The ocean (also the
sea or the world ocean) is the body of
salt water that covers approximately 70.8% of the surface of
Earth
Earth is the third planet from the Sun and the only astronomical object known to harbor life. While large volumes of water can be found throughout the Solar System, only Earth sustains liquid surface water. About 71% of Earth's surfa ...
and contains 97% of
Earth's water.
An ocean can also refer to any of the large bodies of water into which the world ocean is conventionally divided.
["Ocean."](_blank)
''Merriam-Webster.com Dictionary'', Merriam-Webster, https://www.merriam-webster.com/dictionary/ocean. Accessed March 14, 2021. Separate names are used to identify five different areas of the ocean:
Pacific
The Pacific Ocean is the largest and deepest of Earth's five oceanic divisions. It extends from the Arctic Ocean in the north to the Southern Ocean (or, depending on definition, to Antarctica) in the south, and is bounded by the contine ...
(the largest),
Atlantic,
Indian,
Southern (Antarctic), and
Arctic (the smallest).
Seawater covers approximately of the planet. The ocean is the principal component of Earth's
hydrosphere, and therefore integral to
life
Life is a quality that distinguishes matter that has biological processes, such as Cell signaling, signaling and self-sustaining processes, from that which does not, and is defined by the capacity for Cell growth, growth, reaction to Stimu ...
on Earth. Acting as a huge
heat reservoir, the ocean influences
climate and
weather patterns, the
carbon cycle, and the
water cycle.
Oceanographers divide the ocean into different vertical and horizontal zones based on physical and biological conditions. The
pelagic zone consists of the
water column from surface to ocean floor throughout the open ocean. The water column is further categorized in other zones depending on depth and on how much light is present. The
photic zone includes water from the surface to a depth of 1% of the surface light (about 200 m in the open ocean), where
photosynthesis can occur. This makes the photic zone the most
biodiverse
Biodiversity or biological diversity is the variety and variability of life on Earth. Biodiversity is a measure of variation at the genetic ('' genetic variability''), species ('' species diversity''), and ecosystem ('' ecosystem diversity'') ...
. Photosynthesis by plants and microscopic
algae (free floating
phytoplankton) creates organic matter using light, water, carbon dioxide, and nutrients. Ocean photosynthesis creates 50% of the oxygen in earth's atmosphere. This upper sunlit zone is the origin of the food supply which sustains most of the ocean
ecosystem
An ecosystem (or ecological system) consists of all the organisms and the physical environment with which they interact. These biotic and abiotic components are linked together through nutrient cycles and energy flows. Energy enters the syste ...
. Light only penetrates to a depth of a few hundred meters; the remaining ocean below is cold and dark. The
continental shelf where the ocean approaches dry land is more shallow, with a depth of a few hundred meters or less. Human activity has a greater impact on the
continental shelf.
Ocean temperatures depend on the amount of solar radiation reaching the ocean surface. In the tropics,
surface temperatures can rise to over . Near the poles where
sea ice forms, the temperature in equilibrium is about . Deep ocean temperature is between and in all parts of the ocean.
Water continuously circulates in the oceans creating
ocean currents. These directed movements of seawater are generated by forces acting upon the water, including temperature differences,
atmospheric circulation
Atmospheric circulation is the large-scale movement of air and together with ocean circulation is the means by which thermal energy is redistributed on the surface of the Earth. The Earth's atmospheric circulation varies from year to year, bu ...
(wind), the
Coriolis effect
In physics, the Coriolis force is an inertial or fictitious force that acts on objects in motion within a frame of reference that rotates with respect to an inertial frame. In a reference frame with clockwise rotation, the force acts to the ...
and differences in
salinity.
Tidal currents originate from
tides, while surface currents are caused by wind and waves. Major ocean currents include the
Gulf Stream,
Kuroshio Current
The , also known as the Black or or the is a north-flowing, warm ocean current on the west side of the North Pacific Ocean basin. It was named for the deep blue appearance of its waters. Similar to the Gulf Stream in the North Atlantic, the Ku ...
,
Agulhas Current and
Antarctic Circumpolar Current. Collectively, currents move enormous amounts of water and heat around the globe. This circulation significantly impacts global climate and the uptake and redistribution of pollutants such as
carbon dioxide by moving these contaminants from the surface into the deep ocean.
Ocean water contains large quantities of dissolved gases, including
oxygen
Oxygen is the chemical element with the symbol O and atomic number 8. It is a member of the chalcogen group in the periodic table, a highly reactive nonmetal, and an oxidizing agent that readily forms oxides with most elements as ...
,
carbon dioxide and
nitrogen
Nitrogen is the chemical element with the symbol N and atomic number 7. Nitrogen is a nonmetal and the lightest member of group 15 of the periodic table, often called the pnictogens. It is a common element in the universe, estimated at se ...
. This
gas exchange takes place at the ocean surface and solubility depends on the temperature and salinity of the water.
The increasing concentration of
carbon dioxide in the atmosphere due to
fossil fuel combustion leads to higher concentrations in ocean water, resulting in
ocean acidification
Ocean acidification is the reduction in the pH value of the Earth’s ocean. Between 1751 and 2021, the average pH value of the ocean surface has decreased from approximately 8.25 to 8.14. The root cause of ocean acidification is carbon dioxid ...
.
The ocean provides society with important
environmental services, including climate regulation. It also offers a means of
trade and transport and access to food and other
resources. Known to be the
habitat of over 230,000
species, it may contain far more – perhaps over two million species.
However, the ocean is subject to numerous human-caused
environmental threats, including
marine pollution,
overfishing, and
effects of climate change on oceans, such as
ocean warming,
ocean acidification
Ocean acidification is the reduction in the pH value of the Earth’s ocean. Between 1751 and 2021, the average pH value of the ocean surface has decreased from approximately 8.25 to 8.14. The root cause of ocean acidification is carbon dioxid ...
,
sea level rise and many more. The continental shelf and
coastal waters
The coast, also known as the coastline or seashore, is defined as the area where land meets the ocean, or as a line that forms the boundary between the land and the coastline. The Earth has around of coastline. Coasts are important zones in n ...
that are most influenced by human activity are especially vulnerable.
Terminology
Ocean and sea
The terms "the ocean" or "the sea" used without specification refer to the interconnected body of salt water covering the majority of the Earth's surface.
It includes the
Atlantic,
Pacific
The Pacific Ocean is the largest and deepest of Earth's five oceanic divisions. It extends from the Arctic Ocean in the north to the Southern Ocean (or, depending on definition, to Antarctica) in the south, and is bounded by the contine ...
,
Indian,
Southern and
Arctic Oceans. As a general term, "the ocean" and "the sea" are often interchangeable, although speakers of
British English
British English (BrE, en-GB, or BE) is, according to Lexico, Oxford Dictionaries, "English language, English as used in Great Britain, as distinct from that used elsewhere". More narrowly, it can refer specifically to the English language in ...
refer to "the sea" in all cases, even when the body of water is one of the oceans.
Strictly speaking, a "sea''"'' is a body of water (generally a division of the world ocean) partly or fully enclosed by land.
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 sea
This is a list of seas of the World Ocean, including marginal seas, areas of water, various gulfs, bights, bays, and straits.
Terminology
* Ocean – the four to seven largest named bodies of water in the World Ocean, all of which have "Ocea ...
s) or wholly (as
inland seas
An inland sea (also known as an epeiric sea or an epicontinental sea) is a continental body of water which is very large and is either completely surrounded by dry land or connected to an ocean by a river, strait, or "arm of the sea". An inland se ...
) bordered by land.
World ocean
The contemporary concept of the ''World Ocean'' was coined in the early 20th century by the
Russian
Russian(s) refers to anything related to Russia, including:
*Russians (, ''russkiye''), an ethnic group of the East Slavic peoples, primarily living in Russia and neighboring countries
*Rossiyane (), Russian language term for all citizens and peo ...
oceanographer
Yuly Shokalsky
Yuly Mikhailovich Shokalsky (russian: Юлий Михайлович Шокальский; October 17, 1856 in Saint Petersburg – March 26, 1940 in Leningrad) was a Russian oceanographer, cartographer, and geographer.
Career
A grandson of A ...
to refer to the continuous ocean that covers and encircles most of Earth.
The global, interconnected body of salt water is sometimes referred to as the world ocean, ''global ocean'' or ''the great ocean''.
["] The concept of a continuous body of water with relatively free interchange among its parts is of fundamental importance to
oceanography.
Etymology
The word ''ocean'' comes from the figure in
classical antiquity
Classical antiquity (also the classical era, classical period or classical age) is the period of cultural history between the 8th century BC and the 5th century AD centred on the Mediterranean Sea, comprising the interlocking civilizations of ...
