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Sunrise on the Jersey Shore coastline at Spring Lake, New Jersey, U.S.
Rugged coastline of the West Coast Region of New Zealand
Southeast coast of Greenland
Escorca coast, Serra de Tramuntana (Balearic Islands)

A coast (coastline, shoreline, seashore) is the land next to the sea or the line that forms the boundary between the land and the ocean or a lake.[1][2] Coasts are influenced by the topography of the surrounding landscape and by aquatic erosion, such as that caused by waves. The geological composition of rock and soil dictates the type of shore that is created. Earth has about 620,000 km (390,000 mi) of coastline.

Coasts are important zones in natural ecosystems, often home to a wide range of biodiversity. On land, they harbor ecosystems, such as freshwater or estuarine wetlands, that are important for birds and other terrestrial animals. In wave-protected areas, coasts harbor salt marshes, mangroves, and seagrasses, all of which can provide nursery habitat for finfish, shellfish, and other aquatic animals.[3][4] Rocky shores are usually found along exposed coasts and provide habitat for a wide range of sessile animals (e.g. mussels, starfish, barnacles) and various kinds of seaweeds.

In physical oceanography, a shore is the wider fringe that is geologically modified by the action of the body of water past and present, and the beach is at the edge of the shore, including the intertidal zone where there is one.[5] Along tropical coasts with clear, nutrient-poor water, coral reefs can often be found at depths of 1–50 m (3.3–164.0 ft).

According to an atlas prepared by the United Nations, about 44% of the human population lives within 150 km (93 mi) of the sea as of 2013.[6] Due to its importance in society and its high population concentrations, the coast is important for major parts of the global food and economic system, and they provide many ecosystem services to humankind. For example, important human activities happen in port cities. Coastal fisheries (commercial, recreational, and subsistence) and aquaculture are major economic activities and create jobs, livelihoods, and protein for the majority of coastal human populations. Other coastal spaces like beaches and seaside resorts generate large revenues through tourism.

Marine coastal ecosystems can also provide protection against sea level rise and tsunamis. In many countries, mangroves are the primary source of wood for fuel (e.g. charcoal) and building material. Coastal ecosystems like mangroves and seagrasses have a much higher capacity for carbon sequestration than many terrestrial ecosystems, and as such can play a critical role in the near-future to help mitigate climate change effects by uptake of atmospheric anthropogenic carbon dioxide.

However, the economic importance of coasts makes many of these communities vulnerable to climate change, which causes increases in extreme weather and sea level rise, as well as related issues like coastal erosion, saltwater intrusion, and coastal flooding.[7] Other coastal issues, such as marine pollution, marine debris, coastal development, and marine ecosystem destruction, further complicate the human uses of the coast and threaten coastal ecosystems.[7]

The interactive effects of climate change, habitat destruction, overfishing, and water pollution (especially eutrophication) have led to the demise of coastal ecosystem around the globe. This has resulted in population collapse of fisheries stocks, loss of biodiversity, increased invasion of alien species, and loss of healthy habitats. International attention to these issues has been captured in Sustainable Development Goal 14 "Life Below Water", which sets goals for international policy focused on preserving marine coastal ecosystems and supporting more sustainable economic practices for coastal communities.[8] Likewise, the United Nations has declared 2021–2030 the UN Decade on Ecosystem Restoration, but restoration of coastal ecosystems has received insufficient attention.[9]

Since coasts are constantly changing, a coastline's exact perimeter cannot be determined; this measurement challenge is called the coastline paradox. The term coastal zone is used to refer to a region where interactions of sea and land processes occur.[10] Both the terms coast and coastal are often used to describe a geographic location or region located on a coastline (e.g., New Zealand's West Coast, or the East, West, and Gulf Coast of the United States.) Coasts with a narrow continental shelf that are close to the open ocean are called pelagic coast, while other coasts are more sheltered coast in a gulf or bay. A shore, on the other hand, may refer to parts of land adjoining any large body of water, including oceans (sea shore) and lakes (lake shore).

Size

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Somalia has the longest coastline in Africa.[11]

The Earth has approximately 620,000 kilometres (390,000 mi) of coastline. Coastal habitats, which extend to the margins of the continental shelves, make up about 7 percent of the Earth's oceans,[12] but at least 85% of commercially harvested fish depend on coastal environments during at least part of their life cycle.[13] As of October 2010, about 2.86% of exclusive economic zones were part of marine protected areas.[14]

The definition of coasts varies. Marine scientists think of the "wet" (aquatic or intertidal) vegetated habitats as being coastal ecosystems (including seagrass, salt marsh etc.) whilst some terrestrial scientists might only think of coastal ecosystems as purely terrestrial plants that live close to the seashore (see also estuaries and coastal ecosystems).

While there is general agreement in the scientific community regarding the definition of coast, in the political sphere, the delineation of the extents of a coast differ according to jurisdiction.[citation needed][15] Government authorities in various countries may define coast differently for economic and social policy reasons.

Challenges of precisely measuring the coastline

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This section is an excerpt from Coastline paradox.[edit]

The coastline paradox is the counterintuitive observation that the coastline of a landmass does not have a well-defined length. This results from the fractal curve–like properties of coastlines; i.e., the fact that a coastline typically has a fractal dimension. Although the "paradox of length" was previously noted by Hugo Steinhaus,[16] the first systematic study of this phenomenon was by Lewis Fry Richardson,[17][18] and it was expanded upon by Benoit Mandelbrot.[19][20]

The measured length of the coastline depends on the method used to measure it and the degree of cartographic generalization. Since a landmass has features at all scales, from hundreds of kilometers in size to tiny fractions of a millimeter and below, there is no obvious size of the smallest feature that should be taken into consideration when measuring, and hence no single well-defined perimeter to the landmass. Various approximations exist when specific assumptions are made about minimum feature size.

Formation

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Atlantic rocky coastline, showing a surf area. Porto Covo, west coast of Portugal
Seaside in Budelli, Italy. Budelli beach is famous for the color of its sand, which is pink due to the presence of fragments of a microorganism called Miniacina miniacea.[21]

Tides often determine the range over which sediment is deposited or eroded. Areas with high tidal ranges allow waves to reach farther up the shore, and areas with lower tidal ranges produce deposition at a smaller elevation interval. The tidal range is influenced by the size and shape of the coastline. Tides do not typically cause erosion by themselves; however, tidal bores can erode as the waves surge up the river estuaries from the ocean.[22]: 421 

Geologists classify coasts on the basis of tidal range into macrotidal coasts with a tidal range greater than 4 m (13 ft); mesotidal coasts with a tidal range of 2 to 4 m (6.6 to 13 ft); and microtidal coasts with a tidal range of less than 2 m (7 ft). The distinction between macrotidal and mesotidal coasts is more important. Macrotidal coasts lack barrier islands and lagoons, and are characterized by funnel-shaped estuaries containing sand ridges aligned with tidal currents. Wave action is much more important for determining bedforms of sediments deposited along mesotidal and microtidal coasts than in macrotidal coasts.[23]

Waves erode coastline as they break on shore releasing their energy; the larger the wave the more energy it releases and the more sediment it moves. Coastlines with longer shores have more room for the waves to disperse their energy, while coasts with cliffs and short shore faces give little room for the wave energy to be dispersed. In these areas, the wave energy breaking against the cliffs is higher, and air and water are compressed into cracks in the rock, forcing the rock apart, breaking it down. Sediment deposited by waves comes from eroded cliff faces and is moved along the coastline by the waves. This forms an abrasion or cliffed coast.

Sediment deposited by rivers is the dominant influence on the amount of sediment located in the case of coastlines that have estuaries.[24] Today, riverine deposition at the coast is often blocked by dams and other human regulatory devices, which remove the sediment from the stream by causing it to be deposited inland. Coral reefs are a provider of sediment for coastlines of tropical islands.[25]

Like the ocean which shapes them, coasts are a dynamic environment with constant change. The Earth's natural processes, particularly sea level rises, waves and various weather phenomena, have resulted in the erosion, accretion and reshaping of coasts as well as flooding and creation of continental shelves and drowned river valleys (rias).

Importance for humans and ecosystems

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Human settlements

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Further information: Port, island nation, Coastal management, and Coastal development hazards
The Coastal Hazard Wheel system published by UNEP for global coastal management

More and more of the world's people live in coastal regions.[26] According to a United Nations atlas, 44% of all people live within 150 km (93 mi) of the sea.[6] Many major cities are on or near good harbors and have port facilities. Some landlocked places have achieved port status by building canals.

Nations defend their coasts against military invaders, smugglers and illegal migrants. Fixed coastal defenses have long been erected in many nations, and coastal countries typically have a navy and some form of coast guard.

Tourism

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Coasts, especially those with beaches and warm water, attract tourists often leading to the development of seaside resort communities. In many island nations such as those of the Mediterranean, South Pacific Ocean and Caribbean, tourism is central to the economy. Coasts offer recreational activities such as swimming, fishing, surfing, boating, and sunbathing.

Growth management and coastal management can be a challenge for coastal local authorities who often struggle to provide the infrastructure required by new residents, and poor management practices of construction often leave these communities and infrastructure vulnerable to processes like coastal erosion and sea level rise. In many of these communities, management practices such as beach nourishment or when the coastal infrastructure is no longer financially sustainable, managed retreat to remove communities from the coast.

Ecosystem services

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See also: Marine coastal ecosystem
This section is an excerpt from Ecosystem service § Estuarine and coastal ecosystem services.[edit]

Estuarine and marine coastal ecosystems are both marine ecosystems. Together, these ecosystems perform the four categories of ecosystem services in a variety of ways: The provisioning services include forest products, marine products, fresh water, raw materials, biochemical and genetic resources. Regulating services include carbon sequestration (contributing to climate change mitigation) as well as waste treatment and disease regulation and buffer zones. Supporting services of coastal ecosystems include nutrient cycling, biologically mediated habitats and primary production. Cultural services of coastal ecosystems include inspirational aspects, recreation and tourism, science and education.

