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This article focuses on human experience, history and culture of the collective
seas of Earth. For natural science aspects, see more at Ocean. For individual seas,
see List of seas. For other uses, see Sea (disambiguation) and The Sea
(disambiguation).
Waves breaking on the shore
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% of the Earth's surface. Used in a particular
sense the word sea refers to particular seas either as second-order sections of the
ocean, such as the Mediterranean Sea, or 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. Humans harnessing and
studying the sea have been recorded since ancient times, and evidenced well into
prehistory, while its modern scientific study is called oceanography. The most
abundant solid dissolved in seawater is sodium chloride. The water also contains
salts of magnesium, calcium, potassium, and mercury, amongst many other elements,
some in minute concentrations. Salinity varies widely, being lower near the surface
and the mouths of large rivers and higher in the depths of the ocean; however, the
relative proportions of dissolved salts vary little across the oceans.

Winds blowing over the surface of the sea produce waves, which break when they
enter the shallow water near land. Winds also create surface currents through
friction, setting up slow but stable circulations of water throughout the oceans.
The directions of the circulation are governed by several factors including the
shapes of the continents and Earth's rotation (through the Coriolis effect). Deep-
sea currents, known as the global conveyor belt, carry cold water from near the
poles to every ocean. Tides, the generally twice-daily rise and fall of sea levels,
are caused by Earth's rotation and the gravitational effects of the Moon and, to a
lesser extent, of the Sun. Tides may have a very high range in bays or estuaries.
Submarine earthquakes arising from tectonic plate movements under the oceans can
lead to destructive tsunamis, as can volcanoes, huge landslides, or the impact of
large meteorites.

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 provides substantial supplies of food for humans, 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. The sea has been an
integral element for humans throughout history and culture.
Definition
Further information: List of seas
Animated map exhibiting the world's oceanic waters. A continuous body of water
encircling Earth, the World Ocean is divided into a number of principal areas with
relatively uninhibited interchange among them. Five oceanic divisions are usually
defined: Pacific, Atlantic, Indian, Arctic, and Southern; the last two listed are
sometimes consolidated into the first three.
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]: 
90  Seas are
generally larger than lakes and contain salt water, but the Sea of Galilee is a
freshwater lake.[4][a] The United Nations Convention on the Law of the Sea states
that all of the ocean is "sea".[8][9][b]
Physical science
Composite images of the Earth created by NASA in 2001
Further information: Physical oceanography

Earth is the only known planet with seas of liquid water on its surface,[3]: 22 
although Mars possesses ice caps and similar planets in other solar systems may
have oceans.[11] Earth's 1,335,000,000 cubic kilometers (320,000,000 cu mi) of sea
contain about 97.2 percent of its known water[12][c] and cover approximately 71
percent of its surface.[3]: 
7 [17] Another 2.15% 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% 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.[12]

The scientific study of water and Earth's water cycle is hydrology; hydrodynamics
studies the physics of water in motion. The more recent study of the sea in
particular is oceanography. This began as the study of the shape of the ocean's
currents[18] but has since expanded into a large and multidisciplinary field:[19]
it examines the properties of seawater; studies waves, tides, and currents; charts
coastlines and maps the seabeds; and studies marine life.[20] The subfield dealing
with the sea's motion, its forces, and the forces acting upon it is known as
physical oceanography.[21] Marine biology (biological oceanography) studies the
plants, animals, and other organisms inhabiting marine ecosystems. Both are
informed by chemical oceanography, which studies the behavior of elements and
molecules within the oceans: particularly, at the moment, the ocean's role in the
carbon cycle and carbon dioxide's role in the increasing acidification of seawater.
Marine and maritime geography charts the shape and shaping of the sea, while marine
geology (geological oceanography) has provided evidence of continental drift and
the composition and structure of the Earth, clarified the process of sedimentation,
and assisted the study of volcanism and earthquakes.[19]
Seawater
Main article: Seawater
Global salinity map
Salinity map taken from the Aquarius Spacecraft. The rainbow colours represent
salinity levels: red = 40 ‰, purple = 30 ‰
Salinity
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 litre, a salinity of 35 ‰. The Mediterranean Sea is slightly higher at
38 ‰,[22] while the salinity of the northern Red Sea can reach 41‰.[23] 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.[24][25] Seawater is
too saline for humans to drink safely, as the kidneys cannot excrete urine as salty
as seawater.[26]
Major solutes in seawater (3.5% salinity)[25] Solute Concentration (‰) % of
total salts
Chloride 19.3 55
Sodium 10.8 30.6
Sulphate 2.7 7.7
Magnesium 1.3 3.7
Calcium 0.41 1.2
Potassium 0.40 1.1
Bicarbonate 0.10 0.4
Bromide 0.07 0.2
Carbonate 0.01 0.05
Strontium 0.01 0.04
Borate 0.01 0.01
Fluoride 0.001 <0.01
All other solutes <0.001 <0.01

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.
[27] Evaporation and by-product of ice formation (known as "brine rejection")
increase salinity, whereas precipitation, sea ice melt, and runoff from land reduce
it.[27] The Baltic Sea, for example, has many rivers flowing into it, and thus the
sea could be considered as brackish.[28] Meanwhile, the Red Sea is very salty due
to its high evaporation rate.[29]
Temperature