,
Oceanus (; grc-gre, ''Ōkeanós'', ), the elder of the
Titans
In Greek mythology, the Titans ( grc, οἱ Τῑτᾶνες, ''hoi Tītânes'', , ''ho Tītân'') were the pre-Olympian gods. According to the ''Theogony'' of Hesiod, they were the twelve children of the primordial parents Uranus (Sky) and Gai ...
in classical
Greek mythology. Oceanus was believed by the
ancient Greeks and
Romans
Roman or Romans most often refers to:
*Rome, the capital city of Italy
* Ancient Rome, Roman civilization from 8th century BC to 5th century AD
*Roman people, the people of ancient Rome
*''Epistle to the Romans'', shortened to ''Romans'', a lette ...
to be the divine personification of an enormous
river encircling the world.
The concept of Ōkeanós has an
Indo-European connection. Greek Ōkeanós has been compared to the
Vedic epithet ā-śáyāna-, predicated of the dragon Vṛtra-, who captured the cows/rivers. Related to this notion, the Okeanos is represented with a dragon-tail on some early Greek vases.
Natural history
During
planetary formation Earth possibly had
magma ocean Magma oceans exist during periods of Earth's or any planet's Accretion (astrophysics), accretion when the planet is completely or partly molten.
In the early Solar System, magma oceans were formed by the melting of Planetesimal, planetesimals and ...
s. Subsequently
outgassing
Outgassing (sometimes called offgassing, particularly when in reference to indoor air quality) is the release of a gas that was dissolved, trapped, frozen, or absorbed in some material. Outgassing can include sublimation and evaporation (which ...
,
volcanic activity
Volcanism, vulcanism or volcanicity is the phenomenon of eruption of molten rock (magma) onto the surface of the Earth or a solid-surface planet or moon, where lava, pyroclastics, and volcanic gases erupt through a break in the surface called a ...
and
meteorite impact
An impact event is a collision between astronomical objects causing measurable effects. Impact events have physical consequences and have been found to regularly occur in planetary systems, though the most frequent involve asteroids, comets or m ...
s, according to current theories, produced an early atmosphere of
carbon dioxide,
nitrogen
Nitrogen is the chemical element with the symbol N and atomic number 7. Nitrogen is a nonmetal and the lightest member of group 15 of the periodic table, often called the pnictogens. It is a common element in the universe, estimated at se ...
and
water vapor.
The gases and with them the atmosphere are thought to have accumulated over millions of years and after Earth's surface had significantly cooled the
water vapor over time would have condensed, forming Earth's first oceans.
The early oceans might have been significantly hotter than today and appeared green due to high iron content.
Geological evidence helps constrain the time frame for liquid water existing on Earth. A sample of pillow basalt (a type of rock formed during an underwater eruption) was recovered from the
Isua Greenstone Belt and provides evidence that water existed on Earth 3.8 billion years ago.
In the
Nuvvuagittuq Greenstone Belt
The Nuvvuagittuq Greenstone Belt (NGB; Inuktitut: ) is a sequence of metamorphosed mafic to ultramafic Volcanic rock, volcanic and associated sedimentary rocks (a greenstone belt) located on the eastern shore of Hudson Bay, 40 km southeast of ...
, Quebec, Canada, rocks dated at 3.8 billion years old by one study and 4.28 billion years old by another show evidence of the presence of water at these ages.
If oceans existed earlier than this, any geological evidence either has yet to be discovered or has since been destroyed by geological processes like
crustal recycling.
However, more recently, in August 2020, researchers reported that sufficient water to fill the oceans may have always been on the
Earth
Earth is the third planet from the Sun and the only astronomical object known to harbor life. While large volumes of water can be found throughout the Solar System, only Earth sustains liquid surface water. About 71% of Earth's surfa ...
since the beginning of the planet's formation.
In this model, atmospheric
greenhouse gases kept the oceans from freezing when the newly forming Sun
had only 70% of its
current luminosity.
By ,
Earth's magnetic field was established, which helped prevent the atmosphere from being stripped away by the
solar wind
The solar wind is a stream of charged particles released from the upper atmosphere of the Sun, called the corona. This plasma mostly consists of electrons, protons and alpha particles with kinetic energy between . The composition of the sol ...
.
Since its formation the ocean has taken many conditions and shapes with many
past ocean divisions and potentially at times covering the whole globe.
During colder climatic periods, more ice caps and glaciers form, and enough of the global water supply accumulates as ice to lessen the amounts in other parts of the water cycle. The reverse is true during warm periods. During the last ice age, glaciers covered almost one-third of Earth's land mass with the result being that the oceans were about 122 m (400 ft) lower than today. During the last global "warm spell," about 125,000 years ago, the seas were about 5.5 m (18 ft) higher than they are now. About three million years ago the oceans could have been up to 50 m (165 ft) higher.
Geography
The ocean covers ~70% of the Earth, sometimes called the "blue planet"
The entire ocean, containing 97% of Earth's water, spans 70.8% of
Earth
Earth is the third planet from the Sun and the only astronomical object known to harbor life. While large volumes of water can be found throughout the Solar System, only Earth sustains liquid surface water. About 71% of Earth's surfa ...
's surface,
making it Earth's global ocean or ''world ocean''.
This makes Earth, along with its vibrant
hydrosphere a "water world"
or "
ocean world",
particularly in Earth's early history when the ocean is thought to have possibly covered Earth completely.
The ocean is shaped irregularly, dominating
Earth's surface
Earth is the third planet from the Sun and the only astronomical object known to harbor life. While large volumes of water can be found throughout the Solar System, only Earth sustains liquid surface water. About 71% of Earth's surface ...
unevenly, allowing the decernment of Earth's surface into a
water and land hemisphere, as well as the division of the ocean into particular oceans.
Oceanic divisions
The major oceanic divisions – listed below in descending order of area and volume – are so named based on nearest
continent
A continent is any of several large landmasses. Generally identified by convention rather than any strict criteria, up to seven geographical regions
In geography, regions, otherwise referred to as zones, lands or territories, are areas t ...
s, various
archipelagos, and other criteria.
Oceans are fringed with coastlines that run for
360,000 kilometres in total distance. They are also connected to smaller, adjoining bodies of water such as,
seas,
gulfs,
bays,
bights, and
straits.
Seawater covers approximately and is customarily divided into five principal oceans, as below:
Ocean ridges and ocean basins
Every
ocean basin
In hydrology, an oceanic basin (or ocean basin) is anywhere on Earth that is covered by seawater. Geologically, ocean basins are large geologic basins that are below sea level.
Most commonly the ocean is divided into basins fol ...
has a
mid-ocean ridge, which creates a long mountain range beneath the ocean. Together they form the global mid-oceanic ridge system that features the
longest mountain range in the world. The longest continuous mountain range is . This underwater mountain range is several times longer than the longest continental mountain range—the
Andes.
Oceanographers state that less than 20% of the oceans have been mapped.
Formation
The origin of Earth's oceans is unknown. Oceans are thought to have formed in the
Hadean eon and may have been the cause for the
emergence of life. Scientists believe that a sizable quantity of
water
Water (chemical formula ) is an Inorganic compound, inorganic, transparent, tasteless, odorless, and Color of water, nearly colorless chemical substance, which is the main constituent of Earth's hydrosphere and the fluids of all known living ...
would have been in the material that formed the Earth. Water molecules would have escaped Earth's gravity more easily when it was less massive during its formation. This is called
atmospheric escape.
Plate tectonics,
post-glacial rebound
Post-glacial rebound (also called isostatic rebound or crustal rebound) is the rise of land masses after the removal of the huge weight of ice sheets during the last glacial period, which had caused isostatic depression. Post-glacial rebound ...
, and
sea level rise continually change the
coastline and structure of the world ocean. A global ocean has existed in one form or another on Earth for eons.
Physical properties
Volumes
The volume of water in all the oceans together is approximately 1.335 billion cubic kilometers (1.335
sextillion liters, 320.3 million cubic miles).
Depth
The average depth of the oceans is about 4 km. More precisely the average depth is .
Nearly half of the world's marine waters are over deep.
"Deep ocean," which is anything below 200 meters (660 ft.), covers about 66% of Earth's surface.
This figure does not include seas not connected to the World Ocean, such as the
Caspian Sea.
The deepest point in the ocean is the
Mariana Trench
The Mariana Trench is an oceanic trench located in the western Pacific Ocean, about east of the Mariana Islands; it is the deepest oceanic trench on Earth. It is crescent-shaped and measures about in length and in width. The maximum known ...
, located in the Pacific Ocean near the
Northern Mariana Islands.
Its maximum depth has been estimated to be . The British naval vessel ''Challenger II'' surveyed the trench in 1951 and named the deepest part of the trench the "
Challenger Deep
The Challenger Deep is the deepest-known point of the seabed of Earth, with a depth of by direct measurement from deep-diving submersibles, remotely operated underwater vehicles and benthic landers, and (sometimes) slightly more by sonar bathym ...
