Query: If water temperatures stay moderate, the damaged sections of the Great Barrier Reef may be covered with corals again in as few as 10 or 15 years

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Great Barrier Reef <=SEP=> The Great Barrier Reef is the world 's largest coral reef system composed of over 2,900 individual reefs and 900 islands stretching for over 2300 km over an area of approximately 344400 km2 . The reef is located in the Coral Sea , off the coast of Queensland , Australia .   The Great Barrier Reef can be seen from outer space and is the world 's biggest single structure made by living organisms . This reef structure is composed of and built by billions of tiny organisms , known as coral polyps . It supports a wide diversity of life and was selected as a World Heritage Site in 1981 . CNN labelled it one of the seven natural wonders of the world . The Queensland National Trust named it a state icon of Queensland .   A large part of the reef is protected by the Great Barrier Reef Marine Park , which helps to limit the impact of human use , such as fishing and tourism . Other environmental pressures on the reef and its ecosystem include runoff , climate change accompanied by mass coral bleaching , and cyclic population outbreaks of the crown-of-thorns starfish . According to a study published in October 2012 by the Proceedings of the National Academy of Sciences , the reef has lost more than half its coral cover since 1985 .   The Great Barrier Reef has long been known to and used by the Aboriginal Australian and Torres Strait Islander peoples , and is an important part of local groups ' cultures and spirituality . The reef is a very popular destination for tourists , especially in the Whitsunday Islands and Cairns regions . Tourism is an important economic activity for the region , generating over AUD$ 3 billion per year . In November 2014 , Google launched Google Underwater Street View in 3D of the Great Barrier Reef .   A March 2016 report stated that coral bleaching was more widespread than previously thought , seriously affecting the northern parts of the reef as a result of warming ocean temperatures . In October 2016 , Outside published an obituary for the reef ; the article was criticized for being premature and hindering efforts to bolster the resilience of the reef . In March 2017 , the journal Nature published a paper showing that huge sections of a 800-kilometre ( 500 mi ) stretch in the northern part of the reef had died in the course of 2016 due to high water temperatures , an event that the authors put down to the effects of global climate change .

Environmental threats to the Great Barrier Reef <=SEP=> The Great Barrier Reef , the world 's largest reef system , stretching along the East coast of Australia from the northern tip down to the town of Bundaberg , is composed of roughly 2,900 individual reefs and 940 islands and cays that stretch for 2,300 kilometres ( 1,616 mi ) and cover an area of approximately 344,400 km2 . The reef is located in the Coral Sea , off the coast of Queensland in northeast Australia . A large part of the reef is protected by the Great Barrier Reef Marine Park .   According to the 2014 report of the Australian Government 's Great Barrier Reef Marine Park Authority ( GBRMPA ) , says that climate change is the most significant environmental threat to the Great Barrier Reef , while the other major environmental pressures are listed as decreased water quality from land-based runoff , impacts from coastal development and some persistent impacts from fishing activities . The reef is also threatened by storms , coral bleaching and ocean acidification . The 2014 report also shows that , while numerous marine life species have recovered after previous declines , the strength of the dugong population is continuing to decline . Terry Hughes , Federation Fellow , ARC Centre of Excellence for Coral Reef Studies at James Cook University , wrote in a 14 August 2014 Conversation piece that harmful government policies and ongoing conflicts of interest over mining royalties are risks of an equivalent magnitude .   The GBRMPA consider climate change , poor water quality , coastal development , and some impacts from fishing to be the area 's major threats , but reef scientists Jon Day , Bob Pressey , Jon Brodie and Hughes stated that the `` cumulative effects of many combined impacts '' is the real issue .   In a Conversation Article , Mathieu Mongin , a biogeochemical modeller at CSIRO and colleagues mapped parts of the Great Barrier Reef that are most exposed to ocean acidification . This map of pH on the Great Barrier Reef presents the exposure to ocean acidification on each of the 3,581 reefs , providing managers with the information they need to tailor management to individual reefs . The Great Barrier Reef is not a singular reef nor a physical barrier that prevents exchange between reefs ; it is a mixture of thousands of productive reefs and shallow areas lying on a continental shelf with complex oceanic circulation .

Coral bleaching <=SEP=> Coral bleaching occurs when coral polyps expel algae that lives inside their tissues . Normally , coral polyps live in an endosymbiotic relationship with the algae and that relationship is crucial for the coral and hence for the health of the whole reef . Bleached corals continue to live . But as the algae provide the coral with 90 % of its energy , after expelling the algae the coral begins to starve . Above-average sea water temperatures caused by global warming have been identified as a leading cause for coral bleaching worldwide . Between 2014 and 2016 , the longest global bleaching events ever were recorded . According to the United Nations Environment Programme , these bleaching events killed coral on an unprecedented scale . In 2016 , bleaching hit 90 percent of coral on the Great Barrier Reef and killed 29 percent of the reef 's coral . In 2017 , the bleaching further expanded to areas of the park that were previously spared , such as the central one .   __ TOC __

Coral reef <=SEP=> Coral reefs are diverse underwater ecosystems held together by calcium carbonate structures secreted by corals . Coral reefs are built by colonies of tiny animals found in marine waters that contain few nutrients . Most coral reefs are built from stony corals , which in turn consist of polyps that cluster in groups . The polyps belong to a group of animals known as Cnidaria , which also includes sea anemones and jellyfish . Unlike sea anemones , corals secrete hard carbonate exoskeletons which support and protect the coral polyps . Most reefs grow best in warm , shallow , clear , sunny and agitated waters .   Often called `` rainforests of the sea '' , shallow coral reefs form some of the most diverse ecosystems on Earth . They occupy less than 0.1 % of the world 's ocean surface , about half the area of France , yet they provide a home for at least 25 % of all marine species , including fish , mollusks , worms , crustaceans , echinoderms , sponges , tunicates and other cnidarians . Paradoxically , coral reefs flourish even though they are surrounded by ocean waters that provide few nutrients . They are most commonly found at shallow depths in tropical waters , but deep water and cold water corals also exist on smaller scales in other areas .   Coral reefs deliver ecosystem services to tourism , fisheries and shoreline protection . The annual global economic value of coral reefs is estimated between US$ 29.8-375 billion . However , coral reefs are fragile ecosystems , partly because they are very sensitive to water temperature . They are under threat from climate change , oceanic acidification , blast fishing , cyanide fishing for aquarium fish , sunscreen use , overuse of reef resources , and harmful land-use practices , including urban and agricultural runoff and water pollution , which can harm reefs by encouraging excess algal growth .