Sea

A sea is a large body of salt water that forms part of the world's oceans and surrounds continents. The term is used in two ways: in a general sense to mean all the salt water on Earth, and in a specific sense to refer to a smaller sea bordered partly by land or islands, such as the Red Sea or the Mediterranean Sea.

Seas cover a large portion of Earth's surface and consist mostly of ocean water. They play an important role in water circulation, climate change, and the life of organisms. Many people living on coasts depend on seas for fishing, transportation, and trade.

Characteristics of seawater

Seawater is salt water, and the salt in it is predominantly sodium chloride (table salt). The salinity and temperature vary from place to place: in warm, arid seas salinity is higher, whereas in areas where large rivers flow in salinity is lower. Depth also varies, ranging from shallow coasts to deep ocean trenches that descend many kilometres.

Water movement

The sea is not stationary. Wind creates waves on the surface. Large-scale water movements, or currents, result from differences in temperature and salinity, as well as from Earth's rotation. The rise and fall of the sea (tides) depend on the gravitational pull of the moon and sun.

Life in the sea

The sea is home to many different types of organisms: fish, marine mammals such as whales and dolphins, seabirds, shellfish, and tiny organisms such as plankton, which form the base of the food chain. The shallowest areas that receive sunlight, such as coral reefs, contain the most life.

Human use and challenges

The sea is a major route for global commerce, fishing grounds, and a source of oil and minerals. There are challenges related to pollution, especially plastic, overfishing, and rising sea levels linked to climate change.