". In 1960, the
Trieste successfully reached the bottom of the trench, manned by a crew of two men.
Color
Oceanic zones
Oceanographers divide the ocean into different vertical and horizontal zones defined by physical and biological conditions. The
pelagic zone consists of the
water column of the open ocean, and can be divided into further regions categorized by light abundance and by depth.
Grouped by light penetration
* The
photic zone includes the oceans from the surface to a depth of 200 m; it is the region where
photosynthesis can occur and is, therefore, the most
biodiverse
Biodiversity or biological diversity is the variety and variability of life on Earth. Biodiversity is a measure of variation at the genetic ('' genetic variability''), species ('' species diversity''), and ecosystem ('' ecosystem diversity'') ...
. Photosynthesis by plants and microscopic
algae (free floating
phytoplankton) allows the creation of organic matter from chemical precursors including water and carbon dioxide. This organic matter can then be consumed by other creatures. Much of the organic matter created in the photic zone is consumed there but some sinks into deeper waters.
* Below the photic zone is the mesopelagic or twilight zone where there is a very small amount of light. Below that is the aphotic deep ocean to which no surface sunlight at all penetrates. Life that exists deeper than the photic zone must either rely on material sinking from above (see
marine snow
In the deep ocean, marine snow (also known as "ocean dandruff") is a continuous shower of mostly organic detritus falling from the upper layers of the water column. It is a significant means of exporting energy from the light-rich photic zone to ...
) or find another energy source.
Hydrothermal vents are a source of energy in what is known as the
aphotic zone
The aphotic zone (aphotic from Greek prefix + "without light") is the portion of a lake or ocean where there is little or no sunlight. It is formally defined as the depths beyond which less than 1 percent of sunlight penetrates. Above the aphot ...
(depths exceeding 200 m). The pelagic part of the photic zone is known as the
epipelagic
The photic zone, euphotic zone, epipelagic zone, or sunlight zone is the uppermost layer of a body of water that receives sunlight, allowing phytoplankton to perform photosynthesis. It undergoes a series of physical, chemical, and biological proc ...
.
Grouped by depth and temperature
The pelagic part of the aphotic zone can be further divided into vertical regions according to depth and temperature:
* The
mesopelagic
The mesopelagic zone (Greek μέσον, middle), also known as the middle pelagic or twilight zone, is the part of the pelagic zone that lies between the photic epipelagic and the aphotic bathypelagic zones. It is defined by light, and begins at ...
is the uppermost region. Its lowermost boundary is at a
thermocline
A thermocline (also known as the thermal layer or the metalimnion in lakes) is a thin but distinct layer in a large body of fluid (e.g. water, as in an ocean or lake; or air, e.g. an atmosphere) in which temperature changes more drastically with ...
of which generally lies at in the
tropics. Next is the
bathypelagic lying between , typically between and . Lying along the top of the
abyssal plain is the
abyssopelagic
The abyssal zone or abyssopelagic zone is a layer of the pelagic zone of the ocean. "Abyss" derives from the Greek word , meaning bottomless. At depths of , this zone remains in perpetual darkness. It covers 83% of the total area of the ocean ...
, whose lower boundary lies at about . The last and deepest zone is the
hadalpelagic which includes the
oceanic trench
Oceanic trenches are prominent long, narrow topographic depressions of the ocean floor. They are typically wide and below the level of the surrounding oceanic floor, but can be thousands of kilometers in length. There are about of oceanic tren ...
and lies between .
* The
benthic zones are aphotic and correspond to the three deepest zones of the
deep-sea
The deep sea is broadly defined as the ocean depth where light begins to fade, at an approximate depth of 200 metres (656 feet) or the point of transition from continental shelves to continental slopes. Conditions within the deep sea are a combin ...
. The
bathyal zone covers the continental slope down to about . The abyssal zone covers the abyssal plains between 4,000 and 6,000 m. Lastly, the
hadal zone corresponds to the hadalpelagic zone, which is found in oceanic trenches.
Distinct boundaries between ocean surface waters and deep waters can be drawn based on the properties of the water. These boundaries are called
thermocline
A thermocline (also known as the thermal layer or the metalimnion in lakes) is a thin but distinct layer in a large body of fluid (e.g. water, as in an ocean or lake; or air, e.g. an atmosphere) in which temperature changes more drastically with ...
s (temperature),
haloclines (salinity),
chemoclines (chemistry), and
pycnocline
A pycnocline is the cline or layer where the density gradient () is greatest within a body of water. An ocean current is generated by the forces such as breaking waves, temperature and salinity differences, wind, Coriolis effect, and tides cause ...
s (density). If a zone undergoes dramatic changes in temperature with depth, it contains a
thermocline
A thermocline (also known as the thermal layer or the metalimnion in lakes) is a thin but distinct layer in a large body of fluid (e.g. water, as in an ocean or lake; or air, e.g. an atmosphere) in which temperature changes more drastically with ...
, a distinct boundary between warmer surface water and colder deep water. The tropical thermocline is typically deeper than the thermocline at higher latitudes.
Polar waters, which receive relatively little solar energy, are not
stratified by temperature and generally lack a thermocline because surface water at polar latitudes are nearly as cold as water at greater depths. Below the thermocline, water everywhere in the ocean is very cold, ranging from −1 °C to 3 °C. Because this deep and cold layer contains the bulk of ocean water, the average temperature of the world ocean is 3.9 °C. If a zone undergoes dramatic changes in salinity with depth, it contains a
halocline. If a zone undergoes a strong, vertical chemistry gradient with depth, it contains a
chemocline. Temperature and salinity control the density of ocean water, with colder and saltier water being more dense, and this density in turn regulates the global water circulation within the ocean.
The halocline often coincides with the thermocline, and the combination produces a pronounced
pycnocline
A pycnocline is the cline or layer where the density gradient () is greatest within a body of water. An ocean current is generated by the forces such as breaking waves, temperature and salinity differences, wind, Coriolis effect, and tides cause ...
, a boundary between less dense surface water and dense deep water.
Grouped by distance from land
The pelagic zone can be further subdivided into two sub regions based on distance from land: the
neritic zone
The neritic zone (or sublittoral zone) is the relatively shallow part of the ocean above the drop-off of the continental shelf, approximately in depth.
From the point of view of marine biology it forms a relatively stable and well-illuminated ...
and the
oceanic zone
The oceanic zone is typically defined as the area of the ocean lying beyond the continental shelf (such as the Neritic zone), but operationally is often referred to as beginning where the water depths drop to below 200 meters (660 feet), seaward ...
. The neritic zone encompasses the water mass directly above the
continental shelves and hence includes
coastal waters
The coast, also known as the coastline or seashore, is defined as the area where land meets the ocean, or as a line that forms the boundary between the land and the coastline. The Earth has around of coastline. Coasts are important zones in n ...
, whereas the oceanic zone includes all the completely open water.
The
littoral zone covers the region between low and high tide and represents the transitional area between marine and terrestrial conditions. It is also known as the
intertidal
The intertidal zone, also known as the foreshore, is the area above water level at low tide and underwater at high tide (in other words, the area within the tidal range). This area can include several types of habitats with various species ...
zone because it is the area where tide level affects the conditions of the region.
Temperature
Ocean temperatures 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 while near the poles the temperature in equilibrium with the
sea ice is about . 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 ocean water has a temperature between and in all parts of the globe.
Sea ice
Seawater with a typical salinity of 35‰ has a freezing point of about −1.8 °C (28.8 °F).
Because sea ice is less
dense
Density (volumetric mass density or specific mass) is the substance's mass per unit of volume. The symbol most often used for density is ''ρ'' (the lower case Greek letter rho), although the Latin letter ''D'' can also be used. Mathematically ...
than water, it floats on the ocean's surface (as does
fresh water ice, which has an even lower density). Sea ice covers about 7% of the Earth's surface and about 12% of the world's oceans.
Sea ice usually starts to freeze at the very surface, initially as a very thin ice film. As further freezing takes place, this ice film thickens and can form
ice sheet
In glaciology, an ice sheet, also known as a continental glacier, is a mass of glacial ice that covers surrounding terrain and is greater than . The only current ice sheets are in Antarctica and Greenland; during the Last Glacial Period at La ...
s. The ice formed incorporates some
sea salt
Sea salt is salt that is produced by the evaporation of seawater. It is used as a seasoning in foods, cooking, cosmetics and for preserving food. It is also called bay salt, solar salt, or simply salt. Like mined rock salt, production of sea sa ...
, but much less than the seawater it forms from. As the ice forms with low salinity this results in saltier residual seawater. This in turn increases density and promotes vertical sinking of the water.
