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1.) The Great Barrier Reef off northern Australia in the Indo-Pacific is the largest barrier reef in the world. This reef stretches more than 1,240 miles (2,000 km). 2.) Coralline algae, encrusting bryozoans, and minerals cement the dead organic matter, stabilizing the reef structure. 3.) fringing reef, barrier reef, small island. 1.)T he earth receives energy from the sun, which warms the e arth’s surface. As this energy passes through the atmosphere, a certain percentage (about 30) gets scattered. Some part of this energy is reflected back into the atmosphere from the land and ocean surface. The rest (70%) actually remains behind to heat the earth. In order to establish a balance, therefore, the earth must radiate some energy back into the atmosphere. As the earth is much cooler than the sun, it does not emit energy as visible light. It emits through infrared or thermal radiation. However, certain gases in the atmosphere form a sort of blanket around the earth and absorb some of this energy emitted back into the atmosphere. Without this blanket effect, the earth would be around 30 ° C colder than it normally is. These gases like carbon dioxide, methane, and nitrous oxide, along with water vapour, comprise less than one per cent of the atmosphere. They are called 'greenhouse gases' as the working principle is same as that which occurs in a greenhouse. Just as the glass of the greenhouse prevents the radiation of excess energy, this ‘gas blanket’ absorbs some of the energy emitted by the earth and keeps temperature levels intact. This effect was first recognized by a French scientist, Jean-Baptiste Fourier, who pointed out the similarity in what happens in the atmosphere and in a greenhouse. Hence the term the ‘greenhouse effect’. This gas blanket has been in place ever since the creation of the earth. Since the industrial revolution human activities have been releasing more and more of these greenhouse gases into the atmosphere. This leads to the blanket becoming thicker and upsets the ‘natural greenhouse effect’. Activities that generate greenhouse gases are called ‘sources’ and those that remove them are known as ‘sinks’. A balance between ‘sources’ and ‘sinks’ maintains the levels of these greenhouse gases. Humankind upsets this balance when new sources that interfere with the natural sinks are introduced. Carbon dioxide is released when we burn such fuels as coal, oil, and natural gas. And, when we destroy forests, the carbon stored in the trees escapes as carbon dioxide into the atmosphere. Increasing agricultural activities, changes in land-use patterns, and other sources lead to rising levels of methane and nitrous oxide. Industrial processes also release artificial and new greenhouse gases like CFCs (chlorofluorocarbons), while automobile exhaust fumes lead to ozone generation. The resulting enhanced greenhouse effect is more commonly referred to as global warming or climate change.
 * sea world: coral reefs**
 * edugreen**

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