Oceans

Oceans cover 71 percent of planet Earth. The seafloor beneath them holds an abundance of minerals; there are also minerals dissolved in seawater. Oceans contain about 97 percent of Earth’s water. In addition, the preponderance of life on Earth is ocean life. Many modern fishing operations are wasteful of these ocean resources, overexploiting the world's fisheries and causing habitat loss. Finally, the oceans provide avenues of commerce, serve as a sink for wastes, and, most importantly, regulate Earth’s climate.

Background

Water is an excellent solvent, so seawater contains more than sixty dissolved elements or their salts. The major constituent percentages of seawater are water (H₂O), 96.5 percent; table salt, or sodium chloride (NaCl), 2.3 percent; magnesium chloride (MgCl₂), 0.5 percent; sodium sulfate (Na₂SO₄), 0.4 percent; and calcium chloride (CaCl₂), 0.1 percent. This slightly alkaline broth was probably the first home to life on Earth.

Table salt has been evaporated from seawater since antiquity, with sunlight and wind supplying the energy. In the twentieth century, additional processes began producing magnesium, bromine, and iodine. Extracting other minerals from seawater is generally not profitable because the potential resource is highly dilute, requiring more pumping and a processing cost that is not worth the return. Some plants and animals are able to do such extractions, and eventually, genetic engineering may harness such organic processes. Water is the prime constituent of seawater, and commercial desalination (removal of salt from seawater or other salt solutions) began in the 1960s. Desalination was expensive, however, and competing natural sources of freshwater were cheaper except in desert regions. Modern desalination methods, such as reverse osmosis, use more energy-efficient, cost-effective processes and innovative membrane designs, making desalination more practical for widespread use.

Water and the Cycles of Climate

Nature desalts water on a global scale through the hydrologic cycle of evaporation and resulting moisture. This cycle not only waters land plants but also affects the climate in two ways. First, evaporation transfers heat from the oceans to places where the moisture condenses. Second, water flow off the land carries minerals containing a large percentage of calcium oxides that are part of the carbon cycle. Seawater has a smaller percentage of calcium ions than the runoff water because various sea plants and animals extract calcium from seawater and fix carbon dioxide from the air to grow (accrete) calcium carbonate (CaCO₃) shells. Much of the calcium carbonate goes to the seafloor. This process helps balance the other half of the carbon cycle, carbon dioxide entering the air from animal respiration and the burning of fossil fuels. Because atmospheric carbon dioxide is an insulator for Earth (the greenhouse effect), more oceanic life absorbing more carbon dioxide from the air could decrease Earth’s temperatures. It has been suggested that airborne dust from the Himalayan highlands may have fertilized blooms of sea plants, triggering ice ages.

Currents, Climate, and Energy Sources

The ocean waters redistribute heat from the sunlight. The flows of this heat engine control the climate of Earth and hold the potential for energy production many times that used by humankind.

Water near the poles loses heat through evaporation, conduction, and radiation. As it cools, its density increases, and it sinks toward the ocean floor. From there, it flows toward the equator, displacing warmer water as it goes. Meanwhile, water near the equator is warmed, becoming less dense. It tends to flow along the surface toward the higher latitudes to replace the sinking, denser water.

The Gulf Stream is such a current. Warm water from the equatorial Atlantic and the Gulf of Mexico (also called the Gulf of America) flows generally northward, parallel to the coast of North America, and bends gradually to the right (northeast) due to the rotation of the Earth. This tendency to curve (right in the Northern Hemisphere, left in the Southern Hemisphere) is called the Coriolis effect, and it eventually bends the flow northeast past Western Europe, warming and moistening air that, in turn, moderates the climate in the region. The cooled water bends south and west back to the start.

