From New World Encyclopedia
Coastal sea waves at Paracas National Reserve, Ica, Peru

The sea in a general sense refers to the ocean or world ocean, the body of salty water that covers approximately 71 percent of the Earth's surface. Used in a particular sense the word sea denotes sections of water, such as the Mediterranean Sea, as well as certain large, entirely landlocked, saltwater lakes, such as the Caspian Sea. The sea moderates Earth's climate and has important roles in the water, carbon, and nitrogen cycles.

A wide variety of organisms, including bacteria, protists, algae, plants, fungi, and animals, lives in the sea, which offers a wide range of marine habitats and ecosystems, ranging vertically from the sunlit surface and shoreline to the great depths and pressures of the cold, dark abyssal zone, and in latitude from the cold waters under polar ice caps to the warm waters of coral reefs in tropical regions. Many of the major groups of organisms evolved in the sea and life may have started there.

The sea is essential for life, and provides substantial supplies of food for human beings, mainly fish, but also shellfish, mammals, and seaweed, whether caught by fishermen or farmed underwater. Other human uses of the sea include trade, travel, mineral extraction, power generation, warfare, and leisure activities such as swimming, sailing, and scuba diving. Many of these activities create marine pollution, and measures have been taken including international agreements to control dumping of waste as well as efforts to address the impact of rising temperatures on seawater.


Marginal seas as defined by the International Maritime Organization

The sea is the interconnected system of all the Earth's oceanic waters, including the Atlantic, Pacific, Indian, Southern and Arctic Oceans.[1] However, the word "sea" can also be used for many specific, much smaller bodies of seawater, such as the North Sea or the Red Sea. There is no sharp distinction between seas and oceans, though generally seas are smaller, and are often partly (as marginal seas or particularly as the Mediterranean sea) or wholly (as inland seas) enclosed by land.[2] However, an exception to this is the Sargasso Sea which has no coastline and lies within a circular current, the North Atlantic Gyre.[3] Seas are generally larger than lakes and contain salt water, but the Sea of Galilee is a freshwater lake.

There is no accepted technical definition of sea amongst oceanographers. One definition defines a sea as a "land-locked" body of water, adding that the term "sea" is only one of convenience.[4] The United Nations Convention on the Law of the Sea states that all of the ocean is "sea."[5]

Physical characteristics

Composite images of the Earth created by NASA in 2001

Earth is the only known planet with seas of liquid water on its surface, although Mars possesses ice caps and similar planets in other solar systems may have oceans. Earth's 1,335,000,000 cubic kilometers (320,000,000 cu mi) of sea contain about 97.2 percent of its known water and cover approximately 71 percent of its surface[6] and cover approximately 71 percent of its surface.[3] Another 2.15 percent of Earth's water is frozen, found in the sea ice covering the Arctic Ocean, the ice cap covering Antarctica and its adjacent seas, and various glaciers and surface deposits around the world. The remainder (about 0.65 per cent of the whole) form underground reservoirs or various stages of the water cycle, containing the freshwater encountered and used by most terrestrial life: vapor in the air, the clouds it slowly forms, the rain falling from them, and the lakes and rivers spontaneously formed as its waters flow again and again to the sea.[6]


Salinity map taken from the Aquarius Spacecraft. The rainbow colours represent salinity levels: red = 40 ‰, purple = 30 ‰


A characteristic of seawater is that it is salty. Salinity is usually measured in parts per thousand (‰ or per mil), and the open ocean has about 35 grams (1.2 oz) solids per liter, a salinity of 35 ‰. The Mediterranean Sea is slightly higher at 38 ‰, while the salinity of the northern Red Sea can reach 41‰. In contrast, some landlocked hypersaline lakes have a much higher salinity, for example the Dead Sea has 300 grams (11 oz) dissolved solids per litre (300 ‰).

While the constituents of table salt (sodium and chloride) make up about 85 percent of the solids in solution, there are also other metal ions such as magnesium and calcium, and negative ions including sulphate, carbonate, and bromide. Despite variations in the levels of salinity in different seas, the relative composition of the dissolved salts is stable throughout the world's oceans. Seawater is too saline for humans to drink safely, as the kidneys cannot excrete urine as salty as seawater.

Although the amount of salt in the ocean remains relatively constant within the scale of millions of years, various factors affect the salinity of a body of water. Evaporation and by-product of ice formation (known as "brine rejection") increase salinity, whereas precipitation, sea ice melt, and runoff from land reduce it.[7] The Baltic Sea, for example, has many rivers flowing into it, and thus the sea could be considered as brackish.


Sea temperature depends on the amount of solar radiation falling on its surface. In the tropics, with the sun nearly overhead, the temperature of the surface layers can rise to over 30 °C (86 °F) while near the poles the temperature in equilibrium with the sea ice is about −2 °C (28.4 °F). There is a continuous circulation of water in the oceans. Warm surface currents cool as they move away from the tropics, and the water becomes denser and sinks. The cold water moves back towards the equator as a deep sea current, driven by changes in the temperature and density of the water, before eventually welling up again towards the surface. Deep seawater has a temperature between −2 °C (28.4 °F) and 5 °C (41 °F) in all parts of the globe.[8]

Seawater with a typical salinity of 35 ‰ has a freezing point of about −1.8 °C (28.8 °F). When its temperature becomes low enough, ice crystals form on the surface. These break into small pieces and coalesce into flat discs that form a thick suspension known as frazil. In calm conditions this freezes into a thin flat sheet known as nilas, which thickens as new ice forms on its underside. In more turbulent seas, frazil crystals join into flat discs known as pancakes. These slide under each other and coalesce to form floes. In the process of freezing, salt water and air are trapped between the ice crystals.[9]

