The ocean (also the sea or the world ocean) is the body of salt water which covers approximately 71% of the surface of the Earth and contains 97% of Earth's water. Another definition is "any of the large bodies of water into which the great ocean is divided".  Separate names are used to identify five different areas of the ocean: Pacific (the largest) Atlantic, Indian, Southern (Antarctic), and Arctic (the smallest).   Seawater covers approximately 361,000,000 km2 (139,000,000 sq mi) of the planet. The ocean is the principal component of Earth's hydrosphere, and therefore integral to life 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 200 m, where photosynthesis can occur. This makes the photic zone the most biodiverse. Photosynthesis by plants and microscopic algae (free floating phytoplankton) creates organic matter from chemical precursors like water and carbon dioxide. This upper sunlit zone is the origin of the food supply which sustains most of the ocean ecosystem. 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 30 °C (86 °F). Near the poles where sea ice forms, the temperature in equilibrium is about −2 °C (28 °F). Deep seawater temperature is between −2 °C (28 °F) and 5 °C (41 °F) 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 (wind), the Coriolis effect 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, 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, carbon dioxide and nitrogen. This g as 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.  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 230,000 species, it may contain far more--perhaps over two million species.  However, the ocean is subject to numerous environmental threats, including marine pollution, overfishing, ocean acidification and other effects of climate change. The continental shelf and coastal waters that are most influenced by human activity are especially vulnerable.
The phrases "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, Indian, Southern and Arctic Oceans.  As a general term, "the ocean" is mostly interchangeable with "the sea" in American English, but not in British English.  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 seas) or wholly (as inland seas) bordered by land. 
The global, interconnected body of salt water is sometimes referred to as the "World Ocean" or global ocean.   The concept of a continuous body of water with relatively free interchange among its parts is of fundamental importance to oceanography.  The contemporary concept of the World Ocean was coined in the early 20th century by the Russian oceanographer Yuly Shokalsky to refer to the continuous ocean that covers and encircles most of Earth. 
The word ocean comes from the figure in classical antiquity, Oceanus ( //; Greek: Ὠκεανός Ōkeanós,  pronounced [ɔːkeanós]), the elder of the Titans in classical Greek mythology, believed by the ancient Greeks and Romans 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. 
The major oceanic divisions – listed below in descending order of area and volume – are defined in part by the continents, various archipelagos, and other criteria.    Oceans are fringed with coastlines that run for 360,000 kilometres.   They are also connected to smaller, adjoining bodies of water such as, seas, gulfs, bays, bights, and straits. Seawater covers approximately 361,000,000 km2 (139,000,000 sq mi) and is customarily divided into five principal oceans and smaller seas.
|1||Pacific Ocean||Extends between Asia and Australasia and the Americas ||168,723,000
|2||Atlantic Ocean||Extends between the Americas and Europe and Africa ||85,133,000
|3||Indian Ocean||Extends between southern Asia, Africa and Australia ||70,560,000
|4||Southern Ocean||Encircles Antarctica. Sometimes considered an extension of the Pacific, Atlantic and Indian Oceans,  ||21,960,000
|5||Arctic Ocean||Borders northern North America and Eurasia and covers much of the Arctic. Sometimes considered a sea or estuary of the Atlantic.   ||15,558,000
Sources: Encyclopedia of Earth,      International Hydrographic Organization,  Regional Oceanography: an Introduction (Tomczak, 2005),  Encyclopædia Britannica,  and the International Telecommunication Union. 
Ocean ridges and ocean basins
The mid-ocean ridges of the world are connected and form a single global mid-oceanic ridge system that is part of every ocean basin and the longest mountain range in the world. The continuous mountain range is 65,000 km (40,000 mi) long (several times longer than the Andes, the longest continental mountain range). 
The origin of Earth's oceans is unknown; a sizable quantity of water 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 due to atmospheric escape. Oceans are thought to have formed in the Hadean eon and may have been the cause for the emergence of life.
Plate tectonics, post-glacial rebound, and sea level rise continually change the coastline and structure of the world ocean. That said a global ocean has existed in one form or another on Earth for eons.
