Coral bleaching is quickly becoming one of the most infamous effects of global warming and poses a threat to temperate and tropical coral species worldwide.

In fact, coral reefs are so susceptible to changing climatic conditions that the 2018 IPCC report estimated that 99% of coral reefs will be lost if climatic temperatures reach 2°C above average.

Anywhere between 70-90% could still be lost if temperatures reach (and remain) at 1.5°C above average; a target that is considered unlikely by many.

Perhaps the most famous coral reef is the Great Barrier Reef, located off the coast of Queensland, Australia in the Coral Sea.

Just a portion of the Great Barrier Reef. Picture by Jumbo Aerial Photography/AP.

Despite its name, the Great Barrier Reef is actually a reef ‘system’ as opposed to an individual coral reef. Comprising over 2,900 individual reefs and 900 islands, the Great Barrier Reef is the largest reef system on the planet, stretching for over 2,300 kilometres.

In spite of its impressive size, the Great Barrier Reef is still vulnerable to human activity. In fact, the system has already experienced five mass bleaching events since 1998 due to extreme marine heatwaves.

Recent aerial surveys led by the Australian Institute of Marine Science (AIMS) and the Great Barrier Reef Marine Park Authority (GBRMPA), have led to the disturbing discovery of yet another mass bleaching event in this world heritage site.

750 reefs were surveyed by helicopters and fixed-wing aircraft, travelling a total length of 1,200 km. Extensive bleaching was observed across the majority of the sampled area.

Neal Cantin, coral biologist at AIMS, said:

“More than half of the living coral cover that we can see from the air is severely bleached completely white and can have signs of fluorescence in the colours of pink, yellow, and blue. The corals are producing these fluorescent pigments in an attempt to protect their tissue from heat and from the intense sun during these marine heatwaves.”

This bleaching event comes as a surprise to marine scientists, since we are currently in a La Niña phase; a climatic oscillation that typically brings cooler waters along the coast of Australia and therefore should counteract the effects of global warming.

A bleached coral in the Maldives after a mass bleaching event. Picture sourced from Catlin Seaview Survey.

Coral bleaching typically occurs during periods of extreme water temperatures; these periods are usually a direct effect of human-induced climate change.

So, why does bleaching occur?

Corals contain microscopic algae called zooxanthellae within their tissues. These organisms produce organic compounds via photosynthesis, which are then used by the coral polyp for growth and development. In return, the algae is granted protection and access to the compounds needed to thrive.

This is a classic case of mutualistic symbiosis; a positive interaction between two different organisms, in which both parties are benefited.

However, if the water temperatures rise too high, the zooxanthellae will begin to produce dangerous levels of reactive oxygen molecules. In a last-ditch effort, the coral will expel the algae, losing their signature colours and thus appearing a ghostly shade of white.

Whilst expelling zooxanthellae can aid in survival in the short term, the ultimate consequence of disrupting this symbiotic relationship is the loss of a sustainable food source. This, in turn, results in starvation, leaving behind the carbonate coral skeletons.

This mass coral bleaching event is the first since 2020 and the fourth within a six-year timeframe.

However, not all hope is lost.

David Wachenfeld, chief scientist at the GBRMPA assures that coral reefs can recover from bleaching:

“If the water temperature decreases, bleached corals can recover from this stress. It is more important to remember that we had a mass bleaching event in 2020, but there was very low coral mortality.”

The news of the Great Barrier Reef’s struggle against climate change came not long after a new study in Hawaii which suggests that there are several species of corals resistant to warming waters.

A mesocosm experiment spanning 22 months, revealed that by raising the water temperature to 3.6°C above pre-industrial times, there was very little coral mortality – in some cases, the species’ growth rates increased.

Hawaiian corals were placed in four controlled tanks, each simulating varying oceanic conditions. This tank featured the current state of Hawaiian waters. Photo sourced from Oregon State University College of Science.

However, lab-based experiments rarely represent real-world conditions, and the study could not analyse the influence of local stressors such as pollution, overfishing, and disease.

Whilst the presence of resistant coral species on Hawaiian reefs rekindles our hope for the survival of these beautiful underwater worlds, we should never take for granted the devastating impacts of mankind on the oceans.

If our practices do not change, the Great Barrier Reef will be one of many heading towards extinction.

Written by Lucas King