Expert Comment: Global coral reef collapse is not inevitable
Associate Professor Noam Vogt-Vincent, from Oxford University’s Department of Earth Sciences, argues that it may not be too late to save our coral reefs – but it is imperative that we reach net zero as soon as possible.
A coral reef in the Red Sea. Image credit: Vitalii Kalutskyi, Getty Images.
Despite covering a tiny fraction of the sea floor, coral reefs are the most biodiverse ecosystems in the ocean, home to at least a quarter of all marine species. They are also of enormous value to humans through the many ecosystem services they provide, including coastal protection, tourism and reef fisheries. Unfortunately, by the late 20th century, decades of overfishing, pollution, disease and physical damage had already reduced coral cover (the fraction of reefs covered by live coral) to 50% of its historical value, with disastrous consequences for coral reef health as a whole. Since then, this has been further compounded by climate change, most importantly through ocean warming.
Continued warming throughout the 21st century will unequivocally have severe consequences for coral reefs. A major report published last year went a step further by arguing that coral reefs may have already globally crossed a ‘tipping point’, setting in motion an irreversible transition beyond which “we will not retain warm-water reefs on our planet at any meaningful scale”. This would be devastating, but how confident should we be in this prediction?
Coral mortality has long been associated with a metric of thermal stress known as the degree heating week, which reflects both the intensity and duration of a marine heatwave. To calculate degree heating weeks, we first define a ‘stress threshold’, usually taken as the historical average temperature of the warmest month of the year. We then measure the temperature of the marine heatwave above the stress threshold and multiply that temperature anomaly by the marine heatwave’s duration.
“Under current policies, severe global coral reef decline will continue, but long-term recovery may still occur as long as the most pessimistic warming scenarios are avoided...It is not too late to save the planet’s most extraordinary ecosystem, and reaching net zero as soon as possible remains the most effective (and, in all likelihood, only) way of achieving this.”
For example, if the warmest month of the year was historically 28°C, but a marine heatwave with a temperature of 29°C lasted for four weeks, this would result in (29°C – 28°C) x four weeks = four degree heating weeks of thermal stress. While this metric is not a perfect predictor of coral mortality, it is a widely used predictor of ‘bleaching’ (a coral stress response) and mortality at large scales.
Marine heatwaves are becoming increasingly common and intense as the ocean warms. Assuming the relationship between degree heating weeks and coral mortality holds, scientists can predict the impact of continued warming on coral reefs by projecting the intensity and frequency of future marine heatwaves using climate models. Unfortunately, even if we limit warming to 2°C relative to the pre-industrial climate (which is unlikely), climate models show that coral reefs will be experiencing at least eight DHW of thermal stress on a near-annual basis by the end of the century. Coral reefs can recover from marine heatwaves, but this may change if severe bleaching and associated mortality becomes a regular occurrence. This is why some scientists have argued that coral reefs may have already passed a point of no return.
Coral reefs are in crisis, but it might not be too late
However, remember that degree heating weeks measure the severity of marine heatwaves relative to a fixed historical stress threshold. By calculating future thermal stress relative to a static benchmark, we assume that corals have no ability to adapt to a warmer ocean. In fact, corals do have some ability to adapt to environmental change.
Corals are already surviving levels of thermal stress that caused serious mortality half a century ago, and experimental evidence suggests that there is significant scope for further adaptation. Changes in coral community composition are likely and the geological process of reef accretion will be severely impacted. Yet, at least in the Indo-Pacific (the situation in the Atlantic is unfortunately more dire), reef-building corals will likely persist and continue to provide many important ecosystem functions. The geological record shows that coral biodiversity can persist through severe environmental change, even if reef formation temporarily grinds to a halt.
Using ecological models that incorporate processes such as evolution, we recently suggested that coral reefs could begin recovering in the second half of the century if we limit global warming to 2°C, the goal of the Paris Climate Agreement. Other independent studies have reached similar conclusions. We are not on track to limit warming to 2°C but, in stark contrast to suggestions that we have already passed a catastrophic global tipping point, evidence from fieldwork, experiments and state-of-the-art modelling suggests that hope is not yet lost, although every fraction of a degree matters. Under current policies, severe global coral reef decline will continue, but long-term recovery may still occur as long as the most pessimistic warming scenarios are avoided.
“Using ecological models that incorporate processes such as evolution, we recently suggested that coral reefs could begin recovering in the second half of the century if we limit global warming to 2°C, the goal of the Paris Climate Agreement. Other independent studies have reached similar conclusions. ”
It is not too late to save the planet’s most extraordinary ecosystem, and reaching net zero as soon as possible remains the most effective (and, in all likelihood, only) way of achieving this. There are further practical steps we can take to give corals the best chance possible in the challenging century ahead. New technologies such as assisted evolution have shown some potential to support coral reef conservation, but major challenges remain in scaling up selective breeding programmes. Geoengineering strategies such as solar radiation management could greatly reduce future thermal stress for coral reefs, although this remains an environmental and political minefield for other reasons.
At a local level, minimising compound stressors from human activity can improve the capacity of coral reefs to recover from disturbances. For example, herbivorous fish play a vital role in keeping macroalgae under control in coral reefs, and fishing gear restrictions have therefore been associated with greater coral reef resilience. The same study also found that coral reefs benefit from reduced land-sea impacts such as wastewater pollution and nutrient input. Local interventions such as transitioning away from cesspools and rethinking land use to minimise runoff from urban and agricultural areas may therefore have positive impacts on adjacent coral reef ecosystems.
However, while these local strategies are complementary to rapid emissions reductions, they are not, and never will be, a substitute. The science is very clear: the only sustainable solution to the coral reef crisis is net zero. And the faster we can get there, the better chance we have of saving our coral reefs.
The paper ‘Global coral reef collapse is not inevitable’ has been published in Trends in Ecology & Evolution.
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