Similar circular patterns (gyres) occur in all the world’s oceans. The gyre in the North Pacific warms East Asia and cools California. Along the way, the gyres help determine fertile areas in the oceans. Sinking water off Antarctica pushes other nutrient-rich water to the surface; currents flowing south past California cause upwelling, and flow from two gyres meeting and turning west leaves a gap that causes upwelling off Peru. Changes in the gyres, as probably occurred during ice ages, would shift fertile ocean areas as they influence climate change. A weaker Gulf Stream might chill Europe to a climate like that of Siberia. Conversely, a warmer Gulf Stream might completely melt the ice in the Arctic Ocean. Greatly increased evaporation would increase the snowfall of lands around the Arctic, which currently have relatively little snowfall. Glaciers in Canada and Russia could grow in a matter of decades. Sunlight also indirectly causes salinity currents. Areas of high evaporation, such as the Mediterranean Sea, have dense saline water that flows out along the bottom to the open ocean as less saline water flows in along the surface.

Theoretically, at least, turbines could harness these currents. For instance, the Gulf Stream has more energy than all the world’s rivers combined. Estimates suggest that the area off the coast of Florida might yield between 1 and 10 gigawatts of extractable power without an observable change in heat flow to Europe. However, this may negatively impact ecosystems or circulation.

One nonsolar energy input is the tides, bulges of water pulled along by the gravitational attraction of the moon and (to a lesser extent) the Sun. These bulges, which are only a few meters in the open ocean, are funneled by some geographic features into much larger rises. For instance, the Bay of Fundy in Nova Scotia, Canada, has tides as great as 17 meters (55.8 feet). Sites with such high tides are limited.

Another potential energy source is the difference between warm tropical surface waters and near-freezing deep waters. Proposed ocean thermal energy conversion (OTEC) power plants would send a gas through a turbine either by boiling a low-boiling-point fluid, such as ammonia, or by boiling water in a partial vacuum. A large insulated pipe would bring up cold water to chill the working gas back to a liquid. The heat difference is small, however, so efficiency is low and capital cost per kilowatt is high. Thus, it may be some time before these vast resources are competitive with power stations on land. Nonetheless, the energy potential is great, and the raised waters would also be high in nutrients, so they could be used to fertilize surrounding waters.

Continental Shelves and Slopes

The continents are essentially blocks of lighter rock, such as granite, floating on heavier rock, such as basalt. The oceans fill the low spots between and lap at the edges of continents. These edges, the continental shelves, usually slope gently for some distance before the continental slopes plunge into oceanic depths. Globally, the continental shelves, which extend down to roughly 200 meters (660 feet), represent an area equivalent to that of Africa. They include areas such as most of the Baltic Sea, wide areas off eastern North America, and narrower areas off western North America. Because the shelves are close to land nutrients, they usually have the richest marine life but are also most vulnerable to pollution from land.

Land minerals continue out onto the continental shelves. Shelf areas in the South China Sea and the Gulf of Mexico have major petroleum deposits. In addition, water-sorted deposits called placers extend along ancient beaches now covered below sea level. (Sea levels have been several hundred meters higher and lower in different geologic times.)

Some minerals are obtained through tunnel mining. Tunnel mines extend from shore to reach particularly desired ores. Dredging, however, is the most common method of mining shallow ocean deposits. Billions of metric tons of sand, gravel, and shells are dredged worldwide each year. Smaller tonnages are mined for more valuable minerals, such as gold, diamonds, and tin.

The continental slopes are a comparatively small area, with a correspondingly small mineral or fishing potential. However, they are awe-inspiring: Their edges plunge to depths averaging 2.8 kilometers (1.7 miles) in the abyssal seafloor, often via submarine canyons, some of which are comparable in depth to the Grand Canyon. Such changes in marine elevation (or depth) are dangerous for sea-bottom facilities, such as cables or drilling platforms, because landslides on the slopes carry mud, sand, and pebbles in turbidity currents. A 1929 earthquake on the Grand Banks east of Canada caused turbidity currents that moved at 80 kilometers (50 miles) per hour and carried roughly 100 cubic kilometers (24 cubic miles) of material over an area of 100,000 square kilometers (24,000 cubic miles). The speed was clocked by the snapping of transatlantic cables one by one.