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 °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 °F) and 5 °C (41 °F) in all parts of the globe.[30]

Seawater with a typical salinity of 35 ‰ has a freezing point of about −1.8 °C

(28.8 °F).[citation needed] 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. Nilas may have a
salinity of 12–15 ‰, but by the time the sea ice is one year old, this falls to 4–6
‰.[31]
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.[32]

Climate change is likely to reduce levels of oxygen in surface waters, since the
solubility of oxygen in water falls at higher temperatures.[33] Ocean deoxygenation
is projected to increase hypoxia by 10%, and triple suboxic waters (oxygen
concentrations 98% less than the mean surface concentrations), for each 1 °C of
upper ocean warming.[34]
Light

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
metres (3,300 ft). There is insufficient light for photosynthesis and plant growth
beyond a depth of about 200 metres (660 ft).[35]
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]: 
74 
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.
[36] At the last glacial maximum, some 20,000 years ago, the sea level was about
125 metres (410 ft) lower than in present times (2012).[37]

For at least the last 100 years, sea level has been rising at an average rate of
about 1.8 millimetres (0.071 in) per year.[38] 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 metres (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.[39]
Waves
0:13
Movement of molecules as waves pass
Diagram showing wave approaching shore
When the wave enters shallow water, it slows down and its amplitude (height)
increases.
Main article: Wind wave

Wind blowing over the surface of a body of water forms waves that are perpendicular
to the direction of the wind. The friction between air and water caused by a gentle
breeze on a pond causes ripples to form. A strong blow over the ocean causes larger
waves as the moving air pushes against the raised ridges of water. The waves reach
their maximum height when the rate at which they are travelling 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, organised masses of water
called swell roll across the ocean.[3]: 
83–84 
[40][41][d] 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.[40] Constructive
interference can cause individual (unexpected) rogue waves much higher than normal.
[42] Most waves are less than 3 m (10 ft) high[42] and it is not unusual for strong
storms to double or triple that height;[43] offshore construction such as wind
farms and oil platforms use metocean statistics from measurements in computing the
wave forces (due to for instance the hundred-year wave) they are designed against.
[44] Rogue waves, however, have been documented at heights above 25 meters (82 ft).
[45][46]

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, which is 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.[42] This rushes in a sheet up the beach before retreating into
the sea under the influence of gravity.[40]
Tsunami
Tsunami in Thailand
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.[47] 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/h)[48] and usually have a height of less than three feet, so they often
pass unnoticed at this stage.[49] In contrast, ocean surface waves caused by winds
have wavelengths of a few hundred feet, travel at up to 65 miles per hour (105
km/h) and are up to 45 feet (14 metres) high.[49]

As a tsunami moves into shallower water its speed decreases, its wavelength
shortens and its amplitude increases enormously,[49] 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.[47] 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.[50] 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.[47]
Currents
Map showing surface currents
Surface currents: red–warm, blue–cold
Main article: Ocean current
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. Westerly winds are most frequent in the mid-latitudes while
easterlies dominate the tropics.[51] 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. 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.[52]
Global climate and weather forecasts are powerfully affected by the world ocean, so
global climate modelling makes use of ocean circulation models as well as models of
other major components such as the atmosphere, land surfaces, aerosols and sea ice.
[53] Ocean models make use of a branch of physics, geophysical fluid dynamics, that
describes the large-scale flow of fluids such as seawater.[54]
Map showing the global conveyor belt
The global conveyor belt shown in blue with warmer surface currents in red

Surface currents only affect the top few hundred metres 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.[55] 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.[52]

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.[56]
These currents can shift great volumes of sand or pebbles, create spits and make
beaches disappear and water channels silt up.[52] A rip current can occur when
water piles up near the shore from advancing waves and is funnelled 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.[57] 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.[52]
Tides
Main article: Tide
Diagram showing how the sun and moon cause tides
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.[58][59]

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. The Moon's mass is some 27 million times smaller
than the Sun, but it is 400 times closer to the Earth.[60] Tidal force or tide-
raising force decreases rapidly with distance, so the moon has more than twice as
great an effect on tides as the Sun.[60] A bulge is formed in the ocean at the
place where the Earth is closest to the Moon, because it is also where the effect
of the Moon's gravity is stronger. On the opposite side of the Earth, the lunar
force is at its weakest and this causes another bulge to form. As the Moon rotates
around the Earth, so do these ocean bulges move around the Earth. The gravitational
attraction of the Sun is also working on the seas, but its effect on tides is less
powerful than that of the Moon, and when the Sun, Moon and Earth are all aligned
(full moon and new moon), the combined effect results in the high "spring tides".
In contrast, when the Sun is at 90° from the Moon as viewed from Earth, the
combined gravitational effect on tides is less causing the lower "neap tides".[58]

A storm surge can occur when high winds pile water up against the coast in a
shallow area and this, coupled with a low pressure system, can raise the surface of
the sea at high tide dramatically.
Ocean basins
Three types of plate boundary
Main article: Ocean basin