Coasts and their adjacent areas on and offshore are an important part of a local ecosystem. The mixture of fresh water and salt water (brackish water) in estuaries provides many nutrients for marine life. Salt marshes, mangroves and beaches also support a diversity of plants, animals and insects crucial to the food chain. The high level of biodiversity creates a high level of biological activity, which has attracted human activity for thousands of years. Coasts also create essential material for organisms to live by, including estuaries, wetland, seagrass, coral reefs, and mangroves. Coasts provide habitats for migratory birds, sea turtles, marine mammals, and coral reefs.[29]

Types

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Emergent coastline

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Further information: Emergent coastline and Submergent coastline

According to one principle of classification, an emergent coastline is a coastline that has experienced a fall in sea level, because of either a global sea-level change, or local uplift. Emergent coastlines are identifiable by the coastal landforms, which are above the high tide mark, such as raised beaches. In contrast, a submergent coastline is one where the sea level has risen, due to a global sea-level change, local subsidence, or isostatic rebound. Submergent coastlines are identifiable by their submerged, or "drowned" landforms, such as rias (drowned valleys) and fjords

Concordant coastline

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Further information: Concordant coastline and Discordant coastline

According to the second principle of classification, a concordant coastline is a coastline where bands of different rock types run parallel to the shore. These rock types are usually of varying resistance, so the coastline forms distinctive landforms, such as coves. Discordant coastlines feature distinctive landforms because the rocks are eroded by the ocean waves. The less resistant rocks erode faster, creating inlets or bay; the more resistant rocks erode more slowly, remaining as headlands or outcroppings.

High and low energy coasts

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Parts of a coastline can be categorised as high energy coast or low energy coast. The distinguishing characteristics of a high energy coast are that the average wave energy is relatively high so that erosion of small grained material tends to exceed deposition, and consequently landforms like cliffs, headlands and wave-cut terraces develop.[30] Low energy coasts are generally sheltered from waves, or in regions where the average wind wave and swell conditions are relatively mild. Low energy coasts typically change slowly, and tend to be depositional environments.[31]

High energy coasts are exposed to the direct impact of waves and storms, and are generally erosional environments.[31] High energy storm events can make large changes to a coastline, and can move significant amounts of sediment over a short period, sometimes changing a shoreline configuration.[32]

Destructive and constructive waves

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Swash is the shoreward flow after the break, backwash is the water flow back down the beach. The relative strength of flow in the swash and backwash determines what size grains are deposited or eroded. This is dependent on how the wave breaks and the slope of the shore.[33] Depending on the form of the breaking wave, its energy can carry granular material up the beach and deposit it, or erode it by carrying more material down the slope than up it. Steep waves that are close together and break with the surf plunging down onto the shore slope expend much of their energy lifting the sediment. The weak swash does not carry it far up the slope, and the strong backwash carries it further down the slope, where it either settles in deeper water or is carried along the shore by a longshore current induced by an angled approach of the wave-front to the shore. These waves which erode the beach are called destructive waves.[34] Low waves that are further apart and break by spilling, expend more of their energy in the swash which carries particles up the beach, leaving less energy for the backwash to transport them downslope, with a net constrictive influence on the beach.[34]

Rivieras

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The Cinque Terre, along the Italian Riviera

Riviera is an Italian word for "shoreline",[35][36][37] ultimately derived from Latin ripa ("riverbank"). It came to be applied as a proper name to the coast of the Ligurian Sea, in the form riviera ligure, then shortened to riviera. Historically, the Ligurian Riviera extended from Capo Corvo (Punta Bianca) south of Genoa, north and west into what is now French territory past Monaco and sometimes as far as Marseille.[35][38][39] Today, this coast is divided into the Italian Riviera and the French Riviera, although the French use the term "Riviera" to refer to the Italian Riviera and call the French portion the "Côte d'Azur".[36]

As a result of the fame of the Ligurian rivieras, the term came into English to refer to any shoreline, especially one that is sunny, topographically diverse and popular with tourists.[35] Such places using the term include the Australian Riviera in Queensland and the Turkish Riviera along the Aegean Sea.[36]

Other coastal categories

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  • A cliffed coast or abrasion coast is one where marine action has produced steep declivities known as cliffs.
  • A flat coast is one where the land gradually descends into the sea.
  • A graded shoreline is one where wind and water action has produced a flat and straight coastline.
  • A primary coast isone which is mainly undergoing early stage development by major long-term processes such as tectonism and climate change A secondary coast is one where the primary processes have mostly stabilised, and more localised processes have become prominent.[31]
  • An erosional coast is on average undergoing erosion, while a depositional coast is accumulating material.[31]
  • An active coast is on the edge of a tectonic plate, while a passive coast is usually on a substantial continental shelf or away from a plate edge.[31]

Landforms

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The following articles describe some coastal landforms:

Coastal landforms. The feature shown here as a bay would, in certain (mainly southern) parts of Britain, be called a cove. That between the cuspate foreland and the tombolo is a British bay.

Cliff erosion

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  • Much of the sediment deposited along a coast is the result of erosion of a surrounding cliff, or bluff. Sea cliffs retreat landward because of the constant undercutting of slopes by waves. If the slope/cliff being undercut is made of unconsolidated sediment it will erode at a much faster rate than a cliff made of bedrock.[24]
  • A natural arch is formed when a headland is eroded through by waves.
  • Sea caves are made when certain rock beds are more susceptible to erosion than the surrounding rock beds because of different areas of weakness. These areas are eroded at a faster pace creating a hole or crevice that, through time, by means of wave action and erosion, becomes a cave.
  • A stack is formed when a headland is eroded away by wave and wind action or an arch collapses leaving an offshore remnant.
  • A stump is a shortened sea stack that has been eroded away or fallen because of instability.
  • Wave-cut notches are caused by the undercutting of overhanging slopes which leads to increased stress on cliff material and a greater probability that the slope material will fall. The fallen debris accumulates at the bottom of the cliff and is eventually removed by waves.
  • A wave-cut platform forms after erosion and retreat of a sea cliff has been occurring for a long time. Gently sloping wave-cut platforms develop early on in the first stages of cliff retreat. Later, the length of the platform decreases because the waves lose their energy as they break further offshore.[24][clarification needed]

Coastal features formed by sediment

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Coastal features formed by another feature

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Other features on the coast

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Coastal waters

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See also: Eutrophication § Coastal waters
Overview of different zones of coastal waters: Input, production, transport and storage pathway of carbon in marine waters, including movement across maritime zones of national jurisdiction: territorial sea, Exclusive Economic Zone (EEZ), continental shelf, high seas (international waters), and deep seabed.

"Coastal waters" (or "coastal seas") is a term that carries different meanings depending upon the context, ranging from a geographic reference to the waters within a few kilometers of the coast, to describing the entire continental shelf that may stretch for more than a hundred kilometers from land.[40] The term is used in a different manner when describing legal and economic boundaries, such as territorial waters[41] and international waters,[42] or when describing the geography of coastal landforms or the ecological systems operating through the continental shelf (marine coastal ecosystems).

The dynamic fluid nature of the ocean means that all components of the whole ocean system are ultimately connected, although certain regional classifications are useful and relevant. The waters of the continental shelves represent such a region.[43] The term "coastal waters" has been used in a wide variety of different ways in different contexts. In European Union environmental management it extends from the coast to just a few nautical miles[44] while in the United States the US EPA considers this region to extend much further offshore.[45][46]

"Coastal waters" has specific meanings in the context of commercial coastal shipping, and somewhat different meanings in the context of naval littoral warfare.[citation needed] Oceanographers and marine biologists have yet other takes. Coastal waters have a wide range of marine habitats from enclosed estuaries to the open waters of the continental shelf.

Similarly, the term littoral zone has no single definition. It is the part of a sea, lake, or river that is close to the shore.[47] In coastal environments, the littoral zone extends from the high water mark, which is rarely inundated, to shoreline areas that are permanently submerged.

Coastal waters can be threatened by coastal eutrophication and harmful algal blooms.[48][49][50]

In geology

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The identification of bodies of rock formed from sediments deposited in shoreline and nearshore environments (shoreline and nearshore facies) is extremely important to geologists. These provide vital clues for reconstructing the geography of ancient continents (paleogeography). The locations of these beds show the extent of ancient seas at particular points in geological time, and provide clues to the magnitudes of tides in the distant past.[51]

Sediments deposited in the shoreface are preserved as lenses of sandstone in which the upper part of the sandstone is coarser than the lower part (a coarsening upwards sequence). Geologists refer to these are parasequences. Each records an episode of retreat of the ocean from the shoreline over a period of 10,000 to 1,000,000 years. These often show laminations reflecting various kinds of tidal cycles.[51]

Some of the best-studied shoreline deposits in the world are found along the former western shore of the Western Interior Seaway, a shallow sea that flooded central North America during the late Cretaceous Period (about 100 to 66 million years ago). These are beautifully exposed along the Book Cliffs of Utah and Colorado.[52]

Geologic processes

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The following articles describe the various geologic processes that affect a coastal zone:

Wildlife

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See also: Coastal ecology and Marine coastal ecosystem

Animals

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See also: Seashore wildlife

Larger animals that live in coastal areas include puffins, sea turtles and rockhopper penguins, among many others. Sea snails and various kinds of barnacles live on rocky coasts and scavenge on food deposited by the sea. Some coastal animals are used to humans in developed areas, such as dolphins and seagulls who eat food thrown for them by tourists. Since the coastal areas are all part of the littoral zone, there is a profusion of marine life found just off-coast, including sessile animals such as corals, sponges, starfish, mussels, seaweeds, fishes, and sea anemones.

There are many kinds of seabirds on various coasts. These include pelicans and cormorants, who join up with terns and oystercatchers to forage for fish and shellfish. There are sea lions on the coast of Wales and other countries.

Coastal fish

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This section is an excerpt from Coastal fish.[edit]
Schooling threadfin, a coastal species

Coastal fish, also called inshore fish or neritic fish, inhabit the sea between the shoreline and the edge of the continental shelf. Since the continental shelf is usually less than 200 metres (660 ft) deep, it follows that pelagic coastal fish are generally epipelagic fish, inhabiting the sunlit epipelagic zone.[53] Coastal fish can be contrasted with oceanic fish or offshore fish, which inhabit the deep seas beyond the continental shelves.

Coastal fish are the most abundant in the world.[54] They can be found in tidal pools, fjords and estuaries, near sandy shores and rocky coastlines, around coral reefs and on or above the continental shelf. Coastal fish include forage fish and the predator fish that feed on them. Forage fish thrive in inshore waters where high productivity results from upwelling and shoreline run off of nutrients. Some are partial residents that spawn in streams, estuaries and bays, but most complete their life cycles in the zone.[54]

Plants

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Many coastal areas are famous for their kelp beds. Kelp is a fast-growing seaweed that can grow up to half a meter a day in ideal conditions. Mangroves, seagrasses, macroalgal beds, and salt marsh are important coastal vegetation types in tropical and temperate environments respectively.[3][4] Restinga is another type of coastal vegetation.

