This article is about natural science aspects of Earth's oceans. For more on human experience, history and culture of oceans, seeSea. For other uses, seeOcean (disambiguation).
Ocean scientists split the ocean into vertical and horizontal zones based on physical and biological conditions. Horizontally the ocean covers theoceanic crust, which it shapes. Where the ocean meets dry land it covers relatively shallowcontinental shelfs, which are part of Earth'scontinental crust. Human activity is mostly coastal with highnegative impacts onmarine life. Vertically thepelagic zone is the open ocean'swater column from the surface to the ocean floor. The water column is further divided into zones based on depth and the amount of light present. Thephotic zone starts at the surface and is defined to be "the depth at which light intensity is only 1% of the surface value"[14]: 36 (approximately 200 m in the open ocean). This is the zone where photosynthesis can occur. In this process plants and microscopicalgae (free-floatingphytoplankton) use light, water, carbon dioxide, and nutrients to produce organic matter. As a result, the photic zone is the mostbiodiverse and the source of the food supply which sustains most of the oceanecosystem. Light can only penetrate a few hundred more meters; the rest of thedeeper ocean is cold and dark (these zones are calledmesopelagic andaphotic zones).
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 wheresea ice forms, the temperature in equilibrium is about −2 °C (28 °F). In all parts of the ocean, deep ocean temperatures range between −2 °C (28 °F) and 5 °C (41 °F).[15] Constant circulation of water in the ocean createsocean currents. Those currents are caused by forces operating on the water, such as temperature and salinity differences,atmospheric circulation (wind), and theCoriolis effect.[16]Tides create tidal currents, while wind and waves cause surface currents. TheGulf Stream,Kuroshio Current,Agulhas Current andAntarctic Circumpolar Current are all major ocean currents. Such currents transport massive amounts of water, gases, pollutants and heat to different parts of the world, and from the surface into the deep ocean. All this has impacts on the globalclimate system.
The terms "the ocean" or "the sea" used without specification refer to the interconnected body of salt water covering the majority of Earth's surface.[9][10] It includes thePacific,Atlantic,Indian,Southern/Antarctic, andArctic oceans.[20] As a general term, "the ocean" and "the sea" are often interchangeable.[21]
Strictly speaking, a "sea" is a body of water (generally a division of the world ocean) partly or fully enclosed by land.[22] The word "sea" can also be used for many specific, much smaller bodies of seawater, such as theNorth Sea or theRed Sea. There is no sharp distinction between seas and oceans, though generally seas are smaller, and are often partly (asmarginal seas) or wholly (asinland seas) bordered by land.[23]
World Ocean
"World Ocean" redirects here and is not to be confused withocean world."Ocean Sea" redirects here. For the 1993 novel by Alessandro Baricco, seeOcean Sea (novel).
The contemporary concept of theWorld Ocean was coined in the early 20th century by theRussian oceanographerYuly Shokalsky to refer to the continuous ocean that covers and encircles most of Earth.[24][25] The global, interconnected body of salt water is sometimes referred to as the World Ocean,global ocean orthe great ocean.[26][27][28] The concept of a continuous body of water with relatively unrestricted exchange between its components is critical inoceanography.[29]
The concept of Ōkeanós could have anIndo-European connection. Greek Ōkeanós has been compared to theVedic 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.[31]
According toM. L. West, the etymology of Oceanus is "obscure" and "cannot be explained from Greek".[32] The use byPherecydes of Syros of the formŌgenós (Ὠγενός)[33] for the name lends support for the name being aloanword.[34] However, according to West, no "very convincing" foreign models have been found.[35] A Semitic derivation has been suggested by several scholars,[36] whileR. S. P. Beekes has suggested a loanword from the AegeanPre-Greek non-Indo-Europeansubstrate.[37] Nevertheless, Michael Janda sees possible Indo-European connections.[38]
Scientists believe that a sizable quantity ofwater would have been in the material that formed Earth.[39] Water molecules would have escaped Earth's gravity more easily when it was less massive during its formation. This is calledatmospheric escape.
Duringplanetary formation, Earth possibly hadmagma oceans. Subsequently,outgassing,volcanic activity andmeteorite impacts, produced an early atmosphere ofcarbon dioxide,nitrogen andwater vapor, according to current theories.The gases and the atmosphere are thought to have accumulated over millions of years. After Earth's surface had significantly cooled, the water vapor over time would have condensed, forming Earth's first oceans.[40] The early oceans might have been significantly hotter than today and appeared green due to high iron content.[41]
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 theIsua Greenstone Belt and provides evidence that water existed on Earth 3.8 billion years ago.[42] In theNuvvuagittuq Greenstone Belt,Quebec, Canada, rocks dated at 3.8 billion years old by one study[43] and 4.28 billion years old by another[44] show evidence of the presence of water at these ages.[42] If oceans existed earlier than this, any geological evidence either has yet to be discovered, or has since been destroyed by geological processes likecrustal recycling.However, in August 2020, researchers reported that sufficient water to fill the oceans may have always been on theEarth since the beginning of the planet's formation.[45][46][47] In this model, atmosphericgreenhouse gases kept the oceans from freezing when the newly formingSun hadonly 70% of itscurrent luminosity.[48]
Since its formation the ocean has taken many conditions and shapes with manypast ocean divisions and potentially at times covering the whole globe.[49]
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.[50]
World map of the five-ocean model with approximate boundaries
The entire ocean, containing 97% of Earth's water, spans 70.8% ofEarth's surface,[8] making it Earth's global ocean orworld ocean.[24][26] This makes Earth, along with its vibranthydrosphere a "water world"[51][52] or "ocean world",[53][54] particularly in Earth's early history when the ocean is thought to have possibly covered Earth completely.[49] The ocean's shape is irregular, unevenly dominating theEarth's surface. This leads to the distinction of the Earth's surface into awater and land hemisphere, as well as the division of the ocean into different oceans.
