Surveys by scientists from the Australian James Cook University and the Great Barrier Reef Marine Park Authority show that the reef, which is a World Heritage Site, has been exposed to unprecedented mass bleaching due to a summer’s extreme heat. Corals from the far north to the southern tip of the 1,400-mile long ecosystem are badly affected.
It was also one of the worst mass bleaching episodes of the reef in terms of intensity, ranked second after 2016, killing half of all shallow water corals on the northern Great Barrier Reef.
Unlike in the summer of 2016, when an intense wave of sea heat coincided with one of the strongest events in El Niño, the past summer brought a bleaching event without support from the Pacific climate.
El Niño events can increase sea temperatures in this part of the world and make bleaching events more likely. For scientists, this is another clear sign that man-made climate change is the main reason for these devastating events.
Mark Eakin, coordinator of the National Oceanic and Atmospheric Administration’s Coral Reef Watch program, described the repetition rate of these events as “really worrying”. The bleaching from the 2016 event was repeated in 2017, when there was no El Niño either.
“The Great Barrier Reef had its first bleaching events in a row in 2016 and 2017. Now we have the third bleaching event in five years, ”Eakin wrote in an email.
“This is unprecedented on the Great Barrier Reef.”
Heat stress can be fatal to corals
Bleaching is a reaction to heat stress that occurs when corals spend too much time in water that is too hot for them. When the reef-building animals are exposed to heat for a long time, they temporarily expel their zooxanthellae, symbiotic algae that protect the corals in exchange for food.
Since these algae also give corals their bright colors, mild bleaching makes corals pale. Heavily bleached corals turn bone white, and if their algae partners stay away too long, they can starve.
When heat rose over the reef in February, the Great Barrier Reef Marine Park Authority began Bleach was reported in the far north towards the end of the month. By early March, eight or more “degree heating weeks” had accumulated across much of the ecosystem, a metric that scientists used to describe the recent cumulative heat exposure.
At this threshold, reef scientists expect widespread bleaching and dying due to thermal stress, according to the NOAA.
The researchers decided to carry out air and water studies to assess the extent of the damage. The investigations conducted in the last two weeks of March quickly confirmed that the reef has undergone its third mass bleaching event in the past five years.
Further details on the scope and severity of the event are now known. A new map from Terry Hughes, director of the ARC Center of Excellence for Coral Reef Studies at James Cook University, confirms what scientists from NOAA and the Australian Bureau of Meteorology have predicted: this year’s bleaching was further compared to 2016, which pounded the reef spreads northern third and 2017, which hit the middle part of the reef hardest.
This year, around 35 percent of the 1,036 reefs studied by scientists were moderately bleached, while a quarter were bleached heavily. Scientists saw a strong bleach on the coastal reefs from Torres Strait in the far north to the southern border of the Great Barrier Reef Marine Park, which was only overshadowed in 2016.
“For the first time, the south was hot, as was the middle and north,” said Hughes. “After 2016-17, when the north and the middle went, I told someone that our worst nightmare is when the next region to be bleached is the south.”
This is because after escaping the previous two events, the south is relatively unaccustomed to bleaching and contains a large number of heat-sensitive acropora corals, including branched and tabular species that give the reef its three-dimensional structure and habitat for Offer fish.
In the northern and central Great Barrier Reef, these corals were largely destroyed by bleaching in the years 2016-17, whereby large parts of the reef were transformed into a “strongly changed, degraded system” according to one year 2018 Paper in the journal Nature.
The south now seems ready to go into a similar ecological decline. Hughes warned that bleaching does not necessarily lead to mortality, and said he would do repeated examinations in about eight months to determine which corals survived and which did not.
“I have to admit, I am devastated to learn that the southern reefs are suffering such a blow because they were a rare bright spot during the 2016 mass bleaching event,” he said Kim Cobb, a coral reef and climate researcher at the Georgia Institute of Technology, who was not involved in the new survey.
Hughes expects lower mortality rates in the north this year as many of the heat sensitive corals have already been killed. But reefs that have been bleached this year in addition to three or four years ago are likely to reset in terms of recovery, he said.
“Underwater and even from the plane we could see a lot of small corals that have been recruiting on the reef since the last bleaching … so the recovery that was in the early stages was interrupted by this new bleaching event,” said Hughes.
Approaching annual bleaching events
Mass bleaching events have often been associated with El Niño, a recurring climate pattern characterized by above-average sea surface temperatures in the eastern equatorial Pacific, leading to shifts in ocean heat distribution, atmospheric circulation, and weather patterns around the world.
On the other side of the Great Barrier Reef, changes in local El Niño weather conditions, including above-average air and sea temperatures, clear skies, and lots of sunshine, can help fuel bleach.
While the first recorded mass bleaching event on the Great Barrier Reef in 1998 and the most intense recorded mass bleaching event in 1998 coincided with El Niño, this was not the case with mass bleaching in 2002, 2017 and now 2020.
This, along with the fact that the gap between severe bleaching events is widening, suggests that as the summer heats up, the reef regularly experiences heat stress due to climate change, regardless of whether the tropical Pacific is in a favorable condition. Hughes said February 2020 brought the highest monthly sea surface temperatures ever measured across the Great Barrier Reef without El Niño being able to help.
“It is now clear that global climate change alone can have major bleaching events without a tropical drive,” said Eakin, adding that we may see “early signs” of a world in which the reef fades almost annually.
Lesley Hughes, a professor of biology at Macquarie University in Australia, agreed that the prospect of annual bleaching on the Great Barrier Reef, something that could be predicted in the 2030s, “is getting closer”.