an aerial view of a reef with bleached white colonies of coral visible

Coral bleaching events

Like many parts of the world, Australia’s reefs have experienced widespread coral bleaching in the past two decades.

AIMS has been monitoring mass bleaching throughout the Great Barrier Reef since the early 1980s, and reefs in Western Australia since the early 1990s.

Mass bleaching events on the Great Barrier Reef have been documented with full-scale surveys in 1998, 2002,2016, 2017, 2020 and 2022.

In Western Australia, mass bleaching events were documented in 1998, 2011-2013 and 2016, with many smaller bleaching events around those times.

AIMS also studies the coral bleaching response, heat tolerance levels and the ability of different coral species to adapt to heat stress. This lets us better predict and model future scenarios for reef managers.

Coral bleaching. Scott Reef, April 2016

Mass coral bleaching

At a local level, many stressors can cause corals to bleach, including storms, disease, sediments and changes in salinity.

However, the primary cause of regional, or mass, bleaching is increased sea temperatures.

Mass bleaching involves many corals from many different species, and can result in widespread mortality.

The frequency, intensity and area of heat stress causing coral bleaching is increasing over time, as a result of changes to the Earth’s climate.

Mass coral bleaching events do not necessarily affect all reefs equally.

When bleaching becomes common across many sectors of the Great Barrier Reef, including both inshore and offshore reefs, it becomes a mass bleaching event.

In Western Australia, mass bleaching on Kimberley reefs in the far north has historically occurred during El Niño conditions, and La Niña conditions for reefs further south, although that separation has broken down in recent years.

In 2008 and 2011, coral bleaching at the Great Barrier Reef was caused by an influx of freshwater affecting local reefs exposed to the flood plumes.
 

What are degree heating weeks?

NOAA Coral Reef Watch degree heating weeks (or DHW) is a widely used tool used to measure accumulated heat stress in an area and predict coral bleaching risk throughout tropical coral reef ecosystems.

It is calculated by combining the intensity of daily temperature extremes and the total time when daily temperatures exceed the bleaching threshold over the previous three months. Significant coral bleaching is predicted above 4 DHW and coral mortality is expected above 8 DHW.

Degree heating weeks are a NOAA heat stress product.
 

2022

Great Barrier Reef

In 2022, aerial surveys confirmed the fourth mass bleaching event to impact the Great Barrier Reef in seven years. It was the first mass bleaching event caused by heat stress during La Niña conditions, which historically produce cooler summer conditions at the reef, with higher-than-average rainfall and high cloud cover.

Severe bleaching (more than 60% of community coral cover bleached) affected 43% of reefs surveyed. Worst affected was the central region of the reef, where the most heat stress occurred. The majority of the reefs bleached were exposed to moderate levels of heat stress (NOAA Degree Heating Week 5-8°C-weeks), which causes bleaching but lower levels of mortality.

 

Western Australia

In 2021-22, sea surface temperatures in Western Australia remained in the highest 10% of observations since 1900. Minor coral bleaching was observed in the Kimberley and Ningaloo reefs, and severe bleaching was observed in parts of the Pilbara region.

However, the full extent of bleaching during this and other heatwaves is unknown, given the vast and remote nature of the region.

2021

Western Australia

In late 2021, minor bleaching (less than 5% of coral cover) was observed at Scott Reef in the Kimberley region. Regional heat stress increased significantly over the following months and bleaching at Scott Reef likely increased, but there were no observations of the reefs at this time.

The adjacent Ashmore Reef and Rowley Shoals were also exposed to significant heat stress, however there were no observations at these reefs.

2020

Great Barrier Reef

Aerial surveys by the ARC Centre of Excellence for Reef Studies documented the third mass bleaching event in five years to occur throughout the Great Barrier Reef. Sea surface temperatures in February were the highest recorded in 120 years of observations.

Heat stress extended into the southern Great Barrier Reef and caused high levels of community coral bleaching in this region compared to 2016 and 2017. During the 2020 event, 25% of reefs surveyed had severe levels of coral bleaching.

Nearly 80% of the reefs throughout the Great Barrier Reef were exposed to heat stress levels that cause bleaching (above 5°C-weeks). The majority of the reefs (about 60%) were exposed to moderate heat stress (4-8°C-weeks). Follow up surveys by AIMS’ Long-Term Monitoring Program in 2021 found this did not cause substantial mortality, with ongoing recovery of coral cover detected in communities across the reef.
 

Western Australia

In early 2020, patchy bleaching was observed at inshore Kimberley reefs. Moderate bleaching (about 20% of cover cover) was observed at Rowley Shoals, which had previously experienced only minor bleaching.

Other oceanic reefs experienced prolonged heat stress, including those at Scott, Ashmore and Christmas Island. However, there were no surveys of these reefs at the time of heat stress.