The Abyssal Zone

The abyssal zone represents more than three-quarters of the ocean floor. It is an area with water consistently just above freezing. It starts at a depth of 1 to 3 kilometers (0.6 to 1.8 miles) and extends to roughly 6 kilometers (3.7 miles). Because the abyssal zone has no light and depends on scraps falling from above, the biomass per unit volume can be a hundredth or even a thousandth that of surface waters. The life-forms are some of the most alien on the planet—usually small, often luminescent. Animals may have jaws capable of swallowing something twice their size. Abyssal topography is often low rolling hills. However, areas with heavy sediment inflow, such as much of the Atlantic, have underlying topography buried under abyssal plains composed of fine ooze; these slope less than 1 part in 1,000.

These differences have mining implications. Some abyssal plains have sediments several kilometers deep. Under pressure and heat, organic material in these sediments decomposes into hydrocarbons, particularly methane (CH₄) and other hydrocarbons that make up petroleum. Meanwhile, the ocean bottom is only slightly above freezing and is under high pressure. With those conditions, a combination of methane and water called methane hydrate freezes, forming a layer that holds methane and acts as a cap rock to block the escape of other hydrocarbons. Therefore, it is possible that much of the sedimented ocean floor may be underlain by oil and gas deposits, perhaps many times those found to date.

Abyssal Mineral Resources

The waters without heavy land sediments, such as much of the Pacific, have other major potential resources. In tectonically active areas, water seeping down into the seafloor containing magma is heated and eventually expelled back into the ocean. The hydrothermal (water plus heat) vents, or marine vents, that exist where this occurs carry dissolved minerals, especially sulfides of zinc, lead, copper, and silver, along with lesser but still significant amounts of lead, cadmium, cobalt, and gold. Such deposits have been test mined in the Red Sea, where underwater valleys keep rich muds enclosed. In the deep ocean, such deposits make chimneys of metal sulfides that might eventually be mined.

Other, more soluble minerals may be carried hundreds or even thousands of kilometers before precipitating as potato-shaped ferromanganese nodules on the ocean floor. These nodules—commonly called simply manganese nodules—contain mostly oxides of iron and manganese, plus potentially profitable small amounts of copper, nickel, and cobalt. They cover millions of square kilometers and contain billions of metric tons of metal. The nodules accrete slowly and could be easily buried by land sediments (as exist in the Atlantic), so they are more commonly observed in the deep Pacific far from land.

Mining of ferromanganese nodules has been considered but not done for several reasons. (A ship that was once thought to be involved in a serious mining venture, the Glomar Explorer, was actually on a spy operation salvaging a wrecked Soviet submarine.) First, raising material from the ocean floor and processing at sea would be expensive compared with land mining. Second, deep-sea mining controls, according to the proposed 1982 Law of the Sea Treaty, include undetermined taxes and subsidies to potential mining rivals. Finally, life-forms in the cold deep waters might be slow to recover from silt pollution from mining operations. Eventually, however, as offshore oil and gas drilling has demonstrated, effective technologies will evolve as the prices of competing land deposits increase.

Oceanic Ridges and Trenches

Another feature of the deep ocean—and perhaps the largest geographic feature on Earth—is the Mid-Oceanic Ridge (also called the Mid-Atlantic Ridge). This 65,000-kilometer (40,390-mile) mountain range is the area where new seafloor is spreading the ocean apart. It extends from the Arctic Ocean, through the Norwegian Sea, through the North and South Atlantic; it continues around South Africa through the Indian, Antarctic, and South Pacific Oceans. This is an area of intense hydrothermal activity, and it contains hydrothermal deposits similar to those in the Red Sea.

Areas of seafloor spreading are balanced by other areas where tectonic plates are driven under other plates. This process leads either to rising mountains on land (such as the Himalayas and Andes) or trenches at sea, such as the Marianas Trench, often with an arc of volcanic islands beside the trenches. At 11 kilometers (6.8 miles), the Marianas Trench contains the deepest known spot on Earth.

Suggestions have been made that toxic chemicals and radioactive wastes be placed into sediments in deep sea trenches for disposal. The suggestion is based on the idea that trenches are areas where plates are submerged into Earth’s mantle, so the wastes would be entombed. However, an unexpected volcano tens of thousands of years in the future might punch through that heated layer of diving sediments and belch toxic material into the stratosphere, and hence around the globe. Also, the costs of placing material into deep sea trenches would be considerable.

