Sea

A sea is a large body of salt water. The word can mean the whole ocean that covers most of Earth, or it can mean a smaller part of water like the Mediterranean Sea. This water holds lots of dissolved salts, mostly sodium chloride. Many kinds of life live here, from tiny bacteria and algae to big animals. They live in many different marine habitats and ecosystems.

The ocean helps shape Earth's climate. It moves water and air around the planet, helping control temperature and weather. Tides, caused by the pull of the Moon and the Sun, make sea levels rise and fall regularly. Seas have always been important to people. They give us food like fish and shellfish. They help with trade and travel, and they let us enjoy fun activities such as swimming, sailing, and scuba diving.

Definition

Further information: List of seas on Earth

The sea is the huge network of all Earth's ocean waters, including the Atlantic, Pacific, Indian, Southern, and Arctic Oceans. But the word "sea" can also refer to many smaller bodies of salt water, like the North Sea or the Red Sea. Seas are usually smaller than oceans and often partly or fully enclosed by land. One special sea, the Sargasso Sea, has no coastline and lies inside a circular current in the North Atlantic Gyre. Most seas contain salt water, but the Sea of Galilee is a freshwater lake.

Physical science

Further information: Ocean § Physical properties, and Physical oceanography

Earth is the only known planet with seas of liquid water on its surface, although Mars has ice caps and similar planets in other solar systems may have oceans. Earth's sea contains about 97.2 percent of its known water and covers most of its surface. Some 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 around the world. The rest forms underground reservoirs or parts of the water cycle, containing freshwater used by most terrestrial life: vapor in the air, the clouds it forms, the rain from them, and the lakes and rivers that flow to the sea.

The scientific study of water and Earth's water cycle is hydrology; hydrodynamics studies the physics of water in motion. Oceanography is the study of the sea. It began with the study of ocean currents but now includes many areas: it examines seawater; studies waves, tides, and currents; charts coastlines and maps the seabeds; and studies marine life. Marine biology studies the plants, animals, and other organisms in marine ecosystems. Chemical oceanography studies the behavior of elements and molecules in the oceans, especially the ocean's role in the carbon cycle and carbon dioxide's effect on seawater.

Seawater

Seawater is salty. The open ocean has about 35 grams of solids per litre, a salinity of 35 ‰. The Mediterranean Sea is a bit higher at 38 ‰, while the northern Red Sea can reach 41‰. Some landlocked hypersaline lakes have much higher salinity, for example, the Dead Sea has 300 grams of dissolved solids per litre (300 ‰).

Table salt (sodium and chloride) makes up most of the solids in seawater, but there are also other metals such as magnesium and calcium, and other ions including sulphate, carbonate, and bromide. Seawater is too salty for humans to drink safely.

Factors such as evaporation, freezing, precipitation, and runoff from land affect salinity. The Baltic Sea has lower salinity because of rivers flowing into it, while the Red Sea is very salty due to high evaporation.

Sea temperature depends on solar radiation. In the tropics, surface temperatures can be over 30 °C (86 °F) while near the poles it is about −2 °C (28 °F). Warm surface currents cool as they move away from the tropics, and the water becomes denser and sinks. Cold water moves back towards the equator as a deep sea current before rising again. Deep seawater has a temperature between −2 °C (28 °F) and 5 °C (41 °F) everywhere.

Seawater freezes at about −1.8 °C (28.8 °F). When it freezes, ice crystals form and become sea ice. Over time, the salinity of sea ice decreases.

Seawater is slightly alkaline with an average pH of about 8.2. Climate change has increased carbon dioxide in the atmosphere, which is absorbed by the oceans and lowers the pH through ocean acidification. The amount of oxygen in seawater depends on plants, mainly algae including phytoplankton, and some vascular plants such as seagrasses. In daylight, these plants produce oxygen, but at night oxygen levels drop. In the deep sea, where light does not reach, there is very little oxygen.

The amount of light that reaches the sea depends on the sun's angle, weather, and the turbidity of the water. Red light is absorbed quickly, while blue and violet light can reach deep. There is not enough light for plants beyond about 200 metres (660 ft).

Sea level

See also: Sea surface height

Over most of history, sea level has been higher than today. At the Last Glacial Maximum, about 20,000 years ago, sea level was lower than now.

For the past 100 years, sea level has been rising at about 1.8 millimetres (0.071 in) per year. This rise is mostly due to warmer sea temperatures and melting ice from melting snow and glaciers.

