Coral reefs decimated by 2050, Great Barrier Reef's coral 95% dead
Rhett A. Butler,
November 17, 2005

Australia's Great Barrier Reef could lose 95 percent of its living coral by 2050 should ocean temperatures increase by the 1.5 degrees Celsius projected by climate scientists. The startling and controversial prediction, made last year in a report commissioned by the World Worldwide Fund for Nature (WWF) and the Queensland government, is just one of the dire scenarios forecast for reefs in the near future. The degradation and possible disappearance of these ecosystems would have profound socioeconomic ramifications as well as ecological impacts says Ove Hoegh-Guldberg, head of the University of Queensland's Centre for Marine Studies.

Hoegh-Guldberg, speaking at the Carnegie Institution Department of Global Ecology at Stanford University, says the most important threat facing the Great Barrier Reef and other reefs of the world is higher sea temperatures that cause thermal stress for corals.

Corals are tiny animals that live in colonies and derive nourishment and energy from a symbiotic relationship with zooxanthellae algae known as dinoflagellates. Coral reefs are formed over the course of thousands of years as limestone skeletons constructed by corals accumulate and form a structural base for living corals. Research indicates that is takes roughly thousand years for a reef to add a meter of height. Individual corals are capable of faster growth -- about one meter every hundred years -- but wave action and other forms of disturbance moderates overall reef growth.

Great Barrier Reef in Australia
The Great Barrier Reef is the world's largest reef, stretching more than 2,300km along the northeast coast of Australia. Made up of about 2,900 unconnected coral reefs and roughly 900 islands, the Great Barrier Reef is home to over 1,500 species of fish and 400 species of coral making it one of the most important marine ecosystems on Earth. Scientists consider it Earth's largest living organism which makes it the only individual living thing visible from space.

While the Great Barrier Reef is one of the world's healthiest reefs, coral reefs are particularly fragile ecosystems, partly due to their sensitivity to water temperature. When corals are physiologically stressed -- as is the case when water temperatures are elevated -- they may lose much of the their symbiotic algae, an event known as "bleaching." Corals can recover from short-term bleaching, but prolonged bleaching can cause irreversible damage and subsequent death.

The first coral bleaching on record occurred in 1979. Since then, there have been six events, each of which has been progressively more frequent and severe. In the El Niño year of 1998, when tropical sea surface temperatures were the highest yet in recorded history, coral reefs around the world suffered the most severe bleaching on record. 48% of reefs in the Western Indian Ocean suffered bleaching, while 16% of the world's appeared to have died by the end of 1998. 2002 was even worse: 60 to 95 per cent of individual reefs of the 110,000 square mile (284,000 square kilometer) Great Barrier Reef suffered some bleaching, while reefs in Palau, the Seychelles, and Okinawa suffered 70-95% bleaching. Early surveys suggest the Caribbean is currently in the midst of a serious event. While most of these reef ecosystems have recovered to some degree, warmer water temperatures in the future may have a more lasting impact.

Great Barrier Reef in Australia
"An increase in frequency of coral bleaching may be one of the first tangible environmental effects of global warming," said Dr. Arnold Dekker of Australia's Commonwealth Scientific and Industrial Research Organisation (CSIRO) in a European Space Agency news release."The concern is that coral reefs might pass a critical bleaching threshold beyond which they are unable to regenerate."

Hoegh-Guldberg agrees. "By 2050 bleaching may be an annual event, that is, if there are still reefs around to be bleached. If you have bleaching events every four years and they take 15-20 to recover, you will start to see bleached reefs not recovering. They will be dying," he adds.

Acidic Oceans

While rising sea temperatures are likely to have the biggest impact of coral reefs in the future, Hoegh-Guldberg notes are there factors that will affect the health of coral reefs including changes in sea level, elevated storm frequency and intensity, altered ocean circulation, variation in precipitation and land runoff, and increasing ocean acidification.

Ocean acidification is of particular concern to scientists because it is crucial to the formation of coral. Coral and other marine organisms use free carbonate ions in sea water to build calcium carbonate shells and exoskeletons, but as atmospheric carbon dioxide levels rise and more carbon dioxide is absorbed by the world's oceans, sea waters become increasingly acidic by stripping out carbonate ions. Lower carbonate ion concentrations make it more difficult for organisms to form shells, leaving them vulnerable to predators and environmental conditions. In the past, changes in ocean acidity have caused mass extinction events. According to a study published in the September issue of Geology, dramatically warmer and more acidic oceans may have contributed to the worst mass extinction on record, the Permian extinction. During the extinction event, which occurred some 250 million years ago, about 95% of ocean's life forms became extinct. The same fate could befall modern day marine life. In September 2005, a team of scientists writing in Nature warned that by 2100, the amount of carbonate available for marine organisms could drop by 60%. In surface ocean waters, where acidification starts before spreading to the deep sea, there may be too little carbonate for organisms to form shells as soon as 2050.