Commerce

From Carthaginian traders through clipper ships to steam and container ships, ocean commerce has become ever more important in the world economy. Since the onset of steam power in the nineteenth century, power plants and ship structures have steadily improved.

However, until the 1960s, even slower freighters spent a majority of their time in port loading and unloading. Containerization—the use of standard-sized large cargo containers—allows one crane to do in an hour what a crew of laborers might need a day to do. Furthermore, the containers can be loaded onto railcars or trucks for quick movement without tedious manual handling. This advance allows factories on opposite sides of the planet to compete directly, increasing world competition and decreasing wages in developed countries. There are dangers to this expanded commerce. Giant (and underpowered) supertankers have had spectacular oil spills, including the Torrey Canyon (1967) and Exxon Valdez (1989). Although not a tanker, the Deepwater Horizon (2010) spill was much larger, exceeding the Exxon Valdez spill by eighteen times. It was the largest offshore oil spill in US history by both volume (134 to 168 million gallons of oil) and time (eighty-seven days). These accidents and others raised environmental concerns, such as emission regulations and supply chain vulnerabilities.

Sea Life and Food from the Sea

For most of the time since life on Earth began, the majority of life has existed in the ocean. A majority of living tonnage is still there—perhaps as much as 100 billion metric tons. That oceanic life supports high-protein food consumption that approaches 100 million metric tons. Eventually, that figure will be much larger. The oceans, covering an area three and one-half times larger than all the land and never limited by water, have the potential to produce many times the amount of food produced on land when used for carefully planned, sustainable production.

However, some experts predicted that before that happens, the existing fishing industry will collapse. This dire prospect is based on significant differences between food production from the sea and agriculture on land. Except for nearshore plants, such as eelgrass and kelp, oceanic plants are drifting algae, most barely large enough to see without magnification. These phytoplankton support most of the animal life in the oceans and live in the top few hundred meters, where sunlight penetrates. All oceanic photosynthesis (that is, the use of light and nutrients to make food) occurs in this euphotic (lit) zone, and most life depends directly or indirectly on this zone.

The tiny plants of the phytoplankton are eaten by tiny animals (zooplankton), which are food for small shoaling fish such as sardines and anchovies. These fish are, in turn, eaten by higher predators in the food chain, such as mackerel, jack, tuna, sharks, and porpoises. At each stage along the path from phytoplankton to the “top predators,” about 90 percent of the food content is lost.

This situation leads to the first great failing of contemporary fishing: It focuses on those top predators, which is the equivalent of hunting lions and eagles. Harvesting zooplankton yields one-tenth the food of phytoplankton, sardines give one-hundredth, and tuna yields one-thousandth. Land agriculture, in contrast, delivers vegetable matter directly to people (or, with a seven-eighths loss, to cattle, then people).

Second, fishing is essentially a high-technology hunting operation that does virtually nothing to improve the environment or nurture the young of fished species. Fishers who hold back in catching fish to save fish for breeding stock simply lose out to other boats. Third, fishing excesses were overmatched by the size of the oceans until the twentieth century, when power boats, synthetic netting materials, and efficient transport of fish to world markets multiplied fishing yields, leading to an ongoing string of fishery collapses. Kilometers-long seine nets swept large areas of the open ocean clean. The more powerful boats and “rock-hoppers” (able to drag rough bottom areas with less danger of snagging nets) have allowed trawlers to work intensively down nearly to abyssal depths. The habitat for the young of many species and food for many others is being compacted, silted, and ground down to wasteland.

Fourth, the areas closest to land, where countries can exercise limits on overfishing, are often contaminated with pollutants. The Chesapeake Bay produces only a fraction of what the region’s early settlers found. The Black Sea, naturally darkened by anaerobic decomposition (material rotting without oxygen), is blacker because of fertilizer runoff and toxic contaminants. In 1991, three thousand people in Peru died from cholera linked to sewage-contaminated seafood.

These problems have caused a series of “crashes” in production from formerly rich fishing grounds. John Steinbeck’s 1945 novel Cannery Row describes the shoreside support for a fishery off California that no longer exists. In the early 1970s, yearly anchovy production off Peru collapsed and has never fully recovered—though the industry improved slightly in the early twenty-first century. In the 1990s, the Grand Banks (east of Canada) began collapsing. However, production is maintained by various subsidies for larger, more sophisticated boats that go farther and deeper to catch dwindling fish stocks.

