The Great Barrier Reef could be hit with repeat coral bleaching events every two years by 2034 under current greenhouse gas pollution rates, according to a new report by Climate Council, revealing that accelerating climate change has driven a 54% increase in the number of marine heatwave days each year (between 1925-1954 and 1987-2016), placing global reefs at serious risk.
The ‘Lethal Consequences: Climate Change Impacts on the Great Barrier Reef,’ report shows the future survival of coral reefs around the world, including the Great Barrier Reef, depends on how deeply and swiftly greenhouse gas pollution levels are slashed over the coming years and decades.
Key findings
- Unprecedented bleaching events on the Great Barrier Reef in 2016 and 2017 have resulted in mass coral mortality.
- Rising sea surface temperatures over the past century have resulted in more frequent and prolonged global marine heatwaves.
- By 2034, the extreme ocean temperatures that led to the 2016 and 2017 bleaching events may occur every two years. Such a short period between bleaching events is not sustainable as the development of coral assemblages takes at least a decade.
- Between 1925–1954 and 1987–2016 the global average frequency of marine heatwaves increased by 34% and the global average duration increased by 17%.
- Global sea surface temperatures have increased by 0.2°C from 1992 to 2010, increasing the odds of more frequent and prolonged marine heatwaves.
- The return period for global bleaching events has decreased from 27 years in the 1980s to only 5.9 years now.
- In the future, regional-scale bleaching can be expected to occur in hot summers in both El Niño and La Niña years.
The unprecedented bleaching events on the Great Barrier Reef in 2016 and 2017 resulted in mass coral mortality, with the 2016 bleaching event at least 175 times more likely to occur due to intensifying climate change.
During the event, corals started to die almost immediately when heat exposure reached a threshold of 3-4°C-weeks (degree heating weeks – a commonly used measure of heat exposure based on the intensity and duration of thermal stress). Coral mortality during the bleaching event was strongly correlated with the level of heat exposure.
The 2016 bleaching event resulted in the death of 29% of the corals on the Great Barrier Reef. Coral mortality during and after bleaching was strongly correlated with thermal exposure.
For example, heat exposure of 4°C-weeks led to median coral loss of 5% or less, 4-8°C-weeks led to median coral loss of 15.6% and 8°C-weeks led to median coral loss of 20.7% (Hughes et al. 2018a). Coral mortality continued to occur in the aftermath of the bleaching event. Mortality measured in the eight-month period following the bleaching was strongly correlated with both the level of bleaching and heat exposure.
Where bleaching on reefs was less than 25%, post-bleaching mortality was negligible, but above this threshold, loss of corals progressively increased. Reefs exposed to 0-3°C-weeks experienced low loss of coral.
Above this threshold, reefs experienced a progressive decline in coral following the bleaching event. Reefs exposed to 4°C-weeks experienced 40% loss of coral, reefs exposed to 8°C-weeks experienced 66% loss and reefs exposed to 9°C-weeks experienced >80% loss.
There is still hope for the Great Barrier Reef in the long-term. But our actions now will determine its long-term survival and that of all warm-water coral reefs around the world. It is imperative that global average temperature rise does not exceed 1.5°C above pre-industrial temperatures.
Find out more in the following report:
