Sea
A sea is a large body of salt water defined as a division of an ocean that is partially enclosed by land.[1][2] Seas number approximately 50 globally, ranging from vast inland extensions like the Mediterranean Sea, which covers about 2.5 million square kilometers, to smaller marginal bodies such as the Sea of Marmara.[2] Collectively, seas form part of the world ocean system that spans roughly 71 percent of Earth's surface area, totaling around 361 million square kilometers.[3][4] Characteristic features include average salinity of about 35 grams of salt per kilogram of seawater, driven by evaporation exceeding precipitation in many regions, and depths that vary widely but often remain shallower than the open ocean, averaging several thousand meters in deeper marginal seas.[5][6] Seas host diverse marine ecosystems, supporting a substantial fraction of global biodiversity through habitats like coral reefs, kelp forests, and pelagic zones, while driving essential processes such as thermohaline circulation that regulate planetary climate and nutrient distribution.[7][6] Human utilization of seas for navigation, fisheries yielding billions of tons annually, and resource extraction underscores their economic significance, though overexploitation and pollution pose ongoing challenges to their sustainability.[2]Definition and Classification
Etymology and Terminology
The English word sea derives from Old English sǣ, denoting a body of salt water, which traces back to Proto-Germanic *saiwiz.[8] This root is cognate with Old High German sē and Gothic saiws, reflecting a shared Germanic linguistic heritage for terms describing large inland or coastal waters.[9] The earliest recorded uses appear in Old English texts before 1150, initially encompassing broader sheets of water before specializing to saline expanses.[10] In Indo-European languages, cognates suggest an ancestral form linked to flowing or standing water, such as Latin mare (from PIE *móri, meaning sea or lake) and Greek thalassa, which may connect to Semitic roots implying saltiness, as in Greek hals for salt or sea.[11] Old English also used mere for seas, oceans, or lakes, a term later narrowed to small ponds, highlighting historical fluidity in aquatic nomenclature before standardization.[12] Terminologically, a sea denotes a large body of saline water smaller than an ocean, often partially enclosed by landmasses or forming marginal divisions of oceans, such as the Mediterranean Sea or North Sea.[13] This contrasts with ocean, which refers to the principal interconnected saltwater basins covering approximately 71% of Earth's surface, divided into five major extents: Pacific, Atlantic, Indian, Southern, and Arctic.[14] Seas exhibit greater variability in enclosure and depth, with inland seas like the Caspian Sea lacking oceanic connection, while oceanic seas like the Caribbean Sea remain open to broader currents.[15] Distinctions extend to related terms: a gulf is a sea-like indentation of the ocean deeply incising coastlines (e.g., Gulf of Mexico), whereas a bay is shallower and broader.[15] Historically, "Seven Seas" signified varying regional seas in ancient and medieval contexts, such as the Persian Gulf, Arabian Sea, and Bay of Bengal in Arabic traditions, or Mediterranean subdivisions in European usage, rather than a fixed global count. These usages underscore seas' role in navigational and cultural terminology, evolving with exploration but rooted in observable geographic boundaries.[16]Scientific and Hydrological Definitions
In oceanography, a sea constitutes a subdivision of the ocean that is partially enclosed by land, distinguishing it from the broader, less land-constrained oceanic basins. This definition emphasizes seas as smaller, often marginal bodies of saline water integral to global marine systems, with approximately 50 such features identified worldwide based on geomorphological criteria.[2] [1] The International Hydrographic Organization (IHO) standardizes sea boundaries through its publication Limits of Oceans and Seas, which delineates hydrographic limits for navigational and scientific purposes, excluding inland landlocked waters unless historically designated as seas. The 1953 edition, still the primary reference pending updates, specifies limits for over 60 seas by defining connecting straits, coastal extents, and oceanic interfaces, facilitating consistent measurement of physical features like depth and currents. [17] Hydrologically, seas are characterized as saline water bodies with oceanic connectivity or analogous isolation leading to comparable salinity levels, typically around 35 parts per thousand, though varying due to regional evaporation-precipitation balances and fluvial inputs. This connectivity governs their role in the global hydrologic cycle, where seas contribute over 90% of evaporated water flux through air-sea interactions, influencing continental precipitation patterns. Inland seas, such as the Caspian, deviate by lacking oceanic exchange, resulting in brackish conditions from endorheic drainage.[18][19]Types of Seas and Boundaries
