes evolve through a complex interaction of different processes, operating at a range of different time and spatial scales. At longer timescales, processes of weathering convert rock and other parent materials into soils. Soils allow the growth of vegetation and thus further feedbacks between atmospheric and surface processes; in some cases these feedbacks can be seen to provide relative stability, while in others the system can become more fragile (Chapter 20). The latter case often arises as a result of erosion processes of various types. Water erosion and mass movements are a significant element of Mediterranean landscape evolution, occurring in parallel with (in response to, and affecting) tectonic processes that have moulded the configuration of the Earth’s crust (see Chapter 1), producing the unique combination of environmental characteristics of the region. Since the Late Pleistocene, depending on location, human activity has led to an acceleration of many of these processes, with important consequences for the basic ‘life-support system’ of the region and for global environmental cycles. The in situ modification of near-surface materials is typically considered to take place along a continuum relating to the dominance of mechanical or chemical processes (e.g. Birkeland 1999). The simplest control may be considered to be climatic, with mechanical breakdown of particles dominating in cold, dry conditions, and chemical processes dominating in warm, wet conditions. Comparing this model to the present day climate of the Mediterranean suggests, as with other processes, something of a north–south divide in terms of the dominant weathering process. The northern part of the basin (together with the Levant and the north-facing uplands of the Maghreb) would seem to be dominated by moderate chemical weathering; exceptions being the arid areas of south-east Spain, southern Sicily, eastern Cyprus, and parts of the Anatolian plateau as well as areas where low average temperatures would also reduce rates, such as in the Alps and parts of Slovenia and Croatia.

Hydrology, River Regimes, and Sediment Yield

In comparison to the rest of Europe, Africa, and Asia, most rivers arising and flowing within the Mediterranean watershed typically drain small catchments with mountainous headwaters. The hydrology of Mediterranean catchments is strongly influenced by the seasonal distribution of precipitation, catchment geology, vegetation type and extent, and the geomorphology of the slope and channel systems. It is important to appreciate, as the preceding chapters have shown, that the area draining to the Mediterranean Sea is large and enormously variable in terms of the key controls on catchment hydrology outlined above, and it is therefore not possible to define, in hydrological terms, a strict single Mediterranean river type. However, river regimes across the basin do have a marked seasonality that is largely controlled by the climate system (Chapter 3) and, in most basins, the dominant flows occur in winter—but autumn and spring runoff is also important in many areas. These patterns reflect the general water balance of the basin as a whole, but there are key geographical patterns in catchment hydrology and sediment yield and a marked contrast is evident between the more humid north and the semi-arid south and east (Struglia et al. 2004; Chapter 21). Also, because of the long history of vegetation and hillslope modification by human activity and the more recent and widespread implementation of water resource management projects, there are almost no natural river regimes in the Mediterranean region, especially in the middle and lower reaches of river catchments (Cudennec et al. 2007). Runoff generation on hillslopes in the Mediterranean is very closely related to rainfall intensities and land surface properties as discussed in Chapter 6. While this is probably true of most catchments, runoff generation in the Mediterranean is very sensitive to vegetation cover because of the seasonal dynamics of rainfall and the role played by extreme events. The cumulative effect of these characteristics is a specific set of management problems and restoration issues and, although these are rather different in the various socio-political regimes of the region, it can be argued that they are in many ways unique to Mediterranean catchments.