In mid-ocean,magma is constantly being thrust through the seabed between adjoining plates to formmid-oceanic ridges and here convection currents within the mantle tend to drive the two plates apart. Parallel to these ridges and nearer the coasts, one oceanic plate may slide beneath another oceanic plate in a process known assubduction. Deeptrenches are formed here and the process is accompanied by friction as the plates grind together. The movement proceeds in jerks which cause earthquakes, heat is produced and magma is forced up creating underwater mountains, some of which may form chains ofvolcanic islands near to deep trenches. Near some of the boundaries between the land and sea, the slightly denser oceanic plates slide beneath the continental plates and more subduction trenches are formed. As they grate together, the continental plates are deformed and buckle causing mountain building and seismic activity.[65][66]
Every ocean basin has amid-ocean ridge, which creates a long mountain range beneath the ocean. Together they form the global mid-oceanic ridge system that features thelongestmountain range in the world. The longest continuous mountain range is 65,000 km (40,000 mi). This underwater mountain range is several times longer than the longest continental mountain range – theAndes.[67]
Oceanographers of theNippon Foundation-GEBCO Seabed 2030 Project (Seabed 2030) state that as of 2024 just over 26% of theocean floor has been mapped at a higher resolution than provided by satellites, while the ocean as a whole will never be fully explored,[68] with some estimating 5% of it having been explored.[69]
The zone where land meets sea is known as thecoast, and the part between the lowest springtides and the upper limit reached by splashing waves is theshore. Abeach is the accumulation of sand orshingle on the shore.[70] Aheadland is a point of land jutting out into the sea and a largerpromontory is known as acape. The indentation of a coastline, especially between two headlands, is a bay. A small bay with a narrow inlet is acove and a large bay may be referred to as a gulf.[71] Coastlines are influenced by several factors including the strength of the waves arriving on the shore, the gradient of the land margin, the composition and hardness of the coastal rock, the inclination of the off-shore slope and the changes of the level of the land due to local uplift or submergence.[70]
Normally, waves roll towards the shore at the rate of six to eight per minute and these are known as constructive waves as they tend to move material up the beach and have little erosive effect. Storm waves arrive on shore in rapid succession and are known as destructive waves as theswash moves beach material seawards. Under their influence, the sand and shingle on the beach is ground together and abraded. Around high tide, the power of a storm wave impacting on the foot of a cliff has a shattering effect as air in cracks and crevices is compressed and then expands rapidly with release of pressure. At the same time, sand and pebbles have an erosive effect as they are thrown against the rocks. This tends to undercut the cliff, and normalweathering processes such as the action of frost follows, causing further destruction. Gradually, a wave-cut platform develops at the foot of the cliff and this has a protective effect, reducing further wave-erosion.[70]
Material worn from the margins of the land eventually ends up in the sea. Here it is subject toattrition as currents flowing parallel to the coast scour out channels and transport sand and pebbles away from their place of origin. Sediment carried to the sea by rivers settles on the seabed causingdeltas to form in estuaries. All these materials move back and forth under the influence of waves, tides and currents.[70] Dredging removes material and deepens channels but may have unexpected effects elsewhere on the coastline. Governments make efforts to prevent flooding of the land by the building ofbreakwaters,seawalls,dykes and levees and other sea defences. For instance, theThames Barrier is designed to protect London from a storm surge,[72] while the failure of the dykes and levees aroundNew Orleans duringHurricane Katrina created ahumanitarian crisis in the United States.
Physical properties
Color
Oceanchlorophyll concentration is a proxy forphytoplankton biomass. In this map, blue colors represent lower chlorophyll and reds represent higher chlorophyll. Satellite-measured chlorophyll is estimated based onocean color by how green the color of the water appears from space.
Most of the ocean is blue in color, but in some places the ocean is blue-green, green, or even yellow to brown.[73] Blue ocean color is a result of several factors. First, water preferentially absorbs red light, which means that blue light remains and is reflected back out of the water. Red light is most easily absorbed and thus does not reach great depths, usually to less than 50 meters (164 ft). Blue light, in comparison, can penetrate up to 200 meters (656 ft).[74] Second, water molecules and very tiny particles in ocean water preferentially scatter blue light more than light of other colors. Blue light scattering by water and tiny particles happens even in the very clearest ocean water,[75] and is similar toblue light scattering in the sky.
The main substances that affect the color of the ocean includedissolved organic matter, livingphytoplankton withchlorophyll pigments, and non-living particles likemarine snow and mineralsediments.[76] Chlorophyll can be measured bysatellite observations and serves as a proxy for ocean productivity (marine primary productivity) in surface waters. In long term composite satellite images, regions with high ocean productivity show up in yellow and green colors because they contain more (green)phytoplankton, whereas areas of low productivity show up in blue.
The ocean is a major driver of Earth'swater cycle.
