Nearly all of the world's shallow-water coral reefs are now classified as critically endangered and face a high risk of collapse within the next 50 years, according to the first global assessment of these ecosystems by the IUCN Red List of Ecosystems, a scientific risk framework run by the International Union for Conservation of Nature. The report, published on Thursday (22), divided reefs into 127 tropical and subtropical ecoregions and found that 96% of them, or 122 regions, sit at the most severe risk level. They include Australia's Great Barrier Reef, the Coral Triangle in the western Pacific, the Mesoamerican Reef in the Caribbean and the reefs of Florida in the United States.

More than 250 experts took part in the work, which identified heat stress driven by climate change as the main threat. Excessive heat causes bleaching, a process in which corals expel the symbiotic algae that supply most of their food; if the stress persists, the coral falls ill and dies. Without the additional warming projected from greenhouse gas emissions, mainly from burning oil, gas and coal, the share of critically endangered reef ecosystems would be 24%, according to The New York Times reporting carried by Folha de S.Paulo. One region, the Western Galapagos Islands, is already considered collapsed.

Under the IUCN definition, collapse means the near-total or complete loss of live hard corals, to the point where the reef can no longer sustain itself. The tropical and subtropical belt assessed includes the reef ecosystems of the Brazilian coast, from the reef at the mouth of the Amazon River to the Abrolhos Bank off Bahia state. The classification is based on the currently estimated climate trajectory, and the researchers stress that the reefs are not yet lost.

What is at stake

Shallow reefs cover less than 1% of the ocean floor but provide habitat for about a quarter of marine species at some point in their life cycle. Hundreds of millions of people depend on them for subsistence fishing and coastal protection. "If coral reefs collapse, we lose one of the most extraordinary ecosystems on Earth and we risk the food, the livelihoods and the lifeline of coastal communities around the world", Sarah Bedolfe, a senior marine scientist at the NGO Oceana who was not part of the assessment, told Mongabay.

What can still be done

For Marcos Valderrábano, programme manager of the Red List of Ecosystems at IUCN, the biggest lever is cutting emissions and demanding urgent climate action from governments. He says it is also necessary to improve the overall health of corals so they can recover after heat waves: reduce pollution on reefs, protect remaining corals and fight the overfishing of species such as parrotfish and grouper, which makes reef recovery harder. "None of this replaces emissions cuts, and we should not pretend that it does", he told Mongabay.