Seas are classified primarily based on their degree of enclosure by land and proximity to major ocean basins. Marginal seas, also known as peripheral or coastal seas, constitute divisions of the ocean that are partially enclosed by landmasses, islands, or archipelagos, such as the Mediterranean Sea, which is bordered by Europe, Africa, and Asia, and the Bering Sea between Asia and North America.[20] These seas typically overlie continental margins and facilitate exchange of water, heat, and biota with adjacent oceans through straits or open passages. Inland seas, in contrast, lie within continental interiors and are connected to oceans via narrow waterways, exemplified by the Black Sea, which links to the Mediterranean through the Bosporus Strait, measuring approximately 700 meters at its narrowest point.[21] Epicontinental seas, often shallow and overlying continental shelves, represent another category, though many modern examples are subsumed under marginal seas; historically, vast epicontinental seas covered portions of continents during periods of high sea level, such as the Western Interior Seaway in North America during the Late Cretaceous, spanning about 2,500 kilometers. National Geographic identifies three broad types: nearly enclosed seas like the Mediterranean, partly enclosed seas like the Tasman Sea, and hypersaline lakes sometimes termed seas, such as the Dead Sea with salinity exceeding 300 grams per kilogram.[22] Boundaries of seas are delineated through a combination of physical geography and international conventions to standardize nautical charting, scientific study, and resource management. The International Hydrographic Organization (IHO) establishes these limits in its publication Limits of Oceans and Seas (3rd edition, 1953), defining demarcation lines via coordinates, straits, and depth contours; for instance, the northern limit of the Mediterranean Sea runs from Cape Trafalgar in Spain eastward through the Strait of Sicily to the Gulf of Gabes. These hydrographic boundaries differ from legal maritime zones under the United Nations Convention on the Law of the Sea (UNCLOS, 1982), which allocates territorial seas up to 12 nautical miles from baselines and exclusive economic zones extending 200 nautical miles, but IHO limits focus on geophysical divisions rather than sovereignty.[23] Disputes over boundaries, such as those in the South China Sea involving overlapping claims by multiple nations, highlight the interplay between IHO delineations and geopolitical factors, though IHO definitions remain the reference for non-jurisdictional purposes.[24] The IHO's framework ensures consistency, with over 60 seas defined globally, though updates are proposed periodically, as seen in the 2000 draft recognizing the Southern Ocean south of 60°S latitude.[17]Legal and International Frameworks
The primary international legal framework governing the sea is the United Nations Convention on the Law of the Sea (UNCLOS), adopted on December 10, 1982, and entered into force on November 16, 1994, following ratification by 60 states.[23] As of 2025, UNCLOS has 169 parties, including 168 states and the European Union, though major maritime powers such as the United States have signed but not ratified it, adhering instead to many provisions as customary international law.[25] UNCLOS codifies rules on maritime zones, resource exploitation, navigation, environmental protection, and dispute settlement, balancing coastal state rights with freedoms of the high seas. UNCLOS delineates specific maritime zones from baselines, typically the low-water line along coasts. Internal waters lie landward of baselines, where coastal states exercise full sovereignty akin to territory. The territorial sea extends up to 12 nautical miles seaward, granting coastal states sovereignty over waters, seabed, subsoil, and airspace, subject to innocent passage rights for foreign vessels that do not prejudice peace, good order, or security.[26] Beyond this, the contiguous zone reaches 24 nautical miles, allowing enforcement of customs, fiscal, immigration, and sanitary laws. The exclusive economic zone (EEZ) spans up to 200 nautical miles, conferring sovereign rights for exploring, exploiting, conserving, and managing natural resources, including living and non-living, as well as jurisdiction over marine scientific research and environmental protection; other states retain freedoms of navigation, overflight, and cable/pipeline laying.