Oxygen concentration

The amount of oxygen found in seawater depends primarily on the plants growing in it. These are mainly algae, including phytoplankton, with some vascular plants such as seagrasses. In daylight the photosynthetic activity of these plants produces oxygen, which dissolves in the seawater and is used by marine animals. At night, photosynthesis stops, and the amount of dissolved oxygen declines. In the deep sea, where insufficient light penetrates for plants to grow, there is very little dissolved oxygen. In its absence, organic material is broken down by anaerobic bacteria producing hydrogen sulphide.[10]


The amount of light that penetrates the sea depends on the angle of the sun, the weather conditions and the turbidity of the water. Much light gets reflected at the surface, and red light gets absorbed in the top few metres. Yellow and green light reach greater depths, and blue and violet light may penetrate as deep as 1,000 meters (3,300 ft). There is insufficient light for photosynthesis and plant growth beyond a depth of about 200 meters (660 ft).[11]

Sea level

Main articles: Sea level and Sea level rise

Over most of geologic time, the sea level has been higher than it is today.[3] The main factor affecting sea level over time is the result of changes in the oceanic crust, with a downward trend expected to continue in the very long term. At the last glacial maximum, some 20,000 years ago, the sea level was about 125 meters (410 ft) lower than in present times (2012).[12]

For at least the last 100 years, sea level has been rising at an average rate of about 1.8 millimeters (0.071 in) per year. Most of this rise can be attributed to an increase in the temperature of the sea due to climate change, and the resulting slight thermal expansion of the upper 500 meters (1,600 ft) of water. Additional contributions, as much as one quarter of the total, come from water sources on land, such as melting snow and glaciers and extraction of groundwater for irrigation and other agricultural and human needs.[13]


When the wave enters shallow water, it slows down and its amplitude (height) increases.

Wind blowing over the surface of a body of water forms waves that are perpendicular to the direction of the wind. The waves reach their maximum height when the rate at which they are traveling nearly matches the speed of the wind. In open water, when the wind blows continuously as happens in the Southern Hemisphere in the Roaring Forties, long, organized masses of water called swell roll across the ocean.[14][15]

As the waves leave the region where they were generated, the longer ones outpace the shorter because their velocity is greater. Gradually, they fall in with other waves travelling at similar speed—-where different waves are in phase they reinforce each other, and where out of phase they are reduced. Eventually, a regular pattern of high and low waves (or swell) is developed that remains constant as it travels out across the ocean.[3]

If the wind dies down, the wave formation is reduced, but already-formed waves continue to travel in their original direction until they meet land. The size of the waves depends on the fetch, the distance that the wind has blown over the water and the strength and duration of that wind. When waves meet others coming from different directions, interference between the two can produce broken, irregular seas.[14] Most waves are less than 3 m (10 ft) high, although Constructive interference can cause individual (unexpected) rogue waves much higher than normal.[16]

The top of a wave is known as the crest, the lowest point between waves is the trough and the distance between the crests is the wavelength. The wave is pushed across the surface of the sea by the wind, but this represents a transfer of energy and not a horizontal movement of water. As waves approach land and move into shallow water, they change their behavior. If approaching at an angle, waves may bend (refraction) or wrap rocks and headlands (diffraction). When the wave reaches a point where its deepest oscillations of the water contact the seabed, they begin to slow down. This pulls the crests closer together and increases the waves' height, called wave shoaling. When the ratio of the wave's height to the water depth increases above a certain limit, it "breaks," toppling over in a mass of foaming water.[16] This rushes in a sheet up the beach before retreating into the sea under the influence of gravity.[14]


The 2004 tsunami in Thailand
Main article: Tsunami

A tsunami is an unusual form of wave caused by an infrequent powerful event such as an underwater earthquake or landslide, a meteorite impact, a volcanic eruption, or a collapse of land into the sea. These events can temporarily lift or lower the surface of the sea in the affected area, usually by a few feet. The potential energy of the displaced seawater is turned into kinetic energy, creating a shallow wave, a tsunami, radiating outwards at a velocity proportional to the square root of the depth of the water and which therefore travels much faster in the open ocean than on a continental shelf.[17] In the deep open sea, tsunamis have wavelengths of around 80 to 300 miles (130 to 480 km), travel at speeds of over 600 miles per hour (970 km/hr) and usually have a height of less than three feet, so they often pass unnoticed at this stage.[18]

As a tsunami moves into shallower water its speed decreases, its wavelength shortens, and its amplitude increases enormously,[18] behaving in the same way as a wind-generated wave in shallow water, but on a vastly greater scale. Either the trough or the crest of a tsunami can arrive at the coast first. In the former case, the sea draws back and leaves subtidal areas close to the shore exposed which provides a useful warning for people on land. When the crest arrives, it does not usually break but rushes inland, flooding all in its path. Much of the destruction may be caused by the flood water draining back into the sea after the tsunami has struck, dragging debris and people with it. Often several tsunami are caused by a single geological event and arrive at intervals of between eight minutes and two hours. The first wave to arrive on shore may not be the biggest or most destructive.[17]


Surface currents: red–warm, blue–cold

Wind blowing over the surface of the sea causes friction at the interface between air and sea. Not only does this cause waves to form but it also makes the surface seawater move in the same direction as the wind. Although winds are variable, in any one place they predominantly blow from a single direction and thus a surface current can be formed. When water moves in this way, other water flows in to fill the gap and a circular movement of surface currents known as a gyre is formed. There are five main gyres in the world's oceans: two in the Pacific, two in the Atlantic, and one in the Indian Ocean. Other smaller gyres are found in lesser seas and a single gyre flows around Antarctica. These gyres have followed the same routes for millennia, guided by the topography of the land, the wind direction, and the Coriolis effect. Westerly winds are most frequent in the mid-latitudes while easterlies dominate the tropics.[19] The surface currents flow in a clockwise direction in the Northern Hemisphere and anticlockwise in the Southern Hemisphere. The water moving away from the equator is warm, and that flowing in the reverse direction has lost most of its heat. These currents tend to moderate the Earth's climate, cooling the equatorial region and warming regions at higher latitudes.[20]

