Ocean
| Earth's ocean | |
|---|---|
 Pacific Oceanside,Apollo 11,July 1969 | |
| Basincountries | List of countries by length of coastline |
| Surface area | 361,000,000 km2(139,382,879 sq mi) (71% Earth's surface area)[1] |
| Average depth | 3.688 km (2 mi)[2] |
| Max. depth | 11.034 km (6.856 mi) (Challenger Deep)[3] |
| Water volume | 1,370,000,000 km3(328,680,479 cu mi)[1](97.5% of Earth's water) |
| Shore length1 | Low interval calculation: 356,000 km (221,208 mi)[4] High interval calculation: 1,634,701 km (1,015,756 mi)[5][vague] |
| Max. temperature | |
| Min. temperature | |
| Sections/sub-basins | Main divisions(volume %):
|
| Trenches | List of oceanic trenches |
| 1Shore length isnot a well-defined measure. | |
Theoceanis the body ofsalt waterthat covers approx. 70.8% ofEarth.[8]InEnglish,the termoceanalso refers to any of the large bodies of water into which the world ocean is conventionally divided.[9]The following names describe five different areas of the ocean:Pacific,Atlantic,Indian,Antarctic/Southern,andArctic.[10][11]The ocean contains 97% ofEarth's water[8]and is the primary component of Earth'shydrosphere;thus the ocean is essential tolifeon Earth. The ocean influencesclimateandweatherpatterns, thecarbon cycle,and thewater cycleby acting as a hugeheat reservoir.
Ocean scientistssplit the ocean into vertical and horizontal zones based on physical and biological conditions. Thepelagic zoneis the open ocean'swater columnfrom the surface to the ocean floor. The water column is further divided into zones based on depth and the amount of light present. Thephotic zonestarts at the surface and is defined to be "the depth at which light intensity is only 1% of the surface value"[12]: 36 (approximately 200 m in the open ocean). This is the zone wherephotosynthesiscan 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 mostbiodiverseand the source of the food supply which sustains most of the oceanecosystem.Ocean photosynthesis also produces half of the oxygen in the Earth's atmosphere.[13]Light can only penetrate a few hundred more meters; the rest of the deeper ocean is cold and dark (these zones are calledmesopelagicandaphoticzones). Thecontinental shelfis where the ocean meets dry land. It is more shallow, with a depth of a few hundred meters or less. Human activity often hasnegative impactsonmarine lifewithin the continental shelf.
Ocean temperaturesdepend on the amount of solar radiation reaching the ocean surface. In the tropics,surface temperaturescan rise to over 30 °C (86 °F). Near the poles wheresea iceforms, 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).[14]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.[15]Tidescreate tidal currents, while wind and waves cause surface currents. TheGulf Stream,Kuroshio Current,Agulhas CurrentandAntarctic Circumpolar Currentare all major ocean currents. Such currents transport massive amounts of water, gases, pollutants andheatto different parts of the world, and from the surface into the deep ocean. All this has impacts on the globalclimate system.
Ocean water contains dissolved gases, includingoxygen,carbon dioxideandnitrogen.Anexchange of these gasesoccurs at the ocean's surface. The solubility of these gases depends on the temperature and salinity of the water.[16]Thecarbon dioxide concentration in the atmosphereis rising due toCO2emissions,mainly fromfossil fuelcombustion. As the oceans absorb CO2from theatmosphere,a higher concentration leads toocean acidification(a drop inpH value).[17]
The ocean provides many benefits to humans such asecosystem services,access toseafoodand othermarine resources,and a means oftransport.The ocean is known to be thehabitatof over 230,000species,but may hold considerably more – perhaps over two million species.[18]Yet, the ocean faces manyenvironmentalthreats, such asmarine pollution,overfishing,and theeffects of climate change.Those effects includeocean warming,ocean acidification andsea level rise.Thecontinental shelfandcoastal watersare most affected by human activity.