Ocean water represents the largest body of water within the globalwater cycle (oceans contain 97% ofEarth's water). Evaporation from the ocean moves water into the atmosphere to later rain back down onto land and the ocean.[77] Oceans have a significant effect on thebiosphere. The ocean as a whole is thought to cover approximately 90% of the Earth's biosphere.[78] Oceanicevaporation, as a phase of the water cycle, is the source of most rainfall (about 90%),[77] causing a globalcloud cover of 67% and a consistent oceanic cloud cover of 72%.[79]Ocean temperatures affectclimate andwind patterns that affect life on land. One of the most dramatic forms ofweather 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'shydrosphere, it is integral tolife on Earth, forms part of thecarbon cycle and water cycle, and – as a hugeheat reservoir – influences climate and weather patterns.
The motions of the ocean surface, known as undulations orwind waves, are the partial and alternate rising and falling of the ocean surface. The series ofmechanical waves that propagate along the interface between water and air is calledswell – a term used insailing,surfing andnavigation.[80] These motions profoundly affect ships on the surface of the ocean and the well-being of people on those ships who might suffer fromsea 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 causesripples to form. A stronger gust blowing 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 theRoaring Forties, long, organized masses of water called swell roll across the ocean.[81]: 83–84 [82][83] 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 thefetch, 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.[82]
Constructive interference can lead to the formation of unusually highrogue waves.[84] Most waves are less than 3 m (10 ft) high[84] and it is not unusual for strong storms to double or triple that height.[85] Rogue waves, however, have been documented at heights above 25 meters (82 ft).[86][87]
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 horizontal movement of water. As waves approach land andmove 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 theocean floor, they begin to slow down. This pulls the crests closer together and increases thewaves' height, which is calledwave 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.[84] This rushes in a sheet up the beach before retreating into the ocean under the influence of gravity.[88]
Theocean's surface is an important reference point for oceanography and geography, particularly asmean sea level. The ocean surface has globally little, butmeasurable topography, depending on the ocean's volumes.
The ocean surface is a crucial interface for oceanic and atmospheric processes. Allowing interchange of particles, enriching the air and water, as well as grounds by some particles becomingsediments. This interchange has fertilized life in the ocean, on land and air. All these processes and components together make upocean surface ecosystems.
Tides are the regular rise and fall in water level experienced by oceans, primarily driven bythe Moon's gravitationaltidal forces upon the Earth. Tidal forces affect all matter on Earth, but onlyfluids like the ocean demonstrate the effects on human timescales. (For example, tidal forces acting on rock may producetidal locking between two planetary bodies.) Though primarily driven by the Moon's gravity, oceanic tides are also substantially modulated by the Sun's tidal forces, by the rotation of the Earth, and by the shape of the rocky continents blocking oceanic water flow. (Tidal forces vary more with distance than the "base" force of gravity: the Moon's tidal forces on Earth are more than double the Sun's,[91] despite the latter's much stronger gravitational force on Earth. Earth's tidal forces upon the Moon are 20x stronger than the Moon's tidal forces on the Earth.)
The primary effect of lunar tidal forces is to bulge Earth matter towards the near and far sides of the Earth, relative to the moon. The "perpendicular" sides, from which the Moon appears in line with the local horizon, experience "tidal troughs". Since it takes nearly 25 hours for the Earth to rotate under the Moon (accounting for the Moon's 28-day orbit around Earth), tides thus cycle over a course of 12.5 hours. However, the rocky continents pose obstacles for the tidal bulges, so the timing of tidal maxima may not actually align with the Moon in most localities on Earth, as the oceans are forced to "dodge" the continents. Timing and magnitude of tides vary widely across the Earth as a result of the continents. Thus, knowing the Moon's position does not allow a local to predict tide timings, instead requiring precomputedtide tables which account for the continents and the Sun, among others.
During each tidal cycle, at any given place the tidal waters rise to maximum height, high tide, before ebbing away again to the minimum level, low tide. As the water recedes, it gradually reveals theforeshore, also known as the intertidal zone. The difference in height between the high tide and low tide is known as thetidal range or tidal amplitude.[92][93] When the sun and moon are aligned (full moon or new moon), the combined effect results in the higher "spring tides", while the sun and moon misaligning (half moons) result in lesser tidal ranges.[92]
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.[94] Some of the largest tidal ranges in the world occur in theBay of Fundy andUngava Bay in Canada, reaching up to 16 meters.[95] Other locations with record high tidal ranges include theBristol Channel between England and Wales,Cook Inlet in Alaska, and theRío Gallegos in Argentina.[96]
Tides are not to be confused withstorm surges, which 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 dramatically above a typical high tide.
The average depth of the oceans is about 4 km. More precisely the average depth is 3,688 meters (12,100 ft).[82] Nearly half of the world's marine waters are over 3,000 meters (9,800 ft) deep.[28] "Deep ocean," which is anything below 200 meters (660 ft), covers about 66% of Earth's surface.[97] This figure does not include seas not connected to the World Ocean, such as theCaspian Sea.
The deepest region of the ocean is at theMariana Trench, located in the Pacific Ocean near theNorthern Mariana Islands.[98] The maximum depth has been estimated to be 10,971 meters (35,994 ft). The British naval vesselChallenger II surveyed the trench in 1951 and named the deepest part of the trench the "Challenger Deep". In 1960, theTrieste successfully reached the bottom of the trench, manned by a crew of two men.
