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It is difficult to work out the age of the Great Barrier Reef. This is because of the way a reef can grow and shrink as the sea level changes. A coral reef can grow in diameter, that is how wide they are, from 1 to 2 cm per year. They can also grow upwards from 1 to 15 cm per year. Coral can not grow if the water is too deep (150 metres or more) because it needs light from the sun. They cannot grow out of the water (above sea level).[5]

n not grow if the water is too deep (150 metres or more) because it needs light from the sun. They cannot grow out of the water (above sea leve

Researchers at the Australian Institute of Marine Science say the reef has lost half of its coral cover over the past 27 years. There are multiple causes, including a destructive kind of starfish, shown here.

The starfish is about 42 percent. Typhoons, big strong storms another 48 percent, and then coral bleaching is the remaining 10 percent which is caused whenever the water gets too hot.

GWEN IFILL: So when you say it's human-caused, do you mean what we do with our waters, what we do with our fishing, what we do with our runoff from agricultural causes?

Four hundred species of coral have been found there, as well as 4,000 species of mollusk and 2,000 different kinds of fish.The Great Barrier Reef acts as a buffer between the strong waves of the Pacific and the shores of Australia.Many parts of the Great Barrier Reef have islands made of piled up coral sand.

Coral Bleaching - Coral reefs are fragile ecosystems and are extremely sensitive to water temperature; so, any changes can have an effect. When the ocean water temperature rises, it causes coral bleaching. That means that the symbiotic alga inside the coral polyps dies, which causes the polyps to turn white and eventually die. The reefs can recover from only a limited amount of bleaching before they are no longer able to recover.Shelter - Barrier reefs play an important role of sheltering the coastline from the full effect of the ocean’s waves. If the reef crumbles, the coastlines are exposed to damaging waves and the people there would have to deal with erosion and wave action, and low-lying structures could be at risk.