The Great Barrier Reef lies off the coast of Queensland and is one of the seven natural wonders of the world. Climate change has caused mass coral bleaching, which has threatened this hotspot of biodiversity. Bleaching ultimately harms every organism living in the reef ecosystem, from the coral themselves to fish species that call the Great Barrier Reef home.
Coral has a symbiotic relationship with a type of algae called zooxanthellae. Zooxanthellae provides coral with nutrients while coral provides the algae with shelter and access to sunlight which the zooxanthellae need for photosynthesis. When the water temperature is too high, the coral expels the zooxanthellae. Without their symbiotic partner, the coral eventually dies. The algae give the coral colour, and without it they are white. Hence the term “coral bleaching.”
However, some sections of the reef are escaping the disastrous effects of global warming. While other reefs are experiencing serious damage, these few are mostly unscathed. Scientists at CSIRO and the Australian Institute of Marine Science (AIMS) have discovered why.
There are bleaching resistant patches at Ribbon Reefs, Swains Reef Complex, and Pompey Reef Complex. These reefs have a common factor that reduces the effects of global warming. They are environments with localised conditions that allow the coral to survive while corals in other regions are dying off. Areas like this are known as climate refuges. But what are the conditions supporting these reefs?
The answer is upwellings of cooler water. Ribbon, Swains, and Pompey all reside near steep continental drop-offs. Currents pull cool water from the deep[,] over the continental shelf[,] and onto the reefs. How is colder water helping the coral?
The upwellings protect the reef by lowering the water temperature and preventing the coral from forcing out the zooxanthellae.
The researchers from CSIRO, AIMS, and many universities discovered climate refuges via satellite imagery and high-resolution regional ocean modelling. Their model included currents, tides, and ocean topography. An article written by the researchers and published in Science Advances states “To date, no study has investigated future conditions in the GBR using fully three-dimensional ocean general circulation models with a horizontal resolution finer than 10 km.” The resolution of the researchers’ model was four kilometres, making it the highest resolution model yet. This model established that upwellings were creating climate refuges. These refuges are predicted to stay 1°C cooler than the surrounding ocean until at least 2080.
The model was built on the RCP 8.5 greenhouse gas emissions scenario. RCP stands for Representative Concentration Pathway and RCPS are climate models of future atmospheric carbon dioxide levels. RCP 8.5 is a scenario where high emissions continue, and CO2 concentration rises rapidly. Dr. Chaojiao Sun, a CSIRO physical oceanographer, said “I guess the good news is we used this worst-case emissions scenario which we are not tracking on anymore thanks to global climate actions.”
The climate refuges protecting the Great Barrier Reef may allow for the coral to adapt to the rising ocean temperatures. Unfortunately for the coral, eventually the deeper water may be too warm to give any relief.
Sources:
https://www.csiro.au/en/news/All/Articles/2024/December/Great-Barrier-Reef-cool-water
https://www.theguardian.com/environment/2024/mar/08/coral-bleaching-great-barrier-reef-australia
https://www.science.org/doi/10.1126/sciadv.ado6884
https://parks.desi.qld.gov.au/__data/assets/pdf_file/0027/166446/swain-reefs.pdf
https://www.aims.gov.au/reef-monitoring/pompey-sector-2017