Known as one of the Seven Natural Wonders Of the World, the Great Barrier Reef attracts over three million tourists to the Queensland coast every year. If the average global temperature warms by 2°C above current levels, it is predicted
that global coral cover will decrease by 95%—a staggering figure which will have catastrophic impacts on ecological processes and the economies of many countries.
The Great Barrier Reef is currently experiencing its 6th mass bleaching event since 1998, caused by an unseasonal heatwave in Northern Queensland, Australia. Temperatures have been reported to reach over 35⁰C every day and remain above 27⁰C
The reef is a vital habitat for 10% of the planet’s total fish species, hundreds of coral species, 30 species of dolphins and whales, and over 130 species of rays and sharks. Worryingly, current climate trends show that we are directly on
track to see this happen if sufficient climate change mitigation is not implemented immediately.
This is the second mass bleaching event the reef has experienced since 2020, despite La Nina conditions. These conditions typically help to reduce bleaching by bringing cooler temperatures, more rainfall and cloud cover that reflects and
absorbs heat energy, whilst also shading the shallow waters from intense solar radiation. However, in recent years La Nina has been unable to mitigate the effects of global sea temperature rise. Consequently, the reef is suffering.
In addition to bleaching events, UNESCO has reported that the reef is under intense threat due to water pollution and bleaching caused by climate-change-induced sea temperature rise and acidification. The Australian Institute of Marine
Science (AIMS) has shown evidence of bleaching on approximately 750 reef colonies, through aerial imagery, with severe bleaching identified in a stretch of over 500 kilometres of the reef.
This has been confirmed by the Great Barrier Reef Marine Park Authority (GBRMPA), a federal government agency. The spatial distribution of bleaching is closely linked to the areas which experienced the most extreme temperatures.
‘Severe bleaching was identified in a stretch of over 500 kilometres of the reef.’
The worst damage has been reported in the northern and central areas, where fluorescent colours of pink, yellow and orange have been observed, in addition to extreme bleaching and potential mortality of over half of the visible corals.
Corals exist in a symbiotic relationship with various species of algae known as zooxanthellae. These algae provide corals with photosynthate which the coral utilises for energy, in return for shelter and certain nutrients.
However, during periods of high stress, such as elevated sea temperatures experienced during heat waves, the symbiotic relationship breaks down and the algae are expelled from the coral structure, leaving the pale white skeleton associated
with bleached coral. Bright, fluorescent colours have been linked to corals undergoing intense stress and are usually a marker that bleaching is occurring.
It is important to note that bleached corals are not dead; if the symbiosis is re-established within a short period, corals can make a full recovery as has been seen after many bleaching events in the past. However, if the coral remains
without sufficient algal symbionts for an extended period, it will starve to death, leading to irreversible reef loss.
After the 2020 bleaching event, most of the reef managed to recover as the water temperatures decreased, allowing corals to regain their symbionts and avoid starvation. This period of high stress had many detrimental impacts, such as
reducing tropical reef fish biodiversity and abundance and decreasing the diversity of both coral and algal symbiont species present on the reef.
It has been found that during prolonged and intense heat waves, algal strains that are intolerant to the high temperatures are wiped out, leaving a disproportionately high number of more heat-tolerant strains.
This may seem beneficial to corals, as they can return to a normal symbiotic state at an elevated temperature in the short term, however, it creates new problems for the reef ecosystem. Low symbiont diversity means that most corals will be
harbouring the same algal species, increasing their susceptibility to being devastated by a pathogen or disease in the future.
In addition, with the constant rise in ocean temperatures, it is unlikely the heat-tolerant algae will be able to maintain optimal rates of photosynthesis for an extended period.
The increasing frequency of heat waves across the reef is leaving less recovery time for corals between periods of intense stress, which is weakening the ecosystem as a whole. UNESCO reports that this is a critical moment in the health and
functionality of the Great Barrier Reef, as depending on the length and intensity of the heatwave, the entire reef ecosystem may be compromised beyond repair. The Great Barrier Reef is at risk of becoming a dysfunctional coral graveyard.
Despite the damage, it is not too late to act. There are two vital actions that must be taken to prevent this catastrophe. The most important is tackling the root cause, climate change. This can be achieved by cutting greenhouse gas
emissions to inhibit the warming of the ocean, giving corals and algae a better chance at adapting to the changing conditions.
The second is to aid the recovery of reefs through protection and restoration, which can take many forms. Policies and laws must be passed by governments to improve water quality and decrease disturbance on reefs. Furthermore, the funding
of research into increasing the temperature thresholds of coral and algae species is required, as well as funding of active restoration projects such as replanting a range of heat-tolerant corals.
It is a synergy of actions that will help increase the resilience of reefs in the face of climate change.