The Earth is composed of a magnetic central core, a mostly liquid mantle and a hard
rigid outer shell (or lithosphere), which is composed of the Earth's rocky crust
and the deeper mostly solid outer layer of the mantle. On land the crust is known
as the continental crust while under the sea it is known as the oceanic crust. The
latter is composed of relatively dense basalt and is some five to ten kilometres
(three to six miles) thick. The relatively thin lithosphere floats on the weaker
and hotter mantle below and is fractured into a number of tectonic plates.[61] In
mid-ocean, magma is constantly being thrust through the seabed between adjoining
plates to form mid-oceanic ridges and here convection currents within the mantle
tend to drive the two plates apart. Parallel to these ridges and nearer the coasts,
one oceanic plate may slide beneath another oceanic plate in a process known as
subduction. Deep trenches are formed here and the process is accompanied by
friction as the plates grind together. The movement proceeds in jerks which cause
earthquakes, heat is produced and magma is forced up creating underwater mountains,
some of which may form chains of volcanic islands near to deep trenches. Near some
of the boundaries between the land and sea, the slightly denser oceanic plates
slide beneath the continental plates and more subduction trenches are formed. As
they grate together, the continental plates are deformed and buckle causing
mountain building and seismic activity.[62][63]

The Earth's deepest trench is the Mariana Trench which extends for about 2,500
kilometres (1,600 mi) across the seabed. It is near the Mariana Islands, a volcanic
archipelago in the West Pacific. Its deepest point is 10.994 kilometres (nearly 7
miles) below the surface of the sea.[64]
Coasts
Praia da Marinha in Algarve, Portugal
The Baltic Sea in the archipelago of Turku, Finland
Main article: Coast

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.[65] 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.[66]
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.[65]

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.[65] Dredging removes material and deepens channels but may have
unexpected effects elsewhere on the coastline. Governments make efforts to prevent
flooding of the land by the building of breakwaters, seawalls, dykes and levees and
other sea defences. For instance, the Thames Barrier is designed to protect London
from a storm surge,[67] while the failure of the dykes and levees around New
Orleans during Hurricane Katrina created a humanitarian crisis in the United
States.
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.[68] Even
in the Atacama Desert, where little rain ever falls, dense clouds of fog known as
the camanchaca blow in from the sea and support plant life.[69]

In central Asia and other large land masses, there are endorheic basins which have
no outlet to the sea, separated from the ocean by mountains or other natural
geologic features that prevent the water draining away. The Caspian Sea is the
largest one of these. Its main inflow is from the River Volga, there is no outflow
and the evaporation of water makes it saline as dissolved minerals accumulate. The
Aral Sea in Kazakhstan and Uzbekistan, and Pyramid Lake in the western United
States are further examples of large, inland saline water-bodies without drainage.
Some endorheic lakes are less salty, but all are sensitive to variations in the
quality of the inflowing water.[70]
Carbon cycle
Further information: Oceanic carbon cycle and Biological pump
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.[71] 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[72] and it remains
there for much longer periods of time.[73] Thermohaline circulation exchanges
carbon between these two layers.[71]

Carbon enters the ocean as atmospheric carbon dioxide dissolves in the surface
layers and is converted into carbonic acid, carbonate, and bicarbonate:[74]

CO2 (gas) ⇌ CO2 (aq)

CO2 (aq) + H2O ⇌ H2CO3
H2CO3 ⇌ HCO3− + H+
HCO3− ⇌ CO32− + H+

It can also enter through rivers as dissolved organic carbon and 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. It circulates in
this layer for long periods of time before either being deposited as sediment or
being returned to surface waters through thermohaline circulation.[73]
Life in the sea
Coral reefs are among the most biodiverse habitats in the world.
Main article: Marine life
Marine habitats

Coastal habitats
Littoral zone
Intertidal zone
Estuaries
Mangrove forests
Seagrass meadows
Kelp forests
Coral reefs
Continental shelf
Neritic zone

Ocean surface
Surface microlayer
Epipelagic zone

Open ocean
Pelagic zone
Oceanic zone

Sea floor
Seamounts
Hydrothermal vents
Cold seeps
Demersal zone
Benthic zone
Marine sediment

vte

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.[75] 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 metres (98 feet) 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.[76][77]: 
204–229 

Life may have originated in the sea and all the major groups of animals are
represented there. Scientists differ as to precisely where in the sea life arose:
the Miller-Urey experiments suggested a dilute chemical "soup" in open water, but
more recent suggestions include volcanic hot springs, fine-grained clay sediments,
or deep-sea "black smoker" vents, all of which would have provided protection from
damaging ultraviolet radiation which was not blocked by the early Earth's
atmosphere.[3]: 138–140 
Marine habitats
Main article: 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]: 
150–
151 

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.[78] The best-known are tropical coral reefs such as Australia's Great
Barrier Reef, but cold water reefs harbour a wide array of species including corals
(only six of which contribute to reef formation).[3]: 
204–207 
[79]
Algae and plants
See also: Marine primary production

