Safe havens for coral reefs will disappear with 2°C of global warming

Cooler regions of the sea provide safe havens for coral reefs by protecting them from warmer temperatures, but such sanctuaries will be lost if the global atmospheric temperature rises by 2°C before the end of this century.

“We’ve known that climate change will be catastrophic for coral reefs for decades. Now, with better models and a finer resolution, our work shows that corals worldwide will be even more at risk from climate change than we thought,” says Adele Dixon at the University of Leeds in the UK.

Rising sea temperatures can lead to coral bleaching, where corals expel the colourful algae (Symbiodinium) that normally live within their tissue and provide them with nutrients. This means bleached corals are more likely to die.

Regions of cool water called thermal refugia result from local weather events and help to protect corals from bleaching and death. In coastal regions, surface waters can be pushed away from the shore by winds, which pulls deeper, cooler water upwards in a process called upwelling.

This movement of cooler water means that upwelling can give reefs in shallow water temporary respite during hotter times of the year. Strong ocean currents, tides and tropical cyclones can also help to cool down reefs in between hotter periods.

Using temperature measurements from Earth observation satellites, Dixon and her colleagues found that between 1986 and 2019, 84 per cent of shallow-water reefs benefited from thermal refugia. This means they had enough time between marine heatwaves to recover, says Dixon.

The researchers used climate models to predict the fate of thermal refugia after 1.5°C of global warming above pre-industrial levels, the goal set in the 2015 Paris Agreement. With a 1.5°C rise by the end of the century, they found that just 0.2 per cent of the world’s shallow-water coral reefs would benefit from thermal refugia.

If global temperatures rise by 2°C – which we are on track to exceed by the end of this century – all thermal refugia are predicted to disappear.

While there are many ongoing attempts to conserve reefs, such as seeding reefs with more heat-tolerant corals, the findings confirm that limiting the global temperature rise is critical.

“Our results show that global emissions reduction and carbon [capture and storage] are essential to prevent large-scale reef loss,” says Dixon.

“I hope this study opens our eyes to the fact that even global warming of 1˚C is too much,” says Javid Kavousi at the University of Western Brittany in France.

Dixon adds that the study doesn’t predict how reefs will respond to the increased heat exposure, but says that will be the focus of future work. The projections also don’t account for coral reefs in deeper waters, which may respond differently to climate change.

“This study is a great advance in that it has global coverage and it uses fine-scale temperature data,” says Toni Lyn Morelli at the University of Massachusetts Amherst.

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