Scientists have a new and ominous way of quantifying the devastating effect of climate change on the world's coral reefs.
In a study published Thursday in the journal Science, an international team of researchers looked at decades worth of data from 100 coral reefs that span the globe from the Indian Ocean and across the Pacific to the Caribbean Sea. Specifically, the team focused on so-called "bleaching" events, when ocean temperatures spike and exceed the heat threshold of the organisms that build the reefs.
As recently as the early 1980s, the data show that such events were rare, typically occurring about once every 25 to 30 years. The long intervals allowed time for living corals to recolonize reefs that were killed off by bleaching.
Now, as recurring heat waves drive up ocean temperature in different regions of the globe, the characteristic time between bleaching events has shrunk to less than once every six years, the team found.
"Bleaching is now occurring so frequently the coral reefs do not have sufficient time to recover between successive bleaching episodes," said Julia Baum, a marine ecologist at the University of Victoria and the only Canadian member of the study team.
If unchecked, the frequent bleaching portends a major loss of coral reefs sooner than might be expected from the steady but gradual increase in worldwide ocean temperature caused by climate change.
Although reefs can span hundreds of kilometres, they depend on a symbiotic relationship between small coral polyps, which build the elaborate, mineralized structures that reefs are made of, and algae, which provide the polyps with food.
When polyps are overcome by heat they expel the algae, leaving behind only the white-coloured structures of corals that are no longer living. It is this transformation that gives rise to bleaching.
In terms of biodiversity, coral reefs are home to tens of thousands of species, making them the underwater equivalents of tropical rainforests. Scientists warn that when corals can no longer survive, the world will lose much of the lush, underwater ecosystems that the reefs support.
"The corals are ecosystem engineers … and it's these engineers, the very foundation of the reef ecosystem, that are most susceptible to climate change," Dr. Baum said.
She added that the new study should be regarded as a call to action and that the need to address the climate threat to coral is both immediate and serious. Some coral researchers have said that in addition to pressing for more action on climate change, policy makers need to buy time for reefs by swiftly reducing other stresses such as pollution, and enhancing protections for refuge areas where corals have a chance of holding out longest as temperatures warm.
"Now that the scope of the problem is in full view, we need … to pivot to finding science-based solutions in an all-hands-on-deck, all-of-the-above approach," said Kim Cobb, an expert in the long-term climate history of coral reefs at the Georgia Institute of Technology who was not involved in the study.
"Reducing greenhouse-gas emissions is a necessary, but not sufficient, part of the solution, given the level of damage that is currently under way," she said.