Threats

[edit]
See also: Coastal development hazards and marine coastal ecosystem

Coasts also face many human-induced environmental impacts and coastal development hazards. The most important ones are:

Pollution

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See also: Human impact on marine life
A settled coastline in Marblehead, Massachusetts. Once a fishing port, the harbor is now dedicated to tourism and pleasure boating. Observe that the sand and rocks have been darkened by oil slick up to the high-water line.
This stretch of coast in Tanzania's capital Dar es Salaam serves as a public waste dump.
Dead zones occur when phosphorus and nitrogen from fertilizers cause excessive growth of microorganisms, which depletes oxygen and kills fauna.

The pollution of coastlines is connected to marine pollution which can occur from a number of sources: Marine debris (garbage and industrial debris); the transportation of petroleum in tankers, increasing the probability of large oil spills; small oil spills created by large and small vessels, which flush bilge water into the ocean.

Marine pollution

[edit]
This section is an excerpt from Marine pollution.[edit]

Marine pollution occurs when substances used or spread by humans, such as industrial, agricultural, and residential waste; particles; noise; excess carbon dioxide; or invasive organisms enter the ocean and cause harmful effects there. The majority of this waste (80%) comes from land-based activity, although marine transportation significantly contributes as well.[55] It is a combination of chemicals and trash, most of which comes from land sources and is washed or blown into the ocean. This pollution results in damage to the environment, to the health of all organisms, and to economic structures worldwide.[56] Since most inputs come from land, via rivers, sewage, or the atmosphere, it means that continental shelves are more vulnerable to pollution. Air pollution is also a contributing factor, as it carries iron, carbonic acid, nitrogen, silicon, sulfur, pesticides, and dust particles into the ocean.[57] The pollution often comes from nonpoint sources such as agricultural runoff, wind-blown debris, and dust. These nonpoint sources are largely due to runoff that enters the ocean through rivers, but wind-blown debris and dust can also play a role, as these pollutants can settle into waterways and oceans.[58] Pathways of pollution include direct discharge, land runoff, ship pollution, bilge pollution, dredging (which can create dredge plumes), atmospheric pollution and, potentially, deep sea mining.

Different types of marine pollution can be grouped as pollution from marine debris, plastic pollution, including microplastics, ocean acidification, nutrient pollution, toxins, and underwater noise. Plastic pollution in the ocean is a type of marine pollution by plastics, ranging in size from large original material such as bottles and bags, down to microplastics formed from the fragmentation of plastic materials. Marine debris is mainly discarded human rubbish which floats on, or is suspended in the ocean. Plastic pollution is harmful to marine life.

Marine debris

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This section is an excerpt from Marine debris.[edit]

Marine debris, also known as marine litter, is human-created solid material that has deliberately or accidentally been released in seas or the ocean. Floating oceanic debris tends to accumulate at the center of gyres and on coastlines, frequently washing aground, when it is known as beach litter or tidewrack. Deliberate disposal of wastes at sea is called ocean dumping. Naturally occurring debris, such as driftwood and drift seeds, are also present. With the increasing use of plastic, human influence has become an issue as many types of (petrochemical) plastics do not biodegrade quickly, as would natural or organic materials.[59] The largest single type of plastic pollution (~10%) and majority of large plastic in the oceans is discarded and lost nets from the fishing industry.[60] Waterborne plastic poses a serious threat to fish, seabirds, marine reptiles, and marine mammals, as well as to boats and coasts.[61]

Dumping, container spillages, litter washed into storm drains and waterways and wind-blown landfill waste all contribute to this problem. This increased water pollution has caused serious negative effects such as discarded fishing nets capturing animals, concentration of plastic debris in massive marine garbage patches, and increasing concentrations of contaminants in the food chain.

Microplastics

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This section is an excerpt from Marine plastic pollution § Microplastics.[edit]

A growing concern regarding plastic pollution in the marine ecosystem is the use of microplastics. Microplastics are beads of plastic less than 5 millimeters wide,[62] and they are commonly found in hand soaps, face cleansers, and other exfoliators. When these products are used, the microplastics go through the water filtration system and into the ocean, but because of their small size they are likely to escape capture by the preliminary treatment screens on wastewater plants.[63] These beads are harmful to the organisms in the ocean, especially filter feeders, because they can easily ingest the plastic and become sick. The microplastics are such a concern because it is difficult to clean them up due to their size, so humans can try to avoid using these harmful plastics by purchasing products that use environmentally safe exfoliates.

Because plastic is so widely used across the planet, microplastics have become widespread in the marine environment. For example, microplastics can be found on sandy beaches[64] and surface waters[65] as well as in the water column and deep sea sediment. Microplastics are also found within the many other types of marine particles such as dead biological material (tissue and shells) and some soil particles (blown in by wind and carried to the ocean by rivers). Population density and proximity to urban centers have been considered the main factors that influence the abundance of microplastics in the environment.

Sea level rise due to climate change

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This section is an excerpt from Sea level rise.[edit]

The sea level has been rising since the end of the last ice age, which was around 20,000 years ago.[66] Between 1901 and 2018, the average sea level rose by 15–25 cm (6–10 in), with an increase of 2.3 mm (0.091 in) per year since the 1970s.[67]: 1216  This was faster than the sea level had ever risen over at least the past 3,000 years.[67]: 1216  The rate accelerated to 4.62 mm (0.182 in)/yr for the decade 2013–2022.[68] Climate change due to human activities is the main cause.[69]: 5, 8  Between 1993 and 2018, melting ice sheets and glaciers accounted for 44% of sea level rise, with another 42% resulting from thermal expansion of water.[70]: 1576 

Sea level rise lags behind changes in the Earth's temperature by decades, and sea level rise will therefore continue to accelerate between now and 2050 in response to warming that has already happened.[71] What happens after that depends on future human greenhouse gas emissions. If there are very deep cuts in emissions, sea level rise would slow between 2050 and 2100. The reported factors of increase in flood hazard potential are often exceedingly large, ranging from 10 to 1000 for even modest sea-level rise scenarios of 0.5 m or less.[72] It could then reach by 2100 between 30 cm (1 ft) and 1.0 m (3+1⁄3 ft) from now and approximately 60 cm (2 ft) to 130 cm (4+1⁄2 ft) from the 19th century. With high emissions it would instead accelerate further, and could rise by 50 cm (1.6 ft) or even by 1.9 m (6.2 ft) by 2100.[73][69][67]: 1302  In the long run, sea level rise would amount to 2–3 m (7–10 ft) over the next 2000 years if warming stays to its current 1.5 °C (2.7 °F) over the pre-industrial past. It would be 19–22 metres (62–72 ft) if warming peaks at 5 °C (9.0 °F).[69]: 21 

Global goals

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International attention to address the threats of coasts has been captured in Sustainable Development Goal 14 "Life Below Water" which sets goals for international policy focused on preserving marine coastal ecosystems and supporting more sustainable economic practices for coastal communities.[8] Likewise, the United Nations has declared 2021–2030 the UN Decade on Ecosystem Restoration, but restoration of coastal ecosystems has received insufficient attention.[9]

See also

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References

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Further reading

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[edit]
Revisions and contributorsEdit on WikipediaRead on Wikipedia

Coast

View on Grokipedia
from Grokipedia
A coast is the transitional zone of indefinite width where land interfaces with the or , including both the subaerial landforms above the and the shallow areas affected by wave action and . This region is defined by its dynamic , where marine processes such as wave , tidal currents, and interact with terrestrial inputs like river sediment to form diverse features including sandy beaches, rocky cliffs, barrier islands, and deltas. Coasts are geologically active environments shaped by the balance between , which removes material through and abrasion, and deposition, which builds landforms via accumulation during periods of lower energy. Ecologically, they host highly productive systems with elevated due to nutrient influx from both land and , supporting mangroves, salt marshes, and intertidal zones that serve as nurseries for fisheries and buffers against storms. Human settlement concentrates along coasts for access to maritime , ports, and resources, though this exposes populations to hazards like and inundation, with empirical observations showing variable rates of shoreline change driven by local , -level fluctuations, and supply rather than uniform global trends.

Definition and Characteristics

Delineation from Adjacent Zones

The coast is empirically delineated as the transition zone between terrestrial landforms and marine environments, where marine processes such as tidal inundation and wave action directly influence land surface morphology and dynamics. Landward, this boundary is marked by the inland limit of regular tidal exchange, which distinguishes coastal areas from adjacent non-tidal inland zones and can extend several kilometers in deltas or estuaries with pronounced tidal ranges. gradients, typically exceeding 0.5 parts per thousand as a threshold for marine influence, further define this demarcation, as freshwater-dominated systems lack the osmotic and sedimentary signatures of coastal interaction. Seaward, the coast is bounded from open ocean realms by the shoreface profile, extending to the depth of closure—the offshore limit beyond which wave-induced changes in seabed elevation are negligible, often ranging from 5 to 20 meters depending on local conditions. Bathymetric gradients, sediment characteristics (e.g., coarser sands resisting erosion versus finer silts promoting deposition), and wave base—the depth at which oscillatory wave motion fails to mobilize bottom sediments—causally determine this extent, as steeper slopes and cohesive sediments confine active zones shallower while dissipative fine-grained seabeds allow broader influence. These geophysical criteria are verified through precise measurements, including multibeam bathymetry for seabed profiling and shoreline positioning via differential GPS, which achieves sub-meter accuracy in mapping dynamic boundaries. Contemporary delineation integrates altimetry to resolve surface elevations near coasts, correcting for land contamination in returns to track tidal and wave-influenced extents with resolutions approaching 1-2 km offshore. Historically, coastal boundaries were first systematically charted in the early through national hydrographic programs, such as the Coast Survey established in , which used lead-line soundings and tidal observations to standardize mappings amid expanding maritime needs. These efforts supplanted earlier recognitions based on visible tidal bores and distributions, providing the foundational data for causal understanding of coastal dynamics over purely observational limits.