Ocean currents and global climate
Types of ocean currents
An
ocean current is a continuous, directed movement of seawater generated by a number of forces acting upon the water, including
wind, the
Coriolis effect
In physics, the Coriolis force is an inertial or fictitious force that acts on objects in motion within a frame of reference that rotates with respect to an inertial frame. In a reference frame with clockwise rotation, the force acts to the ...
,
temperature
Temperature is a physical quantity that expresses quantitatively the perceptions of hotness and coldness. Temperature is measurement, measured with a thermometer.
Thermometers are calibrated in various Conversion of units of temperature, temp ...
and
salinity differences.
Ocean currents are primarily horizontal water movements. They have different origins, such as tides for tidal currents, or wind and waves for surface currents.
Tidal currents are in phase with the
tide, hence are
quasiperiodic
Quasiperiodicity is the property of a system that displays irregular periodicity. Periodic behavior is defined as recurring at regular intervals, such as "every 24 hours". Quasiperiodic behavior is a pattern of recurrence with a component of unpred ...
; associated with the influence of the moon and sun pull on the ocean water. Tidal currents may form various complex patterns in certain places, most notably around
headlands. Non-periodic or non-tidal currents are created by the action of winds and changes in
density of water. In
littoral zones,
breaking waves are so intense and the depth measurement so low, that maritime currents reach often 1 to 2
knots.
The
wind and
waves create surface currents (designated as "drift currents"). These currents can decompose in one quasi-permanent current (which varies within the hourly scale) and one movement of
Stokes drift
For a pure wave motion (physics), motion in fluid dynamics, the Stokes drift velocity is the average velocity when following a specific fluid parcel as it travels with the fluid flow. For instance, a particle floating at the free surface of wat ...
under the effect of rapid waves movement (which vary on timescales of a couple of seconds). The quasi-permanent current is accelerated by the breaking of waves, and in a lesser governing effect, by the friction of the wind on the surface.
This acceleration of the current takes place in the direction of waves and dominant wind. Accordingly, when the ocean depth increases, the
rotation of the
earth
Earth is the third planet from the Sun and the only astronomical object known to harbor life. While large volumes of water can be found throughout the Solar System, only Earth sustains liquid surface water. About 71% of Earth's surfa ...
changes the direction of currents in proportion with the increase of depth, while friction lowers their speed. At a certain ocean depth, the current changes direction and is seen inverted in the opposite direction with current speed becoming null: known as the
Ekman spiral. The influence of these currents is mainly experienced at the mixed layer of the ocean surface, often from 400 to 800 meters of maximum depth. These currents can considerably change and are dependent on the yearly
seasons. If the mixed layer is less thick (10 to 20 meters), the quasi-permanent current at the surface can adopt quite a different direction in relation to the direction of the wind. In this case, the water column becomes virtually homogeneous above the
thermocline
A thermocline (also known as the thermal layer or the metalimnion in lakes) is a thin but distinct layer in a large body of fluid (e.g. water, as in an ocean or lake; or air, e.g. an atmosphere) in which temperature changes more drastically with ...
.
The wind blowing on the ocean surface will set the water in motion. The global pattern of winds (also called
atmospheric circulation
Atmospheric circulation is the large-scale movement of air and together with ocean circulation is the means by which thermal energy is redistributed on the surface of the Earth. The Earth's atmospheric circulation varies from year to year, bu ...
) creates a global pattern of ocean currents. These are not only driven by the wind but also by the effect of the circulation of the earth (
coriolis force). Theses major ocean currents include the
Gulf Stream,
Kuroshio current
The , also known as the Black or or the is a north-flowing, warm ocean current on the west side of the North Pacific Ocean basin. It was named for the deep blue appearance of its waters. Similar to the Gulf Stream in the North Atlantic, the Ku ...
,
Agulhas current and
Antarctic Circumpolar Current. The Antarctic Circumpolar Current encircles
Antarctica and influences the area's climate as well as connecting currents in several oceans.
Relationship of currents and climate
Collectively, currents move enormous amounts of water and heat around the globe influencing
climate. These wind driven currents are largely confined to the top hundreds of meters of the ocean. At greater depth the drivers of water motion are the
thermohaline circulation
Thermohaline circulation (THC) is a part of the large-scale ocean circulation that is driven by global density gradients created by surface heat and freshwater fluxes. The adjective ''thermohaline'' derives from '' thermo-'' referring to temp ...
(the
Atlantic meridional overturning circulation
The Atlantic meridional overturning circulation (AMOC) is part of a global thermohaline circulation in the oceans and is the zonally integrated component of surface and deep currents in the Atlantic Ocean. It is characterized by a northward fl ...
(AMOC) is part of a global thermoholine circulation). This is driven by the cooling of surface waters at northern and southern polar latitudes creating dense water which sinks to the bottom of the ocean. This cold and dense water moves slowly away from the
poles which is why the waters in the deepest layers of the world ocean are so cold. This deep ocean water circulation is relatively slow and water at the bottom of the ocean can be isolated from the ocean surface and atmosphere for hundreds or even a few thousand years.
This circulation has important impacts on global climate and the uptake and redistribution of pollutants such as
carbon dioxide by moving these contaminants from the surface into the deep ocean.
Ocean currents greatly affect Earth's climate by
transferring heat from the
tropics to the
polar regions
The polar regions, also called the frigid zones or polar zones, of Earth are the regions of the planet that surround its geographical poles (the North and South Poles), lying within the polar circles. These high latitudes are dominated by floa ...
and thereby also affecting air temperature and precipitation in coastal regions and further inland. Surface heat and freshwater
fluxes create global
density gradients that drive the
thermohaline circulation
Thermohaline circulation (THC) is a part of the large-scale ocean circulation that is driven by global density gradients created by surface heat and freshwater fluxes. The adjective ''thermohaline'' derives from '' thermo-'' referring to temp ...
part of large-scale ocean circulation. It plays an important role in supplying heat to the polar regions, and thus in
sea ice regulation.
Oceans moderate the climate of locations where prevailing winds blow in from the ocean. At similar latitudes, a place on Earth with more influence from the ocean will have a more moderate climate than a place with more influence from land. For example, the cities San Francisco (37.8 N) and New York (40.7 N) have different climates because San Francisco has more influence from the ocean. San Francisco, on the west coast of North America, gets
winds from the west over the
Pacific Ocean, and the influence of the ocean water yields a more moderate climate with a warmer winter and a longer, cooler summer, with the warmest temperatures happening later in the year. New York, on the east coast of North America gets
winds from the west over land, so New York has colder winters and hotter, earlier summers than San Francisco.
Warmer ocean currents yield warmer climates in the long term, even at high latitudes. At similar latitudes, a place influenced by warm ocean currents will have a warmer climate overall than a place influenced by cold ocean currents. French Riviera (43.5 N) and Rockland, Maine (44.1 N) have same latitude, but the French Riviera is influenced by warm waters transported by the
Gulf Stream into the
Mediterranean Sea and has a warmer climate overall. Maine is influenced by cold waters transported south by the
Labrador Current
The Labrador Current is a cold current in the North Atlantic Ocean which flows from the Arctic Ocean south along the coast of Labrador and passes around Newfoundland, continuing south along the east coast of Canada near Nova Scotia. Near Nova Scot ...
giving it a colder climate overall.
Changes in the thermohaline circulation are thought to have significant impacts on
Earth's energy budget
Earth's energy budget accounts for the balance between the energy that Earth receives from the Sun and the energy the Earth loses back into outer space. Smaller energy sources, such as Earth's internal heat, are taken into consideration, but m ...
. Since the thermohaline circulation governs the rate at which deep waters reach the surface, it may also significantly influence
atmospheric carbon dioxide
Carbon dioxide in Earth's atmosphere is a trace gas that plays an integral part in the greenhouse effect, carbon cycle, photosynthesis and oceanic carbon cycle. It is one of several greenhouse gases in Earth's atmosphere that are contributin ...
concentrations. Modern observations,
climate simulations and paleoclimate reconstructions suggest that the
Atlantic Meridional Overturning Circulation
The Atlantic meridional overturning circulation (AMOC) is part of a global thermohaline circulation in the oceans and is the zonally integrated component of surface and deep currents in the Atlantic Ocean. It is characterized by a northward fl ...
(AMOC) has weakened since the preindustrial era. The latest climate change projections in 2021 suggest that the AMOC is likely to weaken further over the 21st century.
[IPCC, 2019]
Summary for Policymakers
In
IPCC Special Report on the Ocean and Cryosphere in a Changing Climate
.-O. Pörtner, D.C. Roberts, V. Masson-Delmotte, P. Zhai, M. Tignor, E. Poloczanska, K. Mintenbeck, A. Alegría, M. Nicolai, A. Okem, J. Petzold, B. Rama, N.M. Weyer (eds.) Cambridge University Press, Cambridge, UK and New York, NY, USA. https://doi.org/10.1017/9781009157964.001. Such a weakening could cause large changes to global climate, with the North Atlantic particularly vulnerable.