The major oceanic zones, based on depth and biophysical conditions
Oceanographers classify the ocean into vertical and horizontal zones based on physical and biological conditions. Thepelagic zone consists of thewater column of the open ocean, and can be divided into further regions categorized by light abundance and by depth.
The ocean zones can be grouped by light penetration into (from top to bottom): the photic zone, the mesopelagic zone and the aphotic deep ocean zone:
Thephotic zone is defined to be "the depth at which light intensity is only 1% of the surface value".[14]: 36 This is usually up to a depth of approximately 200 m in the open ocean. It is the region wherephotosynthesis can occur and is, therefore, the mostbiodiverse. Photosynthesis by plants and microscopicalgae (free floatingphytoplankton) 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. The pelagic part of the photic zone is known as theepipelagic.[99] The actual optics of light reflecting and penetrating at the ocean surface are complex.[14]: 34–39
Below the photic zone is themesopelagic or twilight zone where there is a very small amount of light. The basic concept is that with that little light photosynthesis is unlikely to achieve any net growth over respiration.[14]: 116–124
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 (seemarine snow) or find another energy source.Hydrothermal vents are a source of energy in what is known as theaphotic zone (depths exceeding 200 m).[99]
Grouped by depth and temperature
The pelagic part of the aphotic zone can be further divided into vertical regions according to depth and temperature:[99]
Themesopelagic is the uppermost region. Its lowermost boundary is at athermocline of 12 °C (54 °F) which generally lies at 700–1,000 meters (2,300–3,300 ft) in thetropics. Next is thebathypelagic 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 theabyssal plain is theabyssopelagic, whose lower boundary lies at about 6,000 meters (20,000 ft). The last and deepest zone is thehadalpelagic which includes theoceanic trench and lies between 6,000–11,000 meters (20,000–36,000 ft).
Thebenthic zones are aphotic and correspond to the three deepest zones of thedeep-sea. Thebathyal 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, thehadal 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 thermoclines (temperature),haloclines (salinity),chemoclines (chemistry), andpycnoclines (density). If a zone undergoes dramatic changes in temperature with depth, it contains a thermocline, a distinct boundary between warmer surface water and colder deep water. In tropical regions, the thermocline is typically deeper compared to higher latitudes. Unlikepolar waters, where solar energy input is limited, temperaturestratification is less pronounced, and a distinct thermocline is often absent. This is due to the fact that surface waters in polar latitudes are nearly as cold as deeper waters. 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.[100] 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 ocean water density. Colder and saltier water is denser, and this density plays a crucial role in regulating the global water circulation within the ocean.[99] The halocline often coincides with the thermocline, and the combination produces a pronounced pycnocline, 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: theneritic zone and theoceanic zone. The neritic zone covers the water directly above thecontinental shelves, includingcoastal waters. On the other hand, the oceanic zone includes all the completely open water.
Thelittoral zone covers the region between low and high tide and represents the transitional area between marine and terrestrial conditions. It is also known as theintertidal zone because it is the area where tide level affects the conditions of the region.[99]
Volumes
The combined volume of water in all the oceans is roughly 1.335 billion cubic kilometers (1.335sextillion liters, 320.3 million cubic miles).[82][101][102]
It has been estimated that there are 1.386 billioncubic kilometres (333 million cubic miles) of water on Earth.[103][104][105] This includes water in gaseous, liquid and frozen forms as soil moisture,groundwater andpermafrost in theEarth's crust (to a depth of 2 km); oceans andseas,lakes,rivers andstreams,wetlands,glaciers, ice and snow cover on Earth's surface; vapour, droplets and crystals in the air; and part of living plants, animals and unicellular organisms of the biosphere.Saltwater accounts for 97.5% of this amount, whereasfresh water accounts for only 2.5%. Of this fresh water, 68.9% is in the form ofice and permanent snow cover in the Arctic, the Antarctic and mountainglaciers; 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.[106]
The total mass of Earth's hydrosphere is about 1.4 × 1018tonnes, which is about 0.023% of Earth's total mass. At any given time, about 2 × 1013 tonnes of this is in the form ofwater vapor in theEarth's atmosphere (for practical purposes, 1 cubic metre of water weighs 1 tonne). Approximately 71% of Earth's surface, an area of some 361 million square kilometres (139.5 million square miles), is covered by ocean. The averagesalinity of Earth's oceans is about 35 grams ofsalt per kilogram of sea water (3.5%).[107]
Ocean temperatures depends on the amount of solar radiation falling on its surface. In the tropics, with the Sun nearly overhead, thetemperature of the surface layers can rise to over 30 °C (86 °F) while near the poles the temperature in equilibrium with thesea 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 ocean water has a temperature between −2 °C (28 °F) and 5 °C (41 °F) in all parts of the globe.[15]
The temperature gradient over the water depth is related to the way the surface water mixes with deeper water or does not mix (a lack of mixing is calledocean stratification). This depends on the temperature: in the tropics the warm surface layer of about 100 m is quite stable and does not mix much with deeper water, while near the poles winter cooling and storms makes the surface layer denser and it mixes to great depth and then stratifies again in summer. Thephotic depth is typically about 100 m (but varies) and is related to this heated surface layer.[108]
It is clear that the ocean is warming as a result of climate change, and this rate of warming is increasing.[109]: 9 The global ocean was the warmest it had ever been recorded by humans in 2022.[110] This is determined by theocean heat content, which exceeded the previous 2021 maximum in 2022.[110] The steady rise in ocean temperatures is an unavoidable result of theEarth's energy imbalance, which is primarily caused by rising levels of greenhouse gases.[110] Between pre-industrial times and the 2011–2020 decade, the ocean's surface has heated between 0.68 and 1.01 °C.[111]: 1214
Temperature and salinity by region
The temperature and salinity of ocean waters vary significantly across different regions. 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 from one ocean region to another. The table below provides an illustration of the sort of values usually encountered.
