The world's largest coral reef system is officially suffering a “serious” bleaching event, but many marine biologists fear the truth is even worse; the sixth mass bleaching event is underway. Bleaching events have become common over recent years, and this is far from the largest. However, the fact that it coincides with a La Niña year makes it even more alarming than bigger events when the ENSO cycle makes them more likely.
The wonder of coral reefs, which support 25 percent of marine species, is built on the symbiotic relationship between corals and the zooxanthellae microalgae that give them both their color, sugars, and amino acids. When corals become stressed they expel the zooxanthellae and become white (bleached), after which they are in a race against time. If conditions do not improve within a few weeks so they can take in new microalgae the coral will die. Many things can stress a reef, but when bleaching occurs over large areas high water temperatures are usually involved.
These bleaching events are then a measure of the health of the reef. The Great Barrier Reef Marine Park Authority (GBRMPA) conducts regular airborne surveys over the reef's vast length looking out for bleaching events so widespread they indicate large-scale stressors rather than simply local effects. Today GBRMPA has announced “widespread” bleaching in the north and central sections of the reef.
Bleaching events do not always lead to large-scale coral death, and whether this occurs or not usually depends on how quickly temperatures return to normal after heatwaves. On this there is some hope, with GBRMPA noting, “The past week has brought cooler temperatures to the Reef.” However, the release continues: “Despite this, sea surface temperatures remain above average across most of the Marine Park and are expected to remain so until the end of March.” Consequently, the danger remains high.
GBRMPA has not described what they are seeing as a “mass bleaching event”, and there is no agreed definition of when bleaching becomes widespread enough to justify the term. However, Professor Terry Hughes of James Cook University who conducted the surveys before GBRMPA took over tweeted the following before the announcement:
He followed up with:
Irrespective of whether this event sees mass coral die-off, the fact the possibility is there this year indicates how dire the reef's danger is on a decadal scale.
The GBR suffered its first recorded mass bleaching events in 1998 followed by others in 2002, 2016, 2017, and 2020, many of them coinciding with widespread bleaching in other parts of the world. There were also events in 2008 and 2011 where flooding harmed inshore reefs but left more distant coral untouched. The more universal events, however, were associated with El Niño years, when the western Pacific is particularly warm and sunny. To have mass bleaching in a year where Queensland south of the reef was hit by record-breaking floods indicates no year is safe.
The Australian government has been resisting efforts by United Nations scientists to have the reef placed on a list of World Heritage “in danger” sites since 2017, partly because of its resistance to accepting climate change and partly due to the tourism the reef brings in. From Monday, UNESCO representatives will be visiting the reef to see if the government's claims of its health are justified. Their findings will inform a world heritage committee meeting in June this year.
With 2,900 individual reefs spread over 2,300 kilometers (1,400 miles), the Great Barrier Reef is so enormous there are always trend-defying pockets. In 2019, one isolated reef near the GBR's southern end was found to be thriving, but the vast majority of the reefs are in decline, their resilience wearing thin as the gaps between damaging events become too short to allow recovery.
GBRMPA notes the prime tourist reefs around Cairns and Port Douglas have been only lightly affected this year, which will no doubt give politicians and fossil fuel advocates further opportunities to dismiss the threat. However, the most seriously affected reefs are off Townsville, right on the Authority's doorstep making it hard for them to ignore.