What it is, how it happens and what we're doing to help.
#What is coral bleaching?
When corals are under stress, they expel the microscopic algae that live in their tissues. Without these algae, corals' tissues become transparent, exposing their white skeleton. This is called coral bleaching. Bleached corals are not dead, but are more at risk of starvation and disease.
#Facts about coral bleaching
Rising ocean temperatures caused by climate change is the primary cause of coral bleaching.
A temperature increase of just one degree Celsius for only four weeks can trigger bleaching.
During bleaching corals become transparent, revealing their white skeletons.
Changes in water quality, increased sun exposure and extreme low tides can also cause corals to bleach.
#Coral bleaching FAQ's
A relative of jellyfish and sea anemones, corals are invertebrates that belong to a large group of animals called Cnidaria. They are known as colonial organisms, which means many individuals live and grow while connected to one another.
Each individual coral is made of a polyp – a clear, tube-shaped structure attached to a reef at one end and open at the other. The open end has a mouth that is surrounded by a ring of tentacles.
Hard corals extract calcium from seawater to make limestone outer skeletons, which form coral reefs. Soft corals tend to be feathery in appearance such as sea fans and sea feathers.
Corals get most of their bright colours from microscopic algae that grows inside the polyps’ tissues. This algae, called zooxanthellae (pronounced zo-UH-zan-thuh-lay), helps corals by removing waste and using it to produce food in a process known as photosynthesis.
When corals are stressed, they expel the zooxanthellae that live inside their tissues. Without the algae to provide colour, corals appear transparent and reveal their white skeletons. This is called coral bleaching. Bleached corals are not dead, but are more at risk of starvation and disease.
Bleaching occurs when corals are under stress. A primary cause of coral bleaching on the Great Barrier Reef during summer is heat stress from raised water temperatures and increased UV radiation. A temperature increase of just one degree Celsius for only four weeks can trigger bleaching.
Deprived of zooxanthellae and therefore their food source, corals begin to starve. If raised water temperatures persist for long periods (eight weeks or more) corals begin to die.
Yes, corals can recover from bleaching over time, but only if temperatures drop and conditions return to normal.
Coral bleaching is a stress response and individual coral colonies suffer from a degree of bleaching in any given summer. This is a natural process and not of particular concern.
Large-scale marine heatwaves create mass coral bleaching events however, in which large numbers of corals bleach severely over a wide area. These events are typically associated with high levels of coral mortality.
As climate change causes ocean temperatures to rise, marine heatwaves and associated coral bleaching events are becoming more frequent and severe, and the Reef’s natural recovery processes are unable to keep up.
Yes. Severe, widespread coral bleaching is an issue on coral reefs around the world as a result of changes to the Earth’s climate.
No. The Great Barrier Reef is made up of 3,000 reefs and is home to thousands of species of marine life. Reports focusing on “how much of the Reef has died” imply finality. Reefs can be severely affected by bleaching but begin to recover as coral communities re-grow and new coral larvae settle on the reef.
The Great Barrier Reef is one of the richest and most complex natural ecosystems in the world. Climate change, which causes temperatures to rise on our land and in our oceans, is the biggest threat to the future of the Great Barrier Reef and coral reefs around the world.
We need urgent action on climate change to drastically reduce global emissions, but this alone is not enough. Action on climate change requires a dual effort to protect our reefs. While the world works towards reducing greenhouse gas emissions, we must make coral reefs more resilient and help them adapt to the warmer temperatures already caused by climate change.
Coral Bleaching on the Great Barrier Reef
Bleaching Corals Expel Their Zooxanthellae, Revealing a Bright White Skeleton
#Mass bleaching events
The past two decades have seen several incidents of widespread coral bleaching events on many of the world’s coral reefs.
The Great Barrier Reef experienced its most widespread bleaching event on record, with the south of the Reef bleaching extensively for the first time. This marked the third mass bleaching event on the Reef in five years. Aerial analysis found that coastal reefs along the entire length of the Reef - a stretch of about 2,300 kilometers from the Torres Strait in the north, right down to the Reef's southern boundary - were severely bleached.
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 ocean temperatures led to record widespread coral bleaching on the Great Barrier Reef. Bleaching was the most widespread and severe in the Far Northern management area, between Cape York and Port Douglas. Here, bleaching intensity was classed as 'severe’. Bleaching intensity was less pronounced in the southern parts of the Reef. Based on in-water monitoring surveys, overall coral mortality was at 22% for the entire Great Barrier Reef.
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. Other reef systems also suffered bleaching events. For example, mass bleaching affected reefs along the Western Australian coast in 2010–2011. This was the first recorded warm water coral bleaching for several sites including whale shark haven, Ningaloo Reef.
In January and February 2006, a bleaching event took place in the southern Reef, especially around the Keppel Islands. Although bleaching was largely confined to this region, the degree of 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.
The summer of 2001–2002 saw a mass bleaching event that was slightly more severe than the 1997–1998 event. 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. Reef recovery was generally good, with fewer than 5% of the reefs suffering high mortality.
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 showed that 74% of inshore and 21% of offshore reefs had moderate to high levels of bleaching. Most reefs recovered fully, with less 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.