Updated Feb 8, 2022

Severe bleaching to hit all coral when global heating hits 1.5C

According to a new study of the world's reefs, once global heating reaches 1.5C, almost no corals on the planet will escape severe bleaching.

The research suggests that the reefs in areas that are currently regarded as cooler refuges will be overwhelmed at 1.5C of heating, and only 0.2% of reefs will escape at least one bleaching outbreak every decade.

A team of scientists from the University of Leeds, Texas Tech University and James Cook University utilised the latest climate model projections to confirm that 1.5C of global heating will be "catastrophic for coral reefs".

Corals bleach when the ocean temperatures are too high for too long, resulting in algae that provide corals with much of their food and colour to separate from the coral when under heat stresses. Severe bleaching can kill corals, but they can recover from milder outbreaks if there are several years absent from further heat waves. This heating is due mostly to the burning of fossil fuels. 

This study arrives as the Great Barrier Reef off Australia’s Queensland coast is on the verge of another mass coral bleaching event. In the study, the team analysed climate projections across all of the world's shallow-water coral reefs, that constitute the vast majority of reefs and provide habitat, tourism revenue and coastal protection. Around 84% of the world's corals exist in areas that are expected to bleach less than once a decade, and are regarded as "thermal refugia".

The analysis suggests that at 1.5C of global heating, only 0.2% of the area covered by reefs is in water cool enough to avoid bleaching at least once every five years. This frequency is considered too short to allow the corals to recover. The only area for escape, as the study claims, is in a region of the east Indian Ocean in an area of natural upwelling of cooler water. Sadly, even areas with strong currents than can usually protect the corals from the heat, such as those in Panama, Florida and the Lesser Sunda Islands in Indonesia, would be overwhelmed by the heat. 

Co-author of the study and marine scientist at James Cook University, Associate Professor Scott Heron, has said that coral reefs sustained a billion people around the world. The study showed corals worldwide were at even greater risk from climate change than previously believed, meaning it is crucial that conservation work continues on reefs, in order to give corals a fighting chance of survival.

Heron said: "Until we reach the point where I can’t see a single coral, it isn’t too late. We have to keep pursuing our best practices to protect coral reefs. Real people rely on these reefs and that is a driver to keep gathering the best knowledge possible".

Lead author of the study and a PhD student at the University of Leeds, Adele Dixon said that corals in French Polynesia and Southern Indonesia may be exposed to the heat stress less frequently and that the study has identified a small number of "hope sites". She added: "Corals in these locations, such as the eastern Pacific, may be better able to cope with temperature extremes".

Nevertheless one prominent coral reef expert challenged these findings, and doubted that the real-world outcomes would be quite as catastrophic as the research suggests.

Professor Peter Mumby, a reef expert at the University of Queensland, agreed that at 1.5C corals worldwide would be under greater stress, but "I don’t agree that this research provides evidence that corals are untenable under 1.5C of warming".

Mumby said the levels of heat stress used in the study were arguably too low, and the method unlikely to capture local conditions that could protect some reefs. He concluded that even reefs hit by bleaching every five years could include areas of coral that would survive.