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.[80][81] About 45 percent of the
sea's primary production of living material is contributed by diatoms.[82] Much
larger algae, commonly known as seaweeds, are important locally; Sargassum forms
floating drifts, while kelp form seabed forests.[77]: 
246–255  Flowering plants in
the form of seagrasses grow in "meadows" in sandy shallows,[83] mangroves line the
coast in tropical and subtropical regions[84] and salt-tolerant plants thrive in
regularly inundated salt marshes.[85] All of these habitats are able to sequester
large quantities of carbon and support a biodiverse range of larger and smaller
animal life.[86]

Light is only able to penetrate the top 200 metres (660 ft) so this is the only
part of the sea where plants can grow.[35] 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.[87] 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]: 
160–163 
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.[88] 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.[88] 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.[89]

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.[90] 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.[91] 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.
[92] Ships can introduce or spread invasive species through the discharge of
ballast water or the transport of organisms that have accumulated as part of the
fouling community on the hulls of vessels.[93]

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.
The extensive calcareous skeletons they extrude build up into coral reefs which are
an important feature of the seabed. These provide a biodiverse habitat for reef
dwelling organisms. 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.[94] Exploration of
the deep sea by submersibles revealed a new world of creatures living on the seabed
that scientists had not previously known to exist. 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 specialised bivalves, sea
anemones, barnacles, crabs, worms and fish, often found nowhere else.[3]: 212  A
dead whale sinking to the bottom of the ocean provides food for an assembly of
organisms which similarly rely largely on the actions of sulphur-reducing bacteria.
Such places support unique biomes where many new microbes and other lifeforms have
been discovered.[95]
Humans and the sea
History of navigation and exploration
Main articles: History of navigation, History of cartography, Maritime history,
Ancient maritime history, and Ocean exploration
Map showing the seaborne migration and expansion of the Austronesians beginning at
around 3000 BC

Humans have travelled the seas since they first built sea-going craft.
Mesopotamians were using bitumen to caulk their reed boats and, a little later,
masted sails.[96] By c. 3000 BC, Austronesians on Taiwan had begun spreading into
maritime Southeast Asia.[97] 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.[98] Their descendants continued to travel
thousands of miles between tiny islands on outrigger canoes,[99] and in the process
they found many new islands, including Hawaii, Easter Island (Rapa Nui), and New
Zealand.[100]

The Ancient Egyptians and Phoenicians explored the Mediterranean and Red Sea with
the Egyptian Hannu reaching the Arabian Peninsula and the African Coast around 2750
BC.[101] In the first millennium BC, Phoenicians and Greeks established colonies
throughout the Mediterranean and the Black Sea.[102] Around 500 BC, the
Carthaginian navigator Hanno left a detailed periplus of an Atlantic journey that
reached at least Senegal and possibly Mount Cameroon.[103][104] In the early
Mediaeval period, the Vikings crossed the North Atlantic and even reached the
northeastern fringes of North America.[105] Novgorodians had also been sailing the
White Sea since the 13th century or before.[106] Meanwhile, the seas along the
eastern and southern Asian coast were used by Arab and Chinese traders.[107] The
Chinese Ming Dynasty had a fleet of 317 ships with 37,000 men under Zheng He in the
early fifteenth century, sailing the Indian and Pacific Oceans.[3]: 12–13  In the
late fifteenth century, Western European mariners started making longer voyages of
exploration in search of trade. Bartolomeu Dias rounded the Cape of Good Hope in
1487 and Vasco da Gama reached India via the Cape in 1498. Christopher Columbus
sailed from Cadiz in 1492, attempting to reach the eastern lands of India and Japan
by the novel means of travelling westwards. He made landfall instead on an island
in the Caribbean Sea and a few years later, the Venetian navigator John Cabot
reached Newfoundland. The Italian Amerigo Vespucci, after whom America was named,
explored the South American coastline in voyages made between 1497 and 1502,
discovering the mouth of the Amazon River.[3]: 
12–13  In 1519 the Portuguese
navigator Ferdinand Magellan led the Spanish Magellan-Elcano expedition which would
be the first to sail around the world.[3]: 
12–13 
Mercator's map of the world
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 on this projection.

As for the history of navigational instrument, 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 James Cook used it in his voyages of exploration.
[108] Nowadays, the Global Positioning System (GPS) using over thirty satellites
enables accurate navigation worldwide.[108]

With regards to maps that are vital for navigation, 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.[109] Subsequently,
Gerardus Mercator made a practical map of the world in 1538, his map projection
conveniently making rhumb lines straight.[3]: 
12–13  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.[19] Furthermore,
in 1921, the International Hydrographic Organization (IHO) was set up, and it
constitutes the world authority on hydrographic surveying and nautical charting.
[110] A fourth edition draft was published in 1986 but so far several naming
disputes (such as the one over the Sea of Japan) have prevented its ratification.
History of oceanography and deep sea exploration
Main article: Deep-sea exploration

