Coral bleaching events
Coral bleaching. Scott Reef, April 2016.
The past two decades have seen several incidents of widespread 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 study 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, 2006, 2016 and most recently 2017 were caused by unusually warm sea surface temperatures during the summer season. Bleaching in 2008 and 2011 was caused by an influx of freshwater.
Severe coral bleaching affected the central third of the Great Barrier Reef in early 2017 associated with unusually warm sea surface temperatures and accumulated heat stress. This back-to-back (2016 and 2017) mass bleaching was unprecedented and collectively affected two thirds of the Great Barrier Reef. The southern sector was spared in both years.
The spatial extent and intensity of bleaching was documented through aerial surveys.
In 2016, record oceans temperatures have led to record widespread coral bleaching on Australian coral reefs. This bleaching is part of the ongoing third global bleaching event, declared by the National Oceanic and Atmospheric Administration (NOAA) in 2015.
Between February and May, the Great Barrier Reef experienced record warm sea surface temperatures. Extensive field surveys and aerial surveys found bleaching was the most widespread and severe in the Far Northern management area, between Cape York and Port Douglas. Here, bleaching intensity was ‘Severe’ (more than 60% community bleaching). Bleaching intensity decreased along a southerly gradient. While most reefs exhibited some degree of bleaching, this bleaching varied in intensity (from less than 10% to over 90% community bleaching) and was patchy throughout most of the management area. (View the GBRMPA map for more information.)
The impact from this bleaching event, the most widespread and severe ever recorded on the Great Barrier Reef, is still unfolding. Based on in-water monitoring surveys, overall coral mortality is (as of June 2016) at 22% for the entire Great Barrier Reef. Coral mortality is highest in the northern section. Post-bleaching reef monitoring surveys towards the end of the year will provide further clarity on the full extent of coral mortality.
Coral reefs in northwestern Australia, including those near the Kimberley, Christmas Island, Scott and Seringapatam Reefs were bleached by record breaking ocean temperatures in early 2016 also. Bleaching was severe at Scott Reef and Seringapatam Reefs, and mortality was observed during bleaching surveys in April. The full extent of coral mortality on northwestern Australian reefs will be clearer following monitoring surveys towards the end of the year.
In the summers of 2008-2009 and 2010-2011, extreme summer seasons were 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.
In January and February 2006, a 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.
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.
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.
- GBRMPA report: Climate change impacts on corals