In 2007, the Intergovernmental Panel on Climate Change identified coral bleaching as the greatest threat to reef systems around the globe. Mass coral bleaching events have become increasingly common in recent decades due to climate change and rising ocean temperatures, threatening the health and biodiversity of coral reef ecosystems worldwide.
As divers and surfers, we’re deeply connected to coral reefs and understand their importance to the health of our marine life, as well as the recreational opportunities they provide. Coral reefs are responsible for creating some of the world's most legendary waves and without them, the underwater landscapes we explore as divers would be severely diminished.
But what exactly is coral bleaching and what causes it to happen?
In this article, we’ll explain everything you need to know about coral bleaching, including what causes it, the reefs that are most affected and how we can help to prevent mass bleaching events in the future.
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What is coral bleaching?
Coral bleaching is a phenomenon that occurs when coral reefs become stressed due to changes in their environment, such as rising ocean temperatures, pollution or changes in salinity or pH levels. The corals and the symbiotic algae called zooxanthellae that live within their tissues have a mutually beneficial relationship. The algae provide the corals with nutrients through photosynthesis and give them their vibrant colours.
However, when environmental stressors cause the water to warm above tolerable levels, the corals expel the algae from their tissues as a survival response. Without the colourful algae, the coral's white skeleton becomes visible through their transparent tissues, giving them a stark white appearance. Hence the term "bleaching".
Coral bleaching does not mean the corals are dead, but it does leave them in a highly vulnerable state. In order to recover from a bleaching event, the zooxanthellae must re-inhabit the coral polyps and resume their photosynthetic processes. If the coral polyps perish from starvation in the aftermath of bleaching, their tissues will decompose, leaving behind only the white calcium carbonate skeletons of the hard coral species.
Opportunistic algae can colonise these skeletal remains, forming a barrier that prevents the regrowth of new coral. As time passes, the coral skeletons themselves erode and disintegrate, ultimately causing the collapse of the reef and the sustenance it provides to an abundance of marine life.
Coral reef bleaching causes
Causes of coral bleaching
Coral bleaching is primarily caused by rising ocean temperatures resulting from human-induced climate change. As the Earth's atmosphere traps more heat due to the accumulation of greenhouse gases like carbon dioxide from the burning of fossil fuels, a significant portion of this excess heat is absorbed by the oceans. Intense sunlight exacerbates the problem by causing further stress to the coral.
However, coral bleaching is not solely triggered by heat and light. Chemical pollution from industrial runoff, sewage and oil spills can contaminate the water and disrupt the delicate balance coral needs to thrive. Exposure to air during extremely low tides leaves coral vulnerable and prone to bleaching, as does increased sedimentation and reduced water salinity following heavy rains.
Destructive practices like cyanide fishing, which involves spraying cyanide onto reefs to stun and capture fish, poisons the coral, while overfishing can disrupt the marine ecosystem altogether. Coral reefs weakened by these environmental assaults become more susceptible to disease, leading to further bleaching in what is a vicious cycle.
Coral bleaching events are strongly correlated with climate-driven warming events like El Niño and marine heatwaves, which are becoming more intense and frequent as the planet heats up. Ocean acidification and coral bleaching are also entwined. Ocean acidification is a consequence of the oceans absorbing excess atmospheric CO2 and further compounds the stress on coral reefs by reducing the availability of the calcium carbonate minerals they need to build their skeletons.
Ocean acidification and coral bleaching
Mass coral bleaching events
Mass coral bleaching events have become increasingly frequent and severe in recent decades, posing a dire threat to the health and survival of coral reefs worldwide. The primary trigger for mass bleaching events is prolonged exposure to elevated sea surface temperatures, often exacerbated by climate change and strong El Niño conditions.
The extent and severity of bleaching can vary based on factors such as the coral species' stress resistance, tolerance to the absence of zooxanthellae and the presence of local climatic conditions that may mitigate the impact. However, even the most resilient coral colonies struggle to withstand the intense and persistent thermal stress associated with mass bleaching events.
Between 1979 and 1990 alone, 60 major episodes of coral bleaching occurred, with the resulting coral mortality impacting reefs in every corner of the planet. The situation reached a dire tipping point around a decade ago when the longest coral bleaching event ever recorded took place. Spanning from 2014 to 2017, this prolonged and highly destructive bleaching event was triggered by an intense El Niño weather pattern.
The unusually warm ocean temperatures caused by El Niño placed the already vulnerable coral reefs under extreme stress for an extended period. Starved of nutrients and unable to cope, vast swaths of coral colonies bleached bone-white and perished. By the end of this catastrophic event, over 70 percent of Earth's coral reefs had suffered significant damage, many irreversibly.
In 1998, the Great Barrier Reef experienced its first major bleaching event. Since that time, the frequency and severity of bleaching has escalated at an alarming rate. Between 2016 and 2020 alone, the Great Barrier Reef suffered through three separate mass bleaching events and scientists warn that if global temperatures continue to rise, reaching 1.5°C above pre-industrial levels, the reef could face bleaching events three times per decade.