Terminology
Ocean and sea
The terms "the ocean" or "the sea" used without specification refer to the interconnected body of salt water covering the majority of Earth's surface.[10][11]It includes thePacific,Atlantic,Indian,Southern/Antarctic,andArcticoceans.[19]As a general term, "the ocean" and "the sea" are often interchangeable.[20]
Strictly speaking, a "sea" is a body of water (generally a division of the world ocean) partly or fully enclosed by land.[21]The word "sea" can also be used for many specific, much smaller bodies of seawater, such as theNorth Seaor 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.[22]
World Ocean
The contemporary concept of theWorld Oceanwas coined in the early 20th century by theRussianoceanographerYuly Shokalskyto refer to the continuous ocean that covers and encircles most of Earth.[23][24]The global, interconnected body of salt water is sometimes referred to as the World Ocean,global oceanorthe great ocean.[25][26][27]The concept of a continuous body of water with relatively unrestricted exchange between its components is critical inoceanography.[28]
Etymology
The wordoceancomes from the figure inclassical antiquity,Oceanus(/oʊˈsiːənəs/;Greek:ὨκεανόςŌkeanós,[29]pronounced[ɔːkeanós]), the elder of theTitansin classicalGreek mythology.Oceanus was believed by theancient GreeksandRomansto be the divine personification of an enormousriverencircling the world.
The concept of Ōkeanós has anIndo-Europeanconnection. Greek Ōkeanós has been compared to theVedicepithet 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.[30]
Natural history
Origin of water
Scientists believe that a sizable quantity ofwaterwould have been in the material that formed Earth.[31]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 activityandmeteorite impacts,produced an early atmosphere ofcarbon dioxide,nitrogenandwater 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, thewater vaporover time would have condensed, forming Earth's first oceans.[32]The early oceans might have been significantly hotter than today and appeared green due to high iron content.[33]
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 Beltand provides evidence that water existed on Earth 3.8 billion years ago.[34]In theNuvvuagittuq Greenstone Belt,Quebec,Canada, rocks dated at 3.8 billion years old by one study[35]and 4.28 billion years old by another[36]show evidence of the presence of water at these ages.[34]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 theEarthsince the beginning of the planet's formation.[37][38][39]In this model, atmosphericgreenhouse gaseskept the oceans from freezing when the newly formingSunhadonly 70%of itscurrent luminosity.[40]
Ocean formation
The origin of Earth's oceans is unknown. Oceans are thought to have formed in theHadeaneon and may have been the cause for theemergence of life.
Plate tectonics,post-glacial rebound,andsea level risecontinually change thecoastlineand structure of the world ocean. A global ocean has existed in one form or another on Earth for eons.
Since its formation the ocean has taken many conditions and shapes with manypast ocean divisionsand potentially at times covering the whole globe.[41]
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.[42]
Geography
The entire ocean, containing 97% of Earth's water, spans 70.8% ofEarth's surface,[8]making it Earth's global ocean orworld ocean.[23][25]This makes Earth, along with its vibranthydrospherea "water world"[43][44]or "ocean world",[45][46]particularly in Earth's early history when the ocean is thought to have possibly covered Earth completely.[41]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.
Seawatercovers about 361,000,000 km2(139,000,000 sq mi) and the ocean's furthestpole of inaccessibility,known as "Point Nemo",in a region known asspacecraft cemeteryof theSouth Pacific Ocean,at48°52.6′S123°23.6′W/ 48.8767°S 123.3933°W.This point is roughly 2,688 km (1,670 mi) from the nearest land.[47]
Oceanic divisions
There are different customs to subdivide the ocean and are adjourned by smaller bodies of water such as,seas,gulfs,bays,bights,andstraits.
The ocean is customarily divided into five principal oceans – listed below in descending order of area and volume:
| # | Ocean | Location | Area (km2) |
Volume (km3) |
Avg. depth (m) |
Coastline (km)[48] |
|---|---|---|---|---|---|---|
| 1 | Pacific Ocean | BetweenAsiaandAustralasiaand theAmericas[49] | 168,723,000 (46.6%) |
669,880,000 (50.1%) |
3,970 | 135,663 (35.9%) |
| 2 | Atlantic Ocean | Between theAmericasandEuropeandAfrica[50] | 85,133,000 (23.5%) |
310,410,900 (23.3%) |
3,646 | 111,866 (29.6%) |
| 3 | Indian Ocean | Betweensouthern Asia,AfricaandAustralia[51] | 70,560,000 (19.5%) |
264,000,000 (19.8%) |
3,741 | 66,526 (17.6%) |
| 4 | Antarctic/Southern Ocean | BetweenAntarcticaand the Pacific, Atlantic and Indian oceans Sometimes considered an extension of those three oceans.[52][53] |
21,960,000 (6.1%) |
71,800,000 (5.4%) |
3,270 | 17,968 (4.8%) |
| 5 | Arctic Ocean | Between northernNorth AmericaandEurasiain theArctic Sometimes considered amarginal seaof the Atlantic.[54][55][56] |
15,558,000 (4.3%) |
18,750,000 (1.4%) |
1,205 | 45,389 (12.0%) |
| Total | 361,900,000 (100%) |
1.335×109 (100%) |
3,688 | 377,412 (100%) | ||
Sources:Encyclopedia of Earth,[49][50][51][52][56]International Hydrographic Organization,[53]Regional Oceanography: an Introduction(Tomczak, 2005),[54]Encyclopædia Britannica,[55]and theInternational Telecommunication Union.[48]
Ocean basins
The ocean fills Earth'soceanic basins.Earth's oceanic basins cover differentgeologic provincesof Earth'soceanic crustas well ascontinental crust.As such it covers mainly Earth'sstructural basins,but alsocontinental shelfs.
