For the second consecutive year, aerial surveys show severe coral bleaching in the Great Barrier Reef off the coast of Australia.
While severe bleaching events have occurred three other times in the past 20 years — in 1998, 2002 and 2016 — this year marks the first time it's known to have happened two years in a row. Scientists say the damage is caused by higher water temperatures due to global warming.
"It takes at least a decade for a full recovery of even the fastest growing corals, so mass bleaching events 12 months apart offers zero prospect of recovery for reefs that were damaged in 2016," said James Kerry, a senior research officer at the ARC Centre of Excellence for Coral Reef Studies in Queensland, Australia.
It was "shocking" to see the extent of the damage in a different section of the reef, Kerry added.
The damage last year was worst in the northern third of the massive reef. This year, it's most severe in the middle third of the reef. As Kerry explained, those areas of damage overlap somewhat so "some of the reefs now, in the central and northern section, have had a double dose of severe bleaching for two years in a row."
You can see how the impacted region has shifted in this map, released by the ARC Centre:
Aerial footage released by the team shows large swaths of the reef that are now drained of color.
Bleaching happens when coral is exposed to higher than normal water temperatures. The unusual conditions "cause corals to expel tiny photosynthetic algae, called 'zooxanthellae,' " the ARC Centre said. "The loss of these colorful algae causes the corals to turn white, and bleach."
They can recover if the ocean temperature returns to normal, but prolonged stress may cause the corals to eventually die.
James Cook University's Mia Hoogenboom carried out a number of underwater assessments for the survey, and said she documented damage to mound-shaped corals that tend to be more resistant to bleaching.
To make matters worse, Tropical Cyclone Debbie at the end of March "struck a section of the reef that had largely escaped the worst of the bleaching," the ARC Centre added.
In research on last year's bleaching event, the scientists have said that more than 90 percent of a 2,300-kilometer (1,430-mile) stretch of the reef had sustained some form of damage.
A recently published study in Nature stated that local measures can ultimately do little to protect the reefs from bleaching. Securing a future for coral reefs "ultimately requires urgent and rapid action to reduce global warming."