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Coral Bleaching on the Great Barrier Reef

In 1998, sea temperatures in some parts of the Great Barrier Reef were between 10C and 20C above normal temperatures for that period. Globally, the temperatures reached and the extents of bleaching at this time were the highest ever recorded. Aerial surveys of the Great Barrier Reef showed that 87 percent of inshore reefs surveyed were bleached to some extent while bleaching affected 28 percent of surveyed mid-shelf and offshore reefs. Of the bleached reefs, the inshore reefs were the most severely affected, 67 percent with high levels of bleaching and 25 percent with extreme bleaching levels. In comparison, mid-shelf and offshore reefs were less affected with only 14 percent highly bleached and none bleached to extreme levels. Subsequent research found that different coral species were affected to different extents. Corals such as the Acroporids, branching Porites and Pocilloporids suffered extreme bleaching and mortality while Turbinaria corals tended to be unaffected or only slightly bleached. Researchers also found that the effect on any one reef depended on the coral community composition, but also varied according to water depth with the shallowest regions of reefs being the most affected. Post bleaching surveys revealed that inshore reefs suffered the highest mortality rates while mid-shelf and offshore reef corals had generally escaped with minimal mortality. To find out more about the effect of the 1998 coral bleaching event on the Great Barrier Reef, visit the Australian Institute of Marine Science or ReefCRC.

The Great Barrier Reef experienced mass coral bleaching events in 1998 and 2002. The frequency of coral bleaching episodes appears to be increasing and most evidence points to a positive link between global warming and increased coral bleaching

aerial shot
The Great Barrier reef experienced mass coral bleaching events in 1998 and 2002.  The frequency of coral bleaching events appears to be increasing and most evidence points to a link between global warming and increased coral bleaching.

The 2002 mass coral bleaching event was the largest on record for the Great Barrier Reef. Two periods of several weeks of hot weather resulted in seawater temperatures several degrees centigrade higher than long-term seasonal averages. Aerial surveys conducted in March and April revealed that 60 percent of reefs surveyed were bleached. While inshore reefs were again the most affected, a greater proportion of mid-shelf and offshore reefs bleached in 2002 than in 1998. In terms of bleaching severity, 69 percent of inshore reefs had moderate to high levels of bleaching but mid-shelf and offshore reefs were much more affected than in 1998 with 51 percent showing moderate to high levels of bleaching. Surveys conducted by divers later showed that although few reefs escaped bleaching, the impacts varied between reefs. Some inshore reefs such as those off Bowen suffered between 50 percent and 90 percent mortality while inshore reefs of the Frankland Islands were almost completely unaffected. As in 1998, different species were affected to varying extents with the more susceptible Acroporid and Pocilloporid species most affected. Furthermore, corals in shallow regions of the reefs were again the most extensively bleached, however some bleached corals were also found at depths between ten to fifteen metres. While data on mortality of bleached corals is still being analysed, the 2002 event was more severe than that of 1998 with bleaching spread across a much larger area of the Great Barrier Reef.

The Great Barrier Reef, as a whole, was less affected by both bleaching events than many other reefs around the world. Relatively few reefs within the Great Barrier Reef World Heritage Area suffered extensive mortality and in the absence of further pressures, most are expected to recover. Many other reefs around the world have been completely decimated by coral bleaching. The 1998 bleaching event saw catastrophic bleaching with massive mortality occurring on reefs in Bahrain, Maldives, Sri Lanka, Singapore, and parts of Tanzania. While some reefs are showing encouraging signs of recovery, there are also many reefs where recovery is barely evident. Increases in the frequency and severity of coral bleaching will have significant and cumulative effects on the corals of the Great Barrier Reef. Considering its potential to directly affect the entire Great Barrier Reef at one time, and the potential to exacerbate the effects of other pressures, climate change and coral bleaching are likely to pose the greatest long-term risks to the Great Barrier Reef. While the long-term effects of climate change and increased coral bleaching on the Great Barrier Reef are unknown, most scientists agree that the link between climate change and coral bleaching is well established. It is widely accepted that climate change will cause coral reefs to change, the challenge now is to determine what changes are likely to occur, how these changes will occur and what effects these changes will have on reef ecosystems and on people interacting with them.

More information about the scientific consensus about coral bleaching and climate change can be found at the Centre for Coral Reef Biodiversity.

What is GBRMPA doing about coral bleaching?