In mid-ocean,magmais constantly being thrust through the seabed between adjoining plates to formmid-oceanic ridgesand 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.Deeptrenchesare 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 andmagmais forced up creating underwater mountains, some of which may form chains ofvolcanic islandsnear 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.[57][58]
Every ocean basin has amid-ocean ridge,which creates a long mountain range beneath the ocean. Together they form the globalmid-oceanic ridgesystem that features thelongestmountain rangein 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.[59]
Oceanographersstate that less than 20% of the oceans have been mapped.[60][vague]
Interaction with the coast
The zone where land meets sea is known as thecoast,and the part between the lowest springtidesand the upper limit reached by splashing waves is theshore.Abeachis the accumulation of sand orshingleon the shore.[61]Aheadlandis a point of land jutting out into the sea and a largerpromontoryis known as acape.The indentation of a coastline, especially between two headlands, is abay,a small bay with a narrow inlet is acoveand a large bay may be referred to as agulf.[62]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.[61]
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 theswashmoves 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 normalweatheringprocesses 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.[61]
Material worn from the margins of the land eventually ends up in the sea. Here it is subject toattritionas 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 causingdeltasto form in estuaries. All these materials move back and forth under the influence of waves, tides and currents.[61]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 leveesand other sea defences. For instance, theThames Barrieris designed to protect London from a storm surge,[63]while the failure of the dykes and levees aroundNew OrleansduringHurricane Katrinacreated ahumanitarian crisisin the United States.
Physical properties
Color
Most of the ocean is blue in color, but in some places the ocean is blue-green, green, or even yellow to brown.[64]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).[65]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,[66]and is similar toblue light scattering in the sky.
The main substances that affect the color of the ocean includedissolved organic matter,livingphytoplanktonwithchlorophyllpigments, and non-living particles likemarine snowand mineralsediments.[67]Chlorophyll can be measured bysatelliteobservations 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.Water cycle, weather, and rainfall
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.[68]Oceans have a significant effect on thebiosphere.The ocean as a whole is thought to cover approximately 90% of the Earth'sbiosphere.[60]Oceanicevaporation,as a phase of the water cycle, is the source of most rainfall (about 90%),[68]causing a globalcloud coverof 67% and a consistent oceanic cloud cover of 72%.[69]Ocean temperaturesaffectclimateandwindpatterns that affect life on land. One of the most dramatic forms ofweatheroccurs 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 tolifeon Earth, forms part of thecarbon cycleandwater cycle,and – as a hugeheat reservoir– influences climate and weather patterns.
Waves and swell
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 wavesthat propagate along the interface between water and air is calledswell– a term used insailing,surfingandnavigation.[70]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 formswavesthat are perpendicular to the direction of the wind. The friction between air and water caused by a gentle breeze on a pond causesripplesto 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 calledswellroll across the ocean.[71]: 83–84 [72][73]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.[72]
Constructive interferencecan lead to the formation of unusually highrogue waves.[74]Most waves are less than 3 m (10 ft) high[74]and it is not unusual for strong storms to double or triple that height.[75]Rogue waves, however, have been documented at heights above 25 meters (82 ft).[76][77]
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.[74]This rushes in a sheet up the beach before retreating into the ocean under the influence of gravity.[78]
Earthquakes,volcanic eruptionsor other major geological disturbances can set off waves that can lead totsunamisin coastal areas which can be very dangerous.[79][80]
Sea level and surface
Theocean's surfaceis 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
Tides are the regular rise and fall in water level experienced by oceans, primarily driven bythe Moon's gravitationaltidal forcesupon the Earth. Tidal forces affect all matter on Earth, but onlyfluidslike the ocean demonstrate the effects on human timescales. (For example, tidal forces acting on rock may producetidal lockingbetween 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,[81]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 tableswhich 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 rangeor tidal amplitude.[82][83]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.[82]
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.[84]Some of the largest tidal ranges in the world occur in theBay of FundyandUngava Bayin Canada, reaching up to 16 meters.[85]Other locations with record high tidal ranges include theBristol Channelbetween England and Wales,Cook Inletin Alaska, and theRío Gallegosin Argentina.[86]
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.
