The Southern Ocean only became official around the year 2000
Many people learned at school that Earth has four oceans. The familiar five-ocean map settled only recently: the U.S. Board on Geographic Names recognised the Southern Ocean, bounded by the current that circles Antarctica, in 1999, and the International Hydrographic Organization followed in 2000. The water had been there all along.
The divisions are human conventions laid over one continuous body of salt water. The Russian oceanographer Yuly Shokalsky coined the term World Ocean in the early twentieth century to capture that unity, which oceanographers treat as fundamental. Medieval Europeans had a narrower World Sea, the water surrounding the joined landmass of Europe, Asia and Africa, which left out the Mediterranean, Black and Caspian seas. The word ocean itself comes from Oceanus, the Titan whom Greeks and Romans pictured as a vast river flowing around the world; some early Greek vases give him a dragon's tail.
Together the oceans hold 97 percent of Earth's water and cover about 70.8 percent of the surface. The spot farthest from any land, called Point Nemo, lies in the South Pacific roughly 2,688 kilometres from the nearest coast, in a region used as a graveyard for retired spacecraft. Only the top layer, down to about 200 metres in open water, gets enough light for photosynthesis, yet that sunlit film feeds most marine life. Ocean plankton first produced the oxygen in our air and still supply around half of it.
How the oceans formed is still uncertain. One picture has water vapour from volcanic outgassing and impacts condensing once the young planet cooled, possibly into seas hotter than today and tinted green by dissolved iron. Pillow basalt from Greenland's Isua belt, rock that only forms in underwater eruptions, shows liquid water 3.8 billion years ago, and a site in Quebec may push that back further. A 2020 study suggested Earth may have held enough water for its oceans from the very start, kept liquid by greenhouse gases while the young Sun shone at only 70 percent of its present strength.
Sea level has never stood still. During the last ice age, glaciers covered almost a third of the land and the oceans sat about 122 metres lower than now. Around 125,000 years ago, in the last warm interval, they were about 5.5 metres higher, and three million years ago perhaps as much as 50 metres higher.
Source: Ocean