Waves

Main article: Wind wave

Wind over water creates waves. A gentle breeze makes small ripples, while strong winds create larger waves. In open water, waves called swell can travel long distances. When waves meet land, they change and can break, creating foam as they wash ashore.

Tsunami

Main article: Tsunami

A tsunami is a large wave caused by events such as underwater earthquakes or landslides. These events lift or lower the sea surface, creating a wave that travels fast across the ocean. In deep water, tsunamis are small but as they near shore, they grow very large and can flood coastal areas.

Currents

Main article: Ocean current

Wind causes surface currents in the sea. These currents flow in certain patterns called gyres. There are five main gyres in the world's oceans. These currents affect Earth's climate. Deeper currents, driven by differences in water density, also move water around the globe.

Temporary currents such as longshore currents and rip currents can form near shores and can be dangerous for swimmers. Upwelling currents bring deep, nutrient-rich water to the surface, supporting many plants and animals.

Tides

Main article: Tide

Tides are the regular rise and fall of sea levels caused by the gravitational pull of the Moon and the Sun. Most places have two high tides and two low tides each day. The Moon has a stronger effect on tides than the Sun. When the Sun, Moon, and Earth line up, we get higher "spring tides". When they are at an angle, we get lower "neap tides".

A storm surge can cause higher tides when strong winds and low pressure push water toward the shore.

Ocean basins

Main article: Ocean basin

The Earth has a central core, a liquid mantle, and a solid outer layer called the lithosphere. The ocean floor, made of oceanic crust, is part of the lithosphere. It is thinner than continental crust and often has mid-oceanic ridges where new crust forms. Subduction occurs where oceanic plates move under others, creating deep trenches. The deepest trench is the Mariana Trench.

Coasts

Main article: Coast

The area where land meets sea is the coast. A beach is sand or shingle on the shore. Waves and weather shape the coast, creating bays, coves, and capes. Material from the land ends up in the sea, forming features like deltas. Human structures like breakwaters and seawalls help protect land from the sea.

Water cycle

Main article: Water cycle

The sea is part of the water or hydrological cycle. Water evaporates from the ocean, travels as vapour, condenses, falls as rain or snow, and returns to the sea. In dry places like the Atacama Desert, fog from the sea supports plant life.

Some areas, like the Caspian Sea and the Aral Sea, have no outlet to the ocean. Their water evaporates, leaving minerals behind.

Carbon cycle

Further information: Oceanic carbon cycle and Biological pump

Oceans hold a lot of carbon. Carbon from the air dissolves in seawater and changes forms. It can also enter through rivers and be used by plants. This carbon moves through the ocean and can become part of the deep sea or return to the surface.

Life in the sea

Main article: Marine life

The oceans are home to many different kinds of living things. Sunlight only reaches the top layers, so most of the ocean is dark. Every part of the ocean, from the surface to the deep, has its own special plants and animals. Marine life includes huge creatures like whales and tiny organisms like phytoplankton and zooplankton. These plants and animals are important because they help keep our planet healthy by absorbing carbon dioxide and providing food for humans.

Life in the sea may have started in places like hot springs or deep-sea vents. Marine habitats include areas near the shore and the open ocean, with many different places to live such as coral reefs, kelp forests, and seagrass meadows. These habitats support many different kinds of life, from tiny bacteria to large fish and whales. Coral reefs, often called the "rainforests of the sea," are especially rich in different species, even though they cover only a small part of the ocean.

Humans and the sea

Humans have traveled the seas since they built boats. Early groups like the Mesopotamians used a special material to make their boats stronger. Later, people from Taiwan spread out into maritime Southeast Asia. The Ancient Egyptians and Phoenicians explored areas like the Mediterranean and Red Sea.

As time went on, new tools helped people navigate, like the compass and astrolabe. These tools made it easier to find directions at sea. Later, maps became very important for navigation. Famous maps made by Ptolemy and Gerardus Mercator helped explorers. Scientists also began studying the ocean, starting with voyages by Captain James Cook.

Environmental issues

Main articles: Ocean § Threats from human activities, and Human impact on marine life

The sea has many problems because of things people do. These problems include pollution, overfishing, and changes in climate. Pollution comes from waste, plastic, and chemicals. This pollution can hurt animals and plants in the water. Overfishing means there are fewer fish and sea creatures. Climate change makes the water warmer and changes the ocean. These problems can hurt the balance of life in the sea and affect the food we get from it.