Waves and swell
The motions of the ocean surface, known as undulations or
wind waves, are the partial and alternate rising and falling of the ocean surface. The series of
mechanical waves that propagate along the interface between water and air is called
swell – a term used in
sailing,
surfing and
navigation. These motions profoundly affect ships on the surface of the ocean and the well-being of people on those ships who might suffer from
sea sickness.
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
Ripple may refer to:
Science and technology
* Capillary wave, commonly known as ripple, a wave traveling along the phase boundary of a fluid
** Ripple, more generally a disturbance, for example of spacetime in gravitational waves
* Ripple (electri ...
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
The Roaring Forties are strong westerly winds found in the Southern Hemisphere, generally between the latitudes of 40°S and 50°S. The strong west-to-east air currents are caused by the combination of air being displaced from the Equator ...
, long, organized masses of water called
swell roll across the ocean.
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
Fetch may refer to:
Books
* ''Fetch'', a 2012 book by Alan MacDonald and David Roberts
* ''The Fetch'', a 2006 book by Chris Humphreys
* ''The Fetch'', a 2009 book by Laura Whitcomb
* ''The Fetch'', a 1991 book by Robert Holdstock
* ''Fazbear ...
, 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.
Constructive interference
In physics, interference is a phenomenon in which two waves combine by adding their displacement together at every single point in space and time, to form a resultant wave of greater, lower, or the same amplitude. Constructive and destructive ...
can cause individual (unexpected)
rogue wave
Rogue waves (also known as freak waves, monster waves, episodic waves, killer waves, extreme waves, and abnormal waves) are unusually large, unpredictable, and suddenly appearing surface waves that can be extremely dangerous to ships, even to lar ...
s much higher than normal.
[Garrison, Tom (2012)]
''Essentials of Oceanography''
6th ed. pp. 204 ff. Brooks/Cole, Belmont. . Most waves are less than high
and it is not unusual for strong storms to double or triple that height. Rogue waves, however, have been documented at heights above .
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 ocean 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 around rocks and headlands (
diffraction). When the wave reaches a point where its deepest oscillations of the water contact the
ocean floor
The seabed (also known as the seafloor, sea floor, ocean floor, and ocean bottom) is the bottom of the ocean. All floors of the ocean are known as 'seabeds'.
The structure of the seabed of the global ocean is governed by plate tectonics. Most of ...
, 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
Break or Breaks or The Break may refer to:
Time off from duties
* Recess (break), time in which a group of people is temporarily dismissed from its duties
* Break (work), time off during a shift/recess
** Coffee break, a short mid-morning rest ...
", toppling over in a mass of foaming water.
This rushes in a sheet up the beach before retreating into the ocean under the influence of gravity.
Earthquakes,
volcanic eruptions
Several types of volcanic eruptions—during which lava, tephra (ash, lapilli, volcanic bombs and volcanic blocks), and assorted gases are expelled from a volcanic vent or fissure—have been distinguished by volcanologists. These are often ...
or other major geological disturbances can set off waves that can lead to
tsunamis in coastal areas which can be very dangerous.
Tides
Tides are the regular rise and fall in water level experienced by 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.
In the open ocean tidal ranges are less than 1 meter, but in coastal areas these tidal ranges increase to more than 10 meters in some areas. Some of the largest tidal ranges in the world occur in the
Bay of Fundy and
Ungava Bay in Canada, reaching up to 16 meters. Other locations with record high tidal ranges include the
Bristol Channel between England and Wales,
Cook Inlet in Alaska, and the
Río Gallegos in Argentina.
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.
Tidal force
The tidal force is a gravitational effect that stretches a body along the line towards the center of mass of another body due to a gradient (difference in strength) in gravitational field from the other body; it is responsible for diverse phenomen ...
or tide-raising force decreases rapidly with distance, so the moon has more than twice as great an effect on tides as the Sun.
When the sun, moon and Earth are all aligned (full moon and new moon), the combined effect results in the high "spring tides".
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 ocean at high tide dramatically.
Water cycle, weather and rainfall
Ocean water represents the largest body of water within the global
water cycle (oceans contain 97% of
Earth's water). Evaporation from the ocean moves water into the atmosphere to later rain back down onto land and the ocean.
Oceans have a significant effect on the
biosphere
The biosphere (from Greek βίος ''bíos'' "life" and σφαῖρα ''sphaira'' "sphere"), also known as the ecosphere (from Greek οἶκος ''oîkos'' "environment" and σφαῖρα), is the worldwide sum of all ecosystems. It can also ...
. The ocean as a whole is thought to cover approximately 90% of the Earth's
biosphere
The biosphere (from Greek βίος ''bíos'' "life" and σφαῖρα ''sphaira'' "sphere"), also known as the ecosphere (from Greek οἶκος ''oîkos'' "environment" and σφαῖρα), is the worldwide sum of all ecosystems. It can also ...
.
Oceanic
evaporation, as a phase of the water cycle, is the source of most rainfall (about 90%),
causing a global
cloud cover of 67% and a consistent oceanic cloud cover of 72%.
Ocean temperatures affect
climate and
wind patterns that affect life on land. One of the most dramatic forms of
weather occurs over the oceans:
tropical cyclones (also called "typhoons" and "hurricanes" depending upon where the system forms).
As the world's ocean is the principal component of Earth's
hydrosphere, it is integral to
life
Life is a quality that distinguishes matter that has biological processes, such as Cell signaling, signaling and self-sustaining processes, from that which does not, and is defined by the capacity for Cell growth, growth, reaction to Stimu ...
on Earth, forms part of the
carbon cycle and
water cycle, and – as a huge
heat reservoir – influences climate and weather patterns.
Chemical composition of seawater
Salinity
Salinity is a measure of the total amounts of dissolved salts in
seawater. It was originally measured via measurement of the amount of
chloride in seawater and hence termed chlorinity. It is now routinely measured by measuring
electrical conductivity of the water sample. Salinity can be calculated using the chlorinity, which is a measure of the total mass of
halogen ions (includes fluorine, chlorine, bromine, and iodine) in seawater. By international agreement, the following formula is used to determine salinity:
:Salinity (in ‰) = 1.80655 × Chlorinity (in ‰)
The average ocean water chlorinity is about 19.2‰, and, thus, the average salinity is around 34.7‰.
Salinity has a major influence on the density of seawater. A zone of rapid salinity increase with depth is called a
halocline. The temperature of maximum density of
seawater decreases as its salt content increases. Freezing temperature of water decreases with salinity, and boiling temperature of water increases with salinity. Typical seawater freezes at around −2 °C at
atmospheric pressure
Atmospheric pressure, also known as barometric pressure (after the barometer), is the pressure within the atmosphere of Earth. The standard atmosphere (symbol: atm) is a unit of pressure defined as , which is equivalent to 1013.25 millibars, ...
.
Salinity is higher in Earth's oceans where there is more
evaporation and lower where there is more
precipitation. If precipitation exceeds evaporation, as is the case in
polar and some
temperate regions
In geography, the temperate climates of Earth occur in the middle latitudes (23.5° to 66.5° N/S of Equator), which span between the tropics and the polar regions of Earth. These zones generally have wider temperature ranges throughout t ...
, salinity will be lower. If evaporation exceeds precipitation, as is sometimes the case in
tropical regions, salinity will be higher. For example, evaporation is greater than precipitation in the
Mediterranean Sea, which has an average salinity of 38‰, more saline than the global average of 34.7‰.
Thus, oceanic waters in polar regions have lower salinity content than oceanic waters tropical regions.
However, when
sea ice forms at high latitudes,
salt is excluded from the ice as it forms, which can increase the salinity in the residual seawater in polar regions such as the
Arctic Ocean.
Observations of sea surface salinity between 1950 to 2019 indicate that due to the
effects of climate change on oceans regions of high salinity and evaporation have become more saline, while regions of low salinity and more precipitation have become fresher.
[Fox-Kemper, B., H.T. Hewitt, C. Xiao, G. Aðalgeirsdóttir, S.S. Drijfhout, T.L. Edwards, N.R. Golledge, M. Hemer, R.E. Kopp, G. Krinner, A. Mix, D. Notz, S. Nowicki, I.S. Nurhati, L. Ruiz, J.-B. Sallée, A.B.A. Slangen, and Y. Yu, 2021]
Chapter 9: Ocean, Cryosphere and Sea Level Change
I
Climate Change 2021: The Physical Science Basis. Contribution of Working Group I to the Sixth Assessment Report of the Intergovernmental Panel on Climate Change
[Masson-Delmotte, V., P. Zhai, A. Pirani, S.L. Connors, C. Péan, S. Berger, N. Caud, Y. Chen, L. Goldfarb, M.I. Gomis, M. Huang, K. Leitzell, E. Lonnoy, J.B.R. Matthews, T.K. Maycock, T. Waterfield, O. Yelekçi, R. Yu, and B. Zhou (eds.)]. Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA, pp. 1211–1362, doi:10.1017/9781009157896.011. It is very likely that the Pacific and Southern Oceans have freshened while the Atlantic has become more saline.