The global conveyor belt shown in blue with warmer surface currents in red

Surface currents only affect the top few hundred meters of the sea, but there are also large-scale flows in the ocean depths caused by the movement of deep water masses. A main deep ocean current flows through all the world's oceans and is known as the thermohaline circulation or global conveyor belt. This movement is slow and is driven by differences in density of the water caused by variations in salinity and temperature. At high latitudes the water is chilled by the low atmospheric temperature and becomes saltier as sea ice crystallizes out. Both these factors make it denser, and the water sinks. From the deep sea near Greenland, such water flows southwards between the continental landmasses on either side of the Atlantic. When it reaches the Antarctic, it is joined by further masses of cold, sinking water and flows eastwards. It then splits into two streams that move northwards into the Indian and Pacific Oceans. Here it is gradually warmed, becomes less dense, rises towards the surface, and loops back on itself. It takes a thousand years for this circulation pattern to be completed.[20]

Besides gyres, there are temporary surface currents that occur under specific conditions. When waves meet a shore at an angle, a longshore current is created as water is pushed along parallel to the coastline. The water swirls up onto the beach at right angles to the approaching waves but drains away straight down the slope under the effect of gravity. The larger the breaking waves, the longer the beach and the more oblique the wave approach, the stronger is the longshore current. These currents can shift great volumes of sand or pebbles, create spits and make beaches disappear and water channels silt up.[20] A rip current can occur when water piles up near the shore from advancing waves and is funneled out to sea through a channel in the seabed. It may occur at a gap in a sandbar or near a man-made structure such as a groyne. These strong currents can have a velocity of 3 ft (0.9 m) per second, can form at different places at different stages of the tide and can carry away unwary bathers. Temporary upwelling currents occur when the wind pushes water away from the land and deeper water rises to replace it. This cold water is often rich in nutrients and creates blooms of phytoplankton and a great increase in the productivity of the sea.[20]


Main article: Tide
High tides (blue) at the nearest and furthest points of the Earth from the Moon

Tides are the regular rise and fall in water level experienced by seas and oceans in response to the gravitational influences of the Moon and the Sun, and the effects of the Earth's rotation. During each tidal cycle, at any given place the water rises to a maximum height known as "high tide" before ebbing away again to the minimum "low tide" level. As the water recedes, it uncovers more and more of the foreshore, also known as the intertidal zone. The difference in height between the high tide and low tide is known as the tidal range or tidal amplitude.[21]

Most places experience two high tides each day, occurring at intervals of about 12 hours and 25 minutes. This is half the 24 hours and 50 minute period that it takes for the Earth to make a complete revolution and return the Moon to its previous position relative to an observer.


Praia da Marinha in Algarve, Portugal
The Baltic Sea in the archipelago of Turku, Finland

The zone where land meets sea is known as the coast and the part between the lowest spring tides and the upper limit reached by splashing waves is the shore. A beach is the accumulation of sand or shingle on the shore.[22] A headland is a point of land jutting out into the sea and a larger promontory is known as a cape. The indentation of a coastline, especially between two headlands, is a bay, a small bay with a narrow inlet is a cove and a large bay may be referred to as a gulf.[23]

Coastlines are influenced by a number of factors including the strength of the waves arriving on the shore, the gradient of the land margin, the composition and hardness of the coastal rock, the inclination of the off-shore slope and the changes of the level of the land due to local uplift or submergence. Normally, waves roll towards the shore at the rate of six to eight per minute and these are known as constructive waves as they tend to move material up the beach and have little erosive effect. Storm waves arrive on shore in rapid succession and are known as destructive waves as the swash moves beach material seawards. Under their influence, the sand and shingle on the beach is ground together and abraded. Around high tide, the power of a storm wave impacting on the foot of a cliff has a shattering effect as air in cracks and crevices is compressed and then expands rapidly with release of pressure. At the same time, sand and pebbles have an erosive effect as they are thrown against the rocks. This tends to undercut the cliff, and normal weathering processes such as the action of frost follows, causing further destruction. Gradually, a wave-cut platform develops at the foot of the cliff and this has a protective effect, reducing further wave-erosion.[22]

Material worn from the margins of the land eventually ends up in the sea. Here it is subject to attrition as currents flowing parallel to the coast scour out channels and transport sand and pebbles away from their place of origin. Sediment carried to the sea by rivers settles on the seabed causing deltas to form in estuaries. All these materials move back and forth under the influence of waves, tides, and currents.[22] breakwaters, seawalls, dykes and levees and other sea defenses may be constructed to prevent flooding of the land from high tides and storm surge.

Water cycle

Main article: Water cycle

The sea plays a part in the water or hydrological cycle, in which water evaporates from the ocean, travels through the atmosphere as vapour, condenses, falls as rain or snow, thereby sustaining life on land, and largely returns to the sea.[24]

Carbon cycle

Main article: carbon cycle

Oceans contain the greatest quantity of actively cycled carbon in the world and are second only to the lithosphere in the amount of carbon they store. The oceans' surface layer holds large amounts of dissolved organic carbon that is exchanged rapidly with the atmosphere. The deep layer's concentration of dissolved inorganic carbon is about 15 percent higher than that of the surface layer[25] and it remains there for much longer periods of time. Thermohaline circulation exchanges carbon between these two layers.