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]: 14  John Harrison's chronometers supported Cook's accurate
navigation and charting on two of these voyages, permanently improving the standard
attainable for subsequent work.[3]:  14  Other expeditions followed in the nineteenth
century, from Russia, France, the Netherlands and the United States as well as
Britain.[3]: 
15  On HMS Beagle, which provided Charles Darwin with ideas and
materials for his 1859 book On the Origin of Species, the ship's captain, Robert
FitzRoy, charted the seas and coasts and published his four-volume report of the
ship's three voyages in 1839.[3]:  15  Edward Forbes's 1854 book, Distribution of
Marine Life argued that no life could exist below around 600 metres (2,000 feet).
This was proven wrong by the British biologists W. B. Carpenter and C. Wyville
Thomson, who in 1868 discovered life in deep water by dredging.[3]:  15  Wyville
Thompson became chief scientist on the Challenger expedition of 1872–1876, which
effectively created the science of oceanography.[3]:  15 

On her 68,890-nautical-mile (127,580 km) journey round the globe, HMS Challenger
discovered about 4,700 new marine species, and made 492 deep sea soundings, 133
bottom dredges, 151 open water trawls and 263 serial water temperature
observations.[111] In the southern Atlantic in 1898/1899, Carl Chun on the Valdivia
brought many new life forms to the surface from depths of over 4,000 metres (13,000
ft). The first observations of deep-sea animals in their natural environment were
made in 1930 by William Beebe and Otis Barton who descended to 434 metres (1,424
ft) in the spherical steel Bathysphere.[citation needed] This was lowered by cable
but by 1960 a self-powered submersible, Trieste developed by Jacques Piccard, took
Piccard and Don Walsh to the deepest part of the Earth's oceans, the Mariana Trench
in the Pacific, reaching a record depth of about 10,915 metres (35,810 ft),[112] a
feat not repeated until 2012 when James Cameron piloted the Deepsea Challenger to
similar depths.[113] An atmospheric diving suit can be worn for deep sea
operations, with a new world record being set in 2006 when a US Navy diver
descended to 2,000 feet (610 m) in one of these articulated, pressurized suits.
[114]

At great depths, no light penetrates through the water layers from above and the
pressure is extreme. For deep sea exploration it is necessary to use specialist
vehicles, either remotely operated underwater vehicles with lights and cameras or
crewed submersibles. The battery-operated Mir submersibles have a three-person crew
and can descend to 20,000 feet (6,100 m). They have viewing ports, 5,000-watt
lights, video equipment and manipulator arms for collecting samples, placing probes
or pushing the vehicle across the sea bed when the thrusters would stir up
excessive sediment.[115]

Bathymetry is the mapping and study of the topography of the ocean floor. Methods
used for measuring the depth of the sea include single or multibeam echosounders,
laser airborne depth sounders and the calculation of depths from satellite remote
sensing data. This information is used for determining the routes of undersea
cables and pipelines, for choosing suitable locations for siting oil rigs and
offshore wind turbines and for identifying possible new fisheries.[116]

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.[117] Whereas in the 1960s and 1970s
research could focus on taxonomy and basic biology, in the 2010s attention has
shifted to larger topics such as climate change.[118] Researchers make use of
satellite-based remote sensing for surface waters, with research ships, moored
observatories and autonomous underwater vehicles to study and monitor all parts of
the sea.[119]
Law

"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.[120] 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.
[120] The safety of shipping is regulated by the International Maritime
Organization. Its objectives include developing and maintaining a regulatory
framework for shipping, maritime safety, environmental concerns, legal matters,
technical co-operation and maritime security.[121]

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.[120]
War
Main article: Naval warfare
Battle of Gibraltar
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 the naval
blockade of a port can be used to cut off food and supplies in time of war. Battles
have been fought on the sea for more than 3,000 years. In about 1210 B.C.,
Suppiluliuma II, the king of the Hittites, defeated and burned a fleet from
Alashiya (modern Cyprus).[122] In the decisive 480 B.C. Battle of Salamis, the
Greek general Themistocles trapped the far larger fleet of the Persian king Xerxes
in a narrow channel and attacked vigorously, destroying 200 Persian ships for the
loss of 40 Greek vessels.[123] 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.[124]

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 1905, the Japanese fleet decisively defeated the Russian fleet, which had
travelled over 18,000 nautical miles (33,000 km), at the Battle of Tsushima.[125]
Dreadnoughts fought inconclusively in the First World War at the 1916 Battle of
Jutland between the Royal Navy's Grand Fleet and the Imperial German Navy's High
Seas Fleet.[126] 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,[127] 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.[128][129]

Submarines became important in naval warfare in World War I, when German

submarines, known as U-boats, sank nearly 5,000 Allied merchant ships,[130]
including the RMS Lusitania, which helped to bring the United States into the war.
[131] In World War II, almost 3,000 Allied ships were sunk by U-boats attempting to
block the flow of supplies to Britain,[132] but the Allies broke the blockade in
the Battle of the Atlantic, which lasted the whole length of the war, sinking 783
U-boats.[133] Since 1960, several nations have maintained fleets of nuclear-powered
ballistic missile submarines, vessels equipped to launch ballistic missiles with
nuclear warheads from under the sea. Some of these are kept permanently on patrol.
[134][135]
Travel