[27][28] The continental shelf comprises the seabed and subsoil extending from the territorial sea to at least 200 nautical miles or up to 350 nautical miles (or beyond under geological criteria) where the margin qualifies, with coastal states holding sovereign rights for resource exploration and exploitation, including sedentary species and minerals, while seabed beyond national jurisdiction falls under the International Seabed Authority for "common heritage of mankind" administration.[23] High seas, areas beyond EEZs and not under archipelagic baselines, remain open to all states for freedoms including navigation, overflight, fishing, scientific research, and constructing artificial islands, with obligations to cooperate in conservation and prevent piracy or slave trade; no state may claim sovereignty over high seas.[23] Dispute settlement under UNCLOS includes compulsory procedures entailing binding decisions via the International Tribunal for the Law of the Sea (ITLOS), the International Court of Justice, or arbitration; ITLOS, established in 1996 and seated in Hamburg, Germany, adjudicates prompt release of vessels, provisional measures, and advisory opinions on ocean-related disputes, comprising 21 independent judges elected for nine-year terms.[29] Parties may opt out of specific categories like fisheries or seabed disputes but not core obligations. Complementing UNCLOS, the 2023 Agreement on Biodiversity Beyond National Jurisdiction (BBNJ or High Seas Treaty), ratified by over 60 states by September 2025 and entering force January 17, 2026, enhances governance of marine biological diversity in high seas and EEZ "areas beyond national jurisdiction," mandating environmental impact assessments and benefit-sharing from genetic resources.[30]| Maritime Zone | Maximum Extent from Baseline | Key Rights of Coastal State | Freedoms for Other States |
|---|---|---|---|
| Territorial Sea | 12 nautical miles | Full sovereignty over water, seabed, airspace; innocent passage | Navigation (innocent passage), overflight (with limits)[26] |
| Contiguous Zone | 24 nautical miles | Enforcement of customs, fiscal, immigration, sanitary laws | Navigation, overflight[27] |
| Exclusive Economic Zone (EEZ) | 200 nautical miles | Sovereign rights for resources, environment, research jurisdiction | Navigation, overflight, cables/pipelines[28] |
| High Seas | Beyond EEZ | None (res communis) | Navigation, overflight, fishing, research, etc.[23] |
Physical Properties
Seawater Composition and Variability
Seawater is composed primarily of liquid water, with approximately 96.5% by mass consisting of H₂O molecules, while the remaining 3.5% comprises dissolved salts, gases, organic compounds, and trace elements.[31] The average salinity, defined as the total mass of dissolved solids per kilogram of seawater, is 35 grams per kilogram (or 35 practical salinity units, psu), though this is a conservative property maintained through long-term oceanic mixing.[18] The major ions constituting over 99% of the dissolved salts include chloride (Cl⁻, ~55% of salinity), sodium (Na⁺, ~30%), sulfate (SO₄²⁻, ~8%), magnesium (Mg²⁺, ~4%), calcium (Ca²⁺, ~1%), and potassium (K⁺, ~1%), with their relative proportions remarkably constant across global oceans due to the stability of ion ratios established over geological timescales.[32] [33] The following table summarizes typical concentrations of major ions in standard seawater (salinity 35 psu):| Ion | Concentration (mg/kg) | Percentage of Total Salinity |
|---|---|---|
| Chloride (Cl⁻) | 18,980 | 55% |
| Sodium (Na⁺) | 10,556 | 30% |
| Sulfate (SO₄²⁻) | 2,649 | 8% |
| Magnesium (Mg²⁺) | 1,262 | 4% |
| Calcium (Ca²⁺) | 400 | 1% |
| Potassium (K⁺) | 380 | 1% |
Dynamic Phenomena
Ocean waves are primarily generated by the friction between wind and the sea surface, transferring energy from the atmosphere to create oscillatory motion in water particles.[45] Wind-driven surface waves dominate, with wavelengths ranging from centimeters for capillary waves (surface tension-dominated) to hundreds of kilometers for swells that propagate far from their origin.[46] Wave height and period depend on wind speed, duration, and fetch; for instance, sustained winds of 20 m/s over 100 km can produce waves up to 5 m high with periods of 8-10 seconds.[47] Seismic disturbances generate tsunamis, long-wavelength waves (up to 200 km) that travel at speeds exceeding 700 km/h in deep water, with minimal height until shoaling near shore amplifies them destructively, as seen in the 2004 Indian Ocean event displacing 30 m of water vertically.[46] Tides result from the differential gravitational forces exerted by the Moon and Sun on Earth's oceans, producing two bulges: one toward the Moon due to its pull and another on the opposite side from inertial effects as Earth rotates through the tidal field.[48][49] The Moon's proximity makes its tidal force about twice the Sun's, yielding semidiurnal tides (two highs and lows per lunar day of 24 hours 50 minutes) in most regions, with spring tides (higher range) during full/new moons when solar and lunar forces align, and neap tides (lower range) at quarter moons.