Carbon entering the ocean as atmospheric carbon dioxide dissolves in the surface layers and is converted into carbonic acid, carbonate, and bicarbonate.[26]

Carbon can also enter through rivers as dissolved organic carbon where it is converted by photosynthetic organisms into organic carbon. This can either be exchanged throughout the food chain or precipitated into the deeper, more carbon rich layers as dead soft tissue or in shells and bones as calcium carbonate.

Life in the sea

Coral reefs are among the most biodiverse habitats in the world.

The oceans are home to a diverse collection of life forms that use it as a habitat. Since sunlight illuminates only the upper layers, the major part of the ocean exists in permanent darkness. As the different depth and temperature zones each provide habitat for a unique set of species, the marine environment as a whole encompasses an immense diversity of life. Marine habitats range from surface water to the deepest oceanic trenches, including coral reefs, kelp forests, seagrass meadows, tidepools, muddy, sandy and rocky seabeds, and the open pelagic zone. The organisms living in the sea range from whales 30 meters (100 ft) long to microscopic phytoplankton and zooplankton, fungi, and bacteria. Marine life plays an important part in the carbon cycle as photosynthetic organisms convert dissolved carbon dioxide into organic carbon and it is economically important to humans for providing fish for use as food.[27]

Life may have originated in the sea and all the major groups of animals are represented there.

Marine habitats

Marine habitats can be divided horizontally into coastal and open ocean habitats. Coastal habitats extend from the shoreline to the edge of the continental shelf. Most marine life is found in coastal habitats, even though the shelf area occupies only 7 percent of the total ocean area. Open ocean habitats are found in the deep ocean beyond the edge of the continental shelf. Alternatively, marine habitats can be divided vertically into pelagic (open water), demersal (just above the seabed) and benthic (sea bottom) habitats. A third division is by latitude: from polar seas with ice shelves, sea ice and icebergs, to temperate and tropical waters.[3]

Coral reefs, the so-called "rainforests of the sea," occupy less than 0.1 percent of the world's ocean surface, yet their ecosystems include 25 percent of all marine species. The best-known are tropical coral reefs such as Australia's Great Barrier Reef, but cold water reefs harbor a wide array of species including corals (only six of which contribute to reef formation).[3]

Algae and plants

Marine primary producers — plants and microscopic organisms in the plankton — are widespread and very essential for the ecosystem. It has been estimated that half of the world's oxygen is produced by phytoplankton.[28] About 45 percent of the sea's primary production of living material is contributed by diatoms. Much larger algae, commonly known as seaweeds, are important locally; Sargassum forms floating drifts, while kelp form seabed forests. Flowering plants in the form of seagrasses grow in "meadows" in sandy shallows, mangroves line the coast in tropical and subtropical regions and salt-tolerant plants thrive in regularly inundated salt marshes. All of these habitats are able to sequester large quantities of carbon and support a biodiverse range of larger and smaller animal life.

Light is only able to penetrate the top 200 meters (660 ft) so this is the only part of the sea where plants can grow.[11] The surface layers are often deficient in biologically active nitrogen compounds. The marine nitrogen cycle consists of complex microbial transformations which include the fixation of nitrogen, its assimilation, nitrification, anammox and denitrification. Some of these processes take place in deep water so that where there is an upwelling of cold waters, and also near estuaries where land-sourced nutrients are present, plant growth is higher. This means that the most productive areas, rich in plankton and therefore also in fish, are mainly coastal.[3]

Animals and other marine life

A thornback cowfish

There is a broader spectrum of higher animal taxa in the sea than on land, many marine species have yet to be discovered and the number known to science is expanding annually. Some vertebrates such as seabirds, seals and sea turtles return to the land to breed but fish, cetaceans and sea snakes have a completely aquatic lifestyle and many invertebrate phyla are entirely marine. In fact, the oceans teem with life and provide many varying microhabitats. One of these is the surface film which, even though tossed about by the movement of waves, provides a rich environment and is home to bacteria, fungi, microalgae, protozoa, fish eggs and various larvae.[29]

The pelagic zone contains macro- and microfauna and myriad zooplankton which drift with the currents. Most of the smallest organisms are the larvae of fish and marine invertebrates which liberate eggs in vast numbers because the chance of any one embryo surviving to maturity is so minute. The zooplankton feed on phytoplankton and on each other and form a basic part of the complex food chain that extends through variously sized fish and other nektonic organisms to large squid, sharks, porpoises, dolphins, and whales. Some marine creatures make large migrations, either to other regions of the ocean on a seasonal basis or vertical migrations daily, often ascending to feed at night and descending to safety by day.

The demersal zone supports many animals that feed on benthic organisms or seek protection from predators and the seabed provides a range of habitats on or under the surface of the substrate which are used by creatures adapted to these conditions. The tidal zone with its periodic exposure to the dehydrating air is home to barnacles, molluscs, and crustaceans. The neritic zone has many organisms that need light to flourish. Here, among algal encrusted rocks live sponges, echinoderms, polychaete worms, sea anemones and other invertebrates. Corals often contain photosynthetic symbionts and live in shallow waters where light penetrates. There is less sea life on the floor of deeper seas but marine life also flourishes around seamounts that rise from the depths, where fish and other animals congregate to spawn and feed. Close to the seabed live demersal fish that feed largely on pelagic organisms or benthic invertebrates. Some like the detrivores rely on organic material falling to the ocean floor. Others cluster round deep sea hydrothermal vents where mineral-rich flows of water emerge from the seabed, supporting communities whose primary producers are sulphide-oxidising chemoautotrophic bacteria, and whose consumers include specialized bivalves, sea anemones, barnacles, crabs, worms, and fish, often found nowhere else.[3]

Humans and the sea

Map showing the seaborne migration and expansion of the Austronesians beginning at around 3000 B.C.E.

