A sea is a large body of salty water. 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, nearly landlocked bodies of water.
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 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.
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
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 influence 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.
Oceans and marginal seas
as defined by the International Maritime
The sea is the interconnected system of all the Earth's oceanic waters, including the Atlantic, Pacific, Indian, Southern and Arctic Oceans. However, 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 particularly as a mediterranean sea) or wholly (as inland seas) enclosed by land. However, an exception to this is the Sargasso Sea which has no coastline and lies within a circular current, the North Atlantic Gyre.: 90 Seas are generally larger than lakes and contain salt water, but the Sea of Galilee is a freshwater lake.[a] The United Nations Convention on the Law of the Sea states that all of the ocean is "sea".[b]
The law of the sea has at its center the definition of the boundaries of the ocean, clarifying its application in marginal seas. But what bodies of water other than the sea the law applies to is being crucially negotiated in the case of the Caspian Sea and its status as "sea", basically revolving around the issue of the Caspian Sea about either being factually an oceanic sea or only a saline body of water and therefore solely a sea in the sense of the common use of the word, like all other saltwater lakes called sea.
Composite images of the Earth created by NASA in 2001
Earth is the only known
planet with seas of liquid water on its
surface,: 22 although Mars possesses ice
caps and similar planets in other solar
systems may have oceans. Earth's
1,335,000,000 cubic kilometers (320,000,000
cu mi) of sea contain about 97.2 percent of
its known water[c] and cover
approximately 71 percent of its
surface.: 7  Another 2.15% of Earth's
water is frozen, found in the sea ice
covering the Arctic Ocean, the ice cap
covering Antarctica and its adjacent seas,
and various glaciers and surface deposits
around the world. The remainder (about 0.65%
of the whole) form underground reservoirs or
various stages of the water cycle,
containing the freshwater encountered and
used by most
terrestrial life: vapor in the
air, the clouds it slowly forms, the rain
falling from them, and the lakes and rivers
spontaneously formed as its waters flow
again and again to the sea.
The scientific study of water and Earth's water cycle is hydrology; hydrodynamics studies the physics of water in motion. The more recent study of the sea in particular is oceanography. This began as the study of the shape of the ocean's currents but has since expanded into a large and multidisciplinary field: it examines the properties of seawater; studies waves, tides, and currents; charts coastlines and maps the seabeds; and studies marine life. The subfield dealing with the sea's motion, its forces, and the forces acting upon it is known as physical oceanography. Marine biology (biological oceanography) studies the plants, animals, and other organisms inhabiting marine ecosystems. Both are informed by chemical oceanography, which studies the behavior of elements and molecules within the oceans: particularly, at the moment, the ocean's role in the carbon cycle and carbon dioxide's role in the increasing acidification of seawater. Marine and maritime geography charts the shape and shaping of the sea, while marine geology (geological oceanography) has provided evidence of continental drift and the composition and structure of the Earth, clarified the process of sedimentation, and assisted the study of volcanism and earthquakes.
Global salinity map
Salinity map taken from the Aquarius Spacecraft. The rainbow colours represent salinity levels: red = 40 �, purple = 30 �
A characteristic of seawater is that it is salty. Salinity is usually measured in parts per thousand (� or per mil), and the open ocean has about 35 grams (1.2 oz) solids per litre, a salinity of 35 �. The Mediterranean Sea is slightly higher at 38 �, while the salinity of the northern Red Sea can reach 41�. In contrast, some landlocked hypersaline lakes have a much higher salinity, for example, the Dead Sea has 300 grams (11 oz) dissolved solids per litre (300 �).
While the constituents of table salt (sodium and chloride) make up about 85 percent of the solids in solution, there are also other metal ions such as magnesium and calcium, and negative ions including sulphate, carbonate, and bromide. Despite variations in the levels of salinity in different seas, the relative composition of the dissolved salts is stable throughout the world's oceans. Seawater is too saline for humans to drink safely, as the kidneys cannot excrete urine as salty as seawater.
Major solutes in seawater (3.5% salinity) Solute Concentration (�) % of total salts
Although the amount of
salt in the ocean remains relatively
constant within the scale of millions of
factors affect the salinity
of a body of water. Evaporation and
by-product of ice formation (known as "brine
rejection") increase salinity, whereas
precipitation, sea ice melt, and runoff from
land reduce it. The Baltic Sea, for
example, has many rivers flowing into it,
and thus the sea could be considered as
brackish. Meanwhile, the Red Sea is very
salty due to its high evaporation rate.
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.
Seawater with a typical salinity of 35 � has a freezing
point of about −1.8 �C (28.8 �F). 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, but by the time the sea ice is one year old, this
Seawater is slightly alkaline and had an average pH of about 8.2 over the past 300 million years. More recently, climate change has resulted in an increase of the carbon dioxide content of the atmosphere; about 3040% of the added CO2 is absorbed by the oceans, forming carbonic acid and lowering the pH (now below 8.1) through a process called ocean acidification. The extent of further ocean chemistry changes, including ocean pH, will depend on climate change mitigation efforts taken by nations and their governments.
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
Climate change is likely to reduce levels of oxygen in surface waters since the solubility of oxygen in water falls at higher temperatures. Ocean deoxygenation is projected to increase hypoxia by 10%, and triple suboxic waters (oxygen concentrations 98% less than the mean surface concentrations), for each of upper-ocean warming.