Measurement Challenges and Fractal Nature

The coastline paradox, first formalized by mathematician Benoit Mandelbrot in 1967, reveals that coastlines lack a fixed length because they exhibit fractal properties, with self-similar irregularities repeating across scales. As measurement resolution increases—transitioning from large-scale units like 100 km to finer ones like 1 km—the recorded length grows without bound, reflecting statistical self-similarity rather than a smooth Euclidean line. This scale-dependence stems from the inherent roughness of natural boundaries, where smaller features (e.g., bays, inlets, and fjords) mimic larger ones, defying traditional dimensional analysis and implying a fractional dimension between one (line-like) and two (area-like). Empirical measurements illustrate this indeterminacy vividly. For Great Britain's mainland coastline, coarse mapping at a 100 km unit yields approximately 2,800 km, but refining to a 1 km unit extends it beyond 12,000 km, with further subdivision amplifying the figure indefinitely. Globally, estimates vary from roughly 350,000 km in low-resolution surveys (e.g., 1:10 million scale) to over 500,000 km in higher-detail assessments, as complexity in rocky terrains—prevalent in regions like Norway's fjords—contrasts with smoother sandy shores, exacerbating inconsistencies. Tidal fluctuations compound these issues, shifting the instantaneous shoreline by meters to kilometers daily, particularly in macrotidal areas exceeding 4 m range, rendering static lengths provisional. In geographic information systems (GIS), consistency is pursued through standardized protocols, such as fixed-interval transects or baseline approximations at resolutions like 30 m from Landsat-derived data, enabling rate-of-change analyses via tools like the USGS Digital Shoreline Analysis System (DSAS). Recent advancements, including multispectral imagery at sub-30 m resolution, have refined delineations for global datasets, improving mapping and reducing noise in dynamic monitoring. Yet, these efforts underscore the paradox's persistence: no universal length exists absent an arbitrary scale choice, informing cautious interpretations in fields from to impact modeling.

Physical Attributes Influencing Dynamics

Coastal dynamics are shaped by substrate composition, which governs resistance to wave and current forces. Hard rock substrates, such as and , exhibit low rates, with median cliff recession around 2.9 cm per year globally, reflecting their durability against mechanical abrasion. In contrast, unconsolidated sandy substrates are highly mobile, prone to rapid transport and reshaping by littoral drift, supplying to downdrift areas while offering minimal structural resistance. Cohesive clay or coasts erode more readily, with median rates up to 23 cm per year, amplifying retreat in energetic settings due to slumping and undercutting. Tidal range exerts control over hydrodynamic regimes, categorized as (mean spring range <2 m), mesotidal (2–4 m), or (>4 m)./04:_Global_wave_and_tidal_environments/4.04:_Large-scale_variation_in_tidal_characteristics/4.4.01:_Global_tidal_environments) coasts feature amplified currents exceeding 1–2 m/s, driving extensive reworking and barrier breaching, whereas systems experience subdued tidal flows (<0.5 m/s), shifting dominance to wave-driven processes./04:_Global_wave_and_tidal_environments/4.04:_Large-scale_variation_in_tidal_characteristics/4.4.01:_Global_tidal_environments) Wave exposure, determined by fetch length—the unobstructed distance over which wind generates waves—and prevailing wind patterns, quantifies energy input. Longer fetches, as along Atlantic margins, yield higher significant wave heights (typically 2–4 m annually) and power fluxes up to 30–50 kW/m, fostering erosive dominance. Sheltered coasts, such as those in enclosed seas, encounter reduced fetches (<500 km), resulting in wave energies below 10 kW/m and gentler interactions. Destructive waves, with steep profiles (height-to-wavelength ratio >1/20), high frequency (>12 waves per day), and pronounced backwash, erode substrates efficiently, while constructive waves (ratio <1/40, <8 waves per day) feature stronger swash for sediment buildup.

Geological Formation and Processes

Primary Formation Mechanisms

Coasts originate primarily from relative changes in sea level and land elevation, resulting from eustatic variations, tectonic deformation, and isostatic responses to glacial loading. Submergent coasts form when sea levels rise relative to the land, inundating preexisting topography such as river valleys or glacial troughs. This process has been dominant globally since the end of the approximately 20,000 years ago, when melting ice sheets drove a eustatic sea-level rise exceeding 120 meters. Resulting features include rias, which are drowned river mouths with branching estuaries, and fjords, steep-walled inlets carved by glaciers and subsequently flooded. These landforms characterize passive continental margins where subsidence outpaces minimal tectonic uplift, as seen along much of the U.S. East Coast. Emergent coasts arise from land uplift exceeding sea-level rise or from eustatic falls in sea level, exposing former submarine features like wave-cut platforms. Tectonic uplift at convergent plate boundaries, such as along the Pacific Ring of Fire, elevates coastal terrains rapidly, while post-glacial isostatic rebound dominates in formerly glaciated regions. In Scandinavia, GPS measurements indicate ongoing uplift rates of 5–10 mm per year, with higher values up to 10 mm per year in the northern area due to viscoelastic mantle response to ice unloading. This rebound, initiated around 15,000 years ago, has raised shorelines by hundreds of meters since deglaciation, producing raised beaches and marine terraces. Neutral coasts reflect a quasi-equilibrium where relative sea-level changes are minimal over Holocene timescales, often on stable passive margins with low tectonic activity and negligible isostatic adjustment. Sediment core analyses from parts of the U.S. Gulf Coast reveal shoreline stability or progradation over the past few millennia prior to modern subsidence influences, indicating balanced eustatic and local subsidence rates. These coasts lack pronounced drowned valleys or elevated platforms, maintaining configurations shaped by long-term tectonic quiescence rather than dramatic level shifts.

Tectonic and Isostatic Influences

Tectonic plate boundary dynamics profoundly shape coastal morphology through vertical crustal movements and faulting. At convergent margins, where oceanic plates subduct beneath continental ones, ongoing compression and uplift generate steep, cliff-dominated coastlines resistant to erosion, as observed along the Pacific Ring of Fire encompassing the western Americas. Paleoseismological studies document fault slip rates exceeding several millimeters per year in these zones, contributing to episodic coastal deformation via earthquakes that elevate or scar landforms. In contrast, divergent boundaries facilitate crustal thinning and rifting, producing irregular coastlines with fault-block scarps and elongated basins, such as the East African Rift Zone where continental separation exposes rift valley coasts to marine inundation. Isostatic adjustments, driven by the redistribution of mass following glacial unloading, induce differential vertical motions that alter relative sea levels and coastal configurations. In regions formerly burdened by the Laurentide Ice Sheet, such as Hudson Bay, post-glacial rebound manifests as land uplift rates of approximately 1 cm per year, as quantified by GRACE satellite gravimetry data from the 2010s and 2020s, which reveal mass loss signals consistent with ongoing crustal recovery. This uplift locally counteracts eustatic sea-level rise, preserving raised beaches and prograding shorelines, with rates peaking near 11 mm/year in southeastern based on integrated geodetic models. Volcanic activity, often linked to tectonic hotspots or arc systems, contributes bold promontories and extended coastal platforms via effusive lava flows that armor shorelines against wave attack. On Hawaii, Holocene eruptions from shield volcanoes like Kilauea have extruded basaltic flows forming resistant headlands, with radiocarbon dating of intercalated organics confirming activity within the past 10,000 years, including flows dated to 1,500–2,000 years ago that directly impinge on existing coastlines. These endogenous processes underscore the dominance of lithospheric forces in dictating long-term coastal architecture over shorter-term surficial dynamics.

Sedimentary and Erosional Cycles

Coastal sedimentary and erosional cycles involve the continuous transport, deposition, and removal of sediments driven primarily by wave action and currents, maintaining dynamic equilibrium in littoral zones. Longshore drift, the dominant mechanism of sediment movement parallel to the shore, redistributes sand and gravel within defined littoral cells, where inputs from rivers and headland erosion balance outputs via offshore losses and alongshore export. Human interventions, such as jetties at harbor entrances, disrupt this balance by trapping sediment updrift while causing downdrift erosion through reduced transport rates, as observed in numerous inlet-stabilized systems where net longshore flux decreases substantially during storms. Sediment budget models quantify these imbalances, revealing deficits in cells interrupted by such structures, with downdrift beaches experiencing accelerated retreat due to insufficient replenishment. Erosion rates along coasts vary significantly with substrate lithology and exposure, with empirical data indicating medians of 0.029 m/year for hard rocks, 0.10 m/year for medium rocks, and 0.23 m/year for weak rocks based on global cliff recession analyses. In unconsolidated sediments like glacial boulder clay, rates escalate; the Holderness coast in the United Kingdom erodes at an average of 2 m/year, releasing approximately 2 million tonnes of material annually due to the soft, easily weathered nature of these deposits under wave attack. Tracer studies and volumetric assessments confirm that such erosion supplies sediment to adjacent depositional zones, but chronic deficits arise when artificial barriers prevent redistribution, leading to net losses in affected cells. These processes exhibit cyclic patterns, with intense storm events driving rapid offshore sediment removal and cliff undercutting, followed by gradual fair-weather accretion that rebuilds beaches through onshore transport. Sediment cores from saltmarshes and barriers preserve layered evidence of these alternations, showing coarse storm deposits overlain by finer fair-weather laminations, spanning multiple centuries and linking episodic erosion to prevailing wave climates. Dendrochronological records from coastal trees further corroborate this cyclicity, revealing growth anomalies tied to burial during accretional phases or exposure from erosional cutbacks, thus providing proxy data for reconstructing long-term budget fluctuations independent of direct instrumental measurements. Overall, these cycles underscore the resilience of coastal systems to natural variability, though anthropogenic alterations amplify erosional dominance in many locales.

Classifications and Types

Morphological and Genetic Categories

Coasts are morphologically classified according to the degree of concordance between the coastline orientation and the underlying geological structures, a criterion that empirically delineates patterns of exposure to marine forces. Concordant coasts exhibit rock strata, folds, or other structures aligned parallel to the shoreline, producing relatively uniform and linear profiles with limited embayments, as observed in tectonically controlled straight coasts. Discordant coasts, conversely, feature geological structures transverse or perpendicular to the coast, yielding irregular morphologies where resistant and weaker materials alternate, such as in settings with dipping strata intersecting the shore at angles. Genetic classification distinguishes coasts based on dominant formative agents, originating from Eduard Suess's late 19th-century framework that separates primary coasts—shaped chiefly by terrestrial or non-marine processes like tectonic movements, subaerial erosion, volcanism, or fluvial deposition—from secondary coasts, which have been substantially reworked by marine erosion, sedimentation, or biological activity. Primary coasts retain youthful terrestrial signatures, including fault-line scarps from recent tectonic activity or volcanic landforms unmodified by waves, comprising approximately 30-40% of global coastlines according to mid-20th-century assessments refined by later mapping. Secondary coasts, predominant worldwide, reflect overlay of marine modifications on prior landforms, such as smoothed erosional cliffs or accumulated barriers, with transitions evident in regions like the U.S. Atlantic margin where glacial deposits have been reshaped. This dichotomy, while simplified, accommodates empirical updates from remote sensing, highlighting causal primacy of non-marine origins in primary types versus superimposed marine dominance in secondary ones. Within morphological subtypes, rivieras denote steep, rocky coasts where elevated terrain abuts the sea with minimal alluvial plains, typically arising from tectonic uplift and subdued sedimentation, as exemplified by Mediterranean segments with abrupt descents from coastal ranges. These contrast with low-relief depositional shores, featuring resistant bedrock exposures and narrow shelves that limit sediment trapping, fostering profiles resistant to rapid change absent significant subsidence or transgression.

