Ocean

• 1. Introduction

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  The ocean covers approximately 70.8% of Earth and holds 97% of the planet's water. It originated photosynthesis, gave Earth its atmospheric oxygen, and still supplies half of it. The word itself comes from Oceanus, the elder of the Titans in classical Greek mythology. Ancient Greeks and Romans believed him to be the divine personification of an enormous river encircling the world.

  This single body of salt water is the primary component of Earth's hydrosphere. It acts as a huge reservoir of heat for Earth's energy budget. It anchors the carbon cycle and the water cycle, forming the basis for climate and weather worldwide. Most of Earth's animals and protist life shelter within it.

  How did this water arrive, and where does it go? Why is the surface warm while the deep stays near freezing? What lives in water no sunlight reaches, and what happens as humans pour carbon into it? Scientists have mapped just over 26% of the ocean floor at high resolution as of 2024. By some estimates only 5% of it has been explored.

• 2. The Origin Of Water

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  Scientists believe a sizable quantity of water sat in the very material that formed Earth. When the planet was less massive during formation, water molecules escaped its gravity more easily, a process called atmospheric escape. During planetary formation Earth possibly held magma oceans.

  Outgassing, volcanic activity, and meteorite impacts produced an early atmosphere of carbon dioxide, nitrogen, and water vapor. These gases accumulated over millions of years. After Earth's surface cooled significantly, the vapor condensed into the first oceans. Those early oceans might have been far hotter than today and appeared green from high iron content.

  Rock holds the timeline. A sample of pillow basalt, formed during an underwater eruption, came from the Isua Greenstone Belt and shows water existed 3.8 billion years ago. In the Nuvvuagittuq Greenstone Belt in Quebec, Canada, rocks dated by one study at 3.8 billion years and by another at 4.28 billion years carry evidence of water. In August 2020, researchers proposed enough water to fill the oceans may have always been present, with greenhouse gases keeping the seas unfrozen when the young Sun shone at only 70% of its current luminosity.

• 3. Charting The Five Oceans

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  Seawater covers about 361,000,000 square kilometers, and its furthest pole of inaccessibility is Point Nemo. This spot in the South Pacific, known as the spacecraft cemetery, sits roughly 2,688 kilometers from the nearest land. By convention the World Ocean splits into five: the Pacific, Atlantic, Indian, Arctic, and Southern. The Pacific leads in area at 46.6% and the Arctic trails at 4.3%.

  The five-ocean model only fully crystallized in the early 21st century. The Southern Ocean, delineated by the Antarctic Circumpolar Current, won recognition from the U.S. Board on Geographic Names in 1999 and the International Hydrographic Organization in 2000. The contemporary idea of one continuous World Ocean came earlier, coined by the Russian oceanographer Yuly Shokalsky in the early 20th century.

  A sea differs from an ocean by being smaller and partly or wholly bordered by land, like the North Sea or the Red Sea. In medieval Europe the World Sea encircled the mainland of Europe, Asia, and Africa, excluding the Mediterranean, Black, and Caspian Seas. Today the Black and Caspian seas still occupy oceanic basins of their own.

• 4. The Restless Floor

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  In mid-ocean, magma is constantly thrust through the seabed between adjoining plates to build mid-oceanic ridges. Convection currents in the mantle drive the two plates apart. Nearer the coasts, one oceanic plate may slide beneath another in a process called subduction, carving deep trenches. The movement proceeds in jerks that cause earthquakes, while forced-up magma raises underwater mountains and chains of volcanic islands.

  Every ocean basin carries a mid-ocean ridge. Together they form the global mid-oceanic ridge system, the longest mountain range in the world. Its longest continuous stretch runs 65,000 kilometers, several times longer than the Andes, the longest continental range.

