Over the past decade, widespread coral bleaching has occurred on several distinct occasions, known as mass coral bleaching events. AIMS scientists have been monitoring mass bleaching events on the Great Barrier Reef since the early 1980's and studying the impacts to better understand the phenomenon and enable better prediction and future scenario modelling for reef managers.

The summer of 1997-1998 was one of the hottest recorded on the Great Barrier Reef in the 20th Century. Mild bleaching began in late January and intensified by February/March. Extensive aerial surveys of 654 reefs conducted by scientists at the Great Barrier Reef Marine Park Authority (GBRMPA) showed that 21% of offshore reefs had moderate to high levels of bleaching compared to 74% of inshore. Most reefs recovered fully with less than 5% of inshore reefs suffering high mortality. The worst affected reefs were in the Palm Island area where up to 70% of corals died.

During the summer of 2002 a mass bleaching event occurred that was equivalent to or slightly more severe than the 1998 event. The first signs of substantial bleaching were reported in January, with the worst over by April. In response, GBRMPA implemented the world's most comprehensive survey of coral bleaching in collaboration with AIMS, Cooperative Research Centre for the Great Barrier Reef (CRC Reef) and National Oceanographic and Atmospheric Association (NOAA). Aerial surveys revealed bleaching in 54% of the 641 reefs observed. Nearly 41% of offshore and 72% of inshore reefs had moderate or high levels of bleaching. Again, reef recovery was generally good with less than 5% suffering high mortality. The worst affected reefs were in the Bowen area where around 70% of corals died.

In January and February 2006 a further bleaching event took place on the southern end of the Great Barrier Reef, especially around the Keppel Islands. AIMS surveys revealed that although bleaching was largely confined to this region, the extent of the bleaching in this area was even worse than in previous years, with up to 98% corals bleached on some reefs, resulting in nearly 39% mortality on the reef flats and 32% on the reef slopes.

This event was compounded by a freak rain storm 8 months later that killed virtually all corals on the reef flat in the inner Keppel islands. Most of the mortality was due to corals being drenched with freshwater, while they were exposed to air during an extreme low tide. Corals below the reef flat were not affected.

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Coral Bleaching on the GBR