Emergent, Submergent, and Compound Coasts

Emergent coasts form through tectonic uplift or post-glacial isostatic rebound that elevates former submarine or intertidal features above present sea level, as evidenced by stratigraphic sequences of marine sediments overlain by terrestrial deposits and dated via radiocarbon on shells or OSL on associated sands. Key indicators include raised beaches—accumulations of gravel and sand from past shorelines—and elevated wave-cut platforms, which are erosional benches incised by waves into bedrock and subsequently stranded inland. Fossil shorelines, such as shell beds or abrasion notches, further confirm emergence, with relative motion rates inferred from terrace elevations and dated benchmarks; for example, California's Pacific coast exhibits uplifted marine terraces preserving Pliocene-to-Pleistocene wave-cut platforms, reflecting ongoing tectonic compression along the system. These features enable reconstruction of uplift histories, with inner terrace edges tracking fault slip and regional deformation. Submergent coasts arise from eustatic sea-level rise or tectonic subsidence that floods preexisting coastal plains and valleys, identifiable through drowned fluvial morphology in seismic profiles and core samples containing transgressive ravinement surfaces—erosional unconformities marking the landward migration of the shoreline. Prominent landforms include estuaries from inundated river mouths and barrier islands, which develop as sand spits or offshore bars migrate landward during transgression and stabilize with reduced accommodation space. Radiocarbon dating of basal peats or shells in back-barrier marshes dates the onset of flooding; on the U.S. Atlantic coast, Holocene sea-level rise post-~11,000 BP drowned ancestral drainages, forming rias like Delaware Bay, while barrier chains such as the Outer Banks emerged around 7,000–4,000 BP as transgression slowed and sediment supply from rivers and longshore transport built protective spits. This submergence history contrasts with stable or emergent margins by preserving submerged paleochannels detectable via multibeam bathymetry. Compound coasts display superimposed evidence of alternating emergence and submergence phases, often from fluctuating glacio-eustatic cycles or localized tectonics, revealed in vertical stratigraphic stacking where emergent terraces cap submergent paleovalleys or relict dunes bury drowned platforms, dated via integrated U/Th, radiocarbon, and OSL chronologies. These polygenetic margins exhibit hybrid landforms, such as elevated barrier remnants overlying estuarine fills, indicating episodic relative sea-level changes; for instance, southeastern Australian coasts feature Holocene relict foredunes—aeolian sands OSL-dated to ~6,000–2,000 BP—perched atop submerged Pleistocene platforms, reflecting initial postglacial submergence followed by minor emergence or stabilization amid low tectonic rates. Such sequences inform predictive geomorphology by highlighting inheritance effects, where older emergent features modulate responses to subsequent submergence.

Energy Regimes and Wave Interactions

Coastal energy regimes are primarily driven by wave action, with secondary contributions from tides and currents, where wave energy is determined by factors such as fetch—the unobstructed distance over which wind generates waves—and refraction, the bending of wave crests due to varying water depths that concentrates energy on promontories and dissipates it in embayments. Wave power, proportional to the square of wave height and period, classifies coasts into high-energy regimes on exposed fetch-limited shores subject to persistent strong winds, versus low-energy regimes in sheltered areas with short fetch and reduced wind exposure. High-energy coasts feature destructive waves characterized by short periods (typically under 10 seconds), steep profiles, and heights exceeding 2 meters, promoting erosion through dominant backwash that exceeds swash on steep beaches; examples include the storm-prone , where significant wave heights routinely surpass 3 meters in winter under fetch from prevailing westerlies. In contrast, low-energy coasts exhibit constructive waves with longer periods (over 10 seconds), lower heights (under 1 meter), and swash dominance that facilitates sediment deposition on wide, dissipative shores. Empirical breaker indices, defined as the ratio of breaking wave height to water depth (γ ≈ 0.5–1.2 depending on beach slope and wave steepness), quantify these dynamics, with lower γ values indicating spilling breakers in dissipative low-energy settings and higher γ for plunging breakers in reflective high-energy ones. Tidal modulation amplifies or attenuates wave energy in macrotidal regimes (tidal range >4 meters), where strong currents interact with waves to alter breaker characteristics and energy dissipation; in the , tidal ranges reach extremes of 16 meters due to resonant amplification in the funnel-shaped basin, enhancing tidal currents up to 5–6 knots that scour sediments and modulate wave heights during and ebb phases. Such interactions overlay hydrodynamic variability on morphological classifications, explaining differential rates within similar coastal types, as and tidal phasing redistribute energy spatially.

Landforms and Features

Erosional Landforms

Coastal cliffs form through the undercutting action of waves, which erode a notch at the base via hydraulic action, abrasion, and corrosion, leading to gravitational collapse of the overlying material. This process creates steep faces, with erosion rates varying by rock type and wave energy; for instance, the White Cliffs of Dover have retreated at 22-32 cm per year over the past 150 years, accelerated from earlier rates of 2-6 cm per year. Field measurements using LiDAR and photogrammetry confirm short-term variability, with some cliffs showing monthly rates up to 28.8 cm in high-energy events. Further erosion of headlands produces sea caves in weaker rock zones, which enlarge and connect through arches; subsequent collapse of the arch roof isolates stacks as isolated pillars. These features exploit geological joints and bedding planes, with stacks representing advanced stages of subtractive morphology before reduction to stumps. Wave-cut platforms emerge as abrasion bevels the cliff base at or near mean low water level, forming gently sloping benches of exposed rock during low tide. In Australia, such platforms in Victoria's Waratah Bay developed across Paleozoic bedrock during Quaternary sea-level fluctuations, with preserved features indicating Pleistocene formation timelines inferred from stratigraphic correlations. On discordant coastlines, where rock strata are perpendicular to the shore, differential rates sculpt headlands from resistant lithologies and bays from softer materials, as waves refract and concentrate energy on protruding sections. Softer rocks like clay erode faster than harder or , creating indented bays between protruding headlands aligned with fetch directions. This pattern is evident in field observations of varying , with global cliff datasets reporting average rates of 0.3-0.5 m per year in soft sedimentary coasts.

Depositional and Sedimentary Features

Depositional coastal features arise from a positive sediment budget, where supply from rivers, waves, and winds exceeds erosional losses, leading to net accumulation and progradation. These landforms, including beaches, spits, barriers, deltas, and dunes, develop through sediment transport processes such as and aeolian redistribution, often achieving dynamic equilibrium profiles shaped by wave energy and grain characteristics. Sediment budgets dictate their evolution; for instance, fluvial inputs historically drove deltaic advance, while interruptions like induce reversal to . Beaches represent the primary depositional interface, consisting of unconsolidated sediments sorted zonally by hydrodynamic forces, with coarser grains typically dominating steeper profiles under higher wave energy. Grain size influences permeability and backwash efficiency, where coarser sediments reduce offshore transport, promoting retention on reflective beaches. Spits form as linear accumulations extending from headlands via longshore sediment drift, enclosing bays and stabilizing when vegetation colonizes, as evidenced by historical mapping of river-mouth spits governed by predicted transport rates. Beach cusps, rhythmic scalloped patterns along the swash zone, emerge from interference patterns like standing edge waves, which organize sediment deposition into horns of coarser material separated by finer embayments. Barrier islands and cheniers develop through Holocene progradation, where rising sea levels post-glacial maximum allowed barrier migration and spit elongation across subsiding shelves. These features rely on ample sand supply to maintain lagoons and overwash deposits, with wave reworking promoting cyclical accretion in wave-influenced settings. Deltas exemplify fluvial dominance in deposition, as seen in the , which prograded seaward by approximately 50 km over the past 5,000 years via annual flood sediments before the 1964 Aswan High Dam trapped over 95% of supply, shifting to coastal retreat rates exceeding 100 meters per year in exposed sectors. Aeolian dunes, particularly foredunes, form landward of beaches as wind transports sand inland, with vegetation enhancing trapping efficiency and stabilizing accumulations against deflation. Foredunes capture significant portions of beach-derived sand, fostering blowout prevention and inland migration under onshore winds, as demonstrated in field calibrations of transport dynamics. These features underscore the interplay of sediment budgets, where deficits from upstream damming or overwash disrupt growth, while surpluses enable parabolic or transverse dune fields.

Biogenic and Structural Formations

Coral reefs represent prominent biogenic formations, constructed through the calcification of scleractinian , , and other organisms that secrete frameworks. These structures manifest as fringing reefs adjacent to shorelines, barrier reefs separated by lagoons, or atolls encircling subsided volcanic remnants, influencing coastal morphology by promoting trapping and vertical growth. In the , uranium-thorium dating of reef cores indicates mixed framework and accretion rates averaging approximately 12.4 mm per year over timescales. Such accretion enables reefs to maintain elevation relative to sea-level fluctuations, with mid- rates in inshore fringing reefs reaching up to 4.6 mm per year based on averaged paleoenvironmental reconstructions. Mangrove forests contribute to biogenic stabilization in tropical and subtropical intertidal zones, where specialized root systems—such as prop roots of species and pneumatophores of —bind sediments and dissipate hydrodynamic forces. These adaptations trap fine suspended particles during tidal inundation, fostering accretion and elevating substrates while reducing on underlying soils. Mangroves thereby attenuate wave heights and currents, mitigating shoreline retreat, as evidenced by their role in promoting deposition and limiting in storm-prone areas. Salt marshes, dominated by halophytic grasses like Spartina alterniflora, exhibit vertical accretion primarily through belowground organic matter accumulation and episodic mineral sediment inputs during floods. In Louisiana's deltaic marshes, marker horizon studies reveal mean accretion rates of 12.8 mm per year, with variations tied to sediment supply; these rates historically outpaced local before 20th-century construction reduced fluvial inputs. Such biogenic elevation allows marshes to counteract relative sea-level rise, maintaining platform integrity against transgressive tendencies. Structural formations, distinct in their reliance on inherited sediment bodies rather than ongoing biological construction, include tombolos—narrow depositional bars or isthmuses linking offshore islands to adjacent mainland via longshore transport. These features arise where wave and concentrate sediment around resistant topographic highs, forming emergent connections. in Dorset, , exemplifies a tombolo comprising 18 km of graded shingle, initiated from sandy deposits remobilized during post-glacial sea-level rise between 20,000 and 14,000 years ago, with subsequent from eroding cliffs sustaining its profile. Tombolos alter local hydrodynamics by sheltering leeward lagoons, such as the Fleet behind Chesil, while resisting breaching through coarse clast armoring.