Humans have traveled the seas since they first built sea-going craft. Mesopotamians used bitumen to caulk their reed boats and, a little later, masted sails.[30] By c. 3000 B.C.E., Austronesians on Taiwan had begun spreading into maritime Southeast Asia. Subsequently, the Austronesian "Lapita" peoples displayed great feats of navigation, reaching out from the Bismarck Archipelago to as far away as Fiji, Tonga, and Samoa.[31] Their descendants continued to travel thousands of miles between tiny islands on outrigger canoes, and in the process they found many new islands, including Hawaii, Easter Island (Rapa Nui), and New Zealand.

Gerardus Mercator's 1569 world map. The coastline of the old world is quite accurately depicted, unlike that of the Americas. Regions in high latitudes (Arctic, Antarctic) are greatly enlarged.

In the second century, Ptolemy mapped the whole known world from the "Fortunatae Insulae", Cape Verde or Canary Islands, eastward to the Gulf of Thailand. This map was used in 1492 when Christopher Columbus set out on his voyages of discovery. Subsequently, Gerardus Mercator made a practical map of the world in 1538, his map projection conveniently making rhumb lines straight.[3] By the eighteenth century better maps had been made and part of the objective of James Cook on his voyages was to further map the ocean. Scientific study has continued with the depth recordings of the Tuscarora, the oceanic research of the Challenger voyages (1872–1876), the work of the Scandinavian seamen Roald Amundsen and Fridtjof Nansen, the Michael Sars expedition in 1910, the German Meteor expedition of 1925, the Antarctic survey work of Discovery II in 1932, and others since. Furthermore, in 1921, the International Hydrographic Organization (IHO) was set up, and it constitutes the world authority on hydrographic surveying and nautical charting.[32]

A compass was first used by the ancient Greeks and Chinese to show where north lies and the direction in which the ship is heading. The latitude (an angle which ranges from 0° at the equator to 90° at the poles) was determined by measuring the angle between the Sun, Moon or a specific star and the horizon by the use of an astrolabe, Jacob's staff or sextant. The longitude (a line on the globe joining the two poles) could only be calculated with an accurate chronometer to show the exact time difference between the ship and a fixed point such as the Greenwich Meridian. In 1759, John Harrison, a clockmaker, designed such an instrument and Cook used it in his voyages of exploration.[33] Nowadays, the Global Positioning System (GPS) using satellites enables accurate navigation worldwide.

Scientific oceanography began with the voyages of Captain James Cook from 1768 to 1779, describing the Pacific with unprecedented precision from 71 degrees South to 71 degrees North.[3] Ongoing oceanographic research includes marine lifeforms, conservation, the marine environment, the chemistry of the ocean, the studying and modelling of climate dynamics, the air-sea boundary, weather patterns, ocean resources, renewable energy, waves and currents, and the design and development of new tools and technologies for investigating the deep.[34]


"Freedom of the seas" is a principle in international law dating from the seventeenth century. It stresses freedom to navigate the oceans and disapproves of war fought in international waters.[35] Today, this concept is enshrined in the United Nations Convention on the Law of the Sea (UNCLOS), the third version of which came into force in 1994. Article 87(1) states: "The high seas are open to all states, whether coastal or land-locked." Article 87(1) (a) to (f) gives a non-exhaustive list of freedoms including navigation, overflight, the laying of submarine cables, building artificial islands, fishing and scientific research.[35]

UNCLOS defines various areas of water. "Internal waters" are on the landward side of a baseline and foreign vessels have no right of passage in these. "Territorial waters" extend to 12 nautical miles (22 kilometres; 14 miles) from the coastline and in these waters, the coastal state is free to set laws, regulate use, and exploit any resource. A "contiguous zone" extending a further 12 nautical miles allows for hot pursuit of vessels suspected of infringing laws in four specific areas: customs, taxation, immigration and pollution. An "exclusive economic zone" extends for 200 nautical miles (370 kilometres; 230 miles) from the baseline. Within this area, the coastal nation has sole exploitation rights over all natural resources. The "continental shelf" is the natural prolongation of the land territory to the continental margin's outer edge, or 200 nautical miles from the coastal state's baseline, whichever is greater. Here the coastal nation has the exclusive right to harvest minerals and also living resources "attached" to the seabed.[35]

The safety of shipping is regulated by the International Maritime Organization. Its objectives include developing and maintaining a regulatory framework for the safety, security and environmental performance of international shipping.[36]


Naval warfare: The explosion of the Spanish flagship during the Battle of Gibraltar, 25 April 1607 by Cornelis Claesz van Wieringen, formerly attributed to Hendrik Cornelisz Vroom

Control of the sea is important to the security of a maritime nation, and battles have been fought on the sea for more than 3,000 years. The naval blockade of a port can be used to cut off food and supplies in time of war. In about 1210 B.C.E., Suppiluliuma II, the king of the Hittites, defeated and burned a fleet from Alashiya (modern Cyprus).[37] At the end of the Age of Sail, the British Royal Navy, led by Horatio Nelson, broke the power of the combined French and Spanish fleets at the 1805 Battle of Trafalgar.[38]

With steam and the industrial production of steel plate came greatly increased firepower in the shape of the dreadnought battleships armed with long-range guns. In the Second World War, the British victory at the 1940 Battle of Taranto showed that naval air power was sufficient to overcome the largest warships, foreshadowing the decisive sea-battles of the Pacific War including the Battles of the Coral Sea, Midway, the Philippine Sea, and the climactic Battle of Leyte Gulf, in all of which the dominant ships were aircraft carriers.[39]

Submarines became important in naval warfare in World War I, when German submarines, known as U-boats, sank nearly 5,000 Allied merchant ships, including the RMS Lusitania, which helped to bring the United States into the war.[40] Since 1960, several nations have maintained fleets of nuclear-powered submarines equipped to launch ballistic missiles with nuclear warheads from under the sea. Some of these are kept permanently on patrol.[41]


Sailing ships or packets carried mail overseas, one of the earliest being the Dutch service to Batavia in the 1670s. These added passenger accommodation, but in cramped conditions. Later, scheduled services were offered but the time journeys took depended much on the weather. When steamships replaced sailing vessels, ocean-going liners took over the task of carrying people. By the beginning of the twentieth century, crossing the Atlantic took about five days and shipping companies competed to own the largest and fastest vessels.