Sailing ships or packets carried mail overseas, one of the earliest being the Dutch
service to Batavia in the 1670s.[136] 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 Blue Riband was an
unofficial accolade given to the fastest liner crossing the Atlantic in regular
service. The Mauretania held the title with 26.06 knots (48.26 km/h) for twenty
years from 1909.[137] The Hales Trophy, another award for the fastest commercial
crossing of the Atlantic, was won by the United States in 1952 for a crossing that
took three days, ten hours and forty minutes.[138]

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.[139]
Trade
Main articles: Shipping and Trade
Map showing shipping routes
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 BC the Arabs,
Phoenicians, Israelites and Indians traded in luxury goods such as spices, gold,
and precious stones.[140] 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.[141] From the 16th to
the 19th centuries, over a period of 400 years, about 12–13 million Africans were
shipped across the Atlantic to be sold as slaves in the Americas as part of the
Atlantic slave trade.[142][143]: 
194 

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; much trade also passes through the English Channel.
[144] Shipping lanes are the routes on the open sea used by cargo vessels,
traditionally making use of trade winds and currents. Over 60 percent of the
world's container traffic is conveyed on the top twenty trade routes.[145]
Increased melting of Arctic ice since 2007 enables ships to travel the Northwest
Passage for some weeks in summertime, avoiding the longer routes via the Suez Canal
or the Panama Canal.[146] Shipping is supplemented by air freight, a more expensive
process mostly used for particularly valuable or perishable cargoes. Seaborne trade
carries more than US$4 trillion worth of goods each year.[147] Bulk cargo in the
form of liquids, powder or particles are carried loose in the holds of bulk
carriers and include crude oil, grain, coal, ore, scrap metal, sand and gravel.
[148] Other cargo, such as manufactured goods, is usually transported within
standard sized, lockable containers, loaded on purpose-built container ships at
dedicated terminals.[149] Before the rise of containerization in the 1960s, these
goods were loaded, transported and unloaded piecemeal as break-bulk cargo.
Containerization greatly increased the efficiency and decreased the cost of moving
goods by sea, and was a major factor leading to the rise of globalization and
exponential increases in international trade in the mid-to-late 20th century.[150]
Food
Main articles: Fishing, Whaling, Seal hunting, and Seaweed farming
Factory ship
German factory ship, 92 metres (302 ft) long

Fish and other fishery products are among the most widely consumed sources of
protein and other essential nutrients.[151] In 2009, 16.6% of the world's intake of
animal protein and 6.5% of all protein consumed came from fish.[151] 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.[152] In 2011, the total
world production of fish, including aquaculture, was estimated to be 154 million
tonnes, of which most was for human consumption.[151] The harvesting of wild fish
accounted for 90.4 million tonnes, while annually increasing aquaculture
contributes the rest.[151] The north west Pacific is by far the most productive
area with 20.9 million tonnes (27 percent of the global marine catch) in 2010.[151]
In addition, the number of fishing vessels in 2010 reached 4.36 million, whereas
the number of people employed in the primary sector of fish production in the same
year amounted to 54.8 million.[151]

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.[153] It has
been estimated that "industrialized fisheries typically reduced community biomass
by 80% within 15 years of exploitation."[153] In order to avoid overexploitation,
many countries have introduced quotas in their own waters.[154] However, recovery
efforts often entail substantial costs to local economies or food provision.
Fishing boat
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.[155]
Aquaculture
Main article: Aquaculture

About 79 million tonnes (78M long tons; 87M short tons) of food and non-food
products were produced by aquaculture in 2010, an all-time high. About six hundred
species of plants and animals were cultured, some for use in seeding wild
populations. The animals raised included finfish, aquatic reptiles, crustaceans,
molluscs, sea cucumbers, sea urchins, sea squirts and jellyfish.[151] Integrated
mariculture has the advantage that there is a readily available supply of
planktonic food in the ocean, and waste is removed naturally.[156] Various methods
are employed. Mesh enclosures for finfish can be suspended in the open seas, cages
can be used in more sheltered waters or ponds can be refreshed with water at each
high tide. Shrimps can be reared in shallow ponds connected to the open sea.[157]
Ropes can be hung in water to grow algae, oysters and mussels. Oysters can be
reared on trays or in mesh tubes. Sea cucumbers can be ranched on the seabed.[158]
Captive breeding programmes have raised lobster larvae for release of juveniles
into the wild resulting in an increased lobster harvest in Maine.[159] At least 145
species of seaweed – red, green, and brown algae – are eaten worldwide, and some
have long been farmed in Japan and other Asian countries; there is great potential
for additional algaculture.[160] Few maritime flowering plants are widely used for
food but one example is marsh samphire which is eaten both raw and cooked.[161] A
major difficulty for aquaculture is the tendency towards monoculture and the
associated risk of widespread disease. Aquaculture is also associated with
environmental risks; for instance, shrimp farming has caused the destruction of
important mangrove forests throughout southeast Asia.[162]
Leisure
Main articles: Cruising (maritime), Sailing, and Recreational boat fishing

Use of the sea for leisure developed in the nineteenth century, and became a
significant industry in the twentieth century.[163] Maritime leisure activities are
varied, and include self-organized trips cruising, yachting, powerboat racing[164]
and fishing;[165] commercially organized voyages on cruise ships;[166] and trips on
smaller vessels for ecotourism such as whale watching and coastal birdwatching.
[167]
Scuba diver
Scuba diver with face mask, fins and underwater breathing apparatus

Sea bathing became the vogue in Europe in the 18th century after Dr. William Buchan
advocated the practice for health reasons.[168] Surfing is a sport in which a wave
is ridden by a surfer, with or without a surfboard. Other marine water sports
include kite surfing, where a power kite propels a rider on a board across the
water,[169] windsurfing, where the power is provided by a fixed, manoeuvrable
sail[170] and water skiing, where a powerboat is used to pull a skier.[171]

Beneath the surface, freediving is necessarily restricted to shallow descents.