It has been estimated that there are 1.36 billion cubic kilometers (332 million cubic miles) of water on Earth.  This includes water in liquid and frozen forms in groundwater, oceans, lakes and streams. Saltwater accounts for 97.5% of this amount, whereas fresh water accounts for only 2.5%. Of this fresh water, 68.9% is in the form of ice and permanent snow cover in the Arctic, the Antarctic and mountain glaciers; 30.8% is in the form of fresh groundwater; and only 0.3% of the fresh water on Earth is in easily accessible lakes, reservoirs and river systems. The total mass of Earth's hydrosphere is about 1.4 × 1018 tonnes, which is about 0.023% of Earth's total mass. At any given time, about 20 × 1012 tonnes of this is in the form of water vapor in the Earth's atmosphere (for practical purposes, 1 cubic meter of water weighs one tonne). Approximately 71% of Earth's surface, an area of some 361 million square kilometers (139.5 million square miles), is covered by ocean. The average salinity of Earth's oceans is about 35 grams of salt per kilogram of sea water (3.5%). 
The average depth of the oceans is about 4 km - or 3,688 meters (12,100 ft) to be precise.  Nearly half of the world's marine waters are over 3,000 meters (9,800 ft) deep.  The vast expanses of deep ocean (anything below 200 meters or 660 feet) cover about 66% of Earth's surface.  This 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, located in the Pacific Ocean near the Northern Mariana Islands.  Its maximum depth has been estimated to be 10,971 meters (35,994 ft). The British naval vessel Challenger II surveyed the trench in 1951 and named the deepest part of the trench the " Challenger Deep". In 1960, the Trieste successfully reached the bottom of the trench, manned by a crew of two men.
The bluish ocean color is a result of several factors including the preferential absorption of red light by water, meaning that blue light is reflected back into the atmosphere. Red light is absorbed most strongly; therefore it penetrates the least in the ocean (to less than 50 m generally), while blue light is absorbed less well and can penetrate to up to 200 m. 
Prominent additional contributors to ocean color include dissolved organic matter and chlorophyll.  These latter aspects of ocean color can be measured by satellite observations and the assessment of chlorophyll provides a measure of ocean productivity ( marine primary productivity) in surface waters. In long term composite images, regions with high ocean productivity show up in yellow and green colors, and low productivity ones in blue.
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. Photosynthesis by plants and microscopic algae (free floating phytoplankton) allows them to create 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) or find another energy source. Hydrothermal vents are a source of energy in what is known as the aphotic zone (depths exceeding 200 m). The pelagic part of the photic zone is known as the epipelagic. 
Grouped according to depth and temperature
The pelagic part of the aphotic zone can be further divided into vertical regions according to depth and temperature: 
- The mesopelagic is the uppermost region. Its lowermost boundary is at a thermocline of 12 °C (54 °F), which, in the tropics generally lies at 700–1,000 meters (2,300–3,300 ft). Next is the bathypelagic lying between 10 and 4 °C (50 and 39 °F), typically between 700–1,000 meters (2,300–3,300 ft) and 2,000–4,000 meters (6,600–13,100 ft), lying along the top of the abyssal plain is the abyssopelagic, whose lower boundary lies at about 6,000 meters (20,000 ft). The last zone includes the deep oceanic trench, and is known as the hadalpelagic. This lies between 6,000–11,000 meters (20,000–36,000 ft) and is the deepest oceanic zone.
- The benthic zones are aphotic and correspond to the three deepest zones of the deep-sea. The bathyal zone covers the continental slope down to about 4,000 meters (13,000 ft). 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.
If a zone undergoes dramatic changes in temperature with depth, it contains a thermocline. 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.
Grouped according to distance from land
The pelagic zone can be further subdivided into two sub regions based on distance from land: the neritic zone and the oceanic zone. The neritic zone encompasses the water mass directly above the continental shelves and hence includes coastal waters, 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 zone because it is the area where tide level affects the conditions of the region. 
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 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. 
Seawater with a typical salinity of 35‰ has a freezing point of about −1.8°C (28.8°F).  When its temperature becomes low enough, ice crystals form on the surface. These break into small pieces and coalesce into flat discs that form a thick suspension known as frazil. In calm conditions this freezes into a thin flat sheet known as nilas, which thickens as new ice forms on its underside. In more turbulent seas, frazil crystals join into flat discs known as pancakes. These slide under each other and coalesce to form floes. In the process of freezing, salt water and air are trapped between the ice crystals. Nilas may have a salinity of 12–15‰, but by the time the sea ice is one year old, this falls to 4–6‰. 
Ocean warming accounts for 90% of the energy accumulation from climate change between 1971 and 2010.  About one third of that extra heat has been estimated to propagate to depth below 700 meters. 
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, temperature 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; 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 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 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. 
The wind blowing on the ocean surface will set the water in motion. The global pattern of winds (also called atmospheric circulation) 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, 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 thermoahline 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 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 part of large-scale ocean circulation. It plays an important role in supplying heat to the polar regions, and thus in sea ice regulation.
Changes in the thermohaline circulation are thought to have significant impacts on Earth's energy budget. Since the thermohaline circulation governs the rate at which deep waters reach the surface, it may also significantly influence atmospheric carbon dioxide concentrations. However, climate change might result in the shutdown of thermohaline circulation in the future. This would in turn trigger cooling in the North Atlantic, Europe, and North America. 
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 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, organized masses of water called swell roll across the ocean. (pp83–84)   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. 
Constructive interference can cause individual (unexpected) rogue waves much higher than normal.  Most waves are less than 3 m (10 ft) high  and it is not unusual for strong storms to double or triple that height.  Rogue waves, however, have been documented at heights above 25 meters (82 ft).  
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, 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.  This rushes in a sheet up the beach before retreating into the ocean under the influence of gravity. 
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 m but in coastal areas these tidal ranges increase to more than 10 m in some areas. 
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 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), with evaporation from the ocean moving water into the atmosphere to later rain back down onto land and the ocean.  Oceans have a significant effect on the biosphere. The ocean as a whole is thought to cover approximately 90% of the Earth's biosphere.  Oceanic evaporation, as a phase of the water cycle, is the source of most rainfall (about 90%).  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 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 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.  If precipitation exceeds evaporation, as is the case in polar and temperate regions, salinity will be lower. If evaporation exceeds precipitation, as is sometimes the case in tropical regions, salinity will be higher. Thus, oceanic waters in polar regions have lower salinity content than oceanic waters in temperate and tropical regions.  However, the formation of sea ice at high latitudes excludes salt from the ice and thereby increases salinity in the residual seawater in some polar regions. 
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 a lot within ocean regions but the table below provides an illustration of the sort of values usually encountered.
|Characteristic||Polar regions||Temperate regions||Tropical regions|
|Precipitation vs. evaporation||P > E||P > E||E > P|
|Sea surface temperature in winter||−2 °C||5 to 20 °C||20 to 25 °C|
|Average salinity||28‰ to 32‰||35‰||35‰ to 37‰|
|Annual variation of air temperature||≤ 40 °C||10 °C||< 5 °C|
|Annual variation of water temperature||< 5 °C||10 °C||< 5 °C|
Oxygen, carbon dioxide, other gases and the carbon cycle
Ocean water contains large quantities of dissolved gases, including oxygen, carbon dioxide and nitrogen. 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 increasing carbon dioxide concentrations in the atmosphere due to fossil fuel combustion lead to higher concentrations in the ocean waters and ocean acidification.  The dissolving atmospheric carbon dioxide then reacts with bicarbonate and carbonate ions in seawater.
The process of photosynthesis in the surface ocean also consumes some carbon dioxide and releases oxygen which may then return to the atmosphere. This photosynthesis in the ocean is dominated by microscopic phytoplankton, a type of free floating algae. The subsequent bacterial decomposition of 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 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 such as mangroves and saltmarshes, a process sometimes referred to as “ Blue carbon”. The attention on these habitats is because these are strong carbon sinks and also habitats under considerable threat from human activities and environmental degradation.