The great liners were comfortable but expensive in fuel and staff. The age of the trans-Atlantic liners waned as cheap intercontinental flights became available. In 1958, a regular scheduled air service between New York and Paris taking seven hours doomed the Atlantic ferry service to oblivion. One by one the vessels were laid up, some were scrapped, others became cruise ships for the leisure industry and still others floating hotels.[42]


Shipping routes, showing relative density of commercial shipping around the world

Maritime trade has existed for millennia. The Ptolemaic dynasty had developed trade with India using the Red Sea ports and in the first millennium B.C.E. the Arabs, Phoenicians, Israelites and Indians traded in luxury goods such as spices, gold, and precious stones.[43] The Phoenicians were noted sea traders and under the Greeks and Romans, commerce continued to thrive. With the collapse of the Roman Empire, European trade dwindled but it continued to flourish among the kingdoms of Africa, the Middle East, India, China, and southeastern Asia.[44]

Large quantities of goods are transported by sea, especially across the Atlantic and around the Pacific Rim. A major trade route passes through the Pillars of Hercules, across the Mediterranean and the Suez Canal to the Indian Ocean and through the Straits of Malacca. Shipping lanes, the routes on the open sea used by cargo vessels, traditionally make use of trade winds and currents.


Main articles: Fishing, Whaling, Seal hunting, Seaweed farming, and Aquaculture
German factory ship, 92 meters (300 ft) long

Fish and other fishery products are among the most widely consumed sources of protein and other essential nutrients. In order to fulfill this need, coastal countries have exploited marine resources in their exclusive economic zone, although fishing vessels are increasingly venturing further afield to exploit stocks in international waters.[45]

Modern fishing vessels include fishing trawlers with a small crew, stern trawlers, purse seiners, long-line factory vessels, and large factory ships which are designed to stay at sea for weeks, processing, and freezing great quantities of fish. The equipment used to capture the fish may be purse seines, other seines, trawls, dredges, gillnets, and long-lines and the fish species most frequently targeted are herring, cod, anchovy, tuna, flounder, mullet, squid, and salmon. Overexploitation has become a serious concern; it does not only cause the depletion of fish stocks, but also substantially reduce the size of predatory fish populations. In order to avoid overexploitation, many countries have introduced quotas in their own waters.[46] However, recovery efforts often entail substantial costs to local economies or food provision.

Fishing boat in Sri Lanka

Artisan fishing methods include rod and line, harpoons, skin diving, traps, throw nets, and drag nets. Traditional fishing boats are powered by paddle, wind, or outboard motors and operate in near-shore waters. The Food and Agriculture Organization is encouraging the development of local fisheries to provide food security to coastal communities and help alleviate poverty.[47]

Food and non-food products are produced by aquaculture, with as many six hundred species of plants and animals being cultured, some for use in seeding wild populations. Animals raised include finfish, aquatic reptiles, crustaceans, molluscs, sea cucumbers, sea urchins, sea squirts, and jellyfish.[48] Integrated mariculture has the advantage that there is a readily available supply of planktonic food in the ocean, and waste is removed naturally.[49]


Use of the sea for leisure developed in the nineteenth century, and became a significant industry in the twentieth century. Maritime leisure activities are varied, and include self-organized trips cruising, yachting, powerboat racing, and fishing;[50] commercially organized voyages on cruise ships and trips on smaller vessels for ecotourism such as whale watching and coastal birdwatching.[51]

Scuba diver with face mask, fins and underwater breathing apparatus

Sea bathing became the vogue in Europe in the eighteenth century after William Buchan advocated the practice for health reasons.[52] A number of marine water sports have increased in popularity in recent years including Surfing, kite surfing, windsurfing, and water skiing.

Beneath the surface, freediving is necessarily restricted to shallow descents, although Pearl divers can dive to 40 feet (12 m) with baskets to collect oysters.[53] Scuba equipment allows underwater breathing and hence a longer time can be spent beneath the surface.


Power generation

Tidal power: the 1 km Rance Tidal Power Station in Brittany generates 0.5 GW.

The sea offers a very large supply of energy carried by ocean waves, tides, salinity differences, and ocean temperature differences which can be harnessed to generate electricity. Tidal power uses generators to produce electricity from tidal flows, sometimes by using a dam to store and then release seawater. For example, the Rance Tidal Power Station, near St Malo in Brittany opened in 1967, with a barrage, a dam-like structure, 750 m (2,461 ft) long, from Brebis point in the west to Briantais point in the east.[3]

The large and highly variable energy of waves gives them enormous destructive capability, making affordable and reliable wave machines problematic to develop.

Offshore wind power is captured by wind turbines placed out at sea; it has the advantage that wind speeds are higher than on land, though wind farms are more costly to construct offshore.

Extractive industries

The seabed contains large reserves of minerals which can be exploited by dredging.

Minerals precipitated near a hydrothermal vent

Seafloor massive sulphide deposits are potential sources of silver, gold, copper, lead, and zinc and trace metals since their discovery in the 1960s. These form when geothermally heated water is emitted from deep sea hydrothermal vents known as "black smokers." The ores are of high quality but prohibitively costly to extract.

