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 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.

Earth is the only known planet with seas of liquid water on its surface,[3]: 22  although Mars possesses ice caps and similar planets in other solar systems may have oceans.[11] Earth's 1,335,000,000 cubic kilometers (320,000,000 cu mi) of sea contain about 97.2 percent of its known water[12][c] and cover approximately 71 percent of its surface.[3]: 7 [17] 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.[12]

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[18] but has since expanded into a large and multidisciplinary field:[19] it examines the properties of seawater; studies waves, tides, and currents; charts coastlines and maps the seabeds; and studies marine life.[20] The subfield dealing with the sea's motion, its forces, and the forces acting upon it is known as physical oceanography.[21] 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.[19]
Seawater[edit]
Global salinity map
Salinity map taken from the Aquarius Spacecraft. The rainbow colours represent salinity levels: red = 40 �, purple = 30 �
Salinity[edit]

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 �,[22] while the salinity of the northern Red Sea can reach 41�.[23] 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.[24][25] Seawater is too saline for humans to drink safely, as the kidneys cannot excrete urine as salty as seawater.[26]
Major solutes in seawater (3.5% salinity)[25] Solute Concentration (�) % of total salts