Coral bleaching events
The past two decades have seen several incidents of widespread mass coral bleaching events on many of the world’s coral reefs.
AIMS has been monitoring mass bleaching events on the Great Barrier Reef since the early 1980s. We are studying the corals’ response, tolerance levels and ability to adapt heat stress, which will let us better predict and model future scenarios for reef managers.
Mass bleaching events in 1998, 2002 and 2006 were caused by unusually warm sea surface temperatures during the summer season. More recent bleaching (2008–2011) was caused by an influx of freshwater.
The summer of 1997–1998 was one of the hottest recorded on the Reef in the 20th century. Mild bleaching began in late January 1998 and intensified by February/March. Extensive aerial surveys of 654 reefs conducted by scientists from the Great Barrier Reef Marine Park Authority (GBRMPA) showed that 74 per cent of inshore and 21 per cent of offshore reefs had moderate to high levels of bleaching.
Most reefs recovered fully, with less than five per cent of inshore reefs suffering high coral mortality. The most severely affected reefs were in the Palm Island area, where up to 70 per cent of corals died.
The summer of 2001–2002 saw a mass bleaching event that was slightly more severe than the 1997–1998 event. The first signs of substantial bleaching were reported in January 2002, with the worst over by April.
In response, GBRMPA implemented the world's most comprehensive survey of coral bleaching in collaboration with AIMS, the Cooperative Research Centre for the Great Barrier Reef (CRC Reef) and the US National Oceanic and Atmospheric Administration (NOAA). Aerial surveys revealed bleaching in 54 per cent of the 641 reefs observed. Nearly 41 per cent of offshore and 72 per cent of inshore reefs had moderate or high levels of bleaching.
Again, reef recovery was generally good, with fewer than five per cent of the reefs suffering high mortality. The worst affected reefs were in the Bowen area where around 70 per cent of corals died.
In January and February 2006, a further bleaching event took place in the southern Reef, especially around the Keppel Islands. AIMS surveys revealed that, although bleaching was largely confined to this region, the degree of bleaching was worse than in previous years. Up to 98 per cent of corals bleached on some reefs, resulting in nearly 39 per cent mortality on the reef flats and 32 per cent on the reef slopes.
Recent extreme summer seasons (such as 2008–2009 and 2010–2011) have also been associated with extremely high rainfall in Queensland. This led to flooding and the discharge of large amounts of freshwater to nearshore reefs resulting in freshwater bleaching.
The Reef is likely to experience more frequent and more serious thermal and freshwater stresses in the future, given current rates of global warming due to increasing levels of atmospheric greenhouse gases.
Other reef systems have also suffered bleaching events. For example, mass bleaching affected reefs across 12o of latitude along the Western Australian coast in 2010–2011. This was the first recorded coral bleaching for several sites, including Ningaloo reef.
- GBRMPA report: Climate change impacts on corals