Reefs are losing recovery time between bleaching events, experts say

© Keith A. Ellenbogen

The world’s coral reefs are experiencing bleaching events caused by climate change more frequently than previously observed, according to new research, potentially leaving too little time for their ecosystems to recover.

In a sweeping study published in the journal Science, researchers looked at mass bleaching events across 100 reefs from 1980 to 2016. Scientists found that the average time between bleaching events is now less than half of what it was just decades ago, meaning less recovery time for reefs to bounce back in the long run.

Making reef recovery even more difficult: stress on the reefs caused by markedly warmer surface temperatures. According to the new study, tropical sea surface temperatures are warmer now during La Niña conditions, traditionally cooler oceanic periods, than they were during warm-water El Niño events three decades ago.

The finding points to a climate departure of sorts for the ocean, a point at which even current low-temperature cycles are warmer than historic high-temperature cycles.

