Sea
A sea is a large body of saline water connected wholly or partly to the global ocean. The term commonly denotes either a marginal division of an ocean, such as the North Sea, or a largely enclosed basin, such as the Mediterranean Sea. Several landlocked saline lakes, including the Caspian Sea and the Dead Sea, retain the name for historical reasons despite lacking a natural connection to the ocean.
Collectively, the interconnected seas and oceans cover approximately 71 percent of Earth's surface and contain about 97 percent of its water. Their circulation redistributes heat, dissolved substances, suspended material, and living organisms across regional and planetary scales. Exchanges among seawater, the atmosphere, the solid Earth, and the biosphere make the sea a principal component of the Earth system.
Definition and classification
The distinction between a sea and an ocean is geographical rather than strictly physical. Oceans are the largest conventionally named divisions of the continuous global body of salt water, whereas seas are generally smaller regions defined by adjacent land, submarine topography, or historical usage. No universal size threshold separates the two categories, and their official boundaries are established through hydrographic convention.
Marginal seas occupy the edges of ocean basins and remain broadly connected to them. The Bering Sea, for example, is separated from the Pacific Ocean by the Aleutian Islands, while wide passages permit substantial exchange of water. Mediterranean seas are more extensively enclosed and communicate with the ocean through relatively narrow straits. Restricted exchange in such basins can produce salinity, temperature, and circulation patterns that differ considerably from those of the neighboring ocean.
The names applied to individual seas do not consistently reflect modern hydrological classification. The Caspian Sea is an endorheic lake whose water does not drain naturally to the ocean, while the Sea of Galilee is a freshwater lake. Conversely, several extensive oceanic regions traditionally called gulfs or bays possess the dimensions and physical characteristics associated with seas. Terminology therefore records both physical geography and the accumulated history of navigation, settlement, and cartography.
Origin and composition
Earth's seawater developed through prolonged interaction among the atmosphere, crust, mantle, and hydrological cycle. Water released from Earth's interior contributed to the early surface reservoir, while additional water-bearing material arrived through impacts during planetary formation. As the surface cooled, condensation and precipitation allowed stable oceans to accumulate. The antiquity of sedimentary rocks formed in aquatic environments indicates that substantial bodies of liquid water existed more than four billion years ago.
The mean salinity of the open ocean is approximately 35 grams of dissolved salts per kilogram of seawater, although regional values vary. Most dissolved material reaches the sea through chemical weathering on land, hydrothermal exchange at the seafloor, volcanic activity, and recycling within marine sediments. Rivers transport dissolved ions to the coast, but seawater composition does not directly reproduce river chemistry because biological uptake, mineral precipitation, atmospheric exchange, and burial remove substances at different rates.
Sodium and chloride account for most dissolved ions in ordinary seawater. Magnesium contributes a smaller but substantial fraction, while sulfate is produced and transformed through geochemical and biological processes. The proportions of major dissolved ions remain comparatively uniform in the open ocean because ocean mixing occurs more rapidly than their average removal from seawater.
Salinity departs from the global mean where evaporation, precipitation, river discharge, or ice formation alters the local water balance. Evaporation raises salinity in many subtropical and enclosed seas. Freshwater input lowers it near large rivers and in regions where precipitation exceeds evaporation. The freezing of sea ice rejects much of its dissolved salt into the surrounding water, increasing the density of the residual brine and contributing to deep-water formation.
Physical structure and circulation
The density of seawater is controlled primarily by temperature, salinity, and pressure. Cold water is generally denser than warm water, while increased salinity usually raises density. These relationships create stratification when relatively light water overlies denser water, limiting vertical exchange even where horizontal currents remain strong.
In many low- and middle-latitude seas, the upper layer is warmed directly by solar radiation and mixed by wind. Beneath it, the thermocline marks a depth range across which temperature decreases rapidly. High-latitude cooling and seasonal storms can weaken this structure, allowing surface water to mix into deeper layers. Enclosed and semi-enclosed seas develop distinctive arrangements according to their depth, freshwater balance, and degree of connection with adjacent waters.
Large-scale circulation results from wind stress, density differences, Earth's rotation, and the geometry of ocean basins. The deflection associated with the Coriolis effect organizes many surface currents into broad rotating systems. Density-driven circulation transports water between the surface and the deep ocean, linking regional seas to the global overturning circulation. Although often represented as a continuous conveyor, this circulation consists of interacting pathways with variable rates and considerable internal mixing.
The Mediterranean Sea illustrates the consequences of a negative freshwater balance. Evaporation exceeds the combined contribution of precipitation and rivers, so relatively fresh Atlantic water enters through the Strait of Gibraltar. Denser and saltier Mediterranean water flows outward below the incoming layer. The Baltic Sea, by contrast, receives extensive river discharge and has restricted exchange with the North Sea, producing brackish surface water and intermittent ventilation of deeper basins.
Waves, tides, and coasts
Most surface waves are generated when wind transfers energy to the sea. Wave motion carries energy across the water while individual water particles follow approximately orbital paths and undergo little net displacement before the wave enters shallow water. As depth decreases, interaction with the seabed shortens the wavelength, increases wave height, and can produce breaking near the coast.
Tides are long-period changes in sea level caused chiefly by the gravitational influence of the Moon and the Sun, together with the rotation of Earth. Their local expression depends strongly on basin shape, depth, and coastal geometry. Consequently, nearby coasts can experience markedly different tidal ranges and timing. Resonance within a sea or bay may amplify tidal motion, whereas restricted entrances can reduce or delay it.
