For coral reefs, climate change continues to be a huge problem.  The warmer waters cause the coral to bleach – lose their colour by expelling the zooxanthellae algae living within their tissue. These colourful algae provide the corals with energy and nutrition via photosynthesis, in exchange for shelter. The Great Barrier Reef has incurred 7 mass bleaching events over the last 25 years, 5 in just the last 8 years! Curiously, some parts of the Great Barrier Reef, such as the northern Ribbon Reefs and the southern Swains and Pompey reefs, have escaped these mass bleaching events and lie untouched – how are these reefs still alive and thriving?

The answer is simpler than you may think – cool water. A natural phenomenon called “upwelling,” caused by winds and currents, forces the cooler, nutrient-rich water from the deep ocean floor to rise and mix with the warmer surface water, providing the corals a much-needed respite from the heat stress. Scientists refer to these naturally protected reefs as “climate refuges”. The average summer sea surface temperature around these reefs is at least 1 ⁰C cooler than the surrounding areas.

CSIRO’s existing maps and sea modelling suggests that this phenomenon could continue to protect parts of the Great Barrier Reef till 2080, even if ongoing climate change causes surface temperatures to rise by 2-3 ⁰C. Even as surface waters get hotter and marine heatwaves arrive more often, the currents carrying cooler water to the surface will continue. But, if climate change continues to overheat the ocean, even the deeper water will become too warm for the corals to tolerate.

Programs such as the Reef Restoration and the Adaptation Program are already looking at ways corals could adapt to and better tolerate these warmer waters. Examples like the coral reefs of the Red Sea show us that this might be a possibility.  If these refuge corals find continued ways of adapting to marine heatwaves and climate change, it might be possible to use them to help the bleached reefs recover. Bleached coral reefs are not dead, and the zooxanthellae algae can repopulate if the conditions return to normal. Could corals on these protected reefs adapt fast enough? Could these heat-adapted coral larvae help the bleached corals to regrow?

Climate change, although the biggest, is not the only threat faced by the corals of the Great Barrier Reef. Damage from shipping, overfishing, and outbreaks of crown-of-thorn starfish all contribute. We need to continue to fight on all these fronts to protect the Great Barrier Reef and its wonderful biodiversity into the next century.