[50][51] Tidal ranges vary globally; the Bay of Fundy reaches 16 m due to resonant amplification from coastal geometry.[52] Ocean currents encompass wind-driven surface flows in the upper 100-400 m, propelled by drag and deflected by the Coriolis effect into gyres (e.g., the North Atlantic Gyre circulating clockwise), and density-driven thermohaline circulation extending to abyssal depths.[53] Thermohaline currents arise from salinity-temperature variations causing water to sink where denser (e.g., North Atlantic Deep Water forms at 2-4°C and 34.9 psu salinity, sinking to 4 km), driving the global conveyor belt that cycles water on millennial timescales and redistributes heat.[54][55] Surface currents like the Gulf Stream transport 150 million m³/s of water northward at 2 m/s, moderating Europe's climate by 5-10°C.[56] These phenomena interact dynamically; for example, tides modulate currents in coastal zones, enhancing mixing, while waves influence surface current patterns through Stokes drift, a net mass transport of ~1 cm/s for typical seas.[57] Upwelling, where Ekman transport diverges surface water to bring nutrient-rich deep water upward, exemplifies coupled wind-current dynamics, sustaining fisheries like Peru's anchoveta harvest exceeding 10 million tons annually in El Niño-off years.[58]Sea Level Fluctuations
Over geological timescales, sea level has undergone substantial fluctuations driven primarily by variations in continental ice volume, with secondary contributions from tectonic uplift, sedimentation, and thermal expansion of seawater. Proxy records from coral reefs, sediment cores, and oxygen isotope ratios in foraminifera indicate that during the Pleistocene epoch (2.58 million to 11,700 years ago), global mean sea level oscillated by up to 130 meters in response to glacial-interglacial cycles, with minima of approximately 120-130 meters below present levels during glacial maxima, such as around 20,000 years ago when vast ice sheets locked up water on land.[59][60] These changes resulted from the growth and decay of polar ice sheets, influenced by Milankovitch orbital forcings that modulated solar insolation and triggered ice age cycles.[61] Following the Last Glacial Maximum, sea level rose rapidly at rates of 10-20 millimeters per year during meltwater pulses, contributing to a total post-glacial rebound of about 120 meters by the early Holocene (approximately 11,700 to 8,200 years ago), after which rates decelerated to near stability.[62] In the mid-to-late Holocene (roughly 6,000 years ago to the 19th century), global mean sea level remained relatively stable, with average rise rates below 0.5 millimeters per year in many regions, as evidenced by stable coral microatolls and beach ridge sequences, reflecting a balance between residual ice melt and isostatic adjustments from glacial unloading.[63][64] However, regional variations persisted due to glacio-isostatic rebound, where formerly glaciated areas like Scandinavia experienced post-glacial uplift exceeding sea level rise, while subsiding sedimentary basins saw relative increases.[65] In the modern era, since the late 19th century, global mean sea level has risen by 21-24 centimeters, with the rate accelerating from about 1.5-2.0 millimeters per year in the early 20th century to 4.5 millimeters per year as of 2024, based on satellite altimetry data from missions like TOPEX/Poseidon and Jason series.[66][67] Tide gauge records, which measure relative sea level changes at coastal stations and require corrections for vertical land motion, show consistent but sometimes lower global averages (around 1.7-3.0 millimeters per year over the 20th century) due to local subsidence or uplift effects not captured in absolute satellite measurements.[68][69] This recent acceleration breaks the preceding millennial-scale stability, attributed mainly to anthropogenic influences: thermal expansion from ocean warming (accounting for about 40-50% of 20th-century rise) and mass addition from glacier and ice sheet melt, particularly from Greenland and Antarctica.[66][70] Natural variability, including decadal oscillations like the Pacific Decadal Oscillation, modulates short-term rates but does not explain the long-term trend.[71]| Period | Approximate Sea Level Change | Primary Driver | Measurement Method |
|---|---|---|---|
| Pleistocene Glacials | -120 to -130 m | Ice sheet growth | Proxy (oxygen isotopes, reef terraces)[72] |
| Post-Last Glacial Maximum to Early Holocene | +120 m total rise | Ice sheet melt | Proxy and geological records[64] |
| Mid-to-Late Holocene | <0.5 mm/yr stability | Isostatic equilibrium | Coral records, sediment cores[73] |
| 1880-Present | +21-24 cm, accelerating to 4.5 mm/yr | Thermal expansion, ice melt | Tide gauges, satellite altimetry[66][67] |