Trending Science: Aerial surveys show two-thirds of Great Barrier Reef now bleaching at an unprecedented rate

Aerial surveys show the extent of the damage that two years of rising sea temperatures have done to the Great Barrier Reef. An extensive area of the Northern section of the Reef was bleached last year: This year the middle section has been hit the hardest.

‘Since 1998, we have seen four of these events and the gap between them has varied substantially, but this is the shortest gap we have seen,’ said Prof Terry Hughes, director of ARC Centre of Excellence for Coral Reef Studies at James Cook University, Australia.

Fellow researcher Dr James Kerry explains why the scientific community is taking this occurrence of bleaching so seriously, ‘It takes at least a decade for a full recovery of even the fastest growing corals, so mass bleaching events 12 months apart offer zero prospect of recovery for reefs that were damaged in 2016.’

He adds the damage is more worrying than that seen in the past, due to the unrelenting frequency. ‘The central third this year, I would say, was as severe in terms of bleaching as what we saw as the northern third last year,’ he told the BBC. ‘For those reefs that were hit two years in a row, it is effectively a double whammy. They have had no chance to recover from last year''s events.’

The Reef was also damaged by the tropical cyclone Debbie that hit a section in March 2017, including an area that had escaped the worst of the bleaching. An estimated 100 km were impacted by the cyclone.

But cyclones and higher water temperatures caused by last year’s El Nino aside, the researchers are clear that the greatest threat comes from global warming. Professor Hughes commented that as temperatures continue to rise the corals will experience more and more of these events: 1°C of warming so far has already caused four events in the past 19 years.

Oceans are absorbing around 93 % of the increase to the Earth’s temperatures and the resulting rise is putting corals under stress. Corals are given colour and food by algae known as zooxanthellae. When the numbers of the algae are at a healthy level, the corals are vibrantly coloured. But as conditions change, placing them under stress, they eject the algae and begin to starve, leaving them vulnerable to destruction.

The most visible symptom is bleaching as the symbiotic relationship between zooxanthellae and their host corals breaks down. If the algae return to their host and recolonise the areas, the corals can survive and the bleaching reversed. However, that can take years and requires temperatures to fall.