Corals used to frequent temperature swings are more likely to survive the warming of the world's oceans, and may therefore be a good focus for conservation efforts as the global climate changes, new research suggests.

Higher water temperatures often cause corals to lose their colour and die, a phenomenon known as coral bleaching. These die-offs are expected to increase as surface ocean temperatures in the tropics are forecast to rise one to three degrees  by the end of the century, say the Australian, Canadian and U.S. scientists who conducted the study.


Higher water temperatures often cause corals to lose their colour and die, a phenomenon known as coral bleaching. (Ove Hoegh-Guldberg/Centre for Marine Studies/The University of Queensland)

However, some corals may be able to survive this temperature shift. During extended heat waves in 2004 and 2009, corals called Porites in the Republic of Kirabati in the Pacific Ocean were more likely to survive if they were in areas that normally experienced frequent temperature changes due to the effects of El Nino, the temperature anomaly that forms in the Pacific Ocean every three to seven years.

That was something that researchers suspected might be the case, but they didn't have the historical data to prove it until the study published recently in the journal PLoS One.

University of British Columbia researcher Simon Donner, who co-authored the paper, said the group was able to get information about the corals'  growth over time by drilling cores into them.

"It's kind of like drilling into a tree and studying the tree rings," he told CBC's Quirks & Quarks, in an interview that will air Saturday.

The goal of the study was to predict what coral reefs might look like during a warmer future and to help guide which reefs to protect, Donner said.

Coral reefs consist of animals called corals that build the reefs and symbiotic algae that give the reefs their colour and produce food that is shared with the corals. When the water gets too warm, the algae begin to produce oxygen free radicals, chemicals that are poisonous to the corals.

"So the coral either consumes or kicks out the algae, basically," Donner said.

Without its food source, the coral then starves to death.

Corals reefs can tolerate some bleaching, Donner added.

"What we're worried about is bleaching events happening so frequently that the reef just can't grow back."

Donner said the researchers' next step is to try and figure out exactly what makes the heat-tolerant corals better able to survive. They think it partly has to do with the corals' physiology.

Corals can also develop a relationship with more heat tolerant algae, but that may make them less productive, Donner said.

There are other costs associated with heat tolerance in coral, he added — such corals tend to be less physically complex and don't host the same diversity of fish and reef as less heat-tolerant corals.

"The bias in conservation is often to protect the most pristine, undisturbed places," he said. "But our results are starting to say that that may be wrong-headed when you're thinking about climate change. Because it might be the tough and sometimes the uninteresting reefs that are the ones mostly likely to survive."