Hoegh-Guldberg believes an atmospheric carbon dioxide concentration of 500 parts-per-million (ppm) is a key threshold for coral reefs. "Beyond 500 ppm coral reefs may no longer exist. Much of the Pacific Ocean will likely be marginal for coral reefs while net calcification rates will be approaching zero" says Hoegh-Guldberg. Currently the concentration of carbon dioxide in the atmosphere stands around 380 ppm but the Intergovernmental Panel on Climate Change (IPCC) projects that if no precautionary action is taken, carbon dioxide concentrations will rise by 2050 to between 450 and 550 ppm.

Worldwide impact

The degradation and loss of coral ecosystems in will likely have a wide-ranging impact on the world economy. Hoegh-Guldberg points out that more than 500 million people live within 100 kilometers of coral reefs, many of whom rely on reefs and the services they provide for daily subsistence. Should reefs become severely damaged by climate change it could well create a class of ecological refugees in need assistance.

Further reefs play an important role in buffering adjacent shorelines from wave action, erosion, and the impact of storms. For example Moorea in French Polynesian, only experiences a 10 cm tidal range due to its protective barrier reef. Should the reef die and begin to crumble, the island's low-lying structures could be at risk.

Impact of a dying Great Barrier Reef in Australia

Australia may be the best example of the potential ramifications of dying reefs. Though Australia is among the world's most developed countries, a damaged Great Barrier Reef would likely have a significant impact on the country's economy. A recent study found the reef is worth more to Australia as an intact ecosystem than an extractive reserve for fishing.

Each year more than 1.8 million tourists visit the reef, spending an estimated AU$4.3 billion (Australian dollars) on reef-related industries from diving to boat rental to posh island resort stays. Revenue from tourism -- popular activities include snorkeling; scuba diving; fishing; glass-bottomed boat and semi-submersible vessel excursions -- dwarfs the commercial and recreational fishing industries which generate $360 million (Australian dollars) annually. Furthermore, tourism is an important source of employment: in 1998-1999, more than 47,600 people were employed in the sector compared to around 2,000 involved in commercial fishing in the region.

Tourism has given the Australian government an incentive to preserve the reef and last summer it banned all forms of extraction in one-third of the Great Barrier Reef Marine Park, making it the largest fully protected area of ocean in the world. The protected area will also benefit the fishing industry by serving as a nursery for fish-breeding to restock the entire reef.

Great Barrier Reef in Australia
The reef also offers great potential for Australia's nascent but blossoming biotech industry in the form of compounds derived from corals and other organisms that live in the region. Sessile invertebrates -- like corals -- have a special affinity for providing medicinally valuable compounds through their production of toxic chemicals used for defense. Several promising drugs have been developed from coral and other invertebrate species.

There is little doubt that the Great Barrier Reef, as a viable and relatively intact ecosystem, will continue to play an important role in the thriving Australian economy. The big question is, how long will it remain viable and intact?


Some scientists argue that the Great Barrier Reef and other coral ecosystems may be around longer than has been suggested by Hoegh-Guldberg's scenario. Critics say his scenario does not the current level of uncertainty about either the impact of warmer waters on the reefs, or likely climate change -- IPCC projections have been hotly debated.

Since the fossil record for corals is spotty -- the "resolution" for prehistoric dating is only 400 years -- so the impact of abrupt changes on coral are difficult to detect. While corals have certainly persisted through warmer and more acidic periods in Earth's geologic history, Hoegh-Guldberg suspects corals will face a difficult adjustment period in the face of rapidly rising sea temperatures and falling carbonate ion concentrations.

"Biological adaptations can't keep pace with the forecasted level of change," says Hoegh-Guldberg. "In the past the time scale was likely thousands of years, not decades."

Hoegh-Guldberg argues that coral reefs will likely recover in geologic terms, but not in terms of a human lifetime. The short term impact of dying and degraded reefs will be significant.

"For a tour operator, two years of bleached coral can mean the difference in putting food on the table or finding a new job. The tourism industry will be hit especially hard by worsening bleaching events."

Over the longer term, reefs will recover. According to Hoegh-Guldberg model, under the best-case global warming scenario -- where temperatures stabilize around 2100 -- the Great Barrier Reef will recover within a century. Under the pessimistic, it will take at least 500 years for the reef to regenerate, populated by coral species adapted to living in warmer waters. Hoegh-Guldberg says reefs are unlikely to migrate to cooler, higher latitude waters due to other conditions -- including light levels and ion concentrations -- required for their growth.

Despite a bleak future, Hoegh-Guldberg doesn't believe reef conservation and research efforts should be abandoned.

"There is still a lot we don't know about coral reefs. We need to understand these ecosystems to be totally effective in their preservation. Technologies still in their infancy may make it possible for us to moderate some of the effects of climate change on coral reefs."

Rhett A. Butler, (November 17, 2005).

Coral reefs decimated by 2050, Great Barrier Reef's coral 95% dead.