General characteristics of ocean surface waters
The waters in different regions of the ocean have quite different temperature and salinity characteristics. This is due to differences in the local water balance (
precipitation vs.
evaporation) and the "sea to air"
temperature gradients. These characteristics can vary widely among ocean regions. The table below provides an illustration of the sort of values usually encountered.
Dissolved gases
Ocean water contains large quantities of dissolved gases, including
oxygen
Oxygen is the chemical element with the symbol O and atomic number 8. It is a member of the chalcogen group in the periodic table, a highly reactive nonmetal, and an oxidizing agent that readily forms oxides with most elements as ...
,
carbon dioxide and
nitrogen
Nitrogen is the chemical element with the symbol N and atomic number 7. Nitrogen is a nonmetal and the lightest member of group 15 of the periodic table, often called the pnictogens. It is a common element in the universe, estimated at se ...
. These dissolve into ocean water via
gas exchange at the ocean surface, with the solubility of these gases depending on the temperature and salinity of the water.
The four most abundant gases in earth’s atmosphere and oceans are nitrogen, oxygen, argon, and carbon dioxide. In the ocean by volume, the most abundant gases dissolved in seawater are carbon dioxide (including bicarbonate and carbonate ions, 14 mL/L on average), nitrogen (9 mL/L), and oxygen (5 mL/L) at equilibrium at
All gases are more
soluble
In chemistry, solubility is the ability of a substance, the solute, to form a solution with another substance, the solvent. Insolubility is the opposite property, the inability of the solute to form such a solution.
The extent of the solubi ...
– more easily dissolved – in colder water than in warmer water. For example, when salinity and pressure are held constant, oxygen concentration in water almost doubles when the temperature drops from that of a warm summer day to freezing . Similarly, carbon dioxide and nitrogen gases are more
soluble
In chemistry, solubility is the ability of a substance, the solute, to form a solution with another substance, the solvent. Insolubility is the opposite property, the inability of the solute to form such a solution.
The extent of the solubi ...
at colder temperatures, and their solubility changes with temperature at different rates.
Oxygen, photosynthesis and carbon cycling
The process of
photosynthesis in the surface ocean releases oxygen and consumes carbon dioxide. This photosynthesis in the ocean is dominated by
phytoplankton, microscopic free floating algae. After the plants grow, bacterial decomposition of the organic matter formed by photosynthesis in the ocean consumes oxygen and releases carbon dioxide. The sinking and bacterial decomposition of some organic matter in deep ocean water, at depths where the waters are out of contact with the atmosphere, leads to a reduction in oxygen concentrations and increase in carbon dioxide,
carbonate and
bicarbonate.
This
cycling of carbon dioxide in oceans is an important part of the global
carbon cycle.
The increasing
carbon dioxide concentrations in the atmosphere due to
fossil fuel combustion lead to higher concentrations in the ocean waters and
ocean acidification
Ocean acidification is the reduction in the pH value of the Earth’s ocean. Between 1751 and 2021, the average pH value of the ocean surface has decreased from approximately 8.25 to 8.14. The root cause of ocean acidification is carbon dioxid ...
.
[IUCN (2017]
THE OCEAN AND CLIMATE CHANGE
IUCN (International Union for Conservation of Nature) Issues Brief. Dissolving atmospheric carbon dioxide reacts with bicarbonate and carbonate ions in seawater to shift the chemical balance of the water, making it more acidic. The oceans represent a major sink for carbon dioxide taken up from the atmosphere by photosynthesis and by dissolution. There is also increasing attention focused on carbon dioxide uptake in coastal
marine habitats
Marine habitats are habitats that support marine life. Marine life depends in some way on the saltwater that is in the sea (the term ''marine'' comes from the Latin ''mare'', meaning sea or ocean). A habitat is an ecological or environmental ...
such as
mangrove
A mangrove is a shrub or tree that grows in coastal saline or brackish water. The term is also used for tropical coastal vegetation consisting of such species. Mangroves are taxonomically diverse, as a result of convergent evolution in severa ...
s and
saltmarshes, a process sometimes referred to as “
Blue carbon”. Attention is focused on these ecosystems because they are strong carbon sinks as well as ecologically important habitats under considerable threat from human activities and
environmental degradation.
As deep ocean water circulates throughout the globe, it contains gradually less oxygen and gradually more carbon dioxide with more time away from the air at the surface. This gradual decrease in oxygen concentration happens as sinking organic matter continuously gets decomposed during the time the water is out of contact with the atmosphere.
Most of the deep waters of the ocean still contain relatively high concentrations of oxygen sufficient for most animals to survive. However, some ocean areas have very low oxygen due to long periods of isolation of the water from the atmosphere. These oxygen deficient areas, called
oxygen minimum zone
The oxygen minimum zone (OMZ), sometimes referred to as the shadow zone, is the zone in which oxygen saturation in seawater in the ocean is at its lowest. This zone occurs at depths of about , depending on local circumstances. OMZs are found worl ...
s or
hypoxic waters, could be made worse by the
effects of climate change on oceans.
Residence times of chemical elements and ions
The ocean waters contain many
chemical elements as dissolved ions. Elements dissolved in ocean waters have a wide range of concentrations. Some elements have very high concentrations of several grams per liter, such as
sodium and
chloride, together making up the majority of ocean salts. Other elements, such as
iron
Iron () is a chemical element with Symbol (chemistry), symbol Fe (from la, Wikt:ferrum, ferrum) and atomic number 26. It is a metal that belongs to the first transition series and group 8 element, group 8 of the periodic table. It is, Abundanc ...
, are present at tiny concentrations of just a few nanograms (10
−9 grams) per liter.
The concentration of any element depends on its rate of supply to the ocean and its rate of removal. Elements enter the ocean from rivers, the atmosphere and
hydrothermal vents. Elements are removed from ocean water by sinking and becoming buried in
sediment
Sediment is a naturally occurring material that is broken down by processes of weathering and erosion, and is subsequently transported by the action of wind, water, or ice or by the force of gravity acting on the particles. For example, sa ...
s or evaporating to the
atmosphere in the case of water and some gases. Oceanographers consider the balance of input and removal by estimating the
residence time of an element. Residence time is the average time the element would spend dissolved in the ocean before it is removed. Very abundant elements in ocean water like sodium have high rates of input, reflecting high abundance in rocks and relatively rapid rock weathering, coupled to very slow removal from the ocean because sodium ions are rather unreactive and very soluble. In contrast, other elements such as iron and
aluminium are abundant in rocks but very insoluble, meaning that inputs to the ocean are low and removal is rapid. These cycles represent part of the major global cycle of elements that has gone on since the Earth first formed. The residence times of the very abundant elements in the ocean are estimated to be millions of years, while for highly reactive and insoluble elements, residence times are only hundreds of years.
Nutrients
A few elements such as
nitrogen
Nitrogen is the chemical element with the symbol N and atomic number 7. Nitrogen is a nonmetal and the lightest member of group 15 of the periodic table, often called the pnictogens. It is a common element in the universe, estimated at se ...
,
phosphorus,
iron
Iron () is a chemical element with Symbol (chemistry), symbol Fe (from la, Wikt:ferrum, ferrum) and atomic number 26. It is a metal that belongs to the first transition series and group 8 element, group 8 of the periodic table. It is, Abundanc ...
, and
potassium are essential for life, are major components of biological material, and are commonly called “
nutrients”. Nitrate and phosphate have ocean
residence times of 10,000 and 69,000 years, respectively, while potassium is a much more abundant ion in the ocean with a residence time of 12 million years. The biological cycling of these elements means that this represents a continuous removal process from the ocean's water column as degrading organic material sinks to the ocean floor as
sediment
Sediment is a naturally occurring material that is broken down by processes of weathering and erosion, and is subsequently transported by the action of wind, water, or ice or by the force of gravity acting on the particles. For example, sa ...
.
Phosphate from
intensive agriculture and
untreated sewage is transported via runoff to rivers and coastal zones to the ocean where it is metabolized. Eventually, it sinks to the ocean floor and is no longer available to humans as a commercial resource. Production of
rock phosphate, an essential ingredient in inorganic
fertilizer is a slow geological process occurring in some of the world's ocean sediments thus making minable sedimentary
apatite (phosphate) in effect a
non-renewable resource (see
peak phosphorus). This continuous net deposition loss of non-renewable phosphate from human activities may become a resource problem in the future for fertilizer production and
food security.