Depth
The average depth of the oceans is about 4 km. More precisely the average depth is 3,688 meters (12,100 ft).[72]Nearly half of the world's marine waters are over 3,000 meters (9,800 ft) deep.[27]"Deep ocean," which is anything below 200 meters (660 ft), covers about 66% of Earth's surface.[87]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.[88]The maximum depth has been estimated to be 10,971 meters (35,994 ft). The British naval vesselChallenger IIsurveyed the trench in 1951 and named the deepest part of the trench the "Challenger Deep".In 1960, theTriestesuccessfully reached the bottom of the trench, manned by a crew of two men.
Oceanic zones
Oceanographersclassify the ocean into vertical and horizontal zones based on physical and biological conditions. Thepelagic zoneconsists of thewater columnof the open ocean, and can be divided into further regions categorized by light abundance and by depth.
Grouped by light penetration
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 zoneis defined to be "the depth at which light intensity is only 1% of the surface value".[12]: 36 This is usually up to a depth of approximately 200 m in the open ocean. It is the region wherephotosynthesiscan 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.[89]The actual optics of light reflecting and penetrating at the ocean surface are complex.[12]: 34–39
- Below the photic zone is themesopelagicor 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.[12]: 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 ventsare a source of energy in what is known as theaphotic zone(depths exceeding 200 m).[89]
Grouped by depth and temperature
The pelagic part of the aphotic zone can be further divided into vertical regions according to depth and temperature:[89]
- Themesopelagicis the uppermost region. Its lowermost boundary is at athermoclineof 12 °C (54 °F) which generally lies at 700–1,000 meters (2,300–3,300 ft) in thetropics.Next is thebathypelagiclying 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 plainis theabyssopelagic,whose lower boundary lies at about 6,000 meters (20,000 ft). The last and deepest zone is thehadalpelagicwhich includes theoceanic trenchand lies between 6,000–11,000 meters (20,000–36,000 ft).
- Thebenthiczones are aphotic and correspond to the three deepest zones of thedeep-sea.Thebathyal zonecovers 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, thehadalzone 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 calledthermoclines(temperature),haloclines(salinity),chemoclines(chemistry), andpycnoclines(density). If a zone undergoes dramatic changes in temperature with depth, it contains athermocline,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, temperaturestratificationis 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.[90]If a zone undergoes dramatic changes in salinity with depth, it contains ahalocline.If a zone undergoes a strong, vertical chemistry gradient with depth, it contains achemocline.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.[89]The halocline often coincides with the thermocline, and the combination produces a pronouncedpycnocline,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 zoneand 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 zonecovers the region between low and high tide and represents the transitional area between marine and terrestrial conditions. It is also known as theintertidalzone because it is the area where tide level affects the conditions of the region.[89]
Volumes
The combined volume of water in all the oceans is roughly 1.335 billion cubic kilometers (1.335sextillionliters, 320.3 million cubic miles).[72][91][92]
It has been estimated that there are 1.386 billioncubickilometres (333 million cubic miles) of water on Earth.[93][94][95]This includes water in gaseous, liquid and frozen forms as soil moisture,groundwaterandpermafrostin theEarth's crust(to a depth of 2 km); oceans andseas,lakes,riversandstreams,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.Saltwateraccounts for 97.5% of this amount, whereasfresh wateraccounts for only 2.5%. Of this fresh water, 68.9% is in the form oficeand 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.[96]
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 × 1013tonnes of this is in the form ofwater vaporin 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 averagesalinityof Earth's oceans is about 35 grams ofsaltper kilogram of sea water (3.5%).[97]Temperature
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 layerscan rise to over 30 °C (86 °F) while near thepolesthe temperature in equilibrium with thesea iceis 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 temperaturebetween −2 °C (28 °F) and 5 °C (41 °F) in all parts of the globe.[14]
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 mi xing 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 depthis typically about 100 m (but varies) and is related to this heated surface layer.[98]
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 (precipitationvs.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.