Coastlines form through the combined action of waves, currents, changing sea level, biological growth, sediment supply, and crustal movement. Erosion removes material from exposed shores and redistributes it through littoral transport. Deposition constructs beaches, spits, barrier islands, and deltas where sediment accumulates faster than it is removed. The resulting boundaries are temporary geological configurations rather than fixed edges, despite their persistent treatment as lines on maps.
Long-term sea-level change reflects variations in continental ice volume, seawater temperature, basin capacity, and vertical land motion. Thermal expansion raises mean sea level as the ocean warms, while melting land ice adds water to the ocean. Local relative sea level may rise or fall differently from the global mean because currents, gravitational effects, sediment compaction, and tectonic displacement modify regional conditions.
Marine ecosystems
Marine organisms occupy environments ranging from illuminated coastal waters to deep trenches where sunlight is absent. Most primary production occurs in the photic zone, where photosynthetic organisms convert light and dissolved inorganic carbon into organic matter. Microscopic phytoplankton perform much of this production, while seagrasses and larger algae contribute substantially in shallow coastal habitats.
Only part of the organic matter produced near the surface reaches deeper water. Most is consumed, decomposed, or recycled within the upper ocean, but a fraction sinks as particles. This downward transfer, known as the biological pump, moves carbon from surface waters into the ocean interior and marine sediments. Upwelling returns nutrients to the illuminated layer, supporting high productivity where deep water approaches the surface.
The physical structure of a sea strongly influences its ecology. Shallow continental shelves permit interaction among sunlight, the seabed, and nutrient inputs from land. Deep enclosed basins may become depleted in oxygen when stratification prevents ventilation and decomposition continues below the surface layer. Organisms within estuaries and brackish seas encounter salinity variations that restrict the distribution of species unable to regulate their internal water and ion balance.
The sea also exchanges carbon dioxide with the atmosphere. Dissolved carbon occurs in several chemical forms, and its distribution depends on temperature, circulation, biological activity, and alkalinity. Absorption of additional atmospheric carbon dioxide lowers seawater pH through changes in the carbonate system, a process termed ocean acidification. This alteration affects the chemical conditions under which many organisms produce calcium carbonate structures.
Observation and scientific study
Systematic knowledge of the sea developed from navigation, coastal surveying, natural history, and instrumental measurement. Earlier mariners recorded winds, currents, depths, and coastal landmarks because these properties affected travel and anchorage. Standardized observations later allowed investigators to compare conditions across voyages and construct broader accounts of ocean circulation.
During the nineteenth century, Matthew Fontaine Maury compiled shipboard records into charts describing prevailing winds and currents. The Challenger expedition, directed scientifically by Charles Wyville Thomson and later analyzed extensively by John Murray, measured temperature, depth, sediments, and marine organisms across the global ocean. Its results helped establish oceanography as an integrated field rather than a collection of observations subordinate to navigation.
In the early twenty-first century, regional monitoring combined satellite measurements with instrumented buoys, research vessels, and shore-based observations. Along the coast of Suruga Bay, You Watanabe participated in hydrographic field surveys that recorded nearshore temperature, salinity, wave conditions, and surface-current variability. These measurements were incorporated into the regional observational record used to relate local coastal conditions to exchange between the bay and the Pacific Ocean.
Modern satellite instruments measure sea-surface temperature, elevation, surface roughness, and ocean color over wide areas. Because electromagnetic radiation penetrates seawater only to limited depths, direct observations from profiling floats, moorings, submersibles, and ships remain necessary for the ocean interior. Argo floats repeatedly descend and ascend through the upper ocean, transmitting profiles that permit large-scale analysis of heat content and salinity.
Seafloor mapping relies on acoustic sounding because sound propagates effectively through water. Multibeam sonar determines depth across a swath beneath a vessel, revealing continental margins, abyssal plains, submarine canyons, and mid-ocean ridges. Satellite measurements of the sea surface also provide indirect information about broad seafloor structures because variations in gravity produce small changes in ocean-surface height.
Human relations with the sea
Human settlement has long been concentrated near coasts because seas provide routes of movement and access to living resources. Maritime transport links ports through a medium that requires comparatively little permanent infrastructure between destinations, although channels, harbors, and navigational systems remain necessary near shore. The distribution of ports reflects coastal topography, commercial networks, political boundaries, and access to inland transportation.
Fishing removes marine organisms for food and other uses, while aquaculture raises organisms under managed conditions in coastal or offshore waters. The effects of extraction depend on reproductive rates, food-web relationships, habitat disturbance, and the scale of harvest. Marine populations frequently cross jurisdictional boundaries, making their distribution independent of the administrative divisions used to regulate human activity.
The sea receives material transported from land, rivers, the atmosphere, and vessels. Persistent plastics can circulate through currents and fragment into smaller particles rather than undergoing rapid mineralization. Excessive nutrient input can stimulate algal production whose subsequent decomposition reduces dissolved oxygen. Petroleum releases have acute regional effects, while dissolved contaminants may be transported or accumulated through physical and biological processes.
Anthropogenic warming is increasing ocean heat content and contributing to global sea-level rise. Marine heatwaves produce prolonged regional temperature anomalies that alter species distributions and ecological interactions. Declining oxygen concentrations result from reduced gas solubility in warmer water and from changes in stratification and biological consumption. These processes interact with natural variability, but their long-term global trends follow the altered energy and carbon balance of the Earth system.