The great bleach

Coral reefs cover less than 1% of the Earth’s surface, yet sustain 25% of marine biodiversity and directly provide support to over 500 million people worldwide. Coral reefs, among several other coastal ecosystems, represent one of the most crucial ways oceans contribute to climate change adaptation. However, they are susceptible to reaching a tipping point, after which the ecosystem can undergo sudden and self-perpetuating irreversible changes. This suggests that even minor, gradual alterations – such as rising global temperatures – can trigger rapid, extreme, and irreversible reactions within the system.

Over the past three decades, human-induced pressures have led to the loss of 30 to 50% of the world’s coral reefs. Moreover, since the late 1950s, the global capacity of coral reefs to provide ecosystem services has decreased by approximately half. If global warming reaches 1.5°C, it has been predicted that coral reefs will likely undergo a decline ranging from 70 to 90%. This number could escalate to 99% if global warming reaches 2°C. Such declines will have serious implications for the local communities reliant on coral reefs. Moreover, passing a tipping point will lead to cascading and potentially catastrophic consequences on other interconnected systems. This growing threat has prompted an urgent call for immediate and coordinated actions to save coral reefs from future decline.

To briefly recap: In climate science, a tipping point is a critical threshold whose overshooting leads to major and often irreversible changes in the climate system. The behaviour of tipping points can be found in many ecosystems, in ice caps and in the circulation of the ocean and atmosphere. The tipping points of the Greenland ice sheet, the West Antarctic ice sheet and the boreal permafrost have received increasing attention in recent years. Once a tipping point has been reached, it is self-perpetuating. Tipping point systems are believed to be inter-related and thus affect fundamental elements of Earth’s climate system. The domino effect of several tipping points will have a massive impact on human society.

Why are coral reefs so important?

Extending for over 2’300 km along Australia’s northeastern coast, the Great Barrier Reef stands out as one of the planet’s most biodiverse ecosystems. Coral reefs are frequently referred to as the rainforests of the ocean as they support some of the planet’s most diverse ecosystems. Reefs provide ecological niches that support between 1 and 9 million species and sustain a greater number of species per unit area than any other marine environment.

Additionally, coral reefs could be considered the most economically valuable ecosystem on Earth when assessed per unit area. The economic significance and benefits of coral reefs are especially crucial for Small Island Developing States due to their heavy reliance on the services offered by their coastal ecosystems. Caribbean coral reefs alone provide goods and services valued between 5.47 and 8.12 billion USD through activities such as fisheries, dive tourism, and shoreline protection. Globally, coral reef fisheries support the livelihoods of approximately 6 million fishers and their families.

On top of economic benefits, coral reefs play a vital role in enhancing the welfare and safeguarding the physical security of coastal communities, benefiting an estimated 63 to 197 million individuals globally. The ridges within coral reefs serve as natural barriers, capable of diminishing wave energy by as much as 97%, thereby offering vital protection against severe weather occurrences such as tsunamis.

Coral reefs also provide essential ecosystem services in mitigating climate change, by protecting against wind, waves, erosion, and flooding – a role that has become increasingly vital considering climate change impacts. In this context, coral reefs’ carbon storage capacity is also crucial. Moreover, reefs play a critical role in preserving areas such as mangrove forests and seagrass beds, which also serve as crucial nurseries for marine life and carbon sequestration.

Coral reefs and their climate tipping point

In March 2022, the Great Barrier Reef Marine Park Authority issued a warning regarding another extensive bleaching event affecting the Great Barrier Reef, triggered by rising sea temperatures. This marked the fourth occurrence of severe and widespread damage within six years. The Great Barrier Reef is simply an example of global reef collapse. Reefs are increasingly vulnerable to temperature variations and are perceived as tipping points due to their susceptibility to bleaching and irreversible mortality, leading to several ecological shifts.

Climate change-induced stressors, such as ocean warming, rising sea levels, and ocean acidification have emerged as the primary and predominant threat to the operational viability of coral reef ecosystems and their associated services. Combined with anthropogenic activities, such as overfishing, coastal development, and pollution, coral reefs in tropical and subtropical regions face heightened risk. Coupled with recurrent mass mortality events, which are increasingly acknowledged for providing inadequate recovery time for affected populations and ecosystems, these factors collectively lower the tipping thresholds of coral reef ecosystems.

Projections indicate that the thermal tolerance of coral reefs will likely be crossed within the coming decades due to anticipated increases in sea surface temperatures. This will have significant consequences, particularly since the stressors can have cascading impacts on coral reef degradation, notably through events such as marine heatwaves, storm intensity, extreme climate events, and rising sea levels. Moreover, crossing the tipping point not only triggers the loss of corals but also has cascading effects on other interconnected systems dependent upon healthy coral reefs. This can perpetuate the climate crisis by releasing stored carbon, further contributing to ocean acidification, global warming, and the loss of biodiversity and other ecosystem services critical for climate change mitigation. Crossing the tipping point also creates a feedback loop as the loss of coral reefs reduces the capacity of oceans to absorb CO2 emissions from the atmosphere. Ocean acidification further stresses and disrupts marine ecosystems, impacting marine life and compromising the resilience of reefs to environmental stressors.

Globally, millions of individuals would face heightened insecurity. Due to the impacts of climate change, reef communities are projected to see economic losses ranging from 3.95 to 23.78 billion USD. The onset of a tipping point would also impact food security, livelihoods, cultural traditions, economic conditions, and the safeguarding of coastal infrastructure and habitats for millions of individuals. Coastal communities would become more vulnerable to the impacts of extreme weather events, leading to increased economic losses and displacement of populations, exacerbating the socio-economic impacts of climate change.

How to avoid overshooting their tipping point

In the absence of significant intervention, tropical reef ecosystems may face global extinction by the end of the century. The National Oceanic and Atmospheric Administration (NOAA) Coral Reef Conservation Program is currently leading the agency’s coral research, conservation, and restoration initiatives. Through its strategic plan, the program defines a framework aimed at mitigating the primary threats to coral reef ecosystems. The program includes the reintroduction of nursery-grown coral to reefs, ensuring that habitats are conducive to natural coral growth, and enhancing coral resilience to threats.

Despite considerable extensive research on coral reefs reaching a tipping point, there remains a need for more accurate and available data regarding the impacts on connected ecosystems, the efficiency of adaptation measures, and knowledge of the early warning signs of tipping points as well as the threshold levels of key drivers that can cause tipping points. In addition, funding should be directed toward coastal communities to restore marine ecosystems and enhance local capacity building in developing countries. Urgent coordinated global efforts are crucial to alleviating pressures on coral reefs, coupled with the adoption of protective and restoration projects aimed at enhancing settlement, growth, resilience to stress, and biodiversity.

Finally, through climate change mitigation actions to achieve the necessary temperature and GHG concentration levels, we can still make a significant difference. With increased awareness and recognition of the severity of the tipping point, we can implement the necessary response actions to mitigate the consequences.