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Coral reefs may be more capable of adapting to hotter and more acidic oceans than we thought, raising hopes that efforts to slash our greenhouse gas emissions and restore reefs stand a chance of preserving these biodiverse ecosystems.

鈥淭hese results provide us with a possible pathway forward, but it depends entirely on what we decide to do about climate change and what we decide to do about local stressors,鈥� says at the University of Hawai鈥檌 at M膩noa.

Rising levels of carbon dioxide in the atmosphere due to our emissions pose a twin threat to the world鈥檚 coral reefs. Higher temperatures stress corals and this can lead to mass bleaching and death. In addition, atmospheric CO2 is absorbed by the ocean, making the water more acidic聽and weakening coral skeletons. This, along with pollution and overfishing, has already created a crisis for the planet鈥檚 reefs, with extreme heat over the past two years spurring mass bleaching events around the world.

Current models of how reefs will respond to further climate change project an even grimmer future. 鈥淎lmost all warm-water coral reefs are projected to suffer significant losses of area and local extinctions, even if global warming is limited to 1.5潞C,鈥� according to the Intergovernmental Panel on Climate Change, a prediction it labels a 鈥渉igh confidence鈥� one.

However, these models may not capture the extent and speed at which coral species could adapt, says Jury. 鈥淲e haven鈥檛 had the data as to what is realistic.鈥�

To find out more, Jury and his colleagues collected dozens of specimens of eight different coral species from reefs around O鈥檃hu, Hawai鈥檌. They were then grown in an outdoor lab under four different conditions: under today鈥檚 climate, one with temperatures raised by 2掳C but water chemistry unchanged, another with temperatures held but water chemistry made more acidic, and the last with both temperature and water chemistry changed to produce the warmer, more acidic conditions expected in 2075 under a high-emission scenario. 鈥淲e hit them pretty hard,鈥� says Jury.

The exposure lasted nearly six months, far longer than most coral stress tests, but 鈥渢he corals did surprisingly well鈥�, says Jury. All saw lower growth rates 鈥� a measure of coral health 鈥� under the more stressful conditions. Yet individuals of all species survived the combined stressors, with no clear trade-off between tolerance to heat and acidity.

The researchers also compared genetic data of tolerant and more sensitive corals to estimate what proportion of these traits could be passed to future generations, finding between one-quarter and one-half of tolerance was heritable. Based on this, they estimate these coral species have the potential to adapt to between 1 and 1.7掳C of warming over the next 50 years, a rate that would be in line with Paris Agreement emissions targets.

at Newcastle University, UK, who wasn鈥檛 involved with the study, says this is in line with her team鈥檚 modelling based on yet-to-be-published heat-stress experiments. 鈥淚f we do get to the targets, corals have a chance to survive. Which is great news,鈥� she says. However, she says the small number of corals tested still leaves the true potential for adaptation uncertain. 鈥淒oing this type of analysis with corals is extremely challenging.鈥�

Nonetheless, there is building evidence of a significant potential to adapt to changing oceans. For instance, in a second study published this month, at Southern Cross University in Australia and her colleagues analysed heat tolerance of a coral species on the Great Barrier Reef. They found unexpectedly large variation in heat tolerance, even among corals in the same part of the reef. This could be especially valuable for efforts to restore reefs by breeding more tolerant corals, says Naugle.

鈥淚鈥檓 cautiously optimistic,鈥� says at the University of Hawai鈥檌 at M膩noa, who wasn鈥檛 involved with any of the research. But she cautions that any adaptive potential still depends on how much coral is left after extreme events. 鈥淚n a lot of areas around the world, there鈥檚 little coral cover left.鈥