  The Mariana Trench, in the Pacific near the Northern Mariana Islands, holds the deepest region of the ocean. Its maximum depth has been estimated at 10,971 meters. The British naval vessel Challenger II surveyed the trench in 1951 and named the deepest part the Challenger Deep. In 1960, the Trieste reached the bottom, carrying a crew of two men. The average depth of all oceans is 3,688 meters, and nearly half the world's marine waters lie over 3,000 meters deep.

• 5. Light, Cold, And Currents

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  The photic zone starts at the surface and reaches the depth at which light intensity falls to only 1% of the surface value, about 200 meters in the open ocean. Here phytoplankton use light, water, carbon dioxide, and nutrients to make organic matter, which makes this the most biodiverse layer. Below it lies the mesopelagic twilight zone, then the aphotic deep ocean where no sunlight reaches. Hydrothermal vents supply energy in that darkness.

  In the tropics, surface temperatures can rise above 30 degrees Celsius, while near the poles equilibrium with sea ice holds near minus 2 degrees Celsius. Because the cold deep layer contains the bulk of the water, the average temperature of the world ocean is 3.9 degrees Celsius. Boundaries called thermoclines, haloclines, and pycnoclines separate warm, light surface water from dense, cold deep water.

  Constant circulation creates ocean currents, driven by temperature and salinity differences, wind, and the Coriolis effect. The Gulf Stream, Kuroshio Current, Agulhas Current, and Antarctic Circumpolar Current move enormous volumes of water and heat. At depth, the thermohaline circulation takes over, and water at the bottom can stay isolated from the surface for hundreds or even a few thousand years. Modern observations suggest the Atlantic meridional overturning circulation has weakened since the preindustrial era, and projections in 2021 expect it to weaken further across the 21st century.

• 6. A Chemistry Under Strain

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  The average ocean water chlorinity is about 19.2 per mille, giving an average salinity around 34.7 per mille. The Mediterranean Sea, where evaporation outpaces precipitation, runs saltier at 38 per mille. Salinity climbs where evaporation dominates and falls where rain prevails, and observations between 1950 and 2019 show salty regions growing saltier while fresh regions grow fresher.

  Seawater carries dissolved oxygen, carbon dioxide, and nitrogen, all more soluble in colder water than warmer. Photosynthesis at the surface releases oxygen and consumes carbon dioxide, making the oceans a major carbon sink. As deep water circulates the globe, sinking organic matter decomposes, lowering oxygen and raising carbon dioxide and creating oxygen minimum zones.

  The global mean surface pH now sits roughly between 8.05 and 8.08, slightly alkaline. It held near 8.2 across the past 300 million years, then fell from about 8.15 to 8.05 between 1950 and 2020 as atmospheric carbon dioxide passed 410 parts per million in 2020. This is ocean acidification, driven mainly by fossil fuel emissions. Elements linger for vastly different spans: chloride has a residence time of 100,000,000 years, while iron lasts only about 200 years.

• 7. The Human Tide

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  The ocean is known to host over 230,000 species and may hold perhaps over two million. Life within it evolved 3 billion years before life on land. Most animal phyla have marine species, from sponges and corals to octopus, whales, and the seven species of sea turtles. Seabirds, penguins, and pelicans feed on marine animals and spend most of their lives on the water.

  Many of the world's goods move by ship between seaports, especially across the Atlantic and around the Pacific Rim. Containerization, using standard lockable boxes on purpose-built ships, slashed shipping costs and fueled the rise of globalization in the mid-to-late 20th century. The biggest global commercial fishery targets anchovies, Alaska pollock, and tuna. A 2020 report by the FAO stated that in 2017-34 percent of the world's marine fish stocks were classified as overfished.

  Protection is tightening. Marine protected areas guard the ecosystems humans depend on, and in 2021 a framework from 43 expert scientists let researchers evaluate their levels of protection. In March 2023, a legally binding High Seas Treaty was signed and adopted by all 193 United Nations Member States. Its central achievement is the power to create marine protected areas in international waters, opening the path to protect 30% of the oceans by 2030.