Climate change
Marine life
Life within the ocean
evolved 3 billion years prior to life on land. Both the depth and the distance from shore strongly influence the
biodiversity
Biodiversity or biological diversity is the variety and variability of life on Earth. Biodiversity is a measure of variation at the genetic (''genetic variability''), species (''species diversity''), and ecosystem (''ecosystem diversity'') l ...
of the plants and animals present in each region.
The diversity of life in the ocean is immense, including:
*
Animals
Animals are multicellular, eukaryotic organisms in the biological kingdom Animalia. With few exceptions, animals consume organic material, breathe oxygen, are able to move, can reproduce sexually, and go through an ontogenetic stage in ...
: most animal
phyla have species that inhabit the ocean, including many that are only found in marine environments such as
sponges,
Cnidaria (such as
corals and
jellyfish),
comb jellies
Ctenophora (; ctenophore ; ) comprise a phylum of marine invertebrates, commonly known as comb jellies, that inhabit sea waters worldwide. They are notable for the groups of cilia they use for swimming (commonly referred to as "combs"), and ...
,
Brachiopod
Brachiopods (), phylum Brachiopoda, are a phylum of trochozoan animals that have hard "valves" (shells) on the upper and lower surfaces, unlike the left and right arrangement in bivalve molluscs. Brachiopod valves are hinged at the rear end, w ...
s, and
Echinoderms (such as
sea urchins and
sea star
Starfish or sea stars are star-shaped echinoderms belonging to the class Asteroidea (). Common usage frequently finds these names being also applied to ophiuroids, which are correctly referred to as brittle stars or basket stars. Starfish a ...
s). Many other familiar animal groups primarily live in the ocean, including
cephalopods (includes
octopus and
squid),
crustaceans (includes
lobsters,
crabs, and
shrimp),
fish,
sharks
Sharks are a group of elasmobranch fish characterized by a cartilaginous skeleton, five to seven gill slits on the sides of the head, and pectoral fins that are not fused to the head. Modern sharks are classified within the clade Selachimorp ...
,
cetaceans (includes
whales,
dolphins, and
porpoises). In addition, many land animals have adapted to living a major part of their life on the oceans. For instance,
seabirds
Seabirds (also known as marine birds) are birds that are adapted to life within the marine environment. While seabirds vary greatly in lifestyle, behaviour and physiology, they often exhibit striking convergent evolution, as the same envi ...
are a diverse group of birds that have adapted to a life mainly on the oceans. They feed on marine animals and spend most of their lifetime on water, many only going on land for breeding. Other birds that have adapted to oceans as their living space are
penguins,
seagulls and
pelicans
Pelicans (genus ''Pelecanus'') are a genus of large water birds that make up the family Pelecanidae. They are characterized by a long beak and a large throat pouch used for catching prey and draining water from the scooped-up contents before s ...
. Seven species of turtles, the
sea turtles, also spend most of their time in the oceans.
*
Plants
Plants are predominantly photosynthetic eukaryotes of the kingdom Plantae. Historically, the plant kingdom encompassed all living things that were not animals, and included algae and fungi; however, all current definitions of Plantae exclude ...
: including
sea grasses, or
mangroves
*
Algae: algae is a "catch-all" term to include many
photosynthetic,
single-celled eukaryotes, such as
green algae,
diatoms, and
dinoflagellates, but also multicellular algae, such as some
red algae (including organisms like
Pyropia
''Pyropia'' is a genus of red algae eaweedin the family Bangiaceae. It is found around the world in intertidal zones and shallow water. The genus has folding frond-like blades which are either red, brown or green. Some ''Pyropia'' species are u ...
, which is the source of the edible
nori
Nori is a dried edible seaweed used in Japanese cuisine, made from species of the red algae genus '' Pyropia'', including ''P. yezonesis'' and '' P. tenera''. It has a strong and distinctive flavor, and is often used to wrap rolls of sushi or ...
seaweed), and
brown algae
Brown algae (singular: alga), comprising the class Phaeophyceae, are a large group of multicellular algae, including many seaweeds located in colder waters within the Northern Hemisphere. Brown algae are the major seaweeds of the temperate and p ...
(including organisms like
kelp
Kelps are large brown algae seaweeds that make up the order Laminariales. There are about 30 different genera. Despite its appearance, kelp is not a plant - it is a heterokont, a completely unrelated group of organisms.
Kelp grows in "underwa ...
).
*
Bacteria
Bacteria (; singular: bacterium) are ubiquitous, mostly free-living organisms often consisting of one Cell (biology), biological cell. They constitute a large domain (biology), domain of prokaryotic microorganisms. Typically a few micrometr ...
: ubiquitous single-celled
prokaryotes found throughout the world
*
Archaea:
prokaryotes distinct from bacteria, that inhabit many environments of the ocean, as well as many
extreme environments
An extreme environment is a habitat that is considered very hard to survive in due to its considerably extreme conditions such as temperature, accessibility to different energy sources or under high pressure. For an area to be considered an extrem ...
*
Fungi: many
marine fungi
Marine fungi are species of fungi that live in marine or estuarine environments. They are not a taxonomic group, but share a common habitat. Obligate marine fungi grow exclusively in the marine habitat while wholly or sporadically submerged in ...
with diverse roles are found in oceanic environments
Human uses of the oceans
The ocean has been linked to human activity throughout history. These activities serve a wide variety of purposes, including
navigation and exploration,
naval warfare, travel,
shipping and
trade, food production (e.g.
fishing,
whaling,
seaweed farming
Seaweed farming or kelp farming is the practice of cultivating and harvesting seaweed. In its simplest form, it consists of the management of naturally found batches. In its most advanced form, it consists of fully controlling the life cycle ...
,
aquaculture), leisure (
cruising,
sailing,
recreational boat fishing,
scuba diving), power generation (see
marine energy
Marine energy or marine power (also sometimes referred to as ocean energy, ocean power, or marine and hydrokinetic energy) refers to the energy carried by ocean waves, tides, salinity, and ocean temperature differences. The movement of water in ...
and
offshore wind power
Offshore wind power or offshore wind energy is the generation of electricity through wind farms in bodies of water, usually at sea. There are higher wind speeds offshore than on land, so offshore farms generate more electricity per amount of c ...
), extractive industries (
offshore drilling and
deep sea mining
Deep sea mining is a growing subfield of experimental seabed mining that involves the retrieval of minerals and deposits from the ocean floor found at depths of or greater. As of 2021, the majority of marine mining efforts are limited to shal ...
),
freshwater production via
desalination.
Many of the world's goods are moved by
ship between the world's
seaports. Large quantities of goods are transported across the ocean, especially across the Atlantic and around the Pacific Rim. A lot of cargo, such as manufactured goods, is usually transported within
standard sized, lockable containers, loaded on purpose-built
container ships at
dedicated terminals.
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
Globalization, or globalisation (English in the Commonwealth of Nations, Commonwealth English; American and British English spelling differences#-ise, -ize (-isation, -ization), see spelling differences), is the process of foreign relation ...
and exponential increases in
international trade in the mid-to-late 20th century.
Oceans are also the major supply source for the
fishing industry. Some of the major harvests are
shrimp,
fish,
crabs
Crabs are decapod crustaceans of the infraorder Brachyura, which typically have a very short projecting "tail" (abdomen) ( el, βραχύς , translit=brachys = short, / = tail), usually hidden entirely under the thorax. They live in all the ...
, and
lobster.
The biggest commercial fishery globally is for
anchovies
An anchovy is a small, common forage fish of the family Engraulidae. Most species are found in marine waters, but several will enter brackish water, and some in South America are restricted to fresh water.
More than 140 species are placed in 1 ...
,
Alaska pollock
The Alaska pollock or walleye pollock (''Gadus chalcogrammus'') is a marine fish species of the cod genus '' Gadus'' and family Gadidae.
It is a semi-pelagic schooling fish widely distributed in the North Pacific, with largest concentrations ...
and
tuna.
A report by
FAO
The Food and Agriculture Organization of the United Nations (FAO)french: link=no, Organisation des Nations unies pour l'alimentation et l'agriculture; it, Organizzazione delle Nazioni Unite per l'Alimentazione e l'Agricoltura is an intern ...
in 2020 stated that "in 2017, 34 percent of the fish stocks of the world’s marine fisheries were classified as
overfished".
Fish and other fishery products from both
wild fisheries
A wild fishery is a natural body of water with a sizeable free-ranging fish or other aquatic animal (crustaceans and molluscs) population that can be harvested for its commercial value. Wild fisheries can be marine ( saltwater) or lacustrine/r ...
and aquaculture are among the most widely consumed sources of protein and other essential nutrients. Data in 2017 showed that "fish consumption accounted for 17 percent of the global population’s intake of animal proteins".
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.
The ocean offers a very large supply of
energy carried by
ocean wave
In fluid dynamics, a wind wave, water wave, or wind-generated water wave, is a surface wave that occurs on the free surface of bodies of water as a result from the wind blowing over the water surface. The contact distance in the direction of t ...
s,
tides,
salinity differences, and
ocean temperature differences which can be harnessed to
generate electricity.
