April 21 2016
(CNN)A study measuring the extent of coral bleaching in Australia's iconic Great Barrier Reef is branding some of the northern reef's problem as "extreme."
As much as 93% of the 2,300 km (1,429 miles) reef suffers from some level of bleaching, according to the report from the ARC Centre of Excellence for Coral Reef Studies.
"The bleaching is extreme in the 1,000 km (600 mile) region north of Port Douglas all the way up to the northern Torres Strait between Australia and Papua New Guinea," says Andrew Baird of the ARC Centre.
"At some reefs, the final death toll is likely to exceed 90%. When bleaching is this severe, it affects almost all coral species, including old, slow-growing corals that once lost will take decades or longer to return."
Coral bleaching occurs when the marine algae that live inside corals die, either due to increased sea temperatures or extreme weather events.
Dramatic images of Great Barrier Reef dying 01:18
Professor Terry Hughes, who convened the National Coral Bleaching Taskforce, likens the current extent of bleaching in the northern reef as like "10 cyclones have come ashore all at once."
The study surveyed 900 of the individual reefs that make up the Great Barrier Reef, from the air and underwater, building up a picture of a hugely varied extent of damage along the length of the reef.
The worst of the bleaching is in the northern third of the reef, which is one of the country's most important tourist sites. Of the reefs surveyed in this sector, 81% are characterized as "severely bleached."
"Tragically, this is the most remote part of the reef, and its remoteness has protected it from most human pressures: but not climate change," Baird says.
How will climate change impact the oceans? 02:10
"North of Port Douglas, we're already measuring an average of close to 50% mortality of bleached corals. At some reefs, the final death toll is likely to exceed 90%," Baird says.
"When bleaching is this severe, it affects almost all coral species, including old, slow-growing corals that once lost will take decades or longer to return."
The extent of bleaching lessens gradually toward the southern end of the reef, with only 1% of the southern sector, roughly a third of the reef, classed as severely bleached. However, even in this relatively unaffected area, only one quarter of reefs were considered unbleached.
Reef tourism generates an annual income of A$5 billion ($3.9 billion), the report says, and employs nearly 70,000 people.
El Niño has likely played a large part in the coral bleaching events this year in the Great Barrier Reef, as well as in Hawaii and Fiji. El Niño is characterized by warmer than average ocean temperatures in the Pacific Ocean and El Niño years frequently contain the worst bleaching in Pacific region reefs.
Source : http://edition.cnn.com/2016/04/20/asia/great-barrier-reef-coral-bleaching/index.html