This decrease in oxygen concentration increases with the amount of sinking organic matter and 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, there are some ocean areas with water with very low oxygen due to long periods of isolation from the atmosphere, and this oxygen deficiency could be made worse by climate change. 
|Gas||Concentration of seawater, by mass (in parts per million), for the whole ocean||% dissolved gas, by volume, in seawater at the ocean surface|
|Carbon dioxide (CO2)||64 to 107||15%|
|Nitrogen (N2)||10 to 18||48%|
|Oxygen (O2)||0 to 13||36%|
Residence times of chemical elements and ions
The ocean waters contain all of the chemical elements as dissolved ions, but the concentration in which they occur range from some with very high concentrations of several grammes per liter, such as sodium and chloride, to others, such as iron, with tiny concentration of a few ng (10−9) g/l. The concentration of any element depends on its rate of supply to the ocean from rivers, the atmosphere and hydrothermal vents, and the rate of its removal. Hence very abundant elements in ocean water like sodium, have quite high rates of input, reflecting high abundance in rocks and relatively rapid weathering, coupled to very slow removal from the ocean because sodium ions are rather unreactive and very soluble. By contrast some 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. Oceanographers consider the balance of input and removal by estimating the residence time of an element as the average time the element would spend dissolved in the ocean before it is removed. This removal is usually to the sediments, but in the case of water and some gases to the atmosphere. 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 like sodium in the ocean are estimated to be millions of years, while for highly reactive and insoluble elements, residence times are only hundreds of years. 
|Chemical element or ion||Residence time (in years)|
A few elements such as nitrogen, phosphorus and potassium are essential for life and major components of biological material, and are sometimes called “ nutrients”. These elements have intermediate residence times. The biological cycling of these elements means that this represents a major removal route from the ocean as some of the organic material sinks to the ocean floor and is buried. However, rock phosphate is a non-renewable resource (see peak phosphorus) and an essential ingredient in inorganic fertilizer.  These phosphorus components - which are produced through mining - are carried in rivers and then to the ocean because they are released to waterways via intensive agriculture and untreated sewage. Eventually, it sinks to the ocean floor and is no longer available to humans as a resource.  This may become a problem in future for fertilizer production and food security.  
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 of the plants and animals present in each region.  The diversity of life in the ocean is immense, including:
- Animals: 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, Brachiopods, and Echinoderms (such as sea urchins and sea stars). Many other familiar animal groups primarily live in the ocean, including cephalopods (includes octopus and squid), crustaceans (includes lobsters, crabs, and shrimp), fish, sharks, 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 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. Seven species of turtles, the sea turtles, also spend most of their time in the oceans.
- Plants: 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, which is the source of the edible nori seaweed), and brown algae (including organisms like kelp).
- Bacteria: 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
- Fungi: many marine fungi 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, aquaculture), leisure ( cruising, sailing, recreational boat fishing, scuba diving), power generation (see marine energy and offshore wind power), extractive industries ( offshore drilling and deep sea mining), 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 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, and lobster.  The biggest commercial fishery globally is for anchovies, Alaska pollock and tuna. :6 A report by FAO in 2020 stated that "in 2017, 34 percent of the fish stocks of the world’s marine fisheries were classified as overfished". :54 Fish and other fishery products from both wild fisheries 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 waves, 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.   Offshore wind power 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 and drilling rigs 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.  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 International Maritime Organization (IMO), which was ratified in 1958 is responsible mainly for maritime safety, 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. 
Human activities affect marine life and marine habitats through many negative influences, such as marine pollution (including marine debris and microplastics) overfishing, ocean acidification and other effects of climate change on oceans.
Marine pollution occurs when substances used or spread by humans, such as industrial, agricultural and residential waste, particles, noise, excess carbon dioxide or invasive organisms enter the ocean and cause harmful effects there. Since most inputs come from land, either via the rivers, sewage or the atmosphere, it means that continental shelves are more vulnerable to pollution. Air pollution is also a contributing factor by carrying off iron, carbonic acid, nitrogen, silicon, sulfur, pesticides or dust particles into the ocean.  The pollution often comes from nonpoint sources such as agricultural runoff, wind-blown debris, and dust. Land and air pollution have proven to be harmful to marine life and its habitats. 
