All tropical coral reefs could collapse within 50 years

Imagine a world without tropical coral reefs, a world nearly devoid of iconic species like elkhorn and pillar corals and sea creatures like the clownfish from Finding Nemo. That world could come to pass within 50 years due to humanity’s continuing fossil fuel emissions. At least 96 per cent of shallow-water coral reef ecosystems are at extremely high risk of collapsing by 2075, according to an assessment by the International Union for Conservation of Nature (IUCN), the leading authority on endangered species. The remaining ecosystems weren’t evaluated due to lack of data, but are probably just as vulnerable, according to Marcos Valderrábano at the IUCN. “All coral reef ecosystems that we assess, they are all critically endangered, and that’s really surprising,” he says. “In some regions of the world, there is a lot of bleaching, but some other regions of the world are not doing that bad. But what the projections are telling us is that this is going to be affecting all coral ecosystems in the planet in the next 50 years.” Advertisement The IUCN assessment was presented at the biannual UN biodiversity summit now taking place in Armenia. It shows countries where they need to protect reefs by reducing fishing quotas and pollution, according to Valderrábano. Scientists have previously cautioned that warm-water coral reefs are probably crossing a tipping point, but the IUCN warning demonstrates how quickly and massively reef die-offs could unfold. The shallow reefs in tropical and subtropical waters undergird ecosystems that can include thousands of species of corals, molluscs, fish and larger vertebrates like sea turtles and sharks. They cover only 1 per cent of the ocean, but support 25 per cent of marine species. Up to 1 billion people rely on coral reefs for fishing, as well as for tourism income and for protection from erosion and storms. While small pockets of coral could live on after the collapse of larger ecosystems like the Great Barrier Reef, they could become too dispersed for the eggs and sperm they release into the water to cross-fertilise. “If the population size gets so small that you don’t have another coral to mate with nearby, then reproductive failure will happen,” says James Guest at Newcastle University, UK. “That species is in what’s called an extinction vortex.” Coral reefs also exist in deeper, colder waters, including around the UK coast, but they are much less biodiverse and corals there don’t form symbiotic relationships with algae. So far, only one shallow-water coral reef ecosystem has collapsed. The reefs of the western Galapagos Islands lost 95 per cent of their coral – the IUCN’s threshold for ecosystem collapse – after the 1982/83 super El Niño climate phase unleashed a marine heatwave, turning golden brush corals into white skeletons overgrown by string algae. But since then, four mass bleaching events have devastated many other corals. These animals form a symbiotic relationship with algae, which live inside the coral’s tissue and provide it with sugar from photosynthesis. However, algae stressed by warming or polluted waters can give off damaging reactive compounds like free radicals. It is thought that corals temporarily eject the colourful algae in self-defence, leaving the coral a ghostly white. If the warming lasts long enough, the corals essentially starve to death. All these mass bleaching events came after strong El Niños, and experts fear the current record El Niño will usher in another one. Regardless, individual reefs are now bleaching nearly every year due to global warming, according to the US National Oceanic and Atmospheric Administration. Yet another mass-bleaching event could speed up coral collapse, since reef ecosystems don’t have time to recover if they are hit by more than three bleaching events within five years, says Valderrábano. “It would definitely reinforce that perverse loop,” he says. The IUCN projection is based on a “middle-of-the-road” scenario with moderate emissions reductions. But such a future isn’t necessarily a death sentence for all shallow-water reefs because scientists don’t know exactly how fast corals worldwide can adapt to warming, says Guest. His research in the Pacific archipelago of Palau suggests corals there have been developing 0.1°C of additional heat tolerance per decade. “But if we get a really severe heatwave, it’s quite likely that the increase in heat tolerance isn’t going to be enough to protect those reefs,” he says. Some scientists have called for interventions like spreading sunshades over reefs or geoengineering clouds over the ocean to protect species like coral from the record current El Niño. But the best defence for corals is reducing emissions, says Valderrábano. “The more we can do to reduce pressure on them, the better chance they have of using their natural capacity to adapt and recover,” says Guest.