What Is the Difference Between an Ocean and a Sea?

The concept of a sea refers to a large body of salty water. It does not necessarily have to be connected to a larger ocean, although it most often is so. Some people may wonder whether an ocean is bigger than a sea. It is true that seas are smaller bodies of water. Generally speaking, seas have smaller dimensions than oceans. They are not as widely extended and are shallower.

Seas do not usually have natural outlets and are close to large bodies of land. They usually connect land to oceans. Waves are also formed in the waters of seas.

The main seas in the world are the following:

• Mediterranean Sea: the largest sea in the continental interior of the planet, with an approximate extension of 2,500,000 km². It is located between Europe, Africa and Asia. It connects with the Atlantic Ocean through the Strait of Gibraltar.
  
  
• Baltic Sea: it is one of the seas of northeastern Europe and has an extension of 377,000 km². It is an inland sea with brackish water, meaning it has more dissolved salts than fresh water, but than marine water. Learn more with our article on what is brackish water?
  
  
• Aral Sea : it is located in Central Asia and has an extension of 6,800 km². Although it is called a sea, it is technically a lake and was oncee one of the largest lakes in the world. At present its surface has been reduced by more than 10%, which is considered one of the biggest environmental catastrophes on the planet.
  
  
• Dead Sea: it has an area of 605 km² and is located in the Middle East. Due to climate change, the waters of the Dead Sea are reduced by 1 meter every year and the coastal lands are lowered by 15 cm. It is known as the Dead Sea because its high salinity makes it very difficult for many organisms to survive.
  
  
• Caribbean Sea: also known as the Sea of the Antilles, it is located between Central America and South America and has an area of 2,763,800 km². The first known exploration of this sea was by Christopher Columbus in 1492.
  
  
• Caspian Sea: located in the southeast of Europe, it has an area of 371,900 km² and there is debate as to whether it is the smallest sea or the largest lake that exists.
  
  
• Red Sea: it has an area of 450 thousand km² and is located between Africa and Asia. It acts as an important communication channel between Europe and the Far East.
  
  
• Black Sea: lies between Eastern Europe and Western Asia. It has an area of 436,400 km² and a maximum depth of 2,212 meters.

As you can see, some of the large bodies of water referred to as ‘seas’ are actually lakes. This is usually due to their large size and importance to their geographical area. Learn more about how bodies of water are defined with our article on the difference between a lake and a lagoon.

The oceans represent the largest expanses of salt water that are part of the planet's hydrosphere (i.e. the combined water mass of our planet). Oceans cover most of the Earth's surface and separate continents. Historically, there have always been four oceans. This changed in the year 2000 when the International Hydrographic Organization added the Southern (Antarctic) Ocean to this list. For this reason, we can say the world has the following five oceans:

• Atlantic Ocean: this ocean separates the continents of America, Europe and Africa. It has a length of 14,700 meters, from north to south, and a maximum width of 11,800 kilometers, from the Gulf of Mexico to the Black Sea. It reaches a maximum depth of 8,605 meters in the Puerto Rico Trench. It covers approximately 20% of the earth's surface, extending over an area of 81,760,000 km². It is the most important ocean of the five from the point of view of shipping routes, whether for export or import.
  
  
• Pacific Ocean: occupies 166,241,000 km², making it the largest ocean in the world. It has a maximum depth of 10,924 meters in the Mariana Trench and is located between the continents of Asia, America, Oceania and Antarctica. The Pacific and the Atlantic oceans meet at three points. These are the Strait of Magellan, the Panama Canal and the Drake Passage. If you are curious to learn more, you can check out our article on where the Pacific and Atlantic oceans mix.
  
  
• Indian Ocean: occupies almost 74 million km² and bathes the coasts of eastern Africa, the Middle East, South Asia and Australia. It has a maximum width of 10,000 km and its surface occupies 68,556,000 km². It began to be known as the Indian Ocean from the year 1515, before which it was known as ‘the Eastern Ocean’. The first known civilizations were formed around the Indian Ocean.
  
  
• Arctic Ocean: occupies 14,056,000 km² in the Northern Hemisphere, making it the smallest ocean on Earth. Its maximum depth is 5,607 meters in the Molloy Deep. It is largely covered by masses of ice that protect it from atmospheric changes. Its temperatures in winter can reach -50 ºC/-58 ºF and does not exceed 0 ºC/32 ºF. Learn more about our article on the difference between temperature and heat.
  
  
• Antarctic Ocean: also known as the Southern Ocean, it occupies an area of 20,327,000 km² and is located in the Southern Hemisphere. Its maximum depth is 7,235 meters in the South Sandwich Trench.

One of the reasons seas and oceans are confused is due to official and colloquial terminology. The oceans of the world have colloquially been known as the ‘Seven Seas’. This is because the term divides the Pacific and Atlantic Oceans into separate bodies of water which are the northern and southern parts of the same ocean. Despite being called ‘seas’, they are not seas, but oceans.

To add to this confusion, many people refer to any ocean simply as the ‘Sea’. Although we can delineate different oceans by their geography, it is also true to say that the planet Earth has one combined ocean. It is this to which we refer to as the ‘sea’.

With this in mind, we can look at the specific differences between an ocean and a sea:

Extension

The main difference between the two is their extension, since seas are smaller than oceans. In addition, seas are closed bodies of water located between the land and the ocean. Oceans are open, deeper and have strong currents. Due to their extension, there are bodies of water known as seas that are considered to be large saltwater lakes. This group includes the Caspian Sea, the Dead Sea and the Aral Sea.

Depth

Due to their greater depth, oceans reach temperatures lower than seas. This is approximately 4 ºC at any point. As seas are closer to land masses, they receive more solar radiation and reach higher temperatures than oceans, although this varies between different seas. For example, the waters of the Mediterranean Sea are warm and those of the Dead Sea are cold. Due to their proximity to land, seas suffer greater desertification at their coasts as global warming increases, while the oceans have increased their volume of water due to the melting of the poles and glaciers.

Learn more about global warming of the oceans with our article on what is sea ice?

Biodiversity

In terms of marine plant and animal life, seas have a higher biodiversity in number of species than oceans. This is due to the greater solar radiation they receive. Oceans are home to fewer species, but more adapted to depths and low temperatures. Seas are more exposed to environmental pollution than the oceans and, as a consequence, to a greater mortality of species.

Learn about one approach to the world's oceans which can help protect them from pollution and climate change with our article on the definition and examples of the Blue Economy.

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