Mass coral bleaching likely along northern Great Barrier Reef

Extensive stand of severely bleached coral at Lisianski Island in the Papahanaumokuakea Marine National Monument in Hawaii, documented during an August 2014 NOAA research mission. (Credit: NOAA).

Scientists are currently mapping the biological damage caused by global warming

Staff Report

At the end of eastern Australia’s long, hot summer, ocean scientists are once again seeing devastating coral die-backs in the northern reaches of the Great Barrier Reef. Over the next few weeks, they’ll venture underwater to study how the coral communities responded to a second straight year of overheated water.

When temperatures pass a threshold, the coral expels its symbiotic algal partner, leaving underwater wastelands of white-washed reefs. The scientists will also use survey flights above the reef, and even satellite imaging as they mobilize to document one of global warming’s most devastating impacts. There has been a prolonged global mass bleaching under way for the past year, and climate researchers say nearly all the world’s corals will be at risk by mid-century under projected global temperature increases.

Several weeks ago, NOAA’s coral reef watch program issued a bulletin warning of the danger of persistent bleaching in the Southern Hemisphere, where average ocean temperatures have frequently been at or above record highs for the past two years.

The new surveys in Australia also come just as a new study shows the reef’s resilience is fading. Repeated and sustained periods of hot water and bleaching weakens the entire coral reef ecosystem, researchers found. If ocean temperatures cool off the next few weeks (Austral autumn), the damage might not be as bad as last year.

“The severity of the 2016 bleaching was off the chart,” said Prof. Terry Hughes, with ARC Centre of Excellence for Coral Reef Studies. “We have now assessed whether past exposure to bleaching in 1998 and 2002 made reefs any more tolerant in 2016. Sadly, we found no evidence that past bleaching makes the corals any tougher.”

While protecting reefs from fishing, and improving water quality is likely to help bleached reefs recover in the longer term, the study also revealed that it made no difference to the amount of bleaching during the extreme heatwave of 2016.

Study co-author Janice Lough said the pattern was clear in all three previous mass coral bleaching events (1998, 2002, 2016) coral bleaching exactly matches where the warmest water is recorded. “That allows us to predict when and where bleaching is likely to occur this year,” said Lough, a researcher at the Australian Institute of Marine Science

“Global warming is the number one threat to the Reef.  The bleaching in 2016 strongly reinforces the urgent need to limit climate change as agreed by world leaders in the Paris Agreement, and fully implement the Reef 2050 Plan to boost the Reef’s resilience,” said co-author Dr. David Wachenfeld from the Great Barrier Reef Marine Park Authority.

“It broke my heart to see so many corals dying on northern reefs on the Great Barrier Reef in 2016,” said Prof. Hughes, who led the expansive aerial surveys. “With rising temperatures due to global warming, it’s only a matter of time before we see more of these events. A fourth event after only one year is a major blow to the Reef.”

Key points from the study:

  • 2015-2016 saw record temperatures that triggered a massive episode of coral bleaching across the tropics
  • Coral bleaching events should no longer be thought of as individual disturbances to reefs, but as recurring events that threaten the viability of coral reefs globally
  • The Great Barrier Reef has had three major bleaching episodes, in 1998, 2002 and 2016, with the latest being the most severe and with catastrophic levels of bleaching occurring in the northern third of the Reef (a region approximately 800 km or 500 miles in length)
  • The amount of bleaching on individual reefs in 2016 was tightly linked to local heat exposure
  • The cumulative, superimposed footprint of the three mass bleaching events on the Great Barrier Reef has now encompassed virtually all of the Great Barrier Reef
  • Past exposure to bleaching in 1998 and 2002 did not lessen the severity of the bleaching in 2016



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