Pearl divers can dive to 40 feet (12 m) with baskets to collect oysters.[172] Human
eyes are not adapted for use underwater but vision can be improved by wearing a
diving mask. Other useful equipment includes fins and snorkels, and scuba equipment
allows underwater breathing and hence a longer time can be spent beneath the
surface.[173] The depths that can be reached by divers and the length of time they
can stay underwater is limited by the increase of pressure they experience as they
descend and the need to prevent decompression sickness as they return to the
surface. Recreational divers restrict themselves to depths of 100 feet (30 m)
beyond which the danger of nitrogen narcosis increases. Deeper dives can be made
with specialised equipment and training.[173]
Industry
Power generation
Main articles: Marine energy and Offshore wind power

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.[174] Forms of sustainable marine energy include tidal power,
ocean thermal energy and wave power.[174][175] Electricity power stations are often
located on the coast or beside an estuary so that the sea can be used as a heat
sink. A colder heat sink enables more efficient power generation, which is
important for expensive nuclear power plants in particular.[176]
Barrage for tidal power
Tidal power: the 1 km Rance Tidal Power Station in Brittany generates 0.5 GW.

Tidal power uses generators to produce electricity from tidal flows, sometimes by
using a dam to store and then release seawater. The Rance barrage, 1 kilometre
(0.62 mi) long, near St Malo in Brittany opened in 1967; it generates about 0.5 GW,
but it has been followed by few similar schemes.[3]: 
111–112 

The large and highly variable energy of waves gives them enormous destructive
capability, making affordable and reliable wave machines problematic to develop. A
small 2 MW commercial wave power plant, "Osprey", was built in Northern Scotland in
1995 about 300 metres (980 feet) offshore. It was soon damaged by waves, then
destroyed by a storm.[3]: 
112 

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.[177] The first offshore wind farm was installed in
Denmark in 1991,[178] and the installed capacity of worldwide offshore wind farms
reached 34 GW in 2020, mainly situated in Europe.[179]
Extractive industries
Main articles: Offshore drilling and Deep sea mining

The seabed contains large reserves of minerals which can be exploited by dredging.
This has advantages over land-based mining in that equipment can be built at
specialised shipyards and infrastructure costs are lower. Disadvantages include
problems caused by waves and tides, the tendency for excavations to silt up and the
washing away of spoil heaps. There is a risk of coastal erosion and environmental
damage.[180]
Minerals from hydrothermal vent
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. They 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.
[181]

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.[182] Drilling for oil in the sea has environmental
impacts. Animals may be disorientated by seismic waves used to locate deposits, and
there is debate as to whether this causes the beaching of whales.[183] Toxic
substances such as mercury, lead and arsenic may be released. The infrastructure
may cause damage, and oil may be spilt.[184]

Large quantities of methane clathrate exist on the seabed and in ocean sediment, of
interest as a potential energy source.[185] 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 favour of more convenient
sources.[186] 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. In Namibia, more diamonds
are now collected from marine sources than by conventional methods on land.[187]
Desalination plant
Reverse osmosis desalination plant

The sea holds large quantities of valuable dissolved minerals.[188] The most
important, Salt for table and industrial use has been harvested by solar
evaporation from shallow ponds since prehistoric times. Bromine, accumulated after
being leached from the land, is economically recovered from the Dead Sea, where it
occurs at 55,000 parts per million (ppm).[189]
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. The brine
produced as a by-product contains some toxic materials and is returned to the sea.
[190]
Indigenous sea peoples

Several nomadic indigenous groups in Maritime Southeast Asia live in boats and
derive nearly all they need from the sea. The Moken people live on the coasts of
Thailand and Burma and islands in the Andaman Sea.[191] The Bajau people are
originally from the Sulu Archipelago, Mindanao and northern Borneo.[citation
needed] Some Sea Gypsies are accomplished free-divers, able to descend to depths of
30 metres (98 ft), though many are adopting a more settled, land-based way of life.
[192][193]

The indigenous peoples of the Arctic such as the Chukchi, Inuit, Inuvialuit and
Yup'iit hunt marine mammals including seals and whales,[194] and the Torres Strait
Islanders of Australia include the Great Barrier Reef among their possessions. They
live a traditional life on the islands involving hunting, fishing, gardening and
trading with neighbouring peoples in Papua and mainland Aboriginal Australians.
[195]
In culture
Main article: Sea in culture
"Great wave" by Hokusai
Great wave off the coast of Kanagawa by Katsushika Hokusai, c. 1830[3]: 
8 