There are large deposits of petroleum, as oil and natural gas, in rocks beneath the seabed. Offshore platforms and drilling rigs extract the oil or gas and store it for transport to land. Offshore oil and gas production can be difficult due to the remote, harsh environment.

Large quantities of methane clathrate exist on the seabed and in ocean sediment, of interest as a potential energy source. Also on the seabed are manganese nodules formed of layers of iron, manganese, and other hydroxides around a core. In the Pacific these may cover up to 30 percent of the deep ocean floor. The minerals precipitate from seawater and grow very slowly. Their commercial extraction for nickel was investigated in the 1970s but abandoned in favor of more convenient sources. In suitable locations, diamonds are gathered from the seafloor using suction hoses to bring gravel ashore. In deeper waters, mobile seafloor crawlers are used and the deposits are pumped to a vessel above.

Reverse osmosis desalination plant

The sea holds large quantities of valuable dissolved minerals. The most important, Salt for table and industrial use has been harvested by solar evaporation from shallow ponds since prehistoric times.

Fresh water production

Desalination is the technique of removing salts from seawater to leave fresh water suitable for drinking or irrigation. The two main processing methods, vacuum distillation and reverse osmosis, use large quantities of energy. Desalination is normally only undertaken where fresh water from other sources is in short supply or energy is plentiful, as in the excess heat generated by power stations.

In culture

Great wave off the coast of Kanagawa by Katsushika Hokusai, c. 1830

The sea appears in human culture in contradictory ways, as both powerful but serene and as beautiful but dangerous. It has its place in literature, art, poetry, film, theatre, classical music, mythology, and dream interpretation. The Ancients personified it, believing it to be under the control of a deity who needed to be appeased, and symbolically, it has been perceived as a hostile environment populated by fantastic creatures; the Leviathan of the Bible, Scylla in Greek mythology, Umibōzu, or sea priest, in Japanese mythology, the Jörmungandr, or Midgard Serpent, and the kraken of late Norse mythology.[54]

The sea and ships have been depicted in art ranging from simple drawings on the walls of huts in Lamu to seascapes by Joseph Turner. The Japanese artist Katsushika Hokusai created color prints of the moods of the sea, including The Great Wave off Kanagawa.

Music too has been inspired by the ocean, sometimes by composers who lived or worked near the shore and saw its many different aspects. Sea shanties, songs that were chanted by mariners to help them perform arduous tasks, have been woven into compositions and impressions in music have been created of calm waters, crashing waves, and storms at sea.[55]

The Oceanids (The Naiads of the Sea), a painting by Gustave Doré (c. 1860)

As a symbol, the sea has for centuries played a role in literature, poetry, and dreams. Sometimes it is there just as a gentle background but often it introduces such themes as storm, shipwreck, battle, hardship, disaster, the dashing of hopes and death.[55] In his epic poem the Odyssey, written in the eighth century B.C.E., Homer describes the ten-year voyage of the Greek hero Odysseus who struggles to return home across the sea's many hazards after the war described in the Iliad. The sea is a recurring theme in the Haiku poems of the Japanese Edo period poet Matsuo Bashō (松尾 芭蕉) (1644–1694). In the works of psychiatrist Carl Jung, the sea symbolizes the personal and the collective unconscious in dream interpretation, the depths of the sea symbolizing the depths of the unconscious mind.

Environmental issues

Human activities affect marine life and marine habitats through overfishing, habitat loss, the introduction of invasive species, ocean pollution, ocean acidification and ocean warming. These impact marine ecosystems and food webs and may result in consequences as yet unrecognised for the biodiversity and continuation of marine life forms.


Seawater is slightly alkaline and had an average pH of about 8.2 over the past 300 million years. More recently, climate change has resulted in an increase of the carbon dioxide content of the atmosphere; about 30–40 percent of the added CO2 is absorbed by the oceans, forming carbonic acid and lowering the pH through a process called ocean acidification.[56]

Calcium is an important element for the formation of skeletal material in marine animals, but calcium carbonate becomes more soluble with pressure, so carbonate shells and skeletons dissolve below its compensation depth. Calcium carbonate also becomes more soluble at lower pH, so ocean acidification is likely to have profound effects on marine organisms with calcareous shells, such as oysters, clams, sea urchins, and corals, because their ability to form shells is reduced. In tropical regions, corals are severely affected as it becomes more difficult to build their calcium carbonate skeletons,[57]

Marine pollution

Many substances enter the sea as a result of human activities. Combustion products are transported in the air and deposited into the sea by precipitation. Industrial outflows and sewage contribute heavy metals, pesticides, PCBs, disinfectants, household cleaning products, and other synthetic chemicals which become concentrated in the surface film and in marine sediment, especially estuarine mud. Nuclear facilities too can pollute, causing radioactive material to seep into the sea. The result of all this contamination is largely unknown because of the large number of substances involved and the lack of information on their biological effects. The heavy metals of greatest concern are copper, lead, mercury, cadmium, and zinc which may be bio-accumulated by marine organisms and are passed up the food chain.

Much floating plastic rubbish does not biodegrade, instead disintegrating over time and eventually breaking down to the molecular level. In the center of the Pacific gyre there is a permanent floating accumulation of mostly plastic waste and there is a similar garbage patch in the Atlantic.[58]

Oil is dangerous for marine animals. In the short term, oil spills result in wildlife populations being decreased and unbalanced, leisure activities being affected, and the livelihoods of people dependent on the sea being devastated. Fortunately, the marine environment has self-cleansing properties and naturally occurring bacteria will act over time to remove oil from the sea. In the Gulf of Mexico, where oil-eating bacteria are already present, they take only a few days to consume oil.[59]

Run-off of fertilizers from agricultural land is a major source of pollution in some areas and the discharge of raw sewage has a similar effect. The extra nutrients provided by these sources can cause excessive plant growth. Nitrogen is often the limiting factor in marine systems, and with added nitrogen, algal blooms and red tides can lower the oxygen level of the water and kill marine animals.

