A NEW OCEAN has appeared on the maps of the National Geographic Society, an American research and conservation organisation. The Southern Ocean, which encircles Antarctica, will henceforth be given the same status, and typeface, as the Arctic, Atlantic, Indian and Pacific Oceans. But of course the Southern Ocean is not really new. Not only has the body of water been there for around 30m years, since Antarctica and South America moved apart, but what to call it has been mulled and contested by others before. So how many oceans are there? And how is that decided?

All of Earth’s oceans are part of one interconnected system. Mapmakers divide it into different zones, including oceans, which also contain smaller seas. Oceans are usually bordered by whole continents whereas seas are typically surrounded by smaller bits of land. But where two bodies of water converge scientists must look at the conditions in the water to decide their boundaries. Bodies of water often have multiple names. The Southern Ocean is sometimes called the Antarctic Ocean. Sometimes these names reflect competing political claims. For instance, the one between Japan and the Koreas is known as the Sea of Japan in Japan, the East Sea in South Korea and the Korean East Sea in North Korea. Most governments have departments given the task of surveying, mapping and naming oceanographic features but the arbiter of such issues is the International Hydrographic Organisation (IHO), to which 94 countries belong. (June 21st is also the IHO’s World Hydrography Day.)

The definition adopted by the National Geographic Society is that the Southern Ocean includes most of the waters that surround Antarctica to a latitude of 60° south, excluding the Drake Passage and Scotia Sea. Also called the “60th parallel south”, this cartographic line roughly corresponds with the path of the Antarctic Circumpolar Current, which swirls water clockwise from west to east and marks a boundary between the cold, northward-flowing waters of the Antarctic and the warmer subantarctic waters. The waters of the Southern Ocean are colder and less saline than those of the southern Atlantic, Pacific and Indian Oceans. This has allowed a distinct ecosystem to flourish, rich in plant and animal life including krill, penguins, seals, whales and albatrosses. Its icy nature makes it particularly susceptible to the effects of climate change.

The United States Board on Geographic Names already counts the Southern Ocean as distinct from other oceans, also using the 60th parallel definition. So do plenty of other governments, scientists and organisations. But the IHO’s position is trickier. The first edition of its marine mapping bible, “Limits of Oceans and Seas”, was published in 1928. This showed the Southern Ocean reaching Africa, Australia and South America. By the second edition, in 1937, the Southern Ocean’s northern limits were moved southward, no longer touching land. In the third edition, in 1953, the Southern Ocean was omitted entirely as its authors no longer saw a “justification” for applying the term ocean to this body of water, “the northern limits of which are difficult to lay down owing to their seasonal change.” Instead the Atlantic, Indian and Pacific Oceans were extended down to Antarctica. In 2000, when the fourth edition of “Limits of Oceans and Seas” was due to be published, IHO members voted to name the waters below the 60th parallel south the “Southern Ocean”. Argentina objected, as did Australia, which argues that it is the Southern, not the Indian Ocean which laps its southern shores. But other disputes between members, such as what to call the Sea of Japan, stopped the fourth edition from being formally ratified. Which leaves the IHO stuck in 1953, when the Southern Ocean didn’t exist at all.