Even more concerning, if warming reaches 2°C, models predict that bleaching will occur every other year, leaving little time for the coral ecosystems to recover between each event.
Want to discover this natural wonder first-hand? Discover 10 incredible ways to experience the Great Barrier Reef
Coral bleaching in the Indian Ocean
Impacts of coral bleaching events
Not only are bleaching events having devastating impacts on coral reefs around the globe but there are knock-on effects for the species that rely on them for survival. The loss of coral coverage leads to declines in the diversity and abundance of fish species, particularly herbivorous fish species, which graze on algae and seaweeds. Without these fish species to keep growth in check, the algae and seaweed can smother the coral.
As bleaching events become more frequent and widespread, coral reef fish populations become more homogenised, with smaller, specialised species that fill important ecological niches being replaced by more generalist species. This loss of biodiversity and ecological specialisation makes the reef ecosystem less resilient and less able to recover from future disturbances and bleaching events.
Additionally, coral reefs act as natural fisheries, providing a crucial spawning ground and nursery habitat for many commercially valuable fish species. As a result, they support abundant fish populations that have sustained entire local economies and served as a vital source of income, especially for small-scale fishers.
As fish populations dwindle and fishing opportunities disappear as a result of coral bleaching, the economic consequences can be profound. One study estimates the direct losses to fisheries from coral reef death could total $49-69 billion if greenhouse gas emissions remain unchecked. Developing nations in Southeast Asia and the Indian Ocean (where many reefs are located) would bear the brunt of these losses, exacerbating poverty and food insecurity as communities are forced to seek alternative livelihoods.
Coral reefs also provide a vital protective barrier for coastlines, acting as a natural breakwater that reduces the force and impact of waves and storm surges. The complex underwater structures of coral reefs dissipate wave energy, lessening the intensity of waves before they reach the shore. This crucial protective function helps to minimise damage from tropical storms, hurricanes and cyclones, as well as reduce coastal erosion and prevent flooding of low-lying areas.
Countries and communities that depend on healthy coral reefs for protection are at risk of losing this valuable natural asset. Without the wave-breaking benefits of coral reefs, coastlines will be more vulnerable and susceptible to severe damage from storms and flooding. The loss of coral reefs will lead to significant economic losses, as governments will need to spend much more on artificial coastal defences and rebuilding damaged infrastructure.
On top of that, countries will suffer from lost tourism revenue, as coral reefs are a major draw for snorkellers, divers and beach-goers. It’s estimated that the economic benefits of tourism drop by nearly 4% for every 1% decrease in coral cover - a figure that is quite staggering! The combination of these direct and indirect economic impacts resulting from the loss of coral reefs will be immense and will hit coastal and island nations especially hard.
Global warming and coral reefs
Scientific research + individual action
As global temperatures continue to climb, coral reefs worldwide are increasingly at risk of undergoing irreversible damage and ecosystem collapse. This would have devastating ripple effects on the incredible biodiversity they support and the millions of people who depend on them for food, livelihoods and coastal protection.
In response to this dire situation, marine biologists and conservationists are undertaking extensive research and innovative projects aimed at slowing the decline of corals and restoring damaged reef ecosystems. Techniques like micro fragmentation are showing promise in increasing coral cover and diversity in impacted areas. This involves breaking coral colonies into small pieces to stimulate faster growth. Coral farming is also having an impact, with corals cultivated in nurseries before being transplanted onto degraded reefs.
In recent years, scientists have been experimenting with methods to make corals more resilient to stressors, such as selective breeding and assisted evolution. To support this vital work, specialised coral research facilities have been established, featuring large tanks that recreate the conditions found on natural reefs. By carefully controlling factors like water temperature, chemistry and lighting, researchers can test restoration approaches and study coral biology under different future climate scenarios.
On an individual level, we can all play a part by taking steps to minimise runoff of harmful pollutants like fertilisers, herbicides and pesticides that damage coral and disrupt the finely-tuned balance of reef ecosystems. As surfers and divers, we should all be using reef-safe sunscreen (which doesn't contain harmful UV-blocking chemicals) and ensure our recreational ocean activities don't result in any damage to reefs.
But to really make a difference and address the main causes of coral bleaching, we must tackle the urgent crisis of climate change head-on. Rising ocean temperatures driven by escalating greenhouse gas emissions are the root cause fuelling mass bleaching events - something that can't be prevented by a few individual actions alone.
Solving this crisis demands immediate action on a global scale to drastically slash emissions by accelerating the transition to clean, renewable energy and pushing governments worldwide to implement forward-thinking, climate-smart policies before it's too late. The window to save coral reefs from a catastrophic collapse is rapidly closing, but if we come together now with resolve and determination, we still have a chance to preserve these aquatic wonders for future generations.