The dumping of waste (including oil, noxious liquids, sewage, and garbage) at sea is governed by international law. The London Convention (1972) is a United Nations agreement to control ocean dumping which had been ratified by 89 countries by June 2012.[60] MARPOL 73/78 is a convention to minimize pollution of the seas by ships. By May 2013, 152 maritime nations had ratified MARPOL.[61]


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ISBN links support NWE through referral fees

  • Aas, Øystein (ed.). Global Challenges in Recreational Fisheries. Wiley-Blackwell, 2007. ISBN 1405156570
  • Ahrens, C. Donald, Peter L. Jackson, and Christine Jackson. Meteorology Today: An Introduction to Weather, Climate, and the Environment. Nelson Cengage, 2015., ISBN 978-0176530792
  • Bellwood, Peter. The Polynesians – Prehistory of an Island People. Thames & Hudson, 1999. ISBN 978-0500274507
  • Buchan, William. Domestic Medicine. Legare Street Press, 2022 (original 1769). ISBN 978-1017483222
  • Cater, Carl, and Erlet Cater. Marine Ecotourism: Between the Devil and the Deep Blue Sea. CABI, 2007. ISBN 978-1845932596
  • Cattelle, Wallis Richard (ed.). The Pearl: Its Story, Its Charm, and Its Value. Sagwan Press, 2018 (original 1907). ISBN 978-1376475708
  • Curtin, Philip D. Cross-Cultural Trade in World History. Cambridge University Press, 1984. ISBN 978-0521269315
  • D’Amato, Raffaele, and Andrea Salimbeti. Bronze Age Greek Warrior 1600–1100 B.C.E.. Osprey Publishing Company, 2011. ISBN 978-1849081955
  • FAO. Increasing the Contribution of Small-Scale Fisheries To Poverty Alleviation and Food Security. Food and Agriculture Organization of the United Nations, 2007. ISBN 978-9251056646
  • FAO. Integrated Mariculture: A Global Review. Food and Agriculture Organization of the United Nations, 2009. ISBN 978-9251063873
  • FAO. The State of World Fisheries and Aquaculture 2012. Food and Agriculture Organization of the United Nations, 2012. ISBN 978-9251072257
  • Fremont-Barnes, Gregory. Trafalgar 1805: Nelson's Crowning Victory. Osprey Publishing, 2005. ISBN 978-1841768922
  • Garrison, Tom S., and Robert Ellis. Essentials of Oceanography. Cengage Learning, 2017. ISBN 978-1337098649
  • Gattuso, Jean-Pierre, and Lina Hansson. Ocean Acidification. Oxford University Press, 2011. ISBN 978-0199591091
  • Intergovernmental Panel on Climate Change. Climate Change 2007 - The Physical Science Basis. Cambridge University Press, 2007. ISBN 978-0521705967
  • Karleskint, George, Richard Turner, and James Small. Introduction to Marine Biology. Cengage Learning, 2012. ISBN 978-1133364467
  • Levinton, Jeffrey S. Marine Biology: International Edition: Function, Biodiversity, Ecology. Oxford University Press Inc, 2010. ISBN 978-0199766611
  • McSween, Harry, Steven Richardson, and Maria Uhle. Geochemistry: Pathways and Processes. Columbia University Press, 2003. ISBN 978-0231124409
  • Monkhouse, F.J. Principles of Physical Geography. Hodder & Stoughton, 1975. ISBN 978-0340049440
  • Munn, Ted, Michael C. MacCracken, and John S. Perry (eds.). Encyclopedia of Global Environmental Change, Volume 1, The Earth System: Physical and Chemical Dimensions of Global Environmental Change. John Wiley & Sons, 2002. ISBN 978-0471977964
  • Potts, D.T. (ed.). A Companion to the Archaeology of the Ancient Near East. Wiley-Blackwell, 2012. ISBN 978-1405189880
  • Preston, Diana. Wilful Murder: The Sinking of the Lusitania. Black Swan, 2003. ISBN 978-0552998864
  • Sarmiento, Jorge L., and Nicolas Gruber. Ocean Biogeochemical Dynamics. Princeton University Press, 2006. ISBN 978-0691017075
  • Shaw, Ian (ed.). The Oxford History of Ancient Egypt. Oxford University Press, 2004. ISBN 978-0192804587
  • Stow, Dorrick. Encyclopedia of the Oceans. Oxford University Press, 2005. ISBN 978-0198606871
  • Thomas, Evan. Sea of Thunder. Simon and Schuster, 2007. ISBN 978-0743252225
  • Thorne-Miller, Boyce. The Living Ocean: Understanding and Protecting Marine Biodiversity. Island Press, 1999. ISBN 978-1559636773
  • Tymieniecka, Anna-Teresa (ed.). Poetics of the Elements in the Human Condition: The Sea: From Elemental Stirrings to Symbolic Inspiration, Language, and Life-Significance in Literary Interpretation and Theory. Springer, 1985. ISBN 978-9027719065
  • Vukas, Budislav. The Law of the Sea: Selected Writings. Martinus Nijhoff, 2004. ISBN 978-9004138636
  • Whittow, John B. The Penguin Dictionary of Physical Geography. Puffin, 1984. ISBN 978-0140510942
  • Yonge, C M., and Frederick S. Russell. The Seas. Sagwan Press, 2015 (original 1928). ISBN 978-1340310165
  • Young, I.R. Wind Generated Ocean Waves. Elsevier, 1999. ISBN 0080433170

External links

All links retrieved April 22, 2023.


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