Ecosystems and Biodiversity

Coastal Aquatic Habitats

Coastal aquatic habitats encompass the nearshore waters where oceanic and terrestrial influences converge, creating dynamic transitional ecosystems characterized by steep physicochemical gradients in , , , and . These zones exhibit high primary due to nutrient enrichment from riverine inputs and coastal processes, supporting diverse microbial, , and nektonic communities. Empirical measurements from conductivity-temperature-depth (CTD) casts reveal sharp and stratifications, often forming fronts that concentrate and serve as foraging areas for higher trophic levels. Estuaries and coastal lagoons function as mixing zones where freshwater inflows meet marine waters, resulting in elevated from suspended sediments and high loads that fuel blooms. Estuaries retain a substantial portion of incoming riverine sediments, with northern estuaries trapping the bulk of their inputs alongside additional marine-derived materials, thereby stabilizing benthic environments and enhancing deposition. Globally, estuaries contribute approximately 16% of non-oceanic fisheries yields through nursery functions for larval and juvenile fish, underscoring their role in secondary production despite covering less than 1% of the ocean surface. Lagoons, often shallow and semi-enclosed, amplify retention and exhibit seasonal variations driven by precipitation and tidal mixing, promoting eutrophic conditions that boost algal productivity but also risk hypoxia. Benthic zones in coastal soft sediments host infaunal communities of polychaetes, bivalves, and crustaceans that bioturbate substrates, facilitating nutrient cycling and organic decomposition. These assemblages thrive in muddy or silty deposits, where low oxygen penetration depths limit vertical distribution, and sediment grain size dictates species composition—finer particles supporting deposit feeders. Infauna densities can reach thousands per square meter, contributing to engineering by altering porewater chemistry and supporting overlying pelagic food webs. Eastern boundary currents, such as the Humboldt (Peru) Current, drive upwelling that elevates productivity across narrow coastal bands, bringing nutrient-rich deep waters to the photic zone. These systems, spanning less than 1% of ocean area, account for up to 20% of global fisheries catches through sustained phytoplankton blooms that underpin anchoveta and sardine populations. The Humboldt Current specifically supports 10-20% of worldwide marine fish landings, with peak productivity during austral winter upwelling favorable winds. Ocean fronts associated with these gradients act as biodiversity hotspots, aggregating zooplankton and micronekton via convergence and enhanced retention.

Terrestrial and Intertidal Biota

Intertidal zones exhibit distinct vertical banding of biota, driven by tidal exposure gradients that impose varying durations of submersion, , and wave stress. In the upper intertidal, such as Balanus glandula predominate, with their upper distributional limit set by tolerance to aerial exposure and , while their lower boundary is influenced by competition and predation. Algal bands, including spp. in the mid-intertidal, form conspicuous green and brown zones adapted to periodic inundation, providing substrate for epiphytes and grazers. Gastropods like periwinkle snails (Littorina spp.) occupy the high to mid-intertidal, employing behavioral adaptations such as retreating into shell crevices and physiological to maintain internal salt balance during emersion, enabling survival in fluctuating regimes up to 50% concentration. Terrestrial coastal vegetation includes specialized for saline, wind-exposed environments. In salt marshes, smooth cordgrass (Spartina alterniflora) thrives as a facultative , excreting excess salts through glandular structures on leaf blades and tolerating interstitial salinities exceeding 30 ppt via exclusion at . These form dense stands that trap sediments, with root systems enhancing soil accretion rates by 1-5 mm annually in temperate marshes. On dunes, marram grass () stabilizes foredunes through extensive rhizomatous growth, binding sand particles and increasing slope shear resistance; experimental comparisons of vegetated versus bare dunes demonstrate that marram reduces susceptibility during storms by reinforcing substrate cohesion. Coastal areas support migratory shorebirds and select mammals reliant on intertidal foraging. Banding and GPS tracking data reveal that species such as (Tringa flavipes) concentrate along Atlantic and Pacific flyways, with stopover durations averaging 10-20 days at coastal wetlands to replenish fat reserves for non-stop flights exceeding 2,000 km. Terrestrial mammals like coyotes (Canis latrans) and raccoons (Procyon lotor) exhibit coastal distributions influenced by prey availability, with adaptations including enlarged nasal glands for salt excretion in diets incorporating marine carrion, though their ranges extend inland without strict coastal . These biota collectively buffer coastal soils against while depending on zonation-specific tolerances to , , and nutrient pulses from .

Ecological Interactions and Services

Coastal ecosystems mediate critical nutrient cycling processes, retaining terrestrial inputs and exporting to the open , where continental shelves and margins account for approximately 80% of global marine organic carbon burial despite comprising only about 7-10% of the seafloor area. This burial, estimated at 248 Tg C yr⁻¹ in margin sediments, enhances long-term carbon storage and regulates atmospheric CO₂ levels through sedimentary processes that outpace remineralization in deeper waters. retention in coastal zones, including and from riverine sources, supports primary productivity gradients, with shelves facilitating up to 75% of river-supplied delivery to offshore ecosystems while minimizing hotspots. These systems bolster through habitat-mediated services, such as wave energy dissipation by vegetated wetlands, which can reduce heights by factors of 2-10 meters over distances of several kilometers, thereby preserving sediment stability and integrity. Mangroves exemplify high-efficiency , accumulating 2-4 times more carbon per unit area than mature tropical forests, with soil stocks often exceeding 1,000 Mg C ha⁻¹ due to anaerobic conditions that inhibit . Such feedbacks contribute to system by maintaining trophic productivity; for instance, reefs in temperate estuaries enhance via bivalve filtration, with individuals clearing 3-12.5 gallons of water daily under ambient conditions, promoting control and benthic light penetration essential for persistence. Coastal biodiversity hotspots amplify these interactions, concentrating productivity and in shelf and nearshore domains that represent roughly 8% of ocean area but sustain elevated rates of and functional diversity. Keystone trophic webs, including predator-prey dynamics in intertidal zones, regulate population carrying capacities; for example, herbivorous fishes and in coral-adjacent graze , preventing phase shifts to macroalgal dominance and preserving rates that underpin reef accretion at 1-10 mm yr⁻¹. Empirical metrics from resilient systems, such as mangrove-oyster synergies, demonstrate enhanced resilience, with filtration and stabilization collectively supporting turnover rates 3-5 times higher than adjacent unvegetated sediments.

Human Engagement and Economic Role

Population Distribution and Settlements

Approximately 40% of the world's resides within 100 kilometers of a coastline, a figure that has grown from about 2 billion in 1990 to over 3 billion by the late 2010s due to economic opportunities tied to maritime access. This concentration reflects empirical patterns of preference for coastal zones, driven by reliable access to protein-rich fisheries and efficient overland-water transport interfaces that reduce trade costs compared to inland locations. Population densities peak in river deltas and estuaries, where fertile sediments support alongside ; the Ganges-Brahmaputra delta, for instance, sustains over 400 million people across its basin with average densities exceeding 390 people per square kilometer, making it one of the most densely settled coastal regions globally. Similar patterns occur in other deltas like the and , where alluvial plains enable intensive cultivation and fishing, historically drawing settlements despite periodic flooding, as humans adapted through raised structures and systems predating modern . Coastal settlements trace back to the , when Mediterranean ports such as those in the Aegean and emerged as hubs for seafaring and resource exchange, with sites like and Kommos facilitating trade in metals and goods that underpinned early empires. These locations prioritized defensive hilltops near anchorages, balancing vulnerability to raids with advantages in maritime connectivity that inland areas lacked. In the , coastal megacities exemplify this enduring calculus, with Tokyo's metropolitan area—abutting —housing 37 million residents as of the early 2020s, its density sustained by port infrastructure that historically mitigated tidal risks through seawalls and . Such concentrations persist because the caloric and economic yields from adjacent seas—via fisheries providing up to 20% of global animal protein—outweigh unmanaged hazards, with pre-industrial diking and harbor works demonstrating proactive risk reduction.

Resource Extraction and Trade

Coastal fisheries provide a primary extraction activity, with global capture production reaching 92.3 million tonnes in , predominantly from marine waters. Approximately 600 million people worldwide rely on fisheries and aquaculture for their livelihoods, underscoring the sector's role in and employment. These activities concentrate along coastlines, where nearshore stocks support small-scale and industrial operations, contributing to protein needs for over 3 billion individuals globally. Offshore oil and gas extraction represents another major coastal resource, accounting for about 37% of global oil production and 28% of output. In the , commercial production began in 1975 with fields like Forties, transforming regional economies through sustained yields until peaking around 2000. Such operations rely on coastal for platforms, pipelines, and export terminals, highlighting the linkage between marine extraction and land-based . Maritime ports facilitate the bulk of global trade, handling over 80% of goods by volume via containerized and bulk shipments. The , for instance, processed 49 million twenty-foot equivalent units (TEUs) in 2023, exemplifying how coastal hubs serve as arteries for commodities like oil, minerals, and manufactures. These facilities enable efficient transfer from extraction sites to inland distribution, with and expansion maintaining for supertankers and mega-carriers. Coastal mineral extraction includes solar evaporation of in ponds to produce salt, a method yielding commercial-grade through sequential concentration in shallow basins. Aggregates like are dredged from nearshore beds for construction, while phosphate-rich deposits, such as those associated with Morocco's 70% share of global reserves, support production via coastal and export. These activities leverage tidal access and natural gradients, contributing to industrial supply chains without overlapping broader ecological or settlement dynamics.