| Characteristic | Polar regions | Temperate regions | Tropical regions |
|---|---|---|---|
| Precipitationvs.evaporation | Precip > Evap | Precip > Evap | Evap > Precip |
| Sea surface temperaturein winter | −2 °C | 5 to 20 °C | 20 to 25 °C |
| Averagesalinity | 28‰ to 32‰ | 35‰ | 35‰ to 37‰ |
| Annual variation ofair temperature | ≤ 40 °C | 10 °C | < 5 °C |
| Annual variation ofwater temperature | < 5 °C | 10 °C | < 5 °C |
Sea ice
Seawater with a typical salinity of 35‰ has a freezing point of about −1.8 °C (28.8 °F).[89][107]Because sea ice is lessdensethan water, it floats on the ocean's surface (as doesfresh waterice, which has an even lower density). Sea ice covers about 7% of the Earth's surface and about 12% of the world's oceans.[108][109][110]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.[111]
Ocean currents and global climate
Types of ocean currents
Anocean currentis a continuous, directed flow of seawater caused by several forces acting upon the water. These includewind,theCoriolis effect,temperatureandsalinitydifferences.[15]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.[112]Non-periodic or non-tidal currents are created by the action of winds and changes indensity of water.Inlittoral zones,breaking wavesare so intense and the depth measurement so low, that maritime currents reach often 1 to 2knots.[113]
Thewindandwavescreate 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 driftunder 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.[113]
This acceleration of the current takes place in the direction of waves and dominant wind. Accordingly, when the ocean depth increases, therotationof theearthchanges 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 thethermocline.[113]
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 currentandAntarctic Circumpolar Current.The Antarctic Circumpolar Current encirclesAntarcticaand influences the area's climate, connecting currents in several oceans.[113]
Relationship of currents and climate
Collectively, currents move enormous amounts of water and heat around the globe influencingclimate.These wind driven currents are largely confined to the top hundreds of meters of the ocean. At greater depth, thethermohaline circulationdrives 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 thepoleswhich 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.[113]This circulation has important impacts on the globalclimate systemand 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 currentsgreatly affect Earth's climate bytransferring heatfrom thetropicsto thepolar regions.This affects air temperature and precipitation in coastal regions and further inland. Surface heat and freshwaterfluxescreate globaldensity gradients,which drive thethermohaline circulationthat is a part of large-scale ocean circulation. It plays an important role in supplying heat to the polar regions, and thus insea iceregulation.[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 westover thePacific Ocean.New York, on the east coast of North America getswinds from the westover 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 dioxideconcentrations. Modern observations,climate simulationsand 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.[114]: 19 Such a weakening could cause large changes to global climate, with the North Atlantic particularly vulnerable.[114]: 19
Chemical properties
Salinity
Salinityis a measure of the total amounts of dissolved salts inseawater.It was originally measured via measurement of the amount ofchloridein seawater and hence termed chlorinity. It is now standard practice to gauge it by measuringelectrical conductivityof the water sample. Salinity can be calculated using the chlorinity, which is a measure of the total mass ofhalogenions (includes fluorine, chlorine, bromine, and iodine) in seawater. According to an international agreement, the following formula is used to determine salinity:[116]
- Salinity (in ‰) = 1.80655 × Chlorinity (in ‰)
The average ocean water chlorinity is about 19.2‰, and, thus, the average salinity is around 34.7‰.[116]
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,[117]normal seawater freezes at a temperature of about −2 °C.
Salinity is higher in Earth's oceans where there is moreevaporationand lower where there is moreprecipitation.If precipitation exceeds evaporation, as is the case inpolarand 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‰.[118]Thus, oceanic waters in polar regions have lower salinity content than oceanic waters in tropical regions.[116]However, whensea iceforms at high latitudes,salt is excludedfrom the ice as it forms, which can increase the salinity in the residual seawater in polar regions such as theArctic Ocean.[89][119]
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.[120]It is very likely that the Pacific and Antarctic/Southern Oceans have freshened while the Atlantic has become more saline.[120]
Dissolved gases
Ocean water contains large quantities of dissolved gases, includingoxygen,carbon dioxideandnitrogen.These dissolve into ocean water viagas exchangeat the ocean surface, with the solubility of these gases depending on the temperature and salinity of the water.[16]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)[122][123][124]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 moresolubleat colder temperatures, and their solubility changes with temperature at different rates.[122][125]
Oxygen, photosynthesis and carbon cycling
Photosynthesisin 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,carbonateandbicarbonate.[98]Thiscycling of carbon dioxide in oceansis an important part of the globalcarbon cycle.
