Sea

A sea is a large body of salt water that is surrounded in whole or in part by land. More broadly, "the sea" is the interconnected system of Earth's salty, oceanic waters—considered as one global ocean or as several principal oceanic divisions. The sea moderates Earth's climate and has important roles in the water cycle, carbon cycle, and nitrogen cycle. Although the sea has been traveled and explored since prehistory, the modern scientific study of the sea—oceanography—dates broadly to the British Challenger expedition of the 1870s. The sea is conventionally divided into up to five large oceanic sections—including the International Hydrographic Organization's four named oceans (the Atlantic, Pacific, Indian, and Arctic) and the Southern Ocean; smaller, second-order sections, such as the Mediterranean, are known as seas.

Owing to the present state of continental drift, the Northern Hemisphere is now fairly equally divided between land and sea (a ratio of about 2:3) but the South is overwhelmingly oceanic (1:4.7). Salinity in the open ocean is generally in a narrow band around 3.5% by mass, although this can vary in more landlocked waters, near the mouths of large rivers, or at great depths. About 85% of the solids in the open sea are sodium chloride. Deep-sea currents are produced by differences in salinity and temperature. Surface currents are formed by the friction of waves produced by the wind and by tides, the changes in local sea level produced by the gravity of the Moon and Sun. The direction of all of these is governed by surface and submarine land masses and by the rotation of the Earth (the Coriolis effect).

Former changes in sea levels have left continental shelves, shallow areas in the sea close to land. These nutrient-rich waters teem with life, which provide humans with substantial supplies of food—mainly fish, but also shellfish, mammals, and seaweed—which are both harvested in the wild and farmed. The most diverse areas surround great tropical coral reefs. Whaling in the deep sea was once common but whales' dwindling numbers prompted international conservation efforts and finally a moratorium on most commercial hunting. Oceanography has established that not all life is restricted to the sunlit surface waters: even under enormous depths and pressures, nutrients streaming from hydrothermal vents support their own unique ecosystem. Life may have started there and aquatic microbial mats are generally credited with the oxygenation of Earth's atmosphere; both plants and animals first evolved in the sea.

The sea is an essential aspect of human trade, travel, mineral extraction, and power generation. This has also made it essential to warfare and left major cities exposed to earthquakes and volcanoes from nearby faults; powerful tsunami waves; and hurricanes, typhoons, and cyclones produced in the tropics. This importance and duality has affected human culture, from early sea gods to the epic poetry of Homer to the changes induced by the Columbian Exchange, from burial at sea to Basho's haikus to hyperrealist marine art, and inspiring music ranging from the shanties in The Complaynt of Scotland to Rimsky-Korsakov's "The Sea and Sinbad's Ship" to A-mei's "Listen to the Sea". It is the scene of leisure activities including swimming, diving, surfing, and sailing. However, population growth, industrialization, and intensive farming have all contributed to present-day marine pollution. Atmospheric carbon dioxide is being absorbed in increasing amounts, lowering its pH in a process known as ocean acidification. The shared nature of the sea has made overfishing an increasing problem.

Definition
Both senses of sea date to Old English; the larger sense has required a definite article since Early Middle English. As the term has been applied over time, there are no sharp distinctions between seas and oceans, although seas are smaller and are usually bounded by land masses (and ones smaller scale than continents), the singular standard exception being the Sargasso Sea, which is created by the four currents bounding what is termed the North Atlantic Gyre. (p90) Seas are generally larger than lakes and contain salt water. While the defining elements of size and being bounded are generally used, there is no formally accepted technical definition of "sea" among oceanographers. In international law, the United Nations Convention on the Law of the Sea states that all the ocean is "the sea".

