A large dead Acropora coral colony is surrounded by more stress-tolerant species on a Christmas Island reef. Thermal stress affects some coral species more than others. (Image credit: Kim Cobb)

This year’s El Niño event is shaping up to be one of the strongest in recorded history, and its effects are making themselves apparent across the globe in the form of droughts, floods and changes in local weather patterns. A less talked-about consequence of warm ocean temperatures, however, is their effect on coral. Warm water can lead to widespread coral bleaching, and scientists are observing just this effect around one of the world’s most ecologically unique islands.

Christmas Island, also known as Kiritimati, belongs to the collection of atolls forming the Pacific nation of Kiribati and is the largest atoll in the world. (It’s not to be confused with the other Christmas Island, found in the Indian Ocean, which is most famous for its huge population of red crabs). It’s home to a large and diverse array of corals, which scientists say are now suffering the extreme effects of this year’s El Niño event, which are particularly strong in the area of the Pacific — close to the equator — where Christmas Island lies.

But while the pervasive bleaching going on in the area is worrying, scientists say the current conditions at Christmas Island offer a unique opportunity to learn about the way different types of corals respond to environmental stressors, which could lead to better conservation strategies for corals in a warming world.

A team of researchers from the Georgia Institute of Technology, led by professor Kim Cobb, spent two weeks on Christmas Island in early November conducting photographic surveys of the corals there and installing devices to capture data on environmental conditions, such as temperature. While there, they discovered that 50 to 90 percent of the corals they observed had bleached, thanks to the warm-water effects of El Niño. Even more concerning, up to 30 percent were already dead at some sites.

This response was likely caused by the extreme increases in water temperature observed with this El Niño event, Cobb said. While water temperatures normally hover around 81 degrees Fahrenheit, she and her team measured temperatures up to 88 degrees during their expedition.

“The aggravating factors have been that these water temperatures have been a lot warmer than average for a very extended time, stretching back earlier into 2014,” Cobb said, noting that scientists thought for a time that there would be an El Niño event last year. “This has become a very extended warm period for this reef.”

Corals are living animals themselves, but they thrive by maintaining a symbiotic relationship with certain species of algae, who live inside the corals (incidentally giving them their brilliant colors) and provide them with energy and nutrients. But under certain types of environmental stress, including extreme water temperatures, corals will expel their algae, turning white, or “bleaching,” in the process.

Bleaching doesn’t necessarily spell a death sentence for the coral — if conditions return to normal, they can recapture their algae and eventually recover. If conditions become too extreme or too prolonged, however, the coral can die, which is what is starting to happen around Christmas Island.

Luckily, there’s still hope for recovery after the El Niño event ends, Cobb said. But she warned that it could take the reef up to a decade to get back to normal — and even then, scientists predict climate change will continue to ramp up ocean warming and acidification in the coming years, which is certainly “adding pressure,” she said.

Notably, different types of coral seem to respond to environmental stress in different ways, with some species handling it better than others. This effect has already been observed at Christmas Island, said Cobb. “It’s…important to stress that some sites showed relatively minimal impact of this thermal stress, while other sites at the same island have had this devastating effect,” she said.

Scientists aren’t totally sure why some corals seem hardier than others, but they do have some theories. One major idea is that different types of symbiotic algae handle stress better than others, said Julia Baum, an assistant professor of biology at the University of Victoria. It also seems likely that coral already stressed from other factors, such as pollution, are more susceptible to environmental changes like heat stress, she said.

Baum and Cobb have both spent extensive time conducting research on Christmas Island, and see a learning opportunity in the El Niño-related events going on there. They plan to team up for an expedition in March, along with other marine biologists and ecologists, that will conduct a more detailed survey in the hopes of better understanding which types of coral are the most resistant to stress and why.

Christmas Island may be one of the best places in the world for this kind of research, thanks to several key characteristics. First, it’s home to a diverse collection of coral species, which are already showing differing responses to the El Niño event. Second, corals in different areas of the atoll are subject to differing levels of human pressures, such as heavy fishing and pollution, making it convenient to study the ways different stressors compound each other and affect a coral’s ability to stand up to heat stress.

“We have 40 different sites around the entire atoll that basically span an unparalleled gradient in human disturbance, from the most degraded to most pristine,” Baum said. “You can go from site to site and basically see how coral reefs fall apart.”

The March expedition, which will include a diverse interdisciplinary team, according to Cobb, will include detailed measurements on temperature, salinity and other environmental factors. The scientists will also take samples from the corals to analyze in the lab in order to figure out what types of algae live inside them and how heat tolerant they might be. The scientists have already tagged certain corals and taken measurements on them during previous expeditions, so they’ve been able to follow the progress of certain colonies over time.

The insights from the Christmas Island expedition could give conservationists a huge leg up when it comes to protecting corals all over the globe in the future, especially in the context of a warming world.

“I think we need to be strategic about where we’re implementing marine protected areas,” Baum said. “We could use the knowledge that we gain from this stress event to actually think about where are coral reefs likely to survive and thrive over the next 50 years of climate change and ensure that we really strongly protect those coral reefs.”

“And especially if we actually can provide definitive proof that minimizing background stressors helps corals survive these events, that provides a lot of ammunition for protecting coral,” she added.

It’s classic scientific pragmatism in the face of an otherwise worrying situation. Christmas Island, among the world’s other reefs, will only be subject to increasing environmental stress in the future — this year’s El Niño event is a kind of window into what the future may be like for the planet’s marine ecosystems. Nonetheless, the circumstances provide a clear opportunity for the kind of work that might make that future easier.

“In the coral reef world, the situation can seem dire a lot of the time,” Baum said. “And I think that in trying to convey that to the public, what’s happened is that we sell out this very negative situation — which it is, and it’s scary — but I think what we have the opportunity to do here is actually focus on the bright spots.”