The sea, the sea
The deep and surface ocean currents carry far more than heat around the world. They mean that salt, oxygen and other chemicals are distributed more or less evenly around the Earths oceans.
However, many of the Earths most important water masses are seas rather than oceans. What they lack in size, they make up for in importance to humanity and to the Earth as a whole.
Just when a sea is a sea is a matter of judgement. The Red Sea, for example, is about as large as the piece of water variously known as the Gulf of Arabia or the Gulf of Iran. There is no good reason for one to be a sea and the other a gulf. Seas are more prone than oceans to political controversy, not least about their names. Iran and its Arab neighbours have bickered for decades over naming that stretch of water that cowardly cartographers call simply The Gulf. And what separates Korea from Japan? The East Sea for a Korean, but the Sea of Japan for the Japanese.
Usually seas are comparatively shallow, existing on the continental shelf rather than the deep ocean. For example, the Yellow Sea between Korea and China sits on Asia's continental shelf and has a maximum depth of around 150m. However, those that are tectonically active, such as the Red Sea, the Mediterranean or the Caribbean, can be as deep as an ocean. The Caribbean is over 7000m deep in the Cayman Trench. To the purist, these are really micro-oceans rather than seas. Thus the East Sea/Sea of Japan is a deep sea that is effectively a detached piece of the Pacific Ocean.
However, a true sea has to be close to land while being more or less cut off from the full ocean, as with the North Sea, the Baltic or the Mediterranean. But it is probably wrong to think of a body of water as a sea unless it is attached to the rest of the world's oceans. So the Caspian Sea and the Sea of Aral are really inland lakes.
Their semi-detached status means that seas have their own ecology and way of working. The more isolated they are, the more individual they become over time. The ultimate case is the Black Sea. It has only a tenuous connection to the Mediterranean, which in turn has only a narrow passage to the Atlantic.
This means that its physical, chemical and biological nature is driven by the battle between the small amounts of salty water that make their way into it from the Mediterranean and the fresh water that arrives from the Don and the Danube, two of Europe's great rivers. This makes the Black Sea the world's biggest “meromictic” water body, one with a huge variation in salinity in a small region. The salty water builds up at the bottom of the sea and over time has formed a large volume of oxygen-free (or anoxic) water.
However, the connection to the Mediterranean is new. The Black Sea was isolated until about 7000 years ago, when it opened in a huge flood, memories of which may connect to the story of Noah. Until then it had many of its own fish, plants and other species, but now most of the Black Sea's flora and fauna have been replaced by Mediterranean types.
The Mediterranean itself shows that it is not just oceans that have a conveyor belt of their own in action. Its depth goes down to 5000m and it is tectonically active, but it is also being gradually squeezed as the African plate pushes up against Europe.
Until the Suez Canal opened in 1869 the Mediterranean had only one outlet, to the Atlantic at Gibraltar. Mediterranean water is saltier and denser than Atlantic water, so that it flows out below the surface at Gibraltar and is replaced by cooler water in a surface current. Like the global ocean conveyor, the Mediterranean conveyor is in no hurry. The whole cycle from Gibraltar to the east and back takes about seventy years. The escaping water is carried out into the Atlantic by the Coriolis Force. Some makes its way up the western coast of Europe and can bring Mediterranean species to the coast of Ireland.
By contrast the North Sea – the term took over from the previous German Ocean, oddly enough around the time of World War I – is more or less a piece of sunken land. It would not take much of an ice age to lower sea level enough to expose most of it to view. It rarely gets to 200m deep and on the fish-rich Dogger Bank, trawling for fish regularly brings up fossil trees instead. It takes in water from the Atlantic, the Baltic and the English Channel, and rivers such as the Thames and the Rhine. Off their estuaries the salinity can fall to below 3 percent. In the summer, the waters ot the North Sea vary hugely in composition from one side to the other. But its winters are so rough that they mix its waters to a state of
almost complete homogeneity.
The varying composition and origin of these waters is no bit of arcane theory. It drives the North Seas ability to feed those who live near it. In the north, cold-water fish such as haddock abound, but further south, fishing yields warmer-water species such as bass. In addition, as recently flooded land, the North Sea has areas of sand, mud and gravel seabed, each preferred by different species.
But for the North Sea and most others, Homo sapiens is the species that matters most. Take a ship across it, and you will rarely be out of sight of another vessel. Its fishing grounds are the subject of dialogue-of-the-deaf arguments between scientists and the fishing industry over how badly damaged they are and how much fish they can afford to lose for human consumption. Gravel and other materials have been scooped from the sea-bed. The rocks that lie below it have been pumped for oil and gas, and in the era of renewable energy there are proposals to extract energy from it by the use of wind generators and machines to mine its tides and currents.
The confinement that gives different seas their individual nature means that they are prone to damage from over-use. They are easy to pollute and hard to clean up again. This same confinement can make them dangerous too. A stark example was seen in the North Sea in January 1953, when a storm surge swept clockwise round the United Kingdom. A storm surge occurs when low pressure causes winds to drive sea water towards the land. At their worst, this effect is supplemented by a high tide. This one began by sinking the ferry Princess Victoria between Scotland and Ireland, but became even more damaging in the North Sea where it combined with a very high tide. In the full ocean, such a surge could have worked off its energy without doing much damage. But in the confined and narrowing funnel that makes up the southern North Sea, the water could not escape and instead flooded low-lying areas of eastern England and the Netherlands. Over 1800 people were killed in the Netherlands, where 200,000 hectares of land were flooded, and 300 people in England. Waves over 6m high were observed on the normally placid coast of Lincolnshire in England.
Since this disaster, coastal defences have been improved in both countries, while computer systems to forecast such surges, and communications to get warnings to the people that need them, have been developed. The confined and less stormy nature of seas means that they tend to have less craggy and forbidding coastlines than shores facing the full ocean. They tend to be the places to visit if you want to find beaches rather than cliffs, and if you prefer paddling to surfing.
Because there is more ocean than sea in the world, about 75 percent of the seafront on the planet is rocky and cliff-dominated. And as erosion is slower along the edge of a sea than of an ocean, seas, especially small ones, tend not to accumulate massive new quantities of sediment, unless there are rivers full of mud and sand to supply it. So it is at continental margins rather than in seas that new islands and deposits build up en masse.
However, there is a tricky balancing act at work on many sea coasts. Thus the east coast of England has large soft sandy stretches that are placid most of the time. But they are prone to sudden, violent erosion during severe storms that can remove many metres of material in a night.