Coral Bleaching

Looking southeast from Whitsunday Island towards Harold Island

Dear Joshua,

We have had a couple of good holidays sailing around the Whitsunday Islands, off the North Queensland coast, and snorkelling from the beach at Heron Island, off the South Queensland coast.

Both of these places are dependent on the Great Barrier Reef. That is their 'unique selling point.'

Without the reef, they would lose much of their appeal.

Last time that we snorkelled in the Whitsunday Islands, Granddad came back aboard complaining that the coral looked dull, and you asked:

What is Coral Bleaching?

Before we can understand coral bleaching, we have to know what coral is.

Coral is a living organism, made up of hundreds or thousands of individual 'polyps'. As the polyps feed and grow they excrete calcium carbonate which accumulates and builds into a reef. The bigger reefs have more polyps growing on them and excreting more calcium carbonate.

Each of these polyps is a tiny tube shaped animal, with a mouth at the 'sea' end and a stomach in its middle. Because the polyp is anchored to the reef, all of the waste products have to exit through the mouth too.

The coral polyps feed on tiny organisms floating around in the ocean, but these can not provide all the things that the coral needs to grow and survive. Just as we have to eat fruit and vegetables to stay healthy, coral needs extra nutrients. It gets these nutrients from algae which live in its tissue.

Algae are tiny plant cells that take some compounds from the coral and use these to turn sunlight into oxygen and other chemicals that the algae then give back to the coral. Without this exchange of nutrients neither the algae nor hte coral can survive. This is called a symbiotic relationship.

Now that we have established that 'coral' is not just coral, we can look at bleaching.

Olivetta sailing downwind

About bleaching

Just like any family, the coral and algae need healthy conditions to live in. For coral to succeed it needs clean water, the right nutrients and the right amount of sunlight. All three of these have come together at the Great Barrier Reef, leading to some of the most spectacular corals in the world which support a great diversity of fish and other ocean life.

If the balance of living conditions is disrupted, something has to change.

The algae that live together withe the coral polyps have a very narrow temperature band that they can survive in. NOAA says that coral generally needs the water it lives in to be warmer than 18 Celsius. Any colder than this, and it simply can't grow. But the coral is also sensitive to water that is too warm. For some corals, too warm is more than one degree Celsius warmer than the summer average temperature. This temperature is called the 'bleaching threshold,' and will vary depending on the site of the coral.

If the bleaching threshold is exceeded for long enough, then the algae that normally provide nutrients for the coral polyps start to produce toxins and the polyp ejects the algae. The algae take the colour with them, leaving the transparent polyp sitting on white calcium carbonate. This is why the coral appears 'bleached'.

There is a measurement, called a 'Degree Heating Week', that measures the amount of time that the sea temperature was hotter than its usual mean temperature. The stress on a coral colony gets worse if the ocean stays warmer than normal for longer periods. When the 'Degree Heating Week' reaches 4°C-weeks, significant coral bleaching is likely.

Sea Surface Temperature anomaly, 2019-08-13 (Credit NOAA)

How is it happening?

The simple answer is "Climate Change". There is not the time or space here to go into all of the causes of climate change, but the science is in, and humans are changing the planet's climate faster than it can adapt.

The Intergovernmental Panel on Climate Change 2013 report stated that more than 93% of the excess heat generated from greenhouse emissions since the 1970s has been absorbed by the oceans.

The US government Environmental Protection Agency has graphed and mapped a statistically significant increase in sea surface temperature over most of the world's oceans between 1901 and 2015.

Abbot Point coal terminal, Queensland. (Credit

What will the Adani mine do to it?

The concern about the new Adani mine comes in two parts.

Firstly, any coal from the mine which is burned will add to carbon dioxide emissions, which contributes to climate change.

Secondly, the coal terminal at Abbot Point on the Queensland coast is expected to expand to ship an extra 35 million tonnes of coal per year. The concern is that this will affect the water quality on the reef because of run-off water from the site. As was said before, coral needs clear water to thrive. Any cloudiness in the water will change the amount of sunlight reaching the reef.

However, both state and federal governments have signed off the environmental assessment plans and do not see environmental threats from the new mine.

Olivetta at anchor in the Whitsunday Islands

How can we stop it from happening?

If the coral had a perfect habitat then this bleaching would not occur.

Returning the oceans around Australia and the rest of the globe to their previous conditions would provide better living conditions and encourage the coral to thrive.

This will take coordinated effort by governments and communities around the globe. The oceans cover about two thirds of our planet, and it is everyone's responsibility to protect them.

Further reading

If you want to read more about coral, or the work that some people are doing to look after the Great Barrier Reef, follow these links to NOAA and a BBC story about citizen science and tourism on the reef. Your favourite search engine will also give you plenty of links to follow.


The Australian Academy of science has tweeted this link to the "Great Barrier Reef outlook report", which has downgraded the reef's health to "very poor."