The ocean is the body of salt water that covers approximately 70.8% of Earth. The ocean is conventionally divided into large bodies of water, which are also referred to as oceans (in descending order by area: the Pacific Ocean, the Atlantic Ocean, the Indian Ocean, the Antarctic/Southern Ocean, and the Arctic Ocean), and are themselves mostly divided into seas, gulfs and subsequent bodies of water. The ocean contains 97% of Earth's water and is the primary component of Earth's hydrosphere, acting as a huge reservoir of heat for Earth's energy budget, as well as for its carbon cycle and water cycle, forming the basis for climate and weather patterns worldwide. The ocean is essential to life on Earth, harbouring most of Earth's animals and protist life. It originated photosynthesis and therefore Earth's atmospheric oxygen, and still supplies half of it. Ocean scientists split the ocean into vertical and horizontal zones based on physical and biological conditions. Horizontally the ocean covers the oceanic crust, which it shapes. Where the ocean meets dry land it covers relatively shallow continental shelves, which are part of Earth's continental crust. Human activity is mostly coastal with high negative impacts on marine life. Vertically the pelagic zone is the open ocean's water 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. The photic zone starts at the surface and is defined to be "the depth at which light intensity is only 1% of the surface value" (approximately 200 m in the open ocean). This is the zone where photosynthesis can occur. In this process plants and microscopic algae (free-floating phytoplankton) use light, water, carbon dioxide, and nutrients to produce organic matter. As a result, the photic zone is the most biodiverse and the source of the food supply which sustains most of the ocean ecosystem. Light can only penetrate a few hundred more meters; the rest of the deeper ocean is cold and dark (these zones are called mesopelagic and aphotic 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 where sea 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). Constant circulation of water in the ocean creates ocean currents. Those currents are caused by forces operating on the water, such as temperature and salinity differences, atmospheric circulation (wind), and the Coriolis effect. Tides create tidal currents, while wind and waves cause surface currents. The Gulf Stream, Kuroshio Current, Agulhas Current and Antarctic 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 global climate system. Ocean water contains dissolved gases, including oxygen, carbon dioxide and nitrogen. An exchange of these gases occurs at the ocean's surface. The solubility of these gases depends on the temperature and salinity of the water. The carbon dioxide concentration in the atmosphere is rising due to CO2 emissions, mainly from fossil fuel combustion. As the oceans absorb CO2 from the atmosphere, a higher concentration leads to ocean acidification (a drop in pH value). The ocean provides many benefits to humans such as ecosystem services, access to seafood and other marine resources, and a means of transport. The ocean is known to be the habitat of over 230,000 species, but may hold considerably more – perhaps over two million species. Yet, the ocean faces many environmental threats, such as marine pollution, overfishing, and the effects of climate change. Those effects include ocean warming, ocean acidification and sea level rise. The continental shelf and coastal waters are 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, i.e., the world ocean. It includes the Pacific, Atlantic, Indian, Antarctic/Southern, and Arctic oceans. As a general term, "the ocean" and "the sea" are often interchangeable. Strictly speaking, a "sea" is a body of water (generally a division of the world ocean) partly or fully enclosed by land. The word "sea" can also be used for many specific, much smaller bodies of seawater, such as the North Sea or the Red Sea. There is no sharp distinction between seas and oceans, though generally seas are smaller, and are often partly (as marginal seas) or wholly (as inland seas) bordered by land.
World Ocean
In medieval Europe, the World Sea was the body of water that encircled the Continent (the mainland of Europe, Asia and Africa), and thus excluded the Mediterranean, Black and Caspian Seas. The contemporary term World Ocean was coined in the early 20th century by the Russian oceanographer Yuly Shokalsky to refer to the continuous ocean that covers and encircles most of Earth. The global, interconnected body of salt water is sometimes referred to as the World Ocean, the "global ocean" or the "great ocean". The concept of a continuous body of water with relatively unrestricted exchange between its components is critical in oceanography.
Etymology The word ocean comes from the figure in classical antiquity, Oceanus (; Ancient Greek: Ὠκεανός Ōkeanós, pronounced [ɔːkeanós]), the elder of the Titans in classical Greek mythology. Oceanus was believed by the ancient Greeks and Romans to be the divine personification of an enormous river encircling the world. The concept of Ōkeanós could have an Indo-European connection. Greek Ōkeanós has been compared to the Vedic epithet ā-śáyāna-, predicated of the dragon Vṛtra-, who captured the cows/rivers. Related to this notion, the Okeanos is represented with a dragon-tail on some early Greek vases.
Natural history
Origin of water
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