A particular concern is the runoff of nutrients (nitrogen and phosphorus) from intensive agriculture, and the disposal of untreated or partially treated sewage to rivers and subsequently oceans. These nitrogen and phosphorus nutrients (which are also contained in fertilizers) stimulate phytoplankton growth, which can provide more food for other marine life, but in excess can lead to harmful algal blooms ( eutrophication) which can be harmful to humans as well as marine creatures. Such blooms are naturally occurring but may be increasing as a result of anthropogenic inputs or alternatively may be something that is now more closely monitored and so more frequently reported.  A second major concern is that the degradation of algal blooms can lead to depletion of oxygen in coastal waters, a situation that may be exacerbated by climate change as warming reduces vertical mixing of the water column. Many potentially toxic chemicals adhere to tiny particles which are then taken up by plankton and benthic animals, most of which are either deposit feeders or filter feeders. In this way, the toxins are concentrated upward within ocean food chains. When pesticides are incorporated into the marine ecosystem, they quickly become absorbed into marine food webs. Once in the food webs, these pesticides can cause mutations, as well as diseases, which can be harmful to humans as well as the entire food web. Toxic metals can also be introduced into marine food webs. These can cause a change to tissue matter, biochemistry, behavior, reproduction, and suppress growth in marine life. Also, many animal feeds have a high fish meal or fish hydrolysate content. In this way, marine toxins can be transferred to land animals, and appear later in meat and dairy products.
A growing field of concern is the pollution of the ocean by plastic particles ranging in size from large original material such as bottles and bags, down to microplastics formed from the fragmentation of plastic material. This material is only very slowly degraded or removed from the ocean so plastic particles are now widespread throughout the surface ocean and are known to be having deleterious effects on marine life. 
Marine debris, also known as marine litter, is human-created waste that has deliberately or accidentally been released in a sea or ocean. Floating oceanic debris tends to accumulate at the center of gyres and on coastlines,  frequently washing aground, when it is known as beach litter or tidewrack. Deliberate disposal of wastes at sea is called ocean dumping. Naturally occurring debris, such as driftwood and drift seeds, are also present.With the increasing use of plastic, human influence has become an issue as many types of (petrochemical) plastics do not biodegrade quickly, as would natural or organic materials.  The largest single type of plastic pollution (~10 %) and majority of large plastic in the oceans is discarded and lost nets from the fishing industry.  Waterborne plastic poses a serious threat to fish, seabirds, marine reptiles, and marine mammals, as well as to boats and coasts.  Dumping, container spillages, litter washed into storm drains and waterways and wind-blown landfill waste all contribute to this problem. This increased has caused serious negative effects such as ghost nets capturing animals, concentration of plastic debris in massive marine garbage patches, and concentration of debris in the food chain.
Due to their ubiquity in the environment, microplastics are widespread among the different matrices. In marine environments, microplastics have been evidenced in sandy beaches,  surface waters,  the water column, and deep sea sediment. Upon reaching marine environments, the fate of microplastics is subject to naturally occurring drivers, such as winds and surface oceanic currents. Numerical models are able to trace small plastic debris (micro- and mesoplastics) drifting in the ocean,  thus predicting their fate.Microplastics enter waterways through many avenues including deterioration of road paint, tire wear and city dust entering the waterways, plastic pellets spilled from shipping containers, ghost nets and other synthetic textiles dumped into the ocean, cosmetics discharged and laundry products entering sewage water and marine coatings on ships degrading. 
The effects of climate change on oceans include the rise in sea level from ocean warming and ice sheet melting, and changes in ocean stratification and circulation due to changing temperatures leading to changes in oxygen concentrations. There is clear evidence that the Earth is warming due to anthropogenic emissions of greenhouse gases and leading inevitably to ocean warming.  The greenhouse gases taken up by the ocean (via carbon sequestration) help to mitigate climate change but lead to ocean acidification.Physical effects of climate change on oceans include sea level rise which will in particular affect coastal areas, ocean currents, weather and the seafloor. Chemical effects include ocean acidification and reductions in oxygen levels. Furthermore, there will be effects on marine life. The consensus of many studies of coastal tide gauge records is that during the past century sea level has risen worldwide at an average rate of 1–2 mm/yr reflecting a net flux of heat into the surface of the land and oceans. The rate at which ocean acidification will occur may be influenced by the rate of surface ocean warming, because the chemical equilibria that govern seawater pH are temperature-dependent.  Increase of water temperature will also have a devastating effect on different oceanic ecosystems like coral reefs. The direct effect is the coral bleaching of these reefs, which live within a narrow temperature margin, so a small increase in temperature would have a drastic effects in these environments.
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, although there is evidence for oceans' existence elsewhere in the Solar System.
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, based on mathematical modeling studies.  
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