Climate change and coral bleaching
Coral bleaching FAQs
What is coral bleaching and why is it bad?
Coral bleaching occurs when corals become stressed due to factors such as rising ocean temperatures, pollution or changes in water chemistry. Under these stressful conditions, the corals expel the symbiotic algae called zooxanthellae that live within their tissues. These algae are critical to the survival of the coral, as they provide the majority of the coral's energy through photosynthesis and give the coral its vibrant colours.
When the zooxanthellae are expelled, the coral's white skeleton becomes visible through its transparent tissue, giving it a "bleached" appearance. Although bleached corals are not dead, they are highly vulnerable and more susceptible to starvation and disease. Without the algae to provide nourishment, the corals struggle to grow, reproduce and defend against threats.
If the stressful conditions persist and the algae do not return, the bleached corals will eventually die. Mass coral bleaching events can destroy entire reef ecosystems, as many marine species depend on healthy corals for food, shelter and breeding grounds. As bleaching events become more severe and frequent, corals have less time to recover between episodes, leading to widespread reef die-offs. This can have devastating consequences for the incredible biodiversity they support and the half a billion people worldwide who depend on reefs for food, income and coastal protection.
What is causing coral bleaching in Australia?
In Australia and around the world, coral bleaching is primarily driven by the rapidly warming oceans. As heat-trapping greenhouse gases from human activities like burning fossil fuels and deforestation accumulate in the atmosphere, the oceans absorb around 93% of the excess heat.
Prolonged exposure to warmer waters disrupts the coral's symbiotic relationship with the algae that provide them with essential nutrients (and colouration). Without these algae, the coral's white skeleton becomes visible through the transparent tissue, giving the coral a ghostly, bleached appearance.
However, warming waters are not the only trigger for bleaching. Corals live within a narrow range of environmental conditions, so stressors like pollution, sedimentation from coastal development and changes in salinity and pH levels linked to climate change can all cause bleaching.
How much of the Great Barrier Reef is bleached?
Despite being the world’s largest coral reef system, the Great Barrier Reef (located off the east coast of Australia) is facing an unprecedented crisis due to widespread coral bleaching. Recent aerial surveys conducted by the Great Barrier Reef Marine Park Authority have revealed around 73% of the surveyed reefs exhibit signs of prevalent bleaching, defined as more than 10% of the coral cover being bleached.
The current mass bleaching event is the fourth of its kind to impact the Great Barrier Reef since 2016 and scientists warn that the frequency and severity of these events are only expected to increase in the coming years. The loss of coral reefs not only has devastating ecological consequences but also significant economic impacts, as the Great Barrier Reef supports a thriving tourism industry and provides critical habitat for countless marine species.
Can bleached coral recover?
Coral bleaching occurs when corals become stressed, most often due to excessively warm ocean temperatures, and expel the colourful symbiotic algae living within their tissues. If ocean temperatures cool quickly enough, bleached coral can recover as the algae return and the coral may regain its colour within a few months.
When bleaching is confined to a limited area, nearby healthy coral can also help reseed and repopulate the damaged reef. However, severe, prolonged or repeated bleaching events make it much harder for coral to bounce back. Recovery, if it's even possible, can be an extremely slow process spanning many decades.
Hindering the recovery of bleached coral is ocean acidification, another issue resulting from climate change. As the ocean absorbs excess carbon dioxide, it becomes more acidic, reducing the availability of carbonate ions corals need to build their calcium carbonate skeletons.
Where is coral bleaching the worst in the world?
While bleaching events have devastated coral reefs across the globe, the damage has been most severe in the Indian Ocean. Warm sea temperatures, exacerbated by climate change, have permanently damaged large swaths of once thriving coral reef provinces in this region.
The Maldives, Seychelles and other island nations in the Indian Ocean have seen up to 90% of their coral reefs bleached in recent years. Australia’s Great Barrier Reef has also experienced significant bleaching events, with the northern sections being the hardest hit. In Southeast Asia, the Coral Triangle region encompassing Indonesia, Malaysia, Papua New Guinea, Philippines, Solomon Islands and Timor-Leste has also suffered widespread bleaching.
About the authors
We are a team of passionate divers and surfers with decades of combined experience in the water and travelling to all corners of the globe. After years of chasing waves and descending into the deep blue, we've created this resource highlighting sustainably run surf camps, eco-friendly dive resorts and conservation-focused ocean trips to help inspire your next adventure.
Eco Ocean Escapes was born out of a love of the ocean, an obsession with travel and a concern about the impacts of our adventures on the environments we explore. Despite the benefits that surf and dive tourism can bring to local communities, we recognised that ocean-based adventures are not always managed in a sustainable manner.
Through our articles, we hope to inspire those seeking a responsible surf or dive trip that is all about supporting local communities, preserving our coastal environments and the incredible marine species that inhabit our oceans.