The oceans represent a majorcarbon sinkfor 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 habitatssuch asmangrovesandsaltmarshes.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.[98]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 zonesorhypoxicwaters, will generally be made worse by theeffects of climate change on oceans.[127][128]
pH
ThepH valueat the surface of oceans (global mean surface pH) is currently approximately in the range of 8.05[129]to 8.08.[130]This makes it slightlyalkaline.The pH value at the surface used to be about 8.2 during the past 300 million years.[131]However, between 1950 and 2020, the average pH of the ocean surface fell from approximately 8.15 to 8.05.[132]Carbon dioxide emissionsfrom human activities are the primary cause of this process calledocean acidification,withatmospheric carbon dioxide (CO2) levelsexceeding 410 ppm (in 2020).[133]CO2from theatmosphereis 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.[134]It can be as high as 8.4 in surface waters in areas of highbiological productivity.[98]
The definition ofglobal mean surface pHrefers 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 lifeandocean currents(see alsoeffects of climate change on oceans).[98]
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 mi xing is calledocean stratification). This in turn depends on the water temperature and hence is different between the tropics and the polar regions (seeocean#Temperature).[98]
Thechemical propertiesof seawater complicate pH measurement, and several distinct pH scales exist inchemical oceanography.[135]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.[136]
Alkalinity
Alkalinityis 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.[98]
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.[98][134]
The absorption of CO2from the atmosphere does not affect the ocean'salkalinity.[137]: 2252 It does lead to a reduction in pH value though (termedocean acidification).[133]
Residence times of chemical elements and ions
The ocean waters contain manychemical elementsas 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 assodiumandchloride,together making up the majority of ocean salts. Other elements, such asiron,are present at tiny concentrations of just a few nanograms (10−9grams) per liter.[116]
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 insedimentsor evaporating to theatmospherein the case of water and some gases. By estimating theresidence timeof 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 andaluminiumare 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.[116]
| Chemical element or ion | Residence time (years) |
|---|---|
| Chloride(Cl−) | 100,000,000 |
| Sodium(Na+) | 68,000,000 |
| Magnesium(Mg2+) | 13,000,000 |
| Potassium(K+) | 12,000,000 |
| Sulfate(SO42−) | 11,000,000 |
| Calcium(Ca2+) | 1,000,000 |
| Carbonate(CO32−) | 110,000 |
| Silicon(Si) | 20,000 |
| Water(H2O) | 4,100 |
| Manganese(Mn) | 1,300 |
| Aluminum(Al) | 600 |
| Iron(Fe) | 200 |
Nutrients
A few elements such asnitrogen,phosphorus,iron,andpotassiumessential for life, are major components of biological material, and are commonly known as "nutrients".Nitrate and phosphate have oceanresidence timesof 10,000[140]and 69,000[141]years, respectively, while potassium is a much more abundant ion in the ocean with a residence time of 12 million[142]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 assediment.
Phosphate fromintensive agricultureanduntreated sewageis 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.[143]Production ofrock phosphate,an essential ingredient in inorganicfertilizer,[144]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 securityin future.[145][146]
Marine life
Life within the oceanevolved3 billion years prior to life on land. Both the depth and the distance from shore strongly influence thebiodiversityof the plants and animals present in each region.[148]The diversity of life in the ocean is immense, including:
- Animals:most animalphylahave species that inhabit the ocean, including many that are found only in marine environments such assponges,Cnidaria(such ascoralsandjellyfish),comb jellies,Brachiopods,andEchinoderms(such assea urchinsandsea stars). Many other familiar animal groups primarily live in the ocean, includingcephalopods(includesoctopusandsquid),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,seabirdsare 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,seagullsandpelicans.Seven species of turtles, thesea turtles,also spend most of their time in the oceans.
- Plants:includingsea grasses,ormangroves
- Algae:algae is a "catch-all" term to include manyphotosynthetic,single-celledeukaryotes,such asgreen algae,diatoms,anddinoflagellates,but also multicellular algae, such as somered algae(including organisms likePyropia,which is the source of the ediblenoriseaweed), andbrown algae(including organisms likekelp).
