By Dr Emma Kennedy

The Great Barrier Reef (GBR) is currently experiencing its most widespread coral bleaching (a physiological response to heat stress) to date. In January, NOAA satellites detected warmer-than-usual sea surface temperatures, which continued through to March, with February breaking a record as the warmest since records began in 1900. With heat stress accumulating much faster than scientists originally anticipated, survey teams were scrambled from across regions and institutions to assess the impact on corals across the world famous Marine Park’s massive 344,400 sq kmextent. The 2020 mass coral bleaching marks the third major bleaching event in just five years. 

Aerial surveys conducted by Prof Terry Hughes from the ARC Centre of Excellence and the Great Barrier Reef Marine Park Authority’s Dr James Kerry over a period of 9 days in late March gave scientists the best overview of the damage. Flying a light aircraft 150 m above the ocean, the team’s flight path crisscrossed 1036 of the Great Barrier Reefs approx. 2900 reefs, giving researchers an opportunity to eyeball each reef and assign a score (either <1%, 1-10%, 10-30%, 30-60% or >60%) based on the estimated proportion of corals bleached white. Survey scores released in early April matched the footprint of the heat stress, which was concentrated on inshore reefs. While outer reefs and much of the northern area appeared to escape this bleaching, and some central reef areas – including the tourist areas of Cairns and Port Douglas – were spared, south of Cairns the damage was extensive. A quarter of all reefs surveyed were assigned a “severe bleaching” category (where >60% corals affected), and these reefs were found in every sector of the GBR for the first time. This makes the bleaching footprint the most widespread ever reported for the Reef, and just second to the 2016 GBR bleaching in terms of severity. Just 40% of reefs fell into the “no bleaching” >1% bleaching category. Dive teams were deployed to collect underwater field data to help verify aerial scores, although for many researchers – including my own Remote Sensing Research Centrelab trying to get to our reef sites at one of the worst affected areas in the Keppel Islands – COVID-19 restrictions limited our ability to access field sites.

Of particular concern for scientists was the impact on the southern reefs, which had largely escaped the 2016 and 2017 mass bleaching events, and had been thriving despite crown-of-thorns outbreaks in the Swains reefs. Catastrophic declines in the number and recruitment of corals followed the 2016 and 2017 events. Although some recovery has been observed, it is not known how this year’s bleaching has affected new coral recruits. The time between these major disturbances generally does not allow coral assemblages to recover. This kind of heatwave recurrence was not predicted until later into the 2030s. Part of the reason that little bleaching was documented in the Far Northern Reef areas is that most of the reefs there are extremely damaged, making the small number of living corals harder to observe from above, a phenomenon scientists have dubbed “ecological memory”, or “dead reefs can’t bleach”. 

 Particularly concerning for scientists was the impact on the southern reefs, which had escaped recent mass bleachings in 2016 and 2017, and had been thriving despite crown-of-thorns outbreaks in the Swains. UQ’s remote sensing research group surveyed the stunning Hardline Reefs last year as part of their project to map the Great Barrier Reef. Credit: Emma Kennedy

While events on the Great Barrier Reef often focus media attention, the bleaching was not just restricted to the Park: in February our dive team returned from the remote Coral Sea after surveying kilometres of corals. Coral bleaching does not always equate to mortality. Generally, warmer-than-normal temperatures (defined by scientists as “Degree Heating Weeks” DHWs – the period of time water temperature remains >1 degree C above the maximum monthly mean for that that area) that exceed 6 DHWs are associated with significant coral loss. Accumulated heat stress was not as high in 2020 as in 2016. As we transition into winter, temperatures are starting to cool and the stress is beginning to alleviate. Only with time will scientists be able to understand the true long-term impacts of this summer’s heatwave. 

With 64,000 Australian jobs reliant on the Reef, the economic impacts of bleaching could be felt for years to come. The GBR brings in an estimated $6.4 billion to the economy. Dive Instructor Tanya Murphy described it as “Gut wrenching - unless we cut down on carbon pollution urgently, tens of thousands of tourism workers like me who are currently on jobseeker payments due to coronavirus are going to be out of work permanently”. Meanwhile the Morrison Government has announced a $100 million commitment (of the $443 million given to the controversial Great Barrier Reef Foundation) towards a Reef Restoration and Adaptation Program (RRAP) to explore technological solutions from cloud brightening to larval reseeding of damaged areas. 

Bleaching data for the 2016 and 2017 global mass-bleaching event showed damage concentrated north of Cairns, and in the central areas from Cairns to Townsville respectively. Southern reefs had largely escaped recent warming, until this summer. Credit: Terry Hughes, ARC Centre of Excellence for Coral Reef Studies, James Cook University

While these innovations are exciting, without a concerted effort to reduce greenhouse gas emissions both at home and abroad, the future of the reef seems bleak. Corals are extremely vulnerable to elevated ocean temperatures. The 2019 IPCC report warned that limiting global warming to <1.5°C was critical to ensure the survival of functioning reef systems that support over a billion people by providing food, coastal protection and jobs globally. In an online discussion on the scientific findings of the Great Barrier Reef surveys live-streamed to over 300 scientists and reef managers, James Cook University’s Prof Morgan Pratchett, involved in coordinating the bleaching response, said “we need to act to reduce the severity of bleaching events … the only way to do that is to reduce carbon dioxide emissions”

“Our planet’s summer from hell is not over by any means”

As Australia’s long and desperate summer finally draws to an end, seasons shift and the heatwave migrates into the Indian Ocean, researchers in the Maldives are already noting the first signs of bleaching on their reefs. As we begin to survey the aftermath, scientists across the planet are now trying to find a way to check on their local reefs in this difficult time.

Heat stress data from the Bureau of Meteorology shows “Degree Heating Days” an accumulation of above-average warm days (climatology taken from 1993-2003) over the Australian summer (1^stDec to 31^stMarch), derived from IMOS L3S AVHRR sea surface temperature (SST) products. Heat stress accumulated on southern inshore reefs this summer. Credit: BOM

The Great Barrier Reef, indicating location of surveyed reefs that were most severely affected (more than 60% corals estimated to be bleaching red circles) and those that were least impacted (<1% corals bleaching, green circles) by the heatwave. In total, 1036 reefs were surveyed. Credit: Terry Hughes, ARC Centre of Excellence for Coral Reef Studies, James Cook University

Bleaching corals in the Keppel Islands. The Keppel Islands near Rockhampton were one of the worst affected areas, but because of COVID-19 restrictions, University of Queensland scientists were not able to access their reef sites to assess bleaching impact. Credit: Oliver Lanyon