The Great Barrier Reef is an amazingly vast, complex and diverse ecosystem – and like any living, breathing thing, certain factors can upset its natural state of being, health, and resilience. Some of the most talked-about threats to the Great Barrier Reef are climate change and coral bleaching, both of which you’ve no doubt heard about either offhand or in-depth. Climate change has been inextricably linked to impacts on the Reef’s resilience, but what is all the fuss about coral bleaching? What is this phenomenon? How bad is it? Have things already gone too far to be fixed?

We’re here to bust some myths and give you the facts about coral bleaching, including how it’s impacting one of our world’s most beloved natural wonders.

What is coral bleaching?

In scientific terms, coral bleaching is a phenomenon in which corals lose their healthy colours due to expelling the symbiotic algae in their tissues. Basically, algae are the good stuff that provides coral with essential nutrients that keep it healthy (and colourful). In this instance, “symbiotic” refers to a mutually beneficial relationship between the coral and the algae – the algae provide food for the coral and get shelter in return.

Corals have a tolerance level when it comes to temperature – and when things fall outside of their optimal range, they can start to protest. When corals are stressed by changes in conditions, they expel their algae and lose their natural colour.

A bleached coral is not a dead coral – but if the stressful conditions persist, the coral can die as a result.

Causes of coral bleaching

So now we know that corals lose their colour when stressed out – but what are the things that upset them? Below, we explain the main causes of coral bleaching events. Bear in mind that these factors can interact and make the overall effects of coral bleaching more intense.

Heat stress

Climate change is causing an increase in ocean temperatures – and this warmer water is the primary cause of coral bleaching. When the water temperatures exceed the tolerance level of the coral's symbiotic algae we spoke about earlier, the algae get expelled.

Mass bleaching events are when coral bleaching occurs across reefs over large areas and are usually caused by extensive marine heatwaves.

Runoff and pollution

Runoff is the movement of land-based water from spillovers and rainfall, which carries with it a lot of material from the land. These materials include chemicals from agricultural and urban areas, some of which are pollutants.

A pollutant is too much of something in the wrong place, which means fertiliser on a farm (a good input) when transported elsewhere by water can become a pollutant. Such fertilisers and sediments eventually end up in the ocean. Because they are pollutants and thus harmful, they can disrupt the delicate balance of the coral reef ecosystem and cause stress, which contributes to bleaching.

Overexposure to sunlight

Corals rely on sunlight for photosynthesis, but even too much exposure to sunlight can cause more harm than good. Periods of low tides or calm seas cause water levels to drop, so corals experience a thinner layer of water as cover. During extended sunlight exposure, this thinner cover can cause stress to corals, leading to a term called photoinhibition. Photoinhibition is where the high levels of solar radiation damage corals in this manner.

Overexposure to sunlight can further intensify coral bleaching, especially when combined with other stressors like high sea temperatures. This is because there is a hotter environment around the sensitive corals.

Freshwater plumes

During heavy rain events, rivers discharge sediment and freshwater. This creates plumes that stress corals by disrupting the reef environment and reducing their ability to expel harmful algae. These freshwater plumes can change the reef water’s salt levels and temperature. If heavy rain occurs during a low tide, freshwater can directly contact corals, further stressing them and potentially leading to bleaching.

Consequences of coral bleaching

How coral bleaching affects marine life

Coral reefs are biodiversity hotspots due to their complex structure, abundant food sources, and diverse habitats supporting thousands of marine species including fish, invertebrates, and other sea organisms. While some bleached corals can recover, they are still left more prone to secondary impacts that can cause death later down the track (like disease and decreased fertility). Sustained higher water temperatures also affect their chances of bouncing back.

Dead corals mean a loss of habitat and resources for these species, leading to diminished biodiversity, altered fish populations, and unbalanced food webs. The repercussions of this can extend far beyond the reefs themselves, affecting the broader marine environment and the other species that depend on these ecosystems for survival.

How coral bleaching affects humans

The impact that coral bleaching has isn’t limited to the water, plants, and animals – it has a flow-on effect that has consequences for people, too.

As coral reefs decline, the marine ecosystems they support are also compromised. This leads to a decrease in fish populations, which are vital for the food security of millions of people and the global fishing industry. Coral reefs also serve as natural barriers against storm surges and tsunamis, helping to protect coastal communities from extreme weather events.

Additionally, coral reef tourism is a significant economic driver for many regions, attracting divers and visitors from around the world. When reefs bleach, the resulting decline in their aesthetic and ecological value can lead to a decrease in tourism, adversely affecting businesses and livelihoods.

How extensive is the coral bleaching in the Great Barrier Reef?

The Great Barrier Reef is famous for its coral, not to mention it makes up about 10% of the world’s total coral reef systems. Within the Reef, you will find more than 450 types of hard corals and at least 1,000 species of soft corals across a sprawling 344,400 square kilometres. The Reef spans approximately 2,300 kilometres along Queensland’s coast – from the tip of the Cape York Peninsula down to Bundaberg – so there are serious variations in temperature, habitat, and the overall ecosystem that all come into play.

The Reef has experienced five coral bleaching events in the past decade. As the climate, environment, and threats to the Reef fluctuate, advice on Reef health is being constantly updated and improved according to new data. The most updated information on Reef health, including coral bleaching events, can be found directly through the Great Barrier Reef Marine Park Authority and the Australian Institute of Marine Science.

While some areas of the Reef have shown resilience and recovery, the frequency and intensity of these bleaching events are increasing. This is why climate action is such a crucial and immediate part of helping the Reef survive and thrive again.

How are coral bleaching events categorised?

The blanket term “coral bleaching event” can be rather ambiguous without additional context. Much like someone saying they’ve been injured, it’s a broad scope – and coral bleaching events can range in severity from the equivalent of a sprain to a broken bone. To help create some clarity, marine managers and scientists from the Reef Authority, Australian Institute of Marine Science, and James Cook University developed a framework to describe and categorise coral bleaching on the Great Barrier Reef.

This framework provides a clear and consistent way to describe coral bleaching events, allowing for comparisons between years, over time, and across the Reef. It focuses on four key factors to help us get the total picture: exposure, colony response, spatial extent, and prevalence.

Exposure: Satellites, weather stations, and underwater sensors measure the duration and intensity of heat stress.

Colony Response: Underwater surveys assess the coral’s reaction, including the severity of bleaching, loss of zooxanthellae, mortality, and survival rates.

Prevalence: Underwater and aerial surveys determine the affected areas of the Reef, such as the reef flat or larger regions, and calculate the percentage of impact, affected habitats, and depths.

Spatial: Large-scale aerial surveys and underwater observations help identify the scale of impacts, ranging from local to widespread or the entire Reef.

These factors help teams categorise coral bleaching events on a scale of 1 to 5, with one being mild and 5 being severe. The outcomes of each category are based on above-average sea temperatures over increasingly longer periods o time.

Category 1	Partial bleaching of sensitive coral species; no mortality across local, regional, or widespread scale.
Category 2	Partial to full bleaching of some coral species; low mortality rates across a few coral species at either local, regional, or widespread scale.
Category 3	Partial to full bleaching of many coral species; mortality for some coral species at either local, regional, or widespread scale.
Category 4	Full bleaching to most coral species; mortality for many coral species at a range of depths and widespread scales.
Category 5	Full bleaching to all coral species; mortality for most coral species at a widespread scale.

What’s being done to stop coral bleaching in the Great Barrier Reef?

Due to its urgent nature and multifaceted causes, combatting coral bleaching in the Reef requires a team effort. The Reef Joint Field Management Program plays a crucial role in protecting the Great Barrier Reef through conservation actions, monitoring, responding to incidents, upholding compliance and educating visitors on how to enjoy the Reef responsibly.

Climate action: Efforts to reduce greenhouse gas emissions at a national and global level are crucial, as climate change is a primary driver of rising sea temperatures which cause coral bleaching. This is why there are messages from the Australian and Queensland governments around the need to address climate change.

Water quality improvement: Improving water management and implementing best industry practices and standards can reduce pollution run-off from agricultural and urban sources. This helps enhance water quality, which can build reef resilience and help corals resist and recover from bleaching. This is why the Queensland Government sets guidelines for ensuring the water remains of good quality to avoid stressing the corals even further.