Biodiversity and Conservation

The biodiversity of Mediterranean-climate ecosystems is of particular interest and concern, not only because all five of these regions (the Mediterranean basin, California, central Chile, Cape Province of South Africa, western and southern parts of Australia) are among the thirty-four hotspots of species diversity in the world (Mittermeier et al. 2004), but they are also hotspots of human population density and growth (Cincotta and Engelman 2000). This relationship is not surprising because there is often a correlation between the biodiversity of natural systems and the abundance of people (Araùjo 2003; Médail and Diadema 2006) and this, inevitably, raises conservation problems. Within the larger hotspot of the Mediterranean basin as a whole, ten regional hotspots have been identified. They cover about 22 per cent of the basin’s total area and harbour about 44 per cent of Mediterranean endemic plant species (Médail and Quézel 1997, 1999), as well as a large number of rare and endemic animals (Blondel and Aronson 1999). A key feature of these Mediterranean hotspots as a whole is their extraordinarily high topographic diversity with many mountainous and insular areas. Not surprisingly this results in high endemism rates and they contain more than 10 per cent of the total plant richness (see the recent synthesis of Thompson 2005). However, of all the mediterranean-type regions in the world, the Mediterranean basin harbours the lowest percentage (c.5%) of natural vegetation considered to be in ‘pristine condition’ (Médail and Myers 2004; Chapter 7). With an average of as many as 111 people per km2, one may expect a significant decline in biological diversity in the Mediterranean basin—a region that has been managed, modified, and, in places, heavily degraded by humans for millennia (Thirgood 1981; Braudel 1986; McNeill 1992; Blondel and Aronson 1999; Chapter 9). There are two contrasting theories that consider the relationships between humans and ecosystems in the Mediterranean (Blondel 2006, 2008). The first one is the ‘Ruined Landscape or Lost Eden’ theory, first advocated by painters, poets, and historians in the sixteenth and seventeenth centuries, and later by a large number of ecologists.

Water Resources

The geography of natural water resources in the Mediterranean basin cannot simply be reduced to the study of water inputs, water distribution, and the pattern of runoff-generating precipitation determined by climate and relief—although these are, of course, fundamental controls (Margat 1992; Benblidia et al. 1996). Any consideration of basin-wide water resources also needs to consider a range of territorially determined factors affecting water resources. These include: (1) the nature of surface and underground flows, which depends on river basin and hydrogeological characteristics; (2) the natural storage capacity of lakes and aquifers and their role in regulating flows, and any losses from these stores which reduce the resulting flows; (3) the existence of favourable conditions for water management and exploitation such as suitable sites for dam construction and the productivity of aquifers, as these factors dictate accessibility to water resources and the production costs; (4) the natural quality of the water, its vulnerability to pollution and its capacity for self-purification; (5) any constraints imposed for reasons of environmental conservation, which may effectively exclude a proportion of water reserves from the category of exploitable resources. It is important to appreciate that each of these factors influences the assessment of water resources in a given area and each factor has its own geography (Margat 1997; Margat and Vallée 1999a). In spite of the broad similarities in climate and landscape between the different parts of the Mediterranean basin, there are considerable variations between regions that impact upon the availability of water resources. Many of the factors affecting water resources cited above are subject to a similar degree of variation (Grenon and Batisse 1989; Chapter 8) and these are discussed in turn below. Marking the transition between the temperate climate of Europe and the aridity of North Africa and the Near East, the Mediterranean climate contains wide variation, and this is reflected in a highly uneven distribution of rainfall (Benblidia et al. 1996; Margat and Vallée 1999a; Chapter 3). For example, moving from one extreme to another, average annual rainfall ranges from more than 3,000 mm in parts of the Dinaric Alps to less than 50 mm in Libya.

Editorial Introduction

This volume has traced the development of the Mediterranean landscape over very long timescales and has examined modern processes in a wide range of settings. Earlier chapters have explored tectonic processes and the evolution of the topography and biota, the nature and impact of Quaternary climate change, and natural hazards, as well as the increasing role of human activity in shaping geomorphological processes and ecosystems during the course of the postglacial period. A core theme in several chapters is the nature of the relationship between humans and the Mediterranean environment. Over the last one hundred years or so, and especially in the period since the Second World War, this relationship has changed dramatically. Resource exploitation, urban expansion, and rural depopulation have all taken place at unprecedented rates, with major impacts upon the quality of land, water, air, and ecosystems. The final part of this volume examines four key topics of environmental concern; its four chapters explore, respectively, land degradation, water resources, interactions between air quality and the climate system, and biodiversity and conservation. Where possible, it is important to place these issues within an appropriate historical perspective. Many components of the Mediterranean environment have responded in a sensitive way to past environmental changes, but the pressures on land and water resources have never been more intense. Improved monitoring networks and new modelling efforts are needed to predict more effectively the impact of climate and social change on all environmental systems and to help inform policymakers seeking a more sustainable use of the region’s resources. Chapter 20 examines the ecological aspects of land degradation and sets out new ideas on productivity dynamics. It explores some of the interactions between land use change, vegetation dynamics, grazing patterns and wildfires. The uneven geography of water resources and water use are highlighted in Chapter 21. Water resource issues have become an increasingly important factor in the geopolitics of the region against a background of climate change uncertainty, rising demand, and a diminishing resource base. Chapter 22 analyses the interactions between climate, air quality, and the water cycle.