The sea appears in human culture in contradictory ways, as both powerful but serene
and as beautiful but dangerous.[3]: 
10  It has its place in literature, art, poetry,
film, theatre, classical music, mythology and dream interpretation.[196] The
Ancients personified it, believing it to be under the control of a being who needed
to be appeased, and symbolically, it has been perceived as a hostile environment
populated by fantastic creatures; the Leviathan of the Bible,[197] Scylla in Greek
mythology,[198] Isonade in Japanese mythology,[199] and the kraken of late Norse
mythology.[200]
Painting by Ludolf Bakhuizen
Dutch Golden Age painting: The Y at Amsterdam, seen from the Mosselsteiger (mussel
pier) by Ludolf Bakhuizen, 1673[201]

The sea and ships have been depicted in art ranging from simple drawings on the
walls of huts in Lamu[196] to seascapes by Joseph Turner. In Dutch Golden Age
painting, artists such as Jan Porcellis, Hendrick Dubbels, Willem van de Velde the
Elder and his son, and Ludolf Bakhuizen celebrated the sea and the Dutch navy at
the peak of its military prowess.[201][202] The Japanese artist Katsushika Hokusai
created colour prints of the moods of the sea, including The Great Wave off
Kanagawa.[3]: 
8 

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.[203]: 
4–8 
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.[203]: 
45  In his epic poem the Odyssey, written in the eighth century BC,
[204] 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.
[205] The sea is a recurring theme in the Haiku poems of the Japanese Edo period
poet Matsuo Bashō (松尾 芭蕉) (1644–1694).[206] 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.[207]
Environmental issues
Further information: Ocean § Threats, and Human impact on marine life

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.[208]
Acidification
Main articles: Ocean acidification and Effects of climate change on oceans

Seawater is slightly alkaline and had an average pH of about 8.2 over the past 300
million years.[209] More recently, climate change has resulted in an increase of
the carbon dioxide content of the atmosphere; about 30–40% of the added CO2 is
absorbed by the oceans, forming carbonic acid and lowering the pH (now below
8.1[209]) through a process called ocean acidification.[210][211][212] The pH is
expected to reach 7.7 (representing a 3-fold increase in hydrogen ion
concentration) by the year 2100, which is a significant change in a century.[213]
[e]

One important element for the formation of skeletal material in marine animals is
calcium, but calcium carbonate becomes more soluble with pressure, so carbonate
shells and skeletons dissolve below its compensation depth.[215] 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,[216] because their ability to form shells will be
reduced,[217] and the carbonate compensation depth will rise closer to the sea
surface. Affected planktonic organisms will include the snail-like molluscs known
as pteropods, and single-celled algae called coccolithophorids and foraminifera.
All of these are important parts of the food chain and a diminution in their
numbers will have significant consequences. In tropical regions, corals are likely
to be severely affected as it becomes more difficult to build their calcium
carbonate skeletons,[218] in turn adversely impacting other reef dwellers.[213]

The current rate of ocean chemistry change appears to be without precedent in

Earth's geological history, making it unclear how well marine ecosystems will be
able to adapt to the shifting conditions of the near future.[219] Of particular
concern is the manner in which the combination of acidification with the expected
additional stressors of higher temperatures and lower oxygen levels will impact the
seas.[220]
Marine pollution
Main article: 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. These become
concentrated in the surface film and in marine sediment, especially estuarine mud.
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.
[221] 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.[222]

Much floating plastic rubbish does not biodegrade, instead disintegrating over time
and eventually breaking down to the molecular level. Rigid plastics may float for
years.[223] In the centre of the Pacific gyre there is a permanent floating
accumulation of mostly plastic waste[224] and there is a similar garbage patch in
the Atlantic.[225] Foraging sea birds such as the albatross and petrel may mistake
debris for food, and accumulate indigestible plastic in their digestive systems.
Turtles and whales have been found with plastic bags and fishing line in their
stomachs. Microplastics may sink, threatening filter feeders on the seabed.[226]

Most oil pollution in the sea comes from cities and industry.[227] Oil is dangerous
for marine animals. It can clog the feathers of sea birds, reducing their
insulating effect and the birds' buoyancy, and be ingested when they preen
themselves in an attempt to remove the contaminant. Marine mammals are less
seriously affected but may be chilled through the removal of their insulation,
blinded, dehydrated or poisoned. Benthic invertebrates are swamped when the oil
sinks, fish are poisoned and the food chain is disrupted. 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.[228] 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 spilt oil.[229]

Run-off of fertilisers 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. Such
events have created dead zones in the Baltic Sea and the Gulf of Mexico.[227] Some
algal blooms are caused by cyanobacteria that make shellfish that filter feed on
them toxic, harming animals like sea otters.[230] Nuclear facilities too can
pollute. The Irish Sea was contaminated by radioactive caesium-137 from the former
Sellafield nuclear fuel processing plant[231] and nuclear accidents may also cause
radioactive material to seep into the sea, as did the disaster at the Fukushima
Daiichi Nuclear Power Plant in 2011.[232]

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 8
June 2012.[233] MARPOL 73/78 is a convention to minimize pollution of the seas by
ships. By May 2013, 152 maritime nations had ratified MARPOL.