Seawater with a typical salinity of 35‰ has a freezing point of about −1.8 °C (28.8 °F).[99][117] Because sea ice is lessdense 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.[118][119][120] 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 formice sheets. The ice formed incorporates somesea salt, 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.[121]
Anocean current is a continuous, directed flow of seawater caused by several forces acting upon the water. These include wind, theCoriolis effect,temperature andsalinity differences.[16] Ocean currents are primarily horizontal water movements that have different origins such as tides for tidal currents, or wind and waves for surface currents.
Tidal currents are in phase with thetide, hence arequasiperiodic; 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 aroundheadlands.[122] Non-periodic or non-tidal currents are created by the action of winds and changes indensity of water. In littoral zones,breaking waves are so intense and the depth measurement so low, that maritime currents reach often 1 to 2knots.[123]
Thewind andwaves 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 ofStokes 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.[123]
This acceleration of the current takes place in the direction of waves and dominant wind. Accordingly, when the ocean depth increases, therotation of theearth 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 theEkman 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 yearlyseasons. 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.[123]
The wind blowing on the ocean surface will set the water in motion. The global pattern of winds (also calledatmospheric circulation) creates a global pattern of ocean currents. These are driven not only by the wind but also by the effect of the circulation of the earth (coriolis force). These major ocean currents include theGulf Stream,Kuroshio Current,Agulhas Current andAntarctic Circumpolar Current. The Antarctic Circumpolar Current encirclesAntarctica and influences the area's climate, connecting currents in several oceans.[123]
Map of theGulf Stream, a major ocean current that transports heat from the equator to northern latitudes and moderates the climate ofEurope
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, thethermohaline circulation drives water motion. For example, theAtlantic meridional overturning circulation (AMOC) is driven by the cooling of surface waters in the polar latitudes in the north and south, creating dense water which sinks to the bottom of the ocean. This cold and dense water moves slowly away from thepoles 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.[123] This circulation has important impacts on the globalclimate system and on the uptake and redistribution of pollutants and gases such ascarbon dioxide, for example by moving contaminants from the surface into the deep ocean.
Ocean currents greatly affect Earth's climate bytransferring heat from thetropics to the polar regions. This affects air temperature and precipitation in coastal regions and further inland. Surface heat and freshwaterfluxes create globaldensity gradients, which drive thethermohaline circulation that is a part of large-scale ocean circulation. It plays an important role in supplying heat to the polar regions, and thus in sea ice regulation.[citation needed]
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 citiesSan Francisco (37.8 N) andNew 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, getswinds from the west over thePacific Ocean. New York, on the east coast of North America getswinds 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.[citation needed]
Changes in the thermohaline circulation are thought to have significant impacts onEarth's energy budget. Because the thermohaline circulation determines the rate at which deep waters reach the surface, it may also significantly influenceatmospheric carbon dioxide concentrations. Modern observations,climate simulations and paleoclimate reconstructions suggest that theAtlantic meridional overturning circulation (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.[124]: 19 Such a weakening could cause large changes to global climate, with the North Atlantic particularly vulnerable.[124]: 19
Salinity is a measure of the total amounts of dissolved salts inseawater. It was originally measured via measurement of the amount ofchloride in seawater and hence termed chlorinity. It is now standard practice to gauge it by measuringelectrical conductivity of the water sample. Salinity can be calculated using the chlorinity, which is a measure of the total mass ofhalogen ions (includes fluorine, chlorine, bromine, and iodine) in seawater. According to an international agreement, the following formula is used to determine salinity:[126]
Salinity (in ‰) = 1.80655 × Chlorinity (in ‰)
The average ocean water chlorinity is about 19.2‰, and, thus, the average salinity is around 34.7‰.[126]
Salinity has a major influence on the density of seawater. A zone of rapid salinity increase with depth is called ahalocline. Asseawater's salt content increases, so does the temperature at which its maximum density occurs. Salinity affects both the freezing and boiling points of water, with the boiling point increasing with salinity. Atatmospheric pressure,[127] normal seawater freezes at a temperature of about −2 °C.