Tourism, Recreation, and Cultural Value

Coastal tourism constitutes a major component of the global tourism industry, with beach tourism valued at approximately $281 billion in 2024 and projected to reach $466.7 billion by 2033. Coastal and marine tourism accounts for about half of all global tourism expenditures, contributing roughly 10% to world GDP according to World Bank estimates. In the United States, beaches attract 3.4 billion annual visits, generating around $240 billion in tourist spending. Florida's beaches alone provide an annual recreational value of about $50 billion, underscoring their outsized economic role in leisure-driven activities. Recreational pursuits at coasts, such as , sunbathing, and , draw the majority of visitors, far exceeding other attractions; U.S. visits surpass combined attendance at national parks, theme parks, and zoos by over 225%. exemplifies specialized coastal recreation, with events like Hawaii's injecting $21 million into Oahu's economy in 2010 through direct spending on lodging, food, and services. These activities yield intangible benefits, including elevated levels from exposure, which supports , immune function, and mood regulation via endorphin release. Coasts hold enduring cultural value as sites of leisure and inspiration, with Roman-era infrastructure like villas and roads along the laying early foundations for seasonal retreats that evolved into modern lifestyles. From prehistoric settlements onward, coastal zones have symbolized renewal and vitality, fostering traditions of seaside sojourns that persist in contemporary cultural narratives. This heritage enhances tourism's appeal, blending natural allure with historical resonance to sustain visitor interest across millennia.

Management and Engineering Practices

Historical Interventions

The use of durable in coastal structures, incorporating and lime, enabled seawalls and harbors to withstand marine exposure for over 2,000 years, as evidenced by intact examples across the Mediterranean and European coasts where self-healing mechanisms involving lime clasts repaired cracks through reaction with . This material's longevity contrasted with later formulations, demonstrating early engineering's capacity to counter and tidal forces without modern reinforcements. In the medieval period, the Dutch initiated systematic through polders, beginning in the with drainage of marshes and fenlands via dikes and windmills, ultimately reclaiming approximately 17% of the nation's area from the by the —totaling over 7,800 square kilometers since the 1300s. These interventions transformed flood-prone deltas into arable farmland, sustaining and despite subsidence risks, with empirical records showing sustained productivity in low-lying areas below . Nineteenth-century advancements in the United States included the construction of jetties to stabilize harbor entrances against and wave action; for instance, the Galveston jetties, completed in the 1890s, deepened the channel to accommodate vessels drawing 21 feet by 1896, facilitating trade growth prior to the 1900 hurricane that prompted further enhancements. These rubble-mound structures reduced inlet shoaling, as verified by navigational records, exemplifying hydraulic engineering's role in securing commercial ports. The Thames Barrier, operational since 1982 following construction in the 1970s, represents a pivotal 20th-century intervention, with gates closed 221 times by 2024 to avert tidal surges, preventing over 100 potential floods in London and upstream areas based on operational logs. Its movable design has empirically maintained estuarine stability amid rising storm frequencies, underscoring scalable barriers' effectiveness in urban coastal defense.

Contemporary Coastal Protection Techniques

Contemporary coastal protection techniques encompass both hard structures and softer, nature-based methods, with evaluations emphasizing cost-, durability, and long-term dynamics. Hard , dominant since the post-1950s era of intensified coastal development, includes seawalls and groynes designed to directly interrupt wave energy and retain . Seawalls, rigid barriers typically constructed from or rock, provide short-term protection against and flooding by reflecting or dissipating waves, yet they often exacerbate downdrift or flanking by disrupting natural longshore , leading to accelerated loss adjacent to the structure. Groynes, perpendicular barriers extending into the water, trap sand on their updrift side to widen beaches but similarly induce on the downdrift flank, with limited to localized areas and requiring ongoing to mitigate these imbalances. Meta-analyses of shoreline hardening indicate these structures maintain structural for decades under moderate conditions but incur high lifecycle costs due to repair needs and unintended propagation, often necessitating supplementary interventions. Soft engineering approaches, gaining prominence from the onward, prioritize working with natural processes to enhance resilience while minimizing ecological disruption. , a primary soft technique, entails and placing compatible to restore eroded profiles and buffer against storms; , over 1.2 billion cubic meters of have been deployed across 475 communities since , with recent annual volumes supporting extensive renourishment programs at costs averaging under $10 per cubic meter for material alone, though full expenses per meter of beach length range from $10 to $20 depending on site specifics and frequency. This method restores natural beach gradients and habitat value but demands periodic replenishment—typically every 5-10 years—as nourished erodes at rates comparable to native material, with durability enhanced by matching to local conditions. Hybrid strategies integrate soft and hard elements; the ' "Building with Nature" paradigm, formalized in the , exemplifies this through mega-scale suppletions like the 2011 , a 21.5 million cubic meter off Delftland that leverages currents for self-sustaining redistribution over 20 kilometers of coast, potentially halving traditional nourishment frequencies and costs while fostering dune accretion and . Advanced monitoring technologies underpin the efficacy of these techniques by enabling data-driven adjustments and predictive modeling. LiDAR-equipped drones and UAVs facilitate high-resolution topographic surveys of coastal changes, capturing centimeter-level accuracy over kilometers in hours, which surpasses traditional ground-based methods in speed and coverage for erosion hotspot detection. Integration of real-time LiDAR data into management reduces maintenance expenditures by optimizing intervention timing and volumes, with studies reporting efficiency gains that lower overall project costs through minimized unnecessary dredging or repairs. These tools also quantify hard structure performance, such as groyne-induced accretion volumes, allowing cost-benefit analyses that favor hybrids in dynamic environments where pure hard defenses prove less durable against variable wave climates.

Adaptive Strategies for Resilience

Adaptive strategies for coastal resilience emphasize data-driven risk assessments to guide and infrastructure decisions, focusing on long-term viability rather than reactive measures. In the United States, following in 2005, coastal zoning ordinances were strengthened to incorporate setbacks and elevation requirements, limiting development in high-risk zones and mandating structures be raised above base elevations (BFE). For instance, elevating buildings to or above BFE in New Orleans has been shown to mitigate surge and flooding damages by reducing inundation exposure, with empirical models indicating potential reductions in losses exceeding 50% for heights of 2-3 meters in vulnerable low-lying areas. These approaches prioritize probabilistic hazard modeling over worst-case projections, enabling cost-effective avoidance of repeated flood-prone investments. Nature-based solutions, such as , leverage ecological processes for accretion and wave energy dissipation, often proving more sustainable than rigid in tropical settings. Field studies in mangrove systems report average vertical accretion rates of 5-6 mm per year, sufficient to keep pace with observed local sea-level rise in many sites, while root systems and canopies trap sediments and attenuate waves more dynamically than seawalls, which can exacerbate adjacent . In comparative trials, restored mangroves have demonstrated superior long-term resilience by facilitating natural shoreline progradation, contrasting with hard structures that fail under extreme events without ongoing maintenance. Economic mechanisms further support adaptation by aligning incentives with risk realities, such as through premiums reflecting empirical loss probabilities and funds for strategic relocation. The ' Room for the River program, initiated in 2007 after major floods in and , exemplifies this by reallocating areas for via dike relocations and excavations, investing over €2.3 billion across 34 projects to enhance discharge capacity without solely relying on heightening defenses. Economic analyses confirmed the program's viability, yielding benefits through reduced probabilities and preserved development potential in safer zones. These strategies integrate market signals, like risk-based , to discourage maladaptive buildup in hazard-prone coastal strips.

Dynamics, Changes, and Threats

Natural Fluctuations and Cycles

Coastal systems exhibit inherent variability driven by astronomical forcings such as tidal cycles and orbital parameters, as well as meteorological phenomena like sequences and decadal oscillations, all predating significant human modification. These natural fluctuations result in periodic , accretion, and redistribution, establishing a dynamic baseline for shoreline positions independent of anthropogenic influences. Decadal-scale oscillations, particularly those associated with the El Niño-Southern Oscillation (ENSO), induce episodic erosion spikes through altered storm tracks and elevated sea levels. During the 1997–1998 El Niño event, beaches experienced substantial shoreline retreat, with average erosion of up to 55 meters in northern sections like , accompanied by widespread loss of sandy beach areas due to intensified wave action and rainfall. Sea surface temperatures in the eastern Pacific reached record anomalies, correlating with 15–20 cm sea-level elevations along the coast for over six months, enhancing wave energy and offshore. Tidal cycles further modulate sediment flux through asymmetric flood-ebb dynamics, where spring-neap variations drive substantial portions of biogeochemical and particle transport variability, often resulting in net landward or seaward shifts depending on local . Storm sequences, including hurricanes and extratropical cyclones, temporarily reshape coastlines over scales of 1–10 kilometers by mobilizing large volumes during peak events. For instance, post-Hurricane Irma surveys in 2017 documented extensive reconfiguration in , with recovery of dune profiles and washover features occurring within 1–1.5 years, as evidenced by repeat topographic and aerial imagery. Such events fill erosional scars like washout channels in days to weeks via subsequent fair-weather redistribution, though full morphologic restoration can extend to several years based on supply and wave climate. These cycles highlight the resilience of coastal landforms to recurrent high-energy forcings. On millennial timescales, —variations in Earth's , obliquity, and —drive eustatic sea-level fluctuations through ice-volume changes, causing coastline migrations of tens to hundreds of kilometers. Glacial-interglacial transitions, such as the around 20,000 years ago when sea levels were approximately 120 meters lower, advanced continental shelves seaward, with subsequent prompting rapid transgressions that reshaped low-gradient coasts over 100,000-year periods. These long-term oscillations provide the stratigraphic template for , linking insolation forcing to global sea-level envelopes without reliance on tectonic or anthropogenic drivers.

Anthropogenic Modifications

Human interventions, particularly the construction of and reservoirs, have substantially reduced delivery to coastal zones worldwide. Large trap an estimated 1-2 billion metric tons of annually, representing a significant fraction—often exceeding 50% in heavily regulated basins—of the natural fluvial supply that sustains deltas and beaches. This starvation manifests as accelerated ; for instance, in the , upstream constructed since the 1990s have contributed to a shift from net land gain to shrinkage, with rates reaching 20-100 meters per year along vulnerable shorelines and cumulative coastline retreat on the order of several kilometers in affected sectors by the 2010s. Pre-dam fluxes supported delta progradation at rates of several square kilometers per year, but post-intervention data from satellite monitoring show net losses exceeding 0.05 km² annually in recent decades. Urban development along coastlines amplifies erosional forces through the proliferation of impervious surfaces such as and asphalt. These surfaces inhibit infiltration, elevating volumes and peak flows by factors of 2 to 5 times compared to pre-development vegetated landscapes, as quantified in hydrological models from the U.S. Geological Survey and EPA assessments. The resultant concentrated discharges scour coastal soils and dunes, with post-urbanization rates in affected areas like U.S. East Coast estuaries increasing by up to 10 times baseline levels, based on comparative gauging station data before and after suburban expansion in the late . Nutrient pollution from anthropogenic sources, including fertilizers applied since the mid-20th century, has induced in semi-enclosed coastal seas, fostering hypoxic zones that disrupt benthic habitats and fisheries. In the Black Sea, intensified agricultural runoff from the 1960s onward—peaking in the 1980s—triggered widespread anoxia, collapsing shelf fisheries from annual catches of 850,000 tons in the mid-1980s to 250,000 tons by 1991, as oxygen depletion killed demersal stocks and favored blooms. Economic disruptions in the early 1990s reduced nutrient inputs by over 50%, enabling partial recovery with improved oxygenation and fish landings rebounding to pre-eutrophication levels by the , underscoring the direct causal link between human nutrient loading and coastal ecological collapse.

