sea
Sea refers to a large body of salty water, specifically of seawater. There are particular seas and the sea. The sea commonly refers to the ocean, the wider body of seawater. Particular seas are either marginal seas, second-order sections of the oceanic sea (e.g. the Mediterranean Sea), or certain large, entirely landlocked, saltwater lakes (e.g. the Caspian Sea).
The salinity of water bodies varies widely, being lower near the surface and the mouths of large rivers and higher in the depths of the ocean; however, the relative proportions of dissolved salts vary little across the oceans. The most abundant solid dissolved in seawater is sodium chloride. The water also contains salts of magnesium, calcium, potassium, and mercury, amongst many other elements, some in minute concentrations.
The ocean moderates Earth's climate and has important roles in the water, carbon, and nitrogen cycles. The surface of water interacts with the atmosphere, exchanging properties such as particles and temperature, as well as currents. Surface currents are the water currents that are produced by the atmosphere's currents and its winds blowing over the surface of the water, producing wind waves, setting up through drag slow but stable circulations of water, as in the case of the ocean sustaining deep-sea ocean currents. Deep-sea currents, known together as the global conveyor belt, carry cold water from near the poles to every ocean and significantly influencing Earth's climate. Tides, the generally twice-daily rise and fall of sea levels, are caused by Earth's rotation and the gravitational effects of the Moon and, to a lesser extent, of the Sun. Tides may have a very high range in bays or estuaries. Submarine earthquakes arising from tectonic plate movements under the oceans can lead to destructive tsunamis, as can volcanoes, huge landslides, or the impact of large meteorites.
A wide variety of organisms, including bacteria, protists, algae, plants, fungi, and animals, lives in the seas, which offers a wide range of marine habitats and ecosystems, ranging vertically from the sunlit surface and shoreline to the great depths and pressures of the cold, dark abyssal zone, and in latitude from the cold waters under polar ice caps to the warm waters of coral reefs in tropical regions. Many of the major groups of organisms evolved in the sea and life may have started there.
The seas have been an integral element for humans throughout history and culture. Humans harnessing and studying the seas have been recorded since ancient times, and evidenced well into prehistory, while its modern scientific study is called oceanography and maritime space is governed by the law of the sea, with admiralty law regulating human interactions at sea. The seas provide substantial supplies of food for humans, mainly fish, but also shellfish, mammals and seaweed, whether caught by fishermen or farmed underwater. Other human uses of the seas include trade, travel, mineral extraction, power generation, warfare, and leisure activities such as swimming, sailing, and scuba diving. Many of these activities create marine pollution.
Sea temperature depends on the amount of solar radiation falling on its surface. In the tropics, with the sun nearly overhead, the temperature of the surface layers can rise to over 30 °C (86 °F) while near the poles the temperature in equilibrium with the sea ice is 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 seawater has a temperature between −2 °C (28 °F) and 5 °C (41 °F) in all parts of the globe.[30]
Seawater with a typical salinity of 35 ‰ has a freezing point of about −1.8 °C (28.8 °F).[citation needed] When its temperature becomes low enough, ice crystals 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 this freezes into a thin flat sheet known as nilas, which thickens as new ice forms on its underside. In more turbulent seas, frazil crystals join into flat discs known as pancakes. These slide under each other and coalesce to form floes. In the process of freezing, salt water and air are trapped between the ice crystals. Nilas may have a salinity of 12–15 ‰, but by the time the sea ice is one year old, this falls to 4–6 ‰.[31]
Oxygen concentration[edit]
The amount of oxygen found in seawater depends primarily on the plants growing in it. These are mainly algae, including phytoplankton, with some vascular plants such as seagrasses. In daylight the photosynthetic activity of these plants produces oxygen, which dissolves in the seawater and is used by marine animals. At night, photosynthesis stops, and the amount of dissolved oxygen declines. In the deep sea, where insufficient light penetrates for plants to grow, there is very little dissolved oxygen. In its absence, organic material is broken down by anaerobic bacteria producing hydrogen sulphide.[32]
Climate change is likely to reduce levels of oxygen in surface waters, since the solubility of oxygen in water falls at higher temperatures.[33] Ocean deoxygenation is projected to increase hypoxia by 10%, and triple suboxic waters (oxygen concentrations 98% less than the mean surface concentrations), for each 1 °C of upper ocean warming.[34]
Light[edit]
The amount of light that penetrates the sea depends on the angle of the sun, the weather conditions and the turbidity of the water. Much light gets reflected at the surface, and red light gets absorbed in the top few metres. Yellow and green light reach greater depths, and blue and violet light may penetrate as deep as 1,000 metres (3,300 ft). There is insufficient light for photosynthesis and plant growth beyond a depth of about 200 metres (660 ft).[35]
Sea level[edit]
Over most of geologic time, the sea level has been higher than it is today.[3]: 74 The main factor affecting sea level over time is the result of changes in the oceanic crust, with a downward trend expected to continue in the very long term.[36] At the last glacial maximum, some 20,000 years ago, the sea level was about 125 metres (410 ft) lower than in present times (2012).[37]
For at least the last 100 years, sea level has been rising at an average rate of about 1.8 millimetres (0.071 in) per year.[38] Most of this rise can be attributed to an increase in the temperature of the sea due to climate change, and the resulting slight thermal expansion of the upper 500 metres (1,600 ft) of water. Additional contributions, as much as one quarter of the total, come from water sources on land, such as melting snow and glaciers and extraction of groundwater for irrigation and other agricultural and human needs.
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