Salinity is higher in Earth's oceans where there is moreevaporation and lower where there is moreprecipitation. If precipitation exceeds evaporation, as is the case inpolar and sometemperate regions, salinity will be lower. Salinity will be higher if evaporation exceeds precipitation, as is sometimes the case intropical regions. For example, evaporation is greater than precipitation in theMediterranean Sea, which has an average salinity of 38‰, more saline than the global average of 34.7‰.[128] Thus, oceanic waters in polar regions have lower salinity content than oceanic waters in tropical regions.[126] However, whensea 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 theArctic Ocean.[99][129]
Due to theeffects of climate change on oceans, observations of sea surface salinity between 1950 and 2019 indicate that regions of high salinity and evaporation have become more saline while regions of low salinity and more precipitation have become fresher.[130] It is very likely that the Pacific and Antarctic/Southern Oceans have freshened while the Atlantic has become more saline.[130]
Dissolved gases
Sea surface oxygen concentration in moles per cubic meter from the World Ocean Atlas[131]
Ocean water contains large quantities of dissolved gases, includingoxygen,carbon dioxide andnitrogen. These dissolve into ocean water viagas exchange at the ocean surface, with the solubility of these gases depending on the temperature and salinity of the water.[17] 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 24 °C (75 °F)[132][133][134] All gases are moresoluble – 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 30 °C (86 °F) to freezing 0 °C (32 °F). Similarly, carbon dioxide and nitrogen gases are more soluble at colder temperatures, and their solubility changes with temperature at different rates.[132][135]
Diagram of the ocean carbon cycle showing the relative size of stocks (storage) and fluxes[136]
Photosynthesis in the surface ocean releases oxygen and consumes carbon dioxide.Phytoplankton, a type of microscopic free-floating algae, controls this process. After the plants have grown, oxygen is consumed and carbon dioxide released, as a result of bacterial decomposition of the organic matter created by photosynthesis in the ocean. 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 andbicarbonate.[108] Thiscycling of carbon dioxide in oceans is an important part of the globalcarbon cycle.
The oceans represent a majorcarbon sink for carbon dioxide taken up from the atmosphere by photosynthesis and by dissolution (see alsocarbon sequestration). There is also increased attention on carbon dioxide uptake in coastalmarine habitats such asmangroves andsaltmarshes. This process is often referred to as "Blue carbon". The focus is on these ecosystems because they are strong carbon sinks as well as ecologically important habitats under threat from human activities andenvironmental 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.[108] 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, calledoxygen minimum zones orhypoxic waters, will generally be made worse by theeffects of climate change on oceans.[137][138]
ThepH value at the surface of oceans (global mean surface pH) is currently approximately in the range of 8.05[139] to 8.08.[140] This makes it slightlyalkaline. The pH value at the surface used to be about 8.2 during the past 300 million years.[141] However, between 1950 and 2020, the average pH of the ocean surface fell from approximately 8.15 to 8.05.[142]Carbon dioxide emissions from human activities are the primary cause of this process calledocean acidification, withatmospheric carbon dioxide (CO2) levels exceeding 410 ppm (in 2020).[143] CO2 from theatmosphere is absorbed by the oceans. This producescarbonic acid (H2CO3) which dissociates into abicarbonate ion (HCO−3) and ahydrogen ion (H+). The presence of free hydrogen ions (H+) lowers the pH of the ocean.
There is a natural gradient of pH in the ocean which is related to the breakdown of organic matter in deep water which slowly lowers the pH with depth: The pH value of seawater is naturally as low as 7.8 in deep ocean waters as a result of degradation of organic matter there.[144] It can be as high as 8.4 in surface waters in areas of highbiological productivity.[108]
The definition ofglobal mean surface pH refers to the top layer of the water in the ocean, up to around 20 or 100 m depth. In comparison, the average depth of the ocean is about 4 km. The pH value at greater depths (more than 100 m) has not yet been affected by ocean acidification in the same way. There is a large body of deeper water where the natural gradient of pH from 8.2 to about 7.8 still exists and it will take a very long time to acidify these waters, and equally as long to recover from that acidification. But as the top layer of the ocean (thephotic zone) is crucial for its marine productivity, any changes to the pH value and temperature of the top layer can have many knock-on effects, for example onmarine life andocean currents (such as effects of climate change on oceans).[108]
The key issue in terms of the penetration of ocean acidification is the way the surface water mixes with deeper water or does not mix (a lack of mixing is calledocean stratification). This in turn depends on the water temperature and hence is different between the tropics and the polar regions (seeocean#Temperature).[108]
Thechemical properties of seawater complicate pH measurement, and several distinct pH scales exist inchemical oceanography.[145] There is no universally accepted reference pH-scale for seawater and the difference between measurements based on multiple reference scales may be up to 0.14 units.[146]
Alkalinity is the balance of base (proton acceptors) and acids (proton donors) in seawater, or indeed any natural waters. The alkalinity acts as achemical buffer, regulating the pH of seawater. While there are many ions in seawater that can contribute to the alkalinity, many of these are at very low concentrations. This means that the carbonate, bicarbonate and borate ions are the only significant contributors to seawater alkalinity in the open ocean with well oxygenated waters. The first two of these ions contribute more than 95% of this alkalinity.[108]
The chemical equation for alkalinity in seawater is:
AT = [HCO3−] + 2[CO32-] + [B(OH)4−]
The growth of phytoplankton in surface ocean waters leads to the conversion of some bicarbonate and carbonate ions into organic matter. Some of this organic matter sinks into the deep ocean where it is broken down back into carbonate and bicarbonate. This process is related to ocean productivity ormarine primary production. Thus alkalinity tends to increase with depth and also along the global thermohaline circulation from the Atlantic to the Pacific and Indian Ocean, although these increases are small. The concentrations vary overall by only a few percent.[108][144]
The absorption of CO2 from the atmosphere does not affect the ocean's alkalinity.[147]: 2252 It does lead to a reduction in pH value though (termedocean acidification).[143]
The ocean waters contain manychemical 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 assodium and chloride, together making up the majority of ocean salts. Other elements, such as iron, are present at tiny concentrations of just a few nanograms (10−9 grams) per liter.[126]
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 andhydrothermal vents. Elements are removed from ocean water by sinking and becoming buried insediments or evaporating to the atmosphere in the case of water and some gases. By estimating theresidence time of an element, oceanographers examine the balance of input and removal. Residence time is the average time the element would spend dissolved in the ocean before it is removed. Heavily abundant elements in ocean water such as sodium, have high input rates. This reflects high abundance in rocks and rapid rock weathering, paired with very slow removal from the ocean due to sodium ions being comparatively unreactive and highly soluble. In contrast, other elements such as iron andaluminium 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.[126]
Ocean gyres rotate clockwise in the north and counterclockwise in the south.