Climate Variability Impacts

Global mean sea level has risen at an average rate of 3.3 mm per year from 1993 to 2023, as measured by satellite altimetry, with the rate accelerating to about 4.2 mm per year in the 2014-2024 period due to and ice melt contributions from atmospheric warming. This eustatic rise manifests variably along coasts, where local land —often unrelated to —can comprise 30-70% of relative changes in deltaic systems, amplifying inundation risks beyond the global signal. For example, the U.S. Gulf Coast has experienced relative rises of 5-10 mm per year in recent decades, with rates of 3-6 mm per year in subsiding zones like contributing substantially to observed flooding and loss. Warmer sea surface temperatures, a direct outcome of ocean heat uptake from greenhouse gas forcings, have increased evaporation rates, fueling higher atmospheric moisture content and intensifying rainfall within tropical cyclones by 10-15% per degree of warming, per IPCC AR6 assessments. This has led to more extreme precipitation events during coastal storms, exacerbating erosion and surge impacts, though overall tropical cyclone frequency shows no global trend increase. In the U.S., historical records from 1851 to the 2020s indicate no rise in hurricane landfall frequency, averaging 1.7-2 per year, with variability tied to multidecadal cycles like the Atlantic Multidecadal Oscillation rather than monotonic intensification. Regional empirical data reveal that climate-driven variability interacts with coastal morphology; in stable, sediment-rich areas, heightened and storm-driven have occasionally promoted foredune accretion, offsetting minor rises through aeolian sand deposition, as observed in macrotidal embayments where dunes have maintained or grown despite 2-3 mm/year relative changes. However, in subsiding or low-sediment coasts, these feedbacks are insufficient, leading to net retreat and heightened vulnerability to episodic inundation during El Niño-enhanced high s. Such impacts underscore the primacy of local in modulating global climate signals, with historical records showing relative stability in non-subsiding U.S. Atlantic sectors over the satellite era.

Controversies and Empirical Debates

Disputes on Erosion Drivers

Empirical analyses of coastal erosion often prioritize sea-level rise (SLR) as the dominant driver, yet studies reveal that anthropogenic interception of fluvial sediments, particularly via dams, accounts for a larger share of observed shoreline retreat in sediment-starved systems. For instance, global river damming has reduced sand delivery to coasts by trapping up to 50% of pre-industrial sediment loads in major basins, exacerbating erosion rates that exceed those attributable to recent SLR increments of approximately 3-4 mm/year. In California, dams have diminished annual sand flux to beaches by 23%, affecting over 20% of the coastline with downstream erosion unrelated to tidal changes. Similarly, post-dam construction in deltas like the Nile and Mekong has induced retreat rates of 50-100 meters per decade, far outpacing localized SLR contributions estimated at less than 10% of total sediment deficit impacts in 2020s modeling. Local geological factors, including , further complicate attribution, dominating relative sea-level changes in up to 41% of observed trends across subsiding coastal zones, such as the U.S. Gulf and Atlantic margins where anthropogenic groundwater extraction amplifies vertical land motion beyond eustatic SLR. Tide gauge records indicate that rates exceeding 3 mm/year affect broad swaths of vulnerable low-lying coasts, rendering SLR a secondary modulator in these contexts; for example, in tectonically active or deltaic regions comprising over half of global at-risk shorelines, drives 60-80% of effective inundation risk per site-specific assessments. This contrasts with narratives emphasizing uniform SLR primacy, as 's role is often underweighted in global models due to sparse integration. Disputes intensify over predictive modeling, where alarmist projections frequently overestimate erosion by incorporating extrapolated SLR accelerations not corroborated by long-term s, particularly in isostatically uplifting regions like . Norwegian and Baltic data from 1960-2020 show relative sea-level stability or slight declines (0-1 mm/year rise) due to glacial countering eustatic trends, enabling coastal progradation in areas with adequate despite model forecasts of . These discrepancies highlight data gaps in integrating local , with empirical observations revealing overprediction factors of 2-5 times in uplifting terrains when s are benchmarked against altimetry-derived models. Engineering perspectives underscore successful countermeasures against multi-decadal SLR without retreat, as evidenced by the Netherlands, where approximately 25-30 cm of rise since 1900 has been offset by dike reinforcements and land reclamation, maintaining net land gain through systematic sediment management and no widespread abandonment. Proponents argue this validates causal emphasis on controllable factors like sediment bypassing over inexorable SLR, citing historical accretion phases pre-20th-century development in similar temperate coasts. Conversely, retreat advocates, often drawing from consensus-driven IPCC scenarios, downplay such cases by attributing stability to temporary engineering, while overlooking pre-industrial accretion data from sediment cores showing progradational balances disrupted primarily by human interception rather than baseline SLR.

Sea Level Rise Projections and Evidence

Global mean sea level rose by 15–25 cm between 1901 and 2018, equivalent to an average rate of approximately 1.4–2.1 mm per year during the early to mid-20th century, with evident in later decades to around 3 mm per year since the based on reconstructions. This historical rise shows regional variability, influenced by factors such as vertical land motion and ocean dynamics; for instance, some Pacific records indicate rates below the global average or even stability in specific locales when adjusted for local , though broader regional trends in the Southwest Pacific exceed the global mean at 4–5 mm per year since 1993. Projections for future sea level rise under IPCC Representative Concentration Pathway (RCP) scenarios estimate global mean increases of 0.28–0.55 m by 2100 for low-emissions paths (RCP2.6) and 0.63–1.01 m for high-emissions paths (RCP8.5), with median values around 0.4–0.8 m depending on ice sheet response assumptions. These forecasts incorporate semi-empirical models and process-based simulations, but uncertainties remain high due to nonlinear ice sheet dynamics; for example, Greenland ice sheet mass loss models from the 2010s projected higher contributions than subsequently observed in gravity satellite data, with actual losses averaging 250–280 Gt per year during 2010–2018 falling within projected ranges but toward the lower end after accounting for variability in surface melt. Empirical indicators, such as salt marsh accretion rates, suggest ecosystems can adapt to rises up to 5–10 mm per year through sediment trapping and belowground production, challenging assumptions of widespread drowning under moderate scenarios, though accelerated inundation beyond 10 mm per year may exceed limits in low-sediment environments. Debates persist over measurement methodologies and attribution, with tide gauge networks recording historical rates of 1.5–2 mm per year globally since 1900, contrasted against altimetry's 3.3–3.9 mm per year since 1993, a discrepancy partly attributed to improved global coverage and glacial isostatic adjustment corrections but potentially amplified by altimetry biases exceeding 10–15% in volume change estimates from incomplete sampling of near-coast and polar regions. Natural variability complicates anthropogenic attribution, as proxy records indicate rates reached 10–20 mm per year during deglacial phases, while reconstructions show regional stability or slight rises of 0.2–0.5 m over centuries in some North Atlantic sites, rates comparable to or exceeding early 20th-century observations without modern CO2 forcing. These empirical discrepancies underscore the need for integrated gauge- validation and highlight how model sensitivities to ice-ocean interactions have led to past overestimates in high-end scenarios, emphasizing epistemic caution in policy-relevant forecasts.

Policy Efficacy and Overregulation Critiques

The Netherlands has demonstrated the efficacy of robust coastal defense policies through its dike and levee system, which safeguards approximately 26% of its land situated below , preventing widespread flooding despite vulnerability to storm surges. This approach, refined after the 1953 flood that prompted the program, has protected densely populated areas where over 60% of the population resides in flood-prone zones, with maintenance costs offset by avoided damages estimated in the trillions of euros. Empirical outcomes underscore causal factors like sediment management and engineering resilience over retreat, yielding net economic benefits through sustained and . In contrast, the National Flood Insurance Program (NFIP), established in 1968, has been critiqued for subsidizing development in high-risk coastal zones, creating and escalating taxpayer burdens. Post-Hurricane Katrina in 2005, the NFIP incurred over $16 billion in claims, contributing to program deficits averaging $1.4 billion annually as premiums fail to cover losses, with total interest payments exceeding $5.7 billion by 2025. This structure incentivizes building in vulnerable areas via below-actuarial-rate insurance, amplifying exposure rather than promoting risk reduction, as evidenced by repeated claims on the same properties totaling billions since inception. Coastal policies often overemphasize sea-level rise (SLR) projections while underaddressing , a dominant local driver of relative sea-level change in regions like , where subsidence rates exceed global SLR by factors of five due to groundwater extraction, oil production, and starvation. In 's $50 billion Coastal Master Plan, diversions—intended to mimic natural delta-building—have faced scrutiny for underperformance, with projects like the Mid-Barataria diversion halted in 2025 amid concerns over high costs ($3 billion initial, plus ongoing ecosystem disruptions like hypoxia exacerbating fisheries losses) and lower land-building efficiency compared to alternatives. Cost-benefit analyses indicate via pipelines can achieve faster wetland restoration at reduced per-acre expenses in select cases, prioritizing direct delivery over ecologically uncertain diversions. Managed retreat policies, such as voluntary buyouts, encounter empirical barriers including low uptake (e.g., New Jersey's program covering under 1% of at-risk properties) and conflicts with rising property values that signal market resilience rather than inevitable abandonment. Critics argue regulatory mandates for overlook data-driven defenses, imposing overregulation that stifles local ; market-based , adjusting premiums to reflect risks without subsidies, better incentivizes prudent siting than interventions. Recent legislation like the Resilient Coasts and Estuaries Act of 2025 (H.R. 2786) seeks to fund restoration for SLR and flooding resilience but risks bureaucratic expansion without rigorous cost-efficacy mandates, potentially mirroring NFIP's fiscal shortfalls. Pragmatic policies favoring empirical outcomes—such as mitigation via extraction controls—over ideologically driven would enhance causal effectiveness in coastal .

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