Forms of
sustainable marine energy include
tidal power,
ocean thermal energy and
wave power
Wave power is the capture of energy of wind waves to do useful work – for example, electricity generation, water desalination, or pumping water. A machine that exploits wave power is a wave energy converter (WEC).
Waves are generated by win ...
.
Offshore wind power
Offshore wind power or offshore wind energy is the generation of electricity through wind farms in bodies of water, usually at sea. There are higher wind speeds offshore than on land, so offshore farms generate more electricity per amount of c ...
is captured by
wind turbines placed out on the ocean; it has the advantage that wind speeds are higher than on land, though wind farms are more costly to construct offshore. There are large deposits of
petroleum, as oil and
natural gas, in rocks beneath the ocean floor.
Offshore platforms
Offshore may refer to:
Science and technology
* Offshore (hydrocarbons)
* Offshore construction, construction out at sea
* Offshore drilling, discovery and development of oil and gas resources which lie underwater through drilling a well
* Of ...
and
drilling rig
A drilling rig is an integrated system that drills wells, such as oil or water wells, or holes for piling and other construction purposes, into the earth's subsurface. Drilling rigs can be massive structures housing equipment used to drill wa ...
s
extract the oil or gas and store it for transport to land.
"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
The terms international waters or transboundary waters apply where any of the following types of bodies of water (or their drainage basins) transcend international boundaries: oceans, large marine ecosystems, enclosed or semi-enclosed region ...
.
Today, this concept is enshrined in the
United Nations Convention on the Law of the Sea (UNCLOS).
There are two major international legal organizations that are involved in
ocean governance on a global scale, namely the
International Maritime Organization and the
United Nations
The United Nations (UN) is an intergovernmental organization whose stated purposes are to maintain international peace and security, develop friendly relations among nations, achieve international cooperation, and be a centre for harmoniz ...
. The International Maritime Organization (IMO), which was ratified in 1958 is responsible mainly for
maritime safety
Maritime safety as part of and overlapping with water safety is concerned with the protection of life (search and rescue) and property through regulation, management and technology development of all forms of waterborne transportation. The execut ...
, liability and compensation and they have held some conventions on marine pollution related to shipping incidents. Ocean governance is the conduct of the policy, actions and affairs regarding the world's
oceans.
Threats from human activities
Human activities affect
marine life and
marine habitat
Marine habitats are habitats that support marine life. Marine life depends in some way on the saltwater that is in the sea (the term ''marine'' comes from the Latin ''mare'', meaning sea or ocean). A habitat is an ecological or environmental ...
s through many negative influences, such as
marine pollution (including
marine debris and microplastics)
overfishing,
ocean acidification
Ocean acidification is the reduction in the pH value of the Earth’s ocean. Between 1751 and 2021, the average pH value of the ocean surface has decreased from approximately 8.25 to 8.14. The root cause of ocean acidification is carbon dioxid ...
and other
effects of climate change on oceans.
Marine pollution
Plastic pollution
Overfishing
Protection
Protecting Earth's oceans ecosystem/s against its recognized
threats
A threat is a communication of intent to inflict harm or loss on another person. Intimidation is a tactic used between conflicting parties to make the other timid or psychologically insecure for coercion or control. The act of intimidation for co ...
is a major component of
environmental protection and is closely related to
sustainable development. One of its main techniques is the creation and enforcement of
marine protected areas (MPAs). Other techniques may include
standardized
Standardization or standardisation is the process of implementing and developing technical standards based on the consensus of different parties that include firms, users, interest groups, standards organizations and governments. Standardization ...
product certification
Product certification or product qualification is the process of certifying that a certain product has passed performance tests and quality assurance tests, and meets qualification criteria stipulated in contracts, regulations, or specifications ...
s, supply chain transparency requirements policies, policies to prevent marine pollution,
eco-tariffs,
research and development, ecosystem-assistance (e.g.
for coral reefs), support for
sustainable seafood (e.g.
sustainable fishing practices and types of aquaculture), banning and systematically obstructing (e.g. via higher costs policies) unsustainable ocean use and associated industries (e.g.
cruise ship travel,
certain shipping practices),
monitoring, revising
waste management of plastics and
fashion industry pollutants, protection of marine resources and components whose extraction or disturbance would cause substantial harm, engagement of broader publics and impacted communities, novel
decision-making mechanisms, and the development of ocean clean-up projects. Ocean protection serves to i.a. protect human health and to safeguard stable conditions of this natural ecosystem upon which humans depend.
It may be necessary to consider marine protection within a national, regional and international context. Marine protection could also have synergistic effects – for instance, according to a study, a global network of MPAs designed to improve fisheries productivity could substantially increase future catch.
In 2021, 43 expert scientists published the first scientific framework version that – via integration,
review
A review is an evaluation of a publication, product, service, or company or a critical take on current affairs in literature, politics or culture. In addition to a critical evaluation, the review's author may assign the work a rating to indi ...
, clarifications and
standardization – enables the evaluation of levels of protection of
marine protected areas and can serve as a guide for any subsequent efforts to improve, plan and monitor marine protection quality and extents. Examples are the efforts towards the 30%-protection-goal of the "Global Deal For Nature" and the UN's
Sustainable Development Goal 14 ("life below water").
Extraterrestrial oceans
Extraterrestrial oceans may be composed of water or other
elements and
compounds. The only confirmed large stable bodies of extraterrestrial surface
liquids are the
lakes of Titan
Lakes of ethane and methane on Titan, Saturn's largest moon, have been detected by the ''Cassini–Huygens'' space probe, and had been suspected long before. The large ones are known as maria (seas) and the small ones as lacūs (lakes).
History
...
, which are made of hydrocarbons instead of water. However, there is strong evidence for subsurface water oceans' existence elsewhere in the
Solar System. The best-established candidates for subsurface water oceans in the Solar System are Jupiter's moons
Europa,
Ganymede, and
Callisto Callisto most commonly refers to:
*Callisto (mythology), a nymph
*Callisto (moon), a moon of Jupiter
Callisto may also refer to:
Art and entertainment
*''Callisto series'', a sequence of novels by Lin Carter
*''Callisto'', a novel by Torsten Kro ...
; and Saturn's moons
Enceladus
Enceladus is the sixth-largest moon of Saturn (19th largest in the Solar System). It is about in diameter, about a tenth of that of Saturn's largest moon, Titan. Enceladus is mostly covered by fresh, clean ice, making it one of the most refle ...
and
Titan.
Although Earth is the only known
planet with large stable bodies of liquid water on its surface and the only one in the
Solar System, other celestial bodies are thought to have large oceans.
In June 2020,
NASA scientists reported that it is likely that
exoplanets with oceans may be common in the
Milky Way galaxy
The Milky Way is the galaxy that includes our Solar System, with the name describing the galaxy's appearance from Earth: a hazy band of light seen in the night sky formed from stars that cannot be individually distinguished by the naked eye. ...
, based on
mathematical modeling studies.
Supercritical fluid on gas giants
The inner structure of
gas giants
A gas giant is a giant planet composed mainly of hydrogen and helium. Gas giants are also called failed stars because they contain the same basic elements as a star. Jupiter and Saturn are the gas giants of the Solar System. The term "gas giant" ...
remain poorly understood. Scientists suspect that, under extreme pressure,
hydrogen
Hydrogen is the chemical element with the symbol H and atomic number 1. Hydrogen is the lightest element. At standard conditions hydrogen is a gas of diatomic molecules having the formula . It is colorless, odorless, tasteless, non-toxic ...
would act as a
supercritical fluid, hence the likelihood of oceans of liquid hydrogen deep in the interior of gas giants like
Jupiter.
Oceans of liquid
carbon have been hypothesized to exist on
ice giants
An ice giant is a giant planet composed mainly of elements heavier than hydrogen and helium, such as oxygen, carbon, nitrogen, and sulfur. There are two ice giants in the Solar System: Uranus and Neptune.
In astrophysics and planetary science th ...
, notably
Neptune and
Uranus.
See also
*
European Atlas of the Seas
*
Land and water hemispheres
The land hemisphere and water hemisphere are the hemispheres of Earth containing the largest possible total areas of land and ocean, respectively. By definition (assuming that the entire surface can be classed as either "land" or "ocean"), the t ...
*
List of seas
* Marine heatwave
* World Ocean Atlas
* World Oceans Day
References
External links
FAO (Food and Agriculture Organization of the United Nations) Fisheries DivisionNOAA – National Oceanic and Atmospheric Administration (United States)United Nations Decade of Ocean Science for Sustainable Development (2021–2030)
{{Authority control
Oceans,
Oceanography, Oceans
Coastal and oceanic landforms
Bodies of water
Articles containing video clips