The Great Barrier Reef Marine Park Authority considers the greatest threat to the Great Barrier Reef to be climate change, causing ocean warming which increases coral bleaching. Mass coral bleaching events due to elevated ocean temperatures occurred in the summers of 1998, 2002 and 2006, and coral bleaching is expected to become an annual occurrence. As global warming continues, corals will not be able to keep up with increasing ocean temperatures. Coral bleaching events lead to increased disease susceptibility, which causes detrimental ecological effects for reef communities.
In July 2017 UNESCO published in a draft decision, expressing serious concern about the impact of coral bleaching on the Great Barrier Reef. The draft decision also warned Australia that it will not meet the targets of the Reef 2050 report without considerable work to improve water quality. Climate change has implications for other forms of reef life—some fish's preferred temperature range leads them to seek new habitat, thus increasing chick mortality in predatory seabirds. Climate change will also affect the population and sea turtle's available habitat. Bleaching events in benthic coral communities (deeper than 20 metres or 66 feet) in the Great Barrier reef are not as well documented as those at shallower depths, but recent research has shown that benthic communities are just as negatively impacted in the face of rising ocean temperatures. Five Great Barrier Reef species of large benthic corals were found bleached under elevated temperatures, affirming that benthic corals are vulnerable to thermal stress.
Another key threat faced by the Great Barrier Reef is pollution and declining water quality. The rivers of north eastern Australia pollute the Reef during tropical flood events. Over 90% of this pollution comes from farm runoff. 80% of the land adjacent to the Great Barrier Reef is used for farming including intensive cropping of sugar cane, and major beef cattle grazing. Farming practices damage the reef due to overgrazing, increased run-off of agricultural sediments, nutrients and chemicals including fertilisers, herbicides and pesticides representing a major health risk for the coral and biodiversity of the reefs. Sediments containing high levels of copper and other heavy metals sourced from the Ok Tedi Mine in Papua New Guinea are a potential pollution risk for the far northern Great Barrier Reef and Torres Strait regions. Some 67% of corals died in the reef's worst-hit northern section, the ARC Centre of Excellence for Coral Reef Studies report said.
The runoff problem is exacerbated by the loss of coastal wetlands which act as a natural filter for toxins and help deposit sediment. It is thought that the poor water quality is due to increased light and oxygen competition from algae. Farming fertiliser runoff release nitrogen, phosphorus, and potassium into the oceanic ecosystem, these limiting nutrients cause massive algal growth which leads to depletion in oxygen available for other creatures which decreases the biodiversity in the affected areas, altering the species composition. A study by Katharina Fabricius and Glen Death of Australian Institute of Marine Science found that hard corals numbers were almost double on reefs that were far from agricultural areas. Fertilizers also increase the amount of phytoplankton available for the crown-of-thorns starfish larvae to consume. A study showed that a doubling of the chlorophyll in the water leads to a tenfold increase in the crown-of-thorns starfish larvae's survival rate.
Mining company Queensland Nickel discharged nitrate-laden water into the Great Barrier Reef in 2009 and 2011 – on the later occasion releasing 516 tonnes (508 long tons; 569 short tons) of waste water. The Great Barrier Reef Marine Park Authority (GBRMPA) stated "We have strongly encouraged the company to investigate options that do not entail releasing the material to the environment and to develop a management plan to eliminate this potential hazard; however, GBRMPA does not have legislative control over how the Yabulu tailings dam is managed".
The unsustainable overfishing of keystone species, such as the Giant Triton, can disrupt food chains vital to reef life. Fishing also impacts the reef through increased water pollution from boats, by-catch of unwanted species (such as dolphins and turtles) and habitat destruction from trawling, anchors and nets. As of the middle of 2004, approximately one-third of the Great Barrier Reef Marine Park is protected from species removal of any kind, including fishing, without written permission.
Shipping accidents are a pressing concern, as several commercial shipping routes pass through the Great Barrier Reef. Although the route through the Great Barrier Reef is not easy, reef pilots consider it safer than outside the reef in the event of mechanical failure, since a ship can sit safely while being repaired. There have been over 1,600 known shipwrecks in the Great Barrier Reef region. On 3 April 2010, the bulk coal carrier Shen Neng 1 ran aground on Douglas Shoals, spilling up to four tonnes of oil into the water and causing extensive damage to the reef.
In March 2015, the Australian and Queensland's governments formed a plan for the protection and preservation of the reef's universal heritage until 2050. This 35 years plan, titled "Reef 2050 Plan" is a document proposing possible measures for the long-term management of the pollution, climate change and other issues that threaten the life span and value of this global heritage. The plan contains all the elements for measurement and improvements, including; long-term sustainability plan, water quality improvement plan and the investment plan for the protection and preservation of The Reef until 2050. However, whereas the 2050 plan aims to incorporate protective measures such as improving water quality, reef restoration, killing of predatory starfish, it does not incorporate additional measures to address the root cause the problem namely climate change (which is caused by greenhouse gas emissions). As such, experts doubt on whether it will be enough to save the fragile environment.
The Great Barrier Reef has long been known to and used by the Aboriginal Australian and Torres Strait Islander peoples. Aboriginal Australians have been living in the area for at least 40,000 years,and Torres Strait Islanders since about 10,000 years ago. For these 70 or so clan groups, the reef is also an important cultural feature. In 1768 Louis de Bougainville found the reef during an exploratory mission, but did not claim the area for the French. On 11 June 1770, HM Bark Endeavour, captained by explorer James Cook, ran aground on the Great Barrier Reef, sustaining considerable damage. Lightening the ship and re-floating it during an incoming tide eventually saved it. One of the most famous wrecks was HMS Pandora, which sank on 29 August 1791, killing 35 men. The Queensland Museum has led archaeological digs to wreck of Pandora since 1983. Because the reef had no atolls, it was largely unstudied in the 19th century. During this time, some of the reef's islands were mined for deposits of guano, and lighthouses were built as beacons throughout the system. As in Raine Island, the earliest example. In 1922, the Great Barrier Reef Committee began carrying out much of the early research on the reef.
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