The Climate System

The Mediterranean region has a highly distinctive climate due to its position between 30 and 45°N to the west of the Euro-Asian landmass. With respect to the global atmospheric system, it lies between subtropical high pressure systems to the south, and westerly wind belts to the north. In winter, as these systems move equatorward, the Mediterranean basin lies under the influence of, and is exposed to, the westerly wind belt, and the weather is wet and mild. In the summer, as shown in Figure 3.1, the Mediterranean lies under subtropical high pressure systems, and conditions are hot and dry, with an absolute drought that may persist for more than two or three months in drier regions. Climates such as this are relatively rare, and the Mediterranean shares its winter wet/summer dry conditions with locations as distant as central Chile, the southern tip of Cape Province in South Africa, southwest Australia in the Southern Hemisphere, and central California in the Northern Hemisphere. All have in common their mid-latitude position, between subtropical high pressure systems and westerly wind belts. They all lie on the westerly side of continents so that, in winter, when the westerly wind belts dominate over their locations, they are exposed to rain-bearing winds. In the Köppen classification (Köppen 1936), these climates are known as Mediterranean (Type Cs, which is subdivided in turn into maritime Csb and continental Csa). The influence of the Mediterranean Sea means that the Mediterranean-type climate of the region extends much further into the continental landmass than elsewhere, and is not restricted to a narrow ocean-facing strip. Nevertheless, within the Mediterranean region climate is modified by position and topographic influences can be important. The proximity of the western Mediterranean to the Atlantic Ocean gives its climate a maritime flavour, with higher rainfall and milder temperatures throughout the year. The eastern Mediterranean lies closer to the truly continental influences of central Europe and Asia. Its climate is drier, and temperatures are hotter in summer and colder in winter than in the west. Annual rainfall is typically around 750 mm in Rome, but only around 400 mm in Athens.

Editorial Introduction

Catastrophic earthquakes, explosive volcanic eruptions, and devastating storms and floods are intimately bound up within the history and mythology of the Mediterranean world. It is a key region for the study of natural hazards because it offers unrivalled access to long records of hazard occurrence and impact through documentary, archaeological and geological archives. Early texts and archaeological data have provided unique insights into the nature and impact of past eruptions, earthquakes, tsunamis, and other hazards. Notable events were carefully documented in Antiquity and the archaeological record provides insights into the impact of catastrophic events on past human societies. The eruption of Vesuvius in AD 79, for example, was famously documented by Pliny the Younger, and the excavations at Pompeii have provided extraordinarily rich insights into the dynamics and impacts of tephra falls and pyroclastic flows. The significance of environmental hazards in the demise of civilizations such as Minoan Crete (tsunami) and the Early Bronze Age in the Near East (drought) has been vigorously debated for decades. While such events have undoubtedly threatened people in the region since prehistoric times, the actual threat to human society has increased dramatically in the historical and modern periods as urban environments and their populations have rapidly expanded. This part of the volume analyses hazards associated with both endogenic and exogenic Earth processes and the interactions between them. It includes volcanic processes, crustal instability, tsunamis, fluvial floods, extreme weather phenomena, and wildfires. Each chapter explores the basic controls and the geography of a particular hazard and related processes, and, over a range of timescales, magnitude and frequency relationships and the nature of the threat to human society. High-magnitude events are a fundamental part of the physical geography of the Mediterranean and play a key role in long-term landscape evolution and ecosystem change. Even though the processes associated with each hazard typically take place over very short timescales, they can set in motion long-term adjustments to geomorphological and ecosystem processes. Tephra falls can change soil properties and vegetation communities, for example, and earthquakes may trigger base-level change and landslips in river basins that enhance fluvial sediment yields for many centuries.