Research and monitoring: Ongoing research helps us better understand the causes and mechanisms of coral bleaching. The Reef is closely monitored for signs of stress, providing data that can inform management actions. The AIMS Long-Term Monitoring Program is the longest and most comprehensive record of reef condition for the Great Barrier Reef, and is a great example of how historical data can help back up new and ongoing conservation strategies.

The Reef Joint Field Management Program also plays a crucial role in safeguarding the Great Barrier Reef from coral bleaching. Its field officers conduct regular surveys to monitor overall reef health, identify bleaching events, and implement strategies to lessen bleaching impacts.

Direct intervention: Active reef management techniques are being explored, such as shading reefs, as well as breeding and transplanting corals that are more resilient to heat stress.

Legislation and protection: Strengthening existing environmental laws and creating protected areas can minimise local stressors on the Reef, which means better coral health and resistance to bleaching.

Community and stakeholder engagement: Working with the Reef Traditional Owners, local communities, businesses, and other stakeholders is essential for successfully implementing conservation strategies.

Global collaboration: We cooperate with international bodies to share knowledge, experience, and resources for reef conservation, recognising that coral bleaching is a global issue.

It’s important to note that while these actions can curb the impact of bleaching and help with recovery, addressing global climate change is vital to ensuring the long-term survival of coral ecosystems. All the efforts we mention here take time and patience to put into action, so their impacts may not be immediately obvious. This is why we keep such a close eye on the Reef – picking up on the slightest changes can help with early intervention.

How can I help protect the Great Barrier Reef?

Taking direct action to help conserve the Great Barrier Reef is a great way to feel empowered. We’ve got loads of ways you can get involved with the Reef, from engaging in citizen science projects to living a greener lifestyle. Reducing your energy usage, recycling effectively, and cutting down on food waste are all vital steps you can take to help the Reef (and the environment in general). Every step taken helps.

Coral bleaching FAQs

Can coral bleaching be reversed?

Good news – coral can come back from a bleaching event! The coral’s recovery depends on factors like the severity of the bleaching, the duration of the stress, and the health of the surrounding ecosystem. In cases where the stress is temporary and alleviated quickly, corals can return to their normal colour.

Are other reefs around the world being affected by coral bleaching?

While the Great Barrier Reef is one of the most commonly cited locations for coral bleaching events, the phenomenon affects reefs all over the world. Climate change is a global concern that affects reef health worldwide – for example, we're enduring a fourth recorded global bleaching event that is impacting the world’s reefs en masse.

Is the Great Barrier Reef dead?

Thankfully the Great Barrier Reef is still very much alive, but it needs our help to keep up its health. While it’s true that the Reef ecosystem is under pressure from various threats, the Queensland and Australian Governments, conservation groups, communities, and passionate everyday people are working together to minimise and reverse Reef damage. That being said, some parts of the Reef are in worse condition than others, so applying observations in one region to the whole reef is not accurate.

Will the Great Barrier Reef naturally adapt to climate change?

Impacts of climate change – like intense tropical cyclones, ocean water acidification, and high sea surface temperatures – are happening too frequently to allow the Reef to adapt. The window between extreme events is closing rapidly – so we need to act strongly and swiftly.

The Reef’s long-term survival will depend on significant reductions in greenhouse gas emissions. Some amazing minds and resources are going towards helping the Reef adapt to a changing climate. Key research from AIMS into why some corals are more heat-resistant than others is helping scientists investigate ways we can harness these genetics to further restoration activities.

The Reef Restoration and Adaptation Program (RRAP) has tapped top marine scientists to investigate heat-tolerant coral genes to understand their resistance to warming temperatures. This research is helping to develop coral-propagation techniques to produce healthy corals for reef restoration.

The science behind the Reef: more resources to check out

Science is fundamental in Reef policy and decision-making – it helps us make the most informed and fact-based decisions for the health of the Reef. If you want to learn more about the scientific organisations, initiatives, and policies that are helping Reef recovery, we’ve assembled some key resources:

• Protecting the Reef
• The Great Barrier Reef Marine Park Reef Authority (GBRMPA)
• Australian Institute of Marine Science
• 2022 Scientific Consensus Statement