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Coral reef refuges could soon be completely lost

Coral reefs all over the world are threatened by the warming oceans associated with climate change. Management of these iconic ecosystems often involves protecting climate refugia where cooler upwelling conditions or strong ocean currents help to maintain thermal safe havens for coral reef organisms, even when surrounding areas become inhospitable. 

Predictions about the future of coral reefs have repeatedly stressed the importance of protecting these “thermal refugia,” but climate change models are often too coarse to capture the smaller-scale oceanographic features that characterize these areas. In addition, it has not been clear whether these safe havens would actually persist as Earth continues to warm. 

With this in mind, Adele Dixon of the University of Leeds, UK, and colleagues from the US and Australia, made use of the most up-to-date global climate projections from the Coupled Model Intercomparison Project (CMIP6) to investigate the projected changes to coral reef refuges at an unprecedented resolution.

Using a technique known as statistical downscaling, the team combined CMIP6 projections with one-kilometer resolution satellite data on daily sea surface temperatures around the world to identify the locations of refugia in the future.

The researchers found that, whereas about 84 percent of the world’s coral reefs currently lie within a thermal refuge, this will decrease to a mere 0.2 percent if temperatures rise 1.5°C above pre-industrial levels. 

The model also predicts that, if temperatures rise by 2°C, none of the current refuges will be suitable for coral reef ecosystems any more. Very small areas of refugia may remain, particularly in the East Indian Ocean Sumatra-Java region, but only rarely, suggesting that coral reef ecosystems are likely to suffer catastrophic or total losses.

This analysis, published today in the journal PLOS Climate, highlights different effects in different regions. For instance, thermal refugia are predicted to disappear first from higher-latitude reefs. At 1.5°C of warming, thermal refuges will disappear from all but two of the twelve regions worldwide: Polynesia and the Coral Triangle in the western Pacific Ocean. Refugia will be lost from regions such as Oman, the Caribbean, Colombia, and Indonesia.

Although reef communities show various levels of tolerance to heat stress, most of them take a decade or more to recover from a warming event. As warm events are set to increase in frequency, it is unlikely that reef communities will be able to recover from them. And with the suitability of thermal refugia being compromised as ocean waters heat up, it is probable that coral reef species will experience catastrophic extinctions.

The findings provide new information that could help guide efforts to manage reefs and help them survive. In particular, efforts that focus on protecting thermal refugia alone may become less effective as climate change progresses and refugia disappear. Instead, the authors note, multiple types of efforts will be needed to help coral adapt to higher temperatures and migrate to more favorable locations. Prioritizing the management of current thermal refugia may, however, help them act as stepping stones for migrating corals to move to more suitable habitats.

By Alison Bosman, Staff Writer

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