Seawater
Seawater is invariably salty and, although its degree of saltiness (salinity) can vary, about 90% of the water in the ocean has 34–35 g (1.2 oz.) of dissolved solids per liter, producing a salinity between 3.4 and 3.5%. To easily describe small differences, however, oceanographers usually express salinity as a millage (‰) or part per thousand (ppt) instead of using percents. The surface salinity of waters in the Northern Hemisphere are generally closer to the 34‰ mark, while those in the South are closer to 35‰. The solutes in ocean water come both from inflowing river water and from the ocean floor. The relative composition of the solutes is stable throughout the world's oceans: sodium (Na) and chloride (Cl) make up about 85%. Other solutes include metal ions such as magnesium (Mg) and calcium (Ca) and negative ions such as sulfate (SO₄), carbonate (CO₃), and bromides. In the absence of other pollution, seawater would not be harmful to drink except that it is much too saline; similarly, it cannot be used for irrigating most plants without being desalinated. For scientific and technical purposes, a standardized form of artificial seawater is often used.

Variations in salinity are caused by many factors: currents flowing between the seas; incoming freshwater from rivers and glaciers; precipitation; the formation and melting of sea ice; and evaporation, which is in turn affected by temperature, winds, and waves. For example, the upper level of the Baltic Sea has a very low salinity (5 to 8‰ – see Salinity) because the low temperatures of the surrounding climate produce minimal evaporation; it has many inflowing rivers; and its small connection to the North Sea tends to create a cold, dense under-layer that hardly mixes with the surface waters. By contrast, the Red Sea lies between the Sahara and Arabian Deserts; it has high evaporation but little precipitation; it has few (and mostly seasonal) inflowing rivers; and its connection to other seas—the Suez Canal in the north and the Bab-el-Mandeb in the south—are both very narrow. Its salinity averages 40‰. The Mediterranean is a little lower, at 37‰, while some landlocked lakes are much higher: the Dead Sea has 300 grams (11 oz) of dissolved solids per liter (300‰).

Sea temperature chiefly depends on the amount of solar radiation it absorbs. In the tropics where sunlight falls more directly, the temperature of the surface layers can rise to over 30 °C (86 °F); near the poles, the temperature is in equilibrium with the sea ice at its freezing point. Its salinity makes this lower than freshwater's, usually about −1.8 °C (28.8 °F). These temperature differences contribute to the continuous circulation of water through the sea. Warm surface currents cool as they move away from the tropics; as the water becomes denser, it sinks. The cold water in the deep sea moves back towards the equator before welling up again to the surface. Deep seawater has a temperature between −2 and 5 °C (28 and 41 °F) in all parts of the globe. In freezing seas, ice crystals begin to form on the surface. These break into small pieces and coalesce into flat discs that form a thick suspension known as frazil. In calm conditions, frazil will freeze into a thin, flat sheet called nilas, which thickens as new ice forms in the sea beneath it. In turbulent waters, frazil instead join together into larger flat discs known as "pancakes". These slide over and under one another to form floes. During these processes, salt water and air are trapped amid the ice. Nilas forms with a salinity around 12–15‰ and is greyish in color but grows fresher over time: after a year, it is bluish and closer to 4–6‰ saline.

The amount of light that penetrates the sea depends on the angle of the sun, the local weather, and the sea's turbidity. Of the light that reaches the surface of the sea, much of it is reflected at the surface and its red wavelengths are absorbed in the top few meters. Yellow and green reach greater depths, and the longer blue and violet wavelengths may penetrate as deep as 1,000 m (3,300 ft).

The amount of oxygen present in seawater depends primarily upon its temperature and the photosynthetic organisms living in it, particularly algae, phytoplankton, and plants such as seagrass. During the day, their photosynthetic activity produces oxygen, which dissolves into the seawater and is used by marine animals. The water's oxygen saturation is lower during the night and much lower in the deep sea. Below a depth of about 200 m (660 ft), there is insufficient light for photosynthesis and consequently little dissolved oxygen. Below this, anaerobic bacteria break down falling organic material, producing hydrogen sulfide (H₂S). It is projected that global warming will reduce oxygen both in surface and deep waters, due to oxygen's decreased solubility as temperatures increase and increased oceanic stratification
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