A few elements such as nitrogen,phosphorus,iron, andpotassium essential for life, are major components of biological material, and are commonly known as "nutrients". Nitrate and phosphate have ocean residence times of 10,000[150] and 69,000[151] years, respectively, while potassium is a much more abundant ion in the ocean with a residence time of 12 million[152] 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.
Phosphate fromintensive agriculture anduntreated 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.[153] Production ofrock phosphate, an essential ingredient in inorganicfertilizer,[154] is a slow geological process that occurs in some of the world's ocean sediments, rendering mineable sedimentaryapatite (phosphate) anon-renewable resource (seepeak phosphorus). This continual net deposition loss of non-renewable phosphate from human activities, may become a resource issue for fertilizer production andfood security in future.[155][156]
Some representative ocean animals (not drawn to scale) within their approximate depth-defined ecological habitats.Marine microorganisms also exist on the surfaces and within the tissues and organs of the diverse life inhabiting the ocean, across all ocean habitats. The animals rooted to or living on the ocean floor are notpelagic but arebenthic animals.[157]
Life within the oceanevolved 3 billion years prior to life on land. Both the depth and the distance from shore strongly influence thebiodiversity of the plants and animals present in each region.[158] The diversity of life in the ocean is immense, including:
Animals: most animalphyla have species that inhabit the ocean, including many that are found only in marine environments such assponges,Cnidaria (such ascorals andjellyfish),comb jellies,Brachiopods, andEchinoderms (such assea urchins andsea stars). Many other familiar animal groups primarily live in the ocean, includingcephalopods (includesoctopus andsquid),crustaceans (includeslobsters,crabs, andshrimp),fish,sharks,cetaceans (includeswhales,dolphins, andporpoises). 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 going on land only for breeding. Other birds that have adapted to oceans as their living space arepenguins,seagulls andpelicans. Seven species of turtles, thesea turtles, also spend most of their time in the oceans.
Amarine habitat is ahabitat that supportsmarine life. Marine life depends in some way on thesaltwater that is in the sea (the termmarine comes from theLatinmare, meaning sea or ocean). A habitat is anecological orenvironmental area inhabited by one or more livingspecies.[166] The marine environment supports many kinds of these habitats.
Many of the world's goods are moved byship between the world'sseaports.[171] Large quantities of goods are transported across the ocean, especially across the Atlantic and around the Pacific Rim.[172] Many types of cargo including manufactured goods, are typically transported instandard sized, lockable containers that are loaded on purpose-builtcontainer ships atdedicated terminals.[173] Containerization greatly boosted the efficiency and reduced the cost of shipping products by sea. This was a major factor in the rise ofglobalization and exponential increases ininternational trade in the mid-to-late 20th century.[174]
Oceans are also the major supply source for thefishing industry. Some of the major harvests areshrimp,fish,crabs, andlobster.[78] The biggest global commercial fishery is foranchovies,Alaska pollock andtuna.[175]: 6 A report byFAO in 2020 stated that "in 2017, 34 percent of the fish stocks of the world's marine fisheries were classified asoverfished".[175]: 54 Fish and other fishery products from bothwild 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".[175] To fulfill this need, coastal countries have exploited marine resources in theirexclusive economic zone. Fishing vessels are increasingly venturing out to exploit stocks in international waters.[176]
TheInternational Maritime Organization (IMO), which was ratified in 1958, is mainly responsible formaritime safety, liability and compensation, and has held some conventions on marine pollution related to shipping incidents.Ocean governance is the conduct of the policy, actions and affairs regarding the world'soceans.[182]
Human activities affect marine life andmarine 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.