- Bacteria:ubiquitous single-celledprokaryotesfound throughout the world
- Archaea:prokaryotesdistinct from bacteria, that inhabit many environments of the ocean, as well as manyextreme environments
- Fungi:manymarine fungiwith diverse roles are found in oceanic environments
 | This articlecontains too many pictures for its overall length.(December 2023) |
Marine life,sea life, or ocean life is theplants,animals,and otherorganismsthat live in thesalt waterofseasor oceans, or thebrackish waterof coastalestuaries.At a fundamental level, marine life affects the nature of the planet. Marine organisms, mostlymicroorganisms,produce oxygenandsequester carbon.Marine life, in part, shape and protect shorelines, and some marine organisms even help create new land (e.g.coralbuildingreefs).
Marine species range in size from the microscopic likephytoplankton,which can be as small as 0.02micrometres,to hugecetaceanslike theblue whale– the largest known animal, reaching 33 m (108 ft) in length.[150][151]Marine microorganisms, includingprotistsandbacteriaand their associatedviruses,have been variously estimated as constituting about 70%[152]or about 90%[153][149]of the total marinebiomass.Marine life is studied scientifically in bothmarine biologyand inbiological oceanography.The termmarinecomes from theLatinmare,meaning "sea" or "ocean".
Human uses of the oceans
The ocean has been linked to human activity throughout history. These activities serve a wide variety of purposes, includingnavigation and exploration,naval warfare,travel,shippingandtrade,food production (e.g.fishing,whaling,seaweed farming,aquaculture), leisure (cruising,sailing,recreational boat fishing,scuba diving), power generation (seemarine energyandoffshore wind power), extractive industries (offshore drillinganddeep sea mining),freshwaterproduction viadesalination.
Many of the world's goods are moved byshipbetween the world'sseaports.[159]Large quantities of goods are transported across the ocean, especially across the Atlantic and around the Pacific Rim.[160]Many types of cargo including manufactured goods, are typically transported instandard sized, lockable containersthat are loaded on purpose-builtcontainer shipsatdedicated terminals.[161]Containerization greatly boosted the efficiency and reduced the cost of shipping products by sea. This was a major factor in the rise ofglobalizationand exponential increases ininternational tradein the mid-to-late 20th century.[162]
Oceans are also the major supply source for thefishing industry.Some of the major harvests areshrimp,fish,crabs,andlobster.[60]The biggest global commercial fishery is foranchovies,Alaska pollockandtuna.[163]: 6 A report byFAOin 2020 stated that "in 2017, 34 percent of the fish stocks of the world's marine fisheries were classified asoverfished".[163]: 54 Fish and other fishery products from bothwild fisheriesand 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".[163]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.[164]
The ocean has a vast amount ofenergycarried byocean waves,tides,salinitydifferences, andocean temperature differenceswhich can be harnessed togenerate electricity.[165]Forms ofsustainable marine energyincludetidal power,ocean thermal energyandwave power.[165][166]Offshore wind poweris captured bywind turbinesplaced 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.[167]There are large deposits ofpetroleum,as oil andnatural gas,in rocks beneath the ocean floor.Offshore platformsanddrilling rigsextractthe oil or gas and store it for transport to land.[168]
"Freedom of the seas" is a principle ininternational lawdating from the seventeenth century. It stresses freedom to navigate the oceans and disapproves of war fought ininternational waters.[169]Today, this concept is enshrined in theUnited Nations Convention on the Law of the Sea(UNCLOS).[169]
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 governanceis the conduct of the policy, actions and affairs regarding the world'soceans.[170]
Threats from human activities
Human activities affectmarine lifeandmarine habitatsthrough many negative influences, such asmarine pollution(includingmarine debrisandmicroplastics)overfishing,ocean acidificationand othereffects of climate change on oceans.