On the brighter side, as fisheries decline, cultured production is expanding. There is already mariculture of fish on land, which includes the growing of shrimp and other sea creatures. (Unregulated production in poorer countries often has grave environmental costs in pollution and lost mangrove swamps.) Production in existing fisheries must be strictly controlled. Finally, artificial fertilization in the deep ocean away from land might conceivably transform a “blue-water desert” into fertile green zones.

Politics

In 1608, Hugo Grotius defined “freedom of the seas” for the Dutch when they had a powerful fleet to defend their boats fishing in waters near Great Britain. The British did not share the Dutch view, however, and drove the Dutch boats away in a bloody war. Later, the British fleet became the most powerful in the world, and Britain embraced freedom of the seas.

The concept held that territorial waters extended about 5.6 kilometers (3.5 miles) from shore, which was the farthest range of cannons. Beyond territorial waters were international waters where a ship could fish or dump anything. In the twenty-first century, territorial waters extend 22.2 kilometers (13.8 miles; 12 nautical miles) from the coast and often farther where the continental shelf is wide.

In 1982, negotiations concluded on the International Law of the Sea Treaty. It includes the concept of a 370-kilometer (230-mile; 200 nautical miles) exclusive economic zone (EEZ) within which the coastal country has exclusive control of all resources. The United States and many other countries adopted the EEZ but not the treaty itself. Following the doctrine of the EEZ, ownership of the tiniest spit of land confers control of a wide circle of ocean. Where circles overlap, claims conflict, and are resolved in various ways. European countries have carefully negotiated boundaries in the North Sea. In the South China Sea, China and Vietnam negotiated with naval gunfire. The Law of the Sea Treaty remained in effect through the 2020s and was updated periodically throughout its lifespan. Though the United States recognized the treaty as a representative of international law, it chose not to ratify it. In 2023, the United Nations adopted the High Seas Treaty, which involved marine genetic resources, marine protected areas, and environmental impact assessments.

Bibliography

Ballard, Robert D. Explorations: My Quest for Adventure and Discovery Under the Sea. Hyperion, 1995.

Boudreau, Diane, et al. "All about the Ocean." National Geographic, 27 May 2025, education.nationalgeographic.org/resource/all-about-the-ocean. Accessed 22 Nov. 2025.

Carson, Rachel. The Sea around Us. Drawings by Katherine L. Howe. Oxford UP, 1951.

Clarke, Arthur C. The Challenge of the Sea. Holt, Rinehart and Winston, 1960.

Cousteau, Ashlan, et al. Oceans. John Wiley & Sons, 2021.

Earle, Sylvia Alice. Sea Change: A Message of the Oceans. Putnam, 1995.

Goldin, Augusta. Oceans of Energy: Reservoir of Power for the Future. Harcourt Brace Jovanovich, 1980.

"Inside the New High Seas Treaty." Pew, 27 Aug. 2024, www.pewtrusts.org/en/research-and-analysis/issue-briefs/2024/08/inside-the-new-high-seas-treaty. Accessed 22 Nov. 2025.

Messier, Vartan P., and Nandita Batra, editors. This Watery World: Humans and the Sea. Rev. 2nd ed., Newcastle upon Tyne, Cambridge Scholars, 2008.

Newton, David E. World Oceans: A Reference Handbook. ABC-CLIO, 2021.

Pinet, Paul R. Invitation to Oceanography. 8th ed., Jones and Bartlett, 2021.

Robinet, Fabrice. "‘Plenty of Fish in the Sea’? Not Anymore, Say UN Experts in Nice." United Nations, 11 June 2025, news.un.org/en/story/2025/06/1164251. Accessed 22 Nov. 2025.

Rose, Paul, and Anne Laking. Oceans: Exploring the Hidden Depths of the Underwater World. U of California P, 2008.

Smith, Hance D., editor. The Oceans: Key Issues in Marine Affairs. Kluwer Academic, 2004.