Coastal Geomorphology and Sea-Level Change

The intricate shores of the Mediterranean Sea twist and turn for some 46,000 km, with three-quarters of their convoluted length confined to only four countries— Italy, Croatia, Greece, and Turkey. Just over half the coast is rocky, much of it limestone, with the remainder encompassing almost every type of littoral environment (exceptions being coral reefs and mangrove wetlands). Such littoral diversity has long made the seaboard of southern Europe, the Levant, and North Africa a fruitful natural laboratory for studying coastal geomorphology and sea-level change. The virtually enclosed sea ensures that wave processes are generally modest and the tidal range is limited (often less than half a metre), a combination that permits observational evidence of many modern shoreline features to be related precisely to mean sea level. Consequently, relative shifts in the position of now relict coastal features can be used to track the rhythms of relative sea-level change and shoreline evolution. Such rhythms have a bearing on several aspects beyond the physical geography of the Mediterranean basin: they inform archaeological reconstructions of the past settlement and exploitation of a coastal zone that has been an important focus of human activity since Palaeolithic times; they provide testing and fine-tuning for geophysical, geodynamic, and palaeoclimatic models for the region; and they set the backdrop to contemporary societal issues, such as future sea-level rise and coastline adjustments to mass tourism, which threaten the long-term sustainability of the Mediterranean littoral. In this chapter, we review these diverse facets of the Mediterranean coastal realm to provide a synthesis of how these shores have evolved into their present-day appearance. The Mediterranean occupies the convergence zone between two major tectonic plates, Africa and Europe, with a third, Arabia, pressing from the east. Caught within the collisional vice of these great plates are several minor plates and crustal blocks, most notably Anatolia and Apulia. The result is a complex network of plate tectonic structures that define the general configuration of the seaboard. In particular, two major subduction systems partition the Mediterranean basin into a patchwork of minor basins and subsidiary seas (Krijgsman 2002; Chapter 1).

Vegetation and Ecosystem Dynamics

Within the Mediterranean region a number of distinctive vegetation communities can be recognized, comprising some 25,000 species, of which about 50 per cent are endemic. Broadly defined, these originated with the establishment of a mediterranean-type climate about 3.2 million years ago, since when they have been subject to the vicissitudes of glacial–interglacial climate changes, plus the intensification of human impact during the last 10,000 years (Chapters 4 and 9). These communities are dynamic, responding to environmental changes at a variety of scales, both spatial and temporal. This chapter explores the characteristics of these communities and examines the relationships between ecosystem dynamics and biodiversity, and ecosystem response to disturbance. For example, each year fires burn out of control and are the subject of regular news stories during summer months. While fires may be economically devastating and lead to loss of life (Chapter 19), ecologically their incidence is an important dynamic component of Mediterranean ecosystems and may, indeed, be crucial to the successful propagation and spread of plants and communities regarded as typically Mediterranean. Associated animal populations generally recover quickly despite inevitable loss of life in some populations. Thus understanding the role of fire and other disturbance factors such as grazing is key to understanding Mediterranean vegetation communities and ecosystem dynamics. The chapter concludes with an evaluation of the likely response of vegetation communities to potential atmospheric and land use changes. While a number of distinct vegetation communities have been identified, a common characteristic is an ability to survive hot, dry summers and cool, wet winters, together with frequent disturbances. Many of the communities are dominated by shrubs, and Mediterranean evergreen sclerophyllous shrublands are recognized as one of the defined ecosystems of the world (di Castri 1981). Such shrublands are at the centre of a continuum of communities which vary along gradients of moisture availability, temperature, and nutrient availability, usually determined by substrate, and human activity. At the extreme ends of these gradients, but still Mediterranean, are sclerophyllous woodlands, coniferous and deciduous forests, savannas and grasslands grading into steppe and semi-desert shrublands, and heathlands.