Oceans are the largest connected bodies of saline water on Earth and collectively constitute one global ocean surrounding and separating the continental landmasses. The global ocean covers approximately 71 percent of Earth's surface and contains about 97 percent of Earth's water. Although geographic convention divides it into named regions, the major ocean basins are physically interconnected. NOAA explicitly describes one global ocean and notes that its named regional divisions have developed for historical, geographical, cultural and scientific reasons. The commonly recognised named regions are the Pacific, Atlantic, Indian, Arctic and Southern Oceans, although exact boundaries can vary among organisations and countries.
The Pacific Ocean is the largest and deepest of the major oceanic regions. It occupies a vast area between Asia and Australia to the west and the Americas to the east and extends toward Antarctica in the south. The Atlantic Ocean lies between the Americas and Europe and Africa and is connected to the Arctic and Southern regions. The Indian Ocean is bounded broadly by Africa, Asia, Australia and Antarctica.
The Arctic Ocean surrounds the North Polar region and is the smallest and shallowest of the commonly recognised major oceans. The Southern Ocean surrounds Antarctica and is distinguished in many modern geographical systems by circulation and water characteristics associated with the Antarctic region. The boundaries among named oceans are therefore geographic conventions applied to a connected physical water system.
Oceans have a highly structured physical environment extending from the surface to great depths. The seafloor contains continental shelves, continental slopes, abyssal plains, mid ocean ridges, seamounts, trenches and other geological features. Ocean depth influences pressure, temperature, light availability and biological conditions. Sunlight penetrates only the upper portion of the ocean, creating a strongly illuminated surface environment above darker deep water. Temperature and salinity influence density, and differences in density help drive thermohaline circulation. Ocean currents also respond to wind, Earth's rotation, coastline geometry, tides and differences in water density. The result is a complex circulation system that transports heat, dissolved substances, nutrients and organisms around the planet.
Oceans play a central role in Earth's climate because water has a high heat capacity and can store and transport enormous quantities of thermal energy. Ocean currents redistribute heat from lower latitudes toward higher latitudes and interact with the atmosphere through evaporation, precipitation and heat exchange. Much of the water evaporated from oceans eventually falls as precipitation over land or ocean.
Oceans also exchange carbon dioxide with the atmosphere and absorb substantial quantities of anthropogenic carbon dioxide, although this process has consequences for seawater chemistry. Marine biological activity influences carbon cycling through photosynthesis, food webs and the sinking of organic material. Ocean circulation therefore links atmospheric, climatic, geological and biological processes on regional and global scales.
Ocean chemistry is dominated by dissolved salts, although the precise composition varies geographically and with depth. Salinity is influenced by evaporation, precipitation, freshwater input, freezing and melting of ice, and mixing. Temperature and salinity together strongly affect seawater density. Dissolved oxygen is particularly important for marine organisms and varies according to biological activity, temperature and circulation. Nutrients such as nitrogen and phosphorus support marine productivity, especially where deep nutrient rich water reaches the surface. Ocean acidification results when additional atmospheric carbon dioxide dissolves into seawater and alters carbonate chemistry, affecting organisms that construct shells or skeletons from calcium carbonate. Ocean chemistry is therefore a dynamic component of the global Earth system rather than a uniform property.
Oceans support enormous biological diversity and contain ecosystems ranging from shallow coastal environments to deep ocean communities. Coral reefs, kelp forests, estuaries, continental shelf habitats and open ocean ecosystems differ substantially in physical conditions and biological communities. Photosynthetic organisms in the ocean contribute significantly to global primary production and influence atmospheric carbon and oxygen cycles. Deep sea environments can support specialised ecosystems around hydrothermal vents, where chemical energy rather than sunlight forms the basis of primary production. Marine organisms are also connected to terrestrial ecosystems through nutrient movement, food supply, climate effects and human use. Fisheries, shipping, tourism, offshore energy and mineral extraction make oceans economically important while simultaneously creating pressures that require scientific monitoring and management.
Ocean basins are geologically active. New oceanic crust forms at mid ocean ridges where plates diverge, while older oceanic crust can be returned to Earth's interior at subduction zones. The distribution of earthquakes, volcanoes and trenches around ocean margins provides evidence of plate interactions. The ocean floor therefore records aspects of Earth's tectonic history. Magnetic anomalies arranged symmetrically around mid ocean ridges preserve evidence of past reversals of Earth's magnetic field and were important in establishing the theory of seafloor spreading. Ocean basins can open, widen, narrow and eventually close over geological time. Their present arrangement is consequently one stage in a continuing tectonic cycle.
A common misconception is that the named Oceans are five completely separate bodies of water. In physical terms they form a connected global ocean, although geographical boundaries are useful for navigation, science and communication. Another misconception is that the ocean is uniformly deep and biologically similar everywhere. In reality, depth, temperature, salinity, pressure, light, nutrients and circulation vary substantially. The ocean also contains major mountains, plains, valleys and trenches beneath its water. The Mariana Trench reaches roughly 10,911 metres at its deepest measured point, while mid ocean ridges form the longest connected mountain system on Earth. Understanding Oceans therefore requires combining marine geography, geology, chemistry, physics, biology and climate science.