Climate change
There are manyeffects of climate change on oceans.One of the main ones is an increase inocean temperatures.More frequentmarine heatwavesare linked to this. The rising temperature contributes to arise in sea levelsdue to meltingice sheets.Other effects on oceans includesea ice decline,reducingpH valuesandoxygen levels,as well as increasedocean stratification.All this can lead to changes ofocean currents,for example a weakening of theAtlantic meridional overturning circulation(AMOC).[99]The main root cause of these changes are theemissions of greenhouse gasesfrom human activities, mainly burning offossil fuels.Carbon dioxideandmethaneare examples of greenhouse gases. The additionalgreenhouse effectleads toocean warmingbecause the ocean takes up most of the additional heat in theclimate system.[172]The ocean also absorbs some of the extracarbon dioxide that is in the atmosphere.This causes thepH value of the seawater to drop.[173]Scientists estimate that the ocean absorbs about 25% of all human-caused CO2emissions.[173]
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.[174]: 471 Connected to this is a decline in mi xing of the ocean layers, so that warm water stabilises near the surface. A reduction of cold, deepwater circulationfollows. The reduced vertical mi xing 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 cyclonesand other storms. Another result is a decrease innutrientsfor fish in the upper ocean layers. These changes also reduce the ocean's capacity tostore carbon.[175]At the same time, contrasts insalinityare increasing. Salty areas are becoming saltier and fresher areas less salty.[176]
Warmer water cannot contain the same amount of oxygen as cold water. As a result, oxygen from the oceans moves to the atmosphere. Increasedthermal stratificationmay reduce the supply of oxygen from surface waters to deeper waters. This lowers the water's oxygen content even more.[177]The ocean has already lost oxygen throughout itswater column.Oxygen minimum zonesare increasing in size worldwide.[174]: 471
These changes harmmarine ecosystems,and this can lead tobiodiversity lossor changes in species distribution.[99]This in turn canaffect fishingand coastal tourism. For example, rising water temperatures are harming tropicalcoral reefs.The direct effect iscoral bleachingon 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 icehabitats due to warming. This will have severe impacts onpolar bearsand 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.[99]Marine pollution
Marine pollutionoccurs when substances used or spread by humans, such asindustrial,agriculturalandresidentialwaste,particles,noise,excesscarbon dioxideorinvasive organismsenter the ocean and cause harmful effects there. The majority of this waste (80%) comes from land-based activity, althoughmarine transportationsignificantly contributes as well.[178]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.[179]Since most inputs come from land, either via therivers,sewageor the atmosphere, it means thatcontinental shelvesare more vulnerable to pollution.Air pollutionis also a contributing factor by carrying off iron, carbonic acid,nitrogen,silicon, sulfur,pesticidesor dust particles into the ocean.[180]The pollution often comes fromnonpoint sourcessuch as agriculturalrunoff,wind-blowndebris,and dust. These nonpoint sources are largely due to runoff that enters the ocean through rivers, but wind-blowndebrisand dust can also play a role, as these pollutants can settle into waterways and oceans.[181]Pathways of pollution include direct discharge, land runoff,ship pollution,bilge pollution,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 tomicroplasticsformed from the fragmentation of plastic material. 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 treatedsewageto rivers and subsequently oceans. Thesenitrogenandphosphorusnutrients (which are also contained infertilizers) stimulatephytoplanktonandmacroalgalgrowth, 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 reefsand lead toloss of biodiversityand coral health. A second major concern is that the degradation ofalgal bloomscan lead to consumption ofoxygenin coastal waters, a situation that may worsen withclimate changeas warming reduces vertical mi xing of the water column.[182]
Many potentially toxic chemicals adhere to tiny particles which are then taken up byplanktonandbenthic animals,most of which are eitherdeposit feedersorfilter feeders.In this way, the toxins areconcentrated upwardwithin 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 metalscan 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 feedshave a highfish mealorfish hydrolysatecontent. In this way,marine toxinscan be transferred to land animals, and appear later in meat and dairy products.Overfishing
Protection
Ocean protection serves to safeguard the ecosystems in the oceans upon which humans depend.[184][185]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.[186]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,[187]and the development of ocean clean-up projects (removal of marine plastic pollution). Examples of the latter includeClean Oceans InternationalandThe 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 ofmarine protected areasand 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"[188]and the UN'sSustainable Development Goal 14( "life below water" ).[189][190]
In March 2023 aHigh Seas Treatywas 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 30target).[191][192]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.[193]
See also
- European Atlas of the Seas
- Land and water hemispheres
- List of seas
- Marine heatwave
- Ocean (disambiguation)
- Ocean world
- Planetary oceanography
- World Ocean Atlas
- World Oceans Day
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Figure 1f
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Further reading
- Peters, Kimberley, et al. eds.The Routledge Handbook of Ocean Space(2022), how experts in 32 fields study and interpret the oceans,onlne book review
External links
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