oceans aware: inform, inspire, involve

the more you know about the ocean the more you can do to protect and restore it

one ocean

For the ancient Greeks, there was only one ocean: Ōkeanós, the eldest son of Uranus (sky) and Gaia (earth), was believed to be the divine personification of the World Ocean, an enormous river encircling the planet. Until recently stronger regional perspectives persisted: the world ocean was split into three main oceans, the Atlantic, Indian and Pacific, two smaller oceans, the Arctic and Southern (or Antarctic) oceans, and numerous seas and regional basins. Moving back to an more connected view, we are now starting to talk again about one ocean, one body of water. Of course each region, each sea and each individual ocean maintains its own particular qualities, dynamics and challenges but in essence they are still one. In order to properly understand the ocean and to protect and preserve it, it is essential to see the ocean as one connected body of water.

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The largest ocean of all is the Pacific Ocean covering just over 162 million square kilometres and containing 46% of the Earth's water, with a coastline of 135,663 kilometres. The average depth of the Pacific Ocean is 4,080 metres, this goes all the way down to the deepest fault of the world, the Mariana Trench, at a depth of 10,983 metres. The Pacific Ocean is surrounded by the Circumpacific Volcanic Zone or ‘Ring of Fire’, the deep-sea furrows in this area are known to be very active. The highest surface layer temperature of 28°C is found at the Equator between Papua New Guinea, the Philippines and the Marshall Islands, the lowest in the north in the Bering Sea, where it is just 1°C. The average seabed temperature at a depth of 4,000 metres is 4°C, this is lowered by polar waters to 1-2°C. 

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The second largest ocean, the Atlantic, covers just over 106 million square kilometres, has a coastline of 111,866 kilometres and holds about 29% of the Earth's water. Its lowest point is the Puerto Rico Trench at a depth of 8,605 metres, its average depth is 3,646 metres. The water temperature ranges from -1°C at the poles to 28°C at the Equator.

The smallest of the three main oceans, the Indian Ocean, covers about 70 million square kilometres of the planet, has a coastline of 66,526 km and holds about 20% of the Earth’s water, with an average depth of about 3,960 metres and the greatest depth of 7,192 metres in the Java Trench. The temperature of the surface layer of the Indian Ocean lies between 18 and 29°C, making it the warmest ocean of all.

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While similar in size, the polar zones are actually very distinct. The 14 million square kilometre expanse of the Arctic Ocean is covered in ice which fluctuates from a few centimetres to around two metres in thickness, whilst Antarctica is a 14.2 million square kilometre landmass, 98% of which is covered by ice up to 4,700 metres thick, surrounded by the Southern or Antarctic Ocean. The ­average annual temperature at the South Pole is -49.3°C, while at the North Pole it is -18°C. Similar to both ecosystems is the seasonal formation and melting of the sea ice. The fluctuation in ice effects the availability of nutrients: as the ice melts the released nutrients feed phytoplankton, the foundation of the marine food chain, providing nutrition for numerous species  from krill to whales.

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Ocean movement

The ocean is never still, its continuous horizontal and vertical movement is determined by the Earth's rotation and gravity, together with winds, tides and the different densities of the water caused by varying levels of salinity and temperature. Water which is cooler with a high level of salinity sinks as it is denser than warm, less salty, water, which stays on the surface. Horizontal movements within the water are called currents, vertical movements are known as upwellings or downwellings. Waves are generally caused by the friction between the wind and the surface water, as well as by the gravitational pull of the sun and moon on the Earth. They can also be caused by disturbances below the surface such as earthquakes or landslides, causing long powerful and often destructive waves called tsunamis. You can find out more about waves here.

Thermohaline circulation, sometimes called the ocean conveyor belt, transfers water, heat, oxygen, carbon, salt, nutrients and, unfortunately, pollution around the planet from the surface to the ocean depths, from the poles to the Equator, helping to balance the global climate and playing a key role in the hydrological cycle. It takes about a thousand years for one loop of the belt to be completed. Climate change is slowing it down, as explained on the ocean-climate nexus page.

In addition to this global circulation, there are 5 major gyres, the North and South Pacific Subtropical Gyres, the North and South Atlantic Subtropical Gyres and the Indian Ocean Subtropical Gyre. Gyres form when wind-blown water meets the force of the rotation of the planet and its direction is changed, the deflection is circular in motion, clockwise in the Northern Hemisphere and anti-clockwise in the Southern Hemisphere, which is known as the Coriolis Effect. The effect is greatest at the ocean surface: the deeper you go, the weaker the deflection, it causes a spiral of water down to a depth of about 100 metres. Gyres nearer to the Equator, where the Coriolis Effect isn’t present, are not circular but flow in an east-west direction. The circumventing currents of the gyres encompass large areas of calm water, meaning that anything brought to those waters accumulates and remains there due to the low level of movement within. Areas of accumulated waste from the surrounding coastal States or ships, so-called garbage patches, have developed in the Indian, North Atlantic and North Pacific ocean gyres over the years. More information on this can be found on the ocean plastic pages.

Ocean chemistry

The ocean produces somewhere between 50 and 80% of the planet's oxygen. While rainforests have often been championed as the lungs of our planet, in actual fact it is the phytoplankton in the ocean which are responsible for providing most of the Earth’s oxygen. The ocean is also an essential component of the carbon cycle: it absorbs at least 23% of carbon dioxide emissions (its water can contain up to 50 times more carbon than the atmosphere) and soaks up over 90% of the excess heat caused by burning fossil fuels. Microalgae absorb carbon dioxide and lock it into the marine food cycle as they are consumed by marine life from the smallest zooplankton through fish up to the largest marine mammals. When marine species die, and if they aren’t consumed, their carbon-infused bodies sink to the ocean floor and settle as sediment, later decomposing and forming natural carbon reservoirs in the form of methane, coal, oil, natural gas, and limestone. The greatest amount of carbon dioxide is, however, dissolved in the water through respiration at lower depths: carbon dioxide isn’t released into the atmosphere if this occurs at or below a depth of about 800 metres, instead it is stored there. Closer to shore, mangroves, tidal marshes and seagrass habitats cover less than 7% of the total ocean area but account for approximately 50% of the total carbon sequestered in ocean sediments, earning them the name blue carbon. More about these carbon champions can be found here. 

Cinzia Osele Bismarck/Ocean Image Bank


Ocean habitats

The ocean encompasses a multitude of marine habitats which vary dramatically, from tropical coral reefs to the freezing poles, from mangroves to the deep sea; each habitat is shaped by its range of light, temperature, depth, pressure and salinity. Coastal habitats support most marine life and extend up to the edge of the continental shelf; the deep sea, which occupies 95% of the living space found on our planet but remains to this day largely unexplored, lies beyond. Each ecosystem, despite its specific makeup, is part of the interconnected system of the ocean: what happens in one can soon effect another many kilometres away. Find out more about ocean habitats here.

Ocean life

An estimated 1 or perhaps even 2 million species of animals inhabit the oceans, with millions more bacteria, other microbes and viruses. The numbers are transient and about 2,000 new species are discovered and described by marine scientists every year. Marine life varies in size from some types of zooplankton which are no more than 2 micrometres to the largest mammal in the world, the blue whale, which can reach up to 30 metres in length and weigh 150 tonnes. Temperature, light, ocean depth, and distance from the shore determine the types of plants and animals living in specific habitats, with each habitat supporting a wide range of marine life, from ghost octopus in the utter darkness and near freezing temperatures on the deep seabed to parrot fish in the warm waters of Indo-Pacific coral reefs. Find out more about ocean life here.

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