A diver examines recent bleaching on Australia’s Great Barrier Reef.

Greg Torda

Severe bleaching hit the Great Barrier Reef for second year, survey confirms

For the second year in a row, Australian marine scientists have carried out the sad task of surveying the Great Barrier Reef (GBR) to determine the impact of widespread bleaching caused by elevated sea water temperatures. And for the second year in a row, the findings are grim: Severe bleaching occurred on many of the individual reefs in the middle third of the 2300–kilometer-long system, according to the aerial survey results released today.

In 2016, severe bleaching hit the northern third of the reef. Now, surveys show a significant number of reefs in the central GBR have been hit 2 years in a row. Because it takes at least a decade for a full recovery by the fastest growing corals, there is “zero prospect of recovery” for reefs hit in successive years, says James Kerry, a marine biologist at James Cook University in Townsville, Australia. 

Bleaching occurs when elevated sea water temperatures cause corals to turn white by expelling the colorful algae, called zooxanthellae, that live within corals and use photosynthesis to provide nutrients for themselves and their hosts. Corals can recover from bleaching if waters cool off quickly enough. So the full extent of this year’s damage won’t be known until in-water surveys are conducted later. But, “We anticipate high levels of coral loss,” Kerry says.

ARC Centre of Excellence for Coral Reef Studies

Last year, scientists found that 67% of the corals in the northern 700-kilometer section of the GBR died from the bleaching. Only the southern stretches of the reef system have been spared from bleaching over the past 2 years. Scientists have concluded that the only hope of preserving the reef is to reverse the global warming that is raising ocean water temperatures.