The various layers of the oceans have different temperatures. For example, the water is colder towards the bottom of the ocean. This temperature stratification will increase as the ocean surface warms due to rising air temperatures.[186]: 471 Connected to this is a decline in mixing of the ocean layers, so that warm water stabilises near the surface. A reduction of cold, deepwater circulation follows. The reduced vertical mixing makes it harder for the ocean to absorb heat. So a larger share of future warming goes into the atmosphere and land. One result is an increase in the amount of energy available fortropical cyclones and other storms. Another result is a decrease innutrients for fish in the upper ocean layers. These changes also reduce the ocean's capacity tostore carbon.[187] At the same time, contrasts insalinity are increasing. Salty areas are becoming saltier and fresher areas less salty.[188]
Warmer water cannot contain the same amount of oxygen as cold water. As a result, oxygen from the oceans moves to the atmosphere. Increasedthermal stratification may reduce the supply of oxygen from surface waters to deeper waters. This lowers the water's oxygen content even more.[189] The ocean has already lost oxygen throughout itswater column.Oxygen minimum zones are increasing in size worldwide.[186]: 471
These changes harmmarine ecosystems, and this can lead tobiodiversity loss or changes in species distribution.[109] This in turn canaffect fishing and coastal tourism. For example, rising water temperatures are harming tropicalcoral reefs. The direct effect iscoral bleaching on these reefs, because they are sensitive to even minor temperature changes. So a small increase in water temperature could have a significant impact in these environments. Another example is loss ofsea ice habitats due to warming. This will have severe impacts onpolar bears and other animals that rely on it. The effects of climate change on oceans put additional pressures on ocean ecosystems which are already under pressure by otherimpacts from human activities.[109]
Marine pollution occurs when substances used or spread by humans, such asindustrial,agricultural, andresidentialwaste;particles;noise; excesscarbon dioxide; orinvasive organisms enter the ocean and cause harmful effects there. The majority of this waste (80%) comes from land-based activity, althoughmarine transportation significantly contributes as well.[190] It is a combination of chemicals and trash, most of which comes from land sources and is washed or blown into the ocean. This pollution results in damage to the environment, to the health of all organisms, and to economic structures worldwide.[191] Since most inputs come from land, viarivers,sewage, or the atmosphere, it means thatcontinental shelves are more vulnerable to pollution.Air pollution is also a contributing factor, as it carries iron, carbonic acid,nitrogen, silicon, sulfur,pesticides, and dust particles into the ocean.[192] The pollution often comes fromnonpoint sources such as agriculturalrunoff, wind-blowndebris, and dust. These nonpoint sources are largely due to runoff that enters the ocean through rivers, but wind-blown debris and dust can also play a role, as these pollutants can settle into waterways and oceans.[193] Pathways of pollution include direct discharge, land runoff,ship pollution,bilge pollution,dredging (which can createdredge plumes), atmospheric pollution and, potentially,deep sea mining.
The types of marine pollution can be grouped as pollution frommarine debris,plastic pollution, includingmicroplastics,ocean acidification,nutrient pollution, toxins, and underwater noise. Plastic pollution in the ocean is a type of marine pollution byplastics, ranging in size from large original material such as bottles and bags, down tomicroplastics formed from the fragmentation of plastic materials. Marine debris is mainly discarded human rubbish which floats on, or is suspended in the ocean. Plastic pollution is harmful tomarine life.
Another concern is the runoff ofnutrients (nitrogen and phosphorus) fromintensive agriculture, and the disposal of untreated or partially treatedsewage to rivers and subsequently oceans. Thesenitrogen andphosphorus nutrients (which are also contained infertilizers) stimulatephytoplankton andmacroalgal growth, which can lead to harmfulalgal blooms (eutrophication) which can be harmful to humans as well as marine creatures. Excessive algal growth can also smother sensitivecoral reefs and lead toloss of biodiversity and coral health. A second major concern is that the degradation ofalgal blooms can lead to consumption ofoxygen in coastal waters, a situation that may worsen withclimate change as warming reduces vertical mixing of the water column.[194]
Many potentially toxic chemicals adhere to tiny particles which are then taken up byplankton andbenthic animals, most of which are eitherdeposit feeders orfilter feeders. In this way, the toxins areconcentrated upward within oceanfood chains. When pesticides are incorporated into themarine ecosystem, they quickly become absorbed into marinefood webs. Once in the food webs, these pesticides can causemutations, 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, manyanimal feeds have a highfish meal orfish hydrolysate content. In this way,marine toxins can be transferred to land animals, and appear later in meat and dairy products.
Overfishing is the removal of a species offish (i.e.fishing) from abody of water at a rate greater than that the species can replenish itspopulation naturally (i.e. theoverexploitation of thefishery's existingfish stock), resulting in the species becoming increasinglyunderpopulated in that area. Overfishing can occur in water bodies of any sizes, such asponds,wetlands,rivers,lakes or oceans, and can result inresource depletion, reduced biological growth rates and lowbiomass levels. Sustained overfishing can lead tocritical depensation, where the fish population is no longer able to sustain itself. Some forms of overfishing, such as theoverfishing of sharks, has led to the upset of entiremarine ecosystems.[195] Types of overfishing include growth overfishing, recruitment overfishing, and ecosystem overfishing. Overfishing not only causes negative impacts on biodiversity and ecosystem functioning, but also reduces fish production, which subsequently leads to negative social and economic consequences.[196]
Ocean protection serves to safeguard the ecosystems in the oceans upon which humans depend.[197][198] Protecting these ecosystems from threats is a major component ofenvironmental protection. One of protective measures is the creation and enforcement ofmarine protected areas (MPAs). Marine protection may need to be considered within a national, regional and international context.[199] Other measures include supply chain transparency requirement policies, policies to prevent marine pollution, ecosystem-assistance (e.g.for coral reefs) and support forsustainable seafood (e.g.sustainable fishing practices and types of aquaculture). There is also the protection of marine resources and components whose extraction or disturbance would cause substantial harm, engagement of broader publics and impacted communities,[200] and the development of ocean clean-up projects (removal of marine plastic pollution). Examples of the latter includeClean Oceans International andThe Ocean Cleanup.
In 2021, 43 expert scientists published the first scientific framework version that – via integration,review, clarifications andstandardization – 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"[201] and the UN'sSustainable Development Goal 14 ("life below water").[202][203]
In March 2023 aHigh Seas Treaty was signed. It is legally binding. The main achievement is the new possibility to create marine protected areas in international waters. By doing so the agreement now makes it possible to protect 30% of the oceans by 2030 (part of the30 by 30 target).[204][205] The treaty has articles regarding the principle "polluter-pays", and different impacts of human activities including areas beyond the national jurisdiction of the countries making those activities. The agreement was adopted by the 193 United Nations Member States.[206]
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