coral reef

Bleached staghorn corals in the Pacific Ocean near Fiji

Helmut Corneli/Alamy

Some corals can swap out the algae that live inside their tissues for different strains that are more heat tolerant – and these coral species have a better chance of surviving global climate change in the coming decades.

When sea temperatures are too high, corals expel the microscopic algae living in their tissues. This is what occurs during coral bleaching. Losing algae in this way is harmful for the corals because the algae normally provide oxygen for them and remove their waste products. However, marine biologists have previously discovered that when some corals are exposed to warmer temperatures, they can swap the algae inside their tissues for strains that have a higher thermal tolerance.

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Cheryl Logan at California State University in Monterey Bay and her colleagues have developed a model to simulate how these corals – and other coral species – will respond to global warming and ocean acidification. They applied their model to 1925 coral reefs around the world under four different climate scenarios.

These scenarios were taken from the 2019 report by the Intergovernmental Panel on Climate Change. Each one models the effects of climate change on the planet until 2100, dependent on different levels of greenhouse gas emissions.

The researchers found that the coral species that are able to swap their algae for more heat-resistant strains are more likely to survive until 2100 by resisting bleaching. But this was only the case in scenarios in which greenhouse gas emissions are kept low and ocean warming is restricted to below 2°C.

“With a low-emissions scenario, which aligns with the targets for the Paris Agreement, we see these types of coral persisting until the end of the century,” says Logan. These corals are better able to withstand temperature increases of up to 1.5°C than the corals without this adaptation.

“The outlook of corals is generally pretty dire under climate change, but we found a glimmer of hope,” says Logan. However, the researchers found that essentially all corals would die by 2050 in the two scenarios with high greenhouse gas emissions – the associated temperature increase was too high for even heat-resistant algae to withstand.

Their model also revealed that the coral reefs could survive until 2300 if any anthropogenic change and human-induced warming of the planet were removed. “Corals are extremely important for biodiversity as they provide a place where up to a quarter of marine fishes live at some point,” says Logan.

The researchers believe this could help inform conservation efforts to focus on these special types of coral over riskier techniques. “Instead of introducing genes into coral populations to artificially increase their thermal tolerance, we could just focus on the natural adaptive ability that these corals have,” says Logan.

Journal reference: Nature Climate Change, DOI: 10.1038/s41558-021-01037-2

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