Read: With no work in lockdown, tour operators helped find coral bleaching on Western Australia's remote reefs (July 2020, The Conversation)
 

2017

Great Barrier Reef

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 mass bleaching in 2016 and 2017 was unprecedented, and collectively affected two-thirds of the Great Barrier Reef. The southern sector avoided intense heat stress and substantial bleaching in both years.

The extent and intensity of bleaching in 2016 and 2017 were documented through the combination of aerial surveys, in-water surveys at the peak of heat stress, and long-term surveys by AIMS’ Long-Term Monitoring Program to assess change in coral cover.

Intense heat stress (above 8°C-weeks) capable of causing widespread coral mortality occurred during the 2017 event, mostly in the central Great Barrier Reef.

Bleaching intensity was severe at 20% of the surveyed reefs. Some bleaching was observed in the southern region, but Long-Term Monitoring Program surveys during the next few years only detected declines in coral cover in reef communities in the northern region (during 2015-17 surveys) and central regions (during 2017-19 surveys), aligning with the intense heat stress footprint.

2016

In 2016, record ocean temperatures led to widespread coral bleaching on Australian reefs. This was part of the third global bleaching event declared by the National Oceanic and Atmospheric Administration (NOAA) in 2015, and the most intense to impact the Great Barrier Reef.
 

Great Barrier Reef

Between February and May, the Great Barrier Reef experienced record warm sea surface temperatures. Extensive field and aerial surveys found the bleaching was most widespread and severe in the far northern management area, between Cape York and Port Douglas. Bleaching intensity was severe (more than 60% of coral cover) at 38% of surveyed reefs, mostly in the northern Great Barrier Reef. 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 more than 90% of coral cover) and was patchy throughout most of the central and southern management areas.

In 2016, intense prolonged heat stress capable of causing coral mortality affected more than 30% of the reef. Overall coral mortality documented with in-water surveys (as of June 2016) was 22% for the entire Great Barrier Reef. Coral mortality was highest in the northern section, where heat stress was most intense.
 

Western Australia

Coral reefs in northwest Australia, including inshore Kimberley, Christmas Island, Scott and Seringapatam reefs, Ashmore and Cartier reefs, and the Rowley Shoals, were bleached by record-breaking ocean temperatures in early 2016-2017. Bleaching was severe (more than 60% of coral cover) at the Scott and Ashmore reef systems, causing widespread mortality.
 

A film about coral bleaching in Australia's north west up until 2016.

Read: AIMS northwest Australian coral bleaching update (May 2016)
 

2008–2011

Great Barrier Reef

Extreme summers in 2008-2009 and 2010-2011 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 localised 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.
 

Western Australia

West Australian reefs also bleached through the transition from El Niño to La Niña conditions. The El Niño caused moderate bleaching at some reefs (Scott Reefs) in the far north, while the La Niña conditions caused mass bleaching affected reefs across 12o of latitude along the Western Australian coast in 2010–2011. This was the first recorded warm water coral bleaching for several reef locations in Western Australia, including Ningaloo reef.
 

2005–2006

Great Barrier Reef

In January and February 2006, a small localised bleaching event took place in the southern Reef, especially around the inshore Keppel Islands. AIMS surveys revealed that, although bleaching was largely confined to this local area, the degree of thermal bleaching was worse than in previous years. Up to 98% of corals bleached on some reefs, resulting in nearly 39% mortality on the reef flats and 32% on the reef slopes.
 

Western Australia

Heat stress in 2005 caused moderate (less than 20% of cover cover) bleaching at the iconic Rowley Shoals, the first recorded bleaching of this reef system.
 

2001–2002

Great Barrier Reef

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, the Great Barrier Reef Marine Park Authority (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% of the 641 reefs observed. Nearly 41% of offshore and 72% of inshore reefs had some level of bleaching. Only 10% of reefs surveyed were documented to have severe levels of community bleaching.

Again, reef recovery was generally good, with fewer than 5% of the reefs suffering high mortality. The worst affected reefs were in the Bowen area, where about 70% of corals died.

1997–1998

Great Barrier Reef

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 GBRMPA showed that 74% of inshore and 21% of offshore reefs had some level of bleaching. Only 11% of the reefs surveyed were documented to have severe levels of community bleaching.

Most reefs recovered fully, with fewer than 5% of inshore reefs suffering high coral mortality. The most severely affected reefs were in the Palm Island area, where up to 70% of corals died.
 

Western Australia

The heatwave in 1998 caused the first, and most severe, recorded bleaching in a Western Australian coral reef. The mass bleaching at the Scott Reefs affected most corals and reduced coral cover by approximately 80%. Less severe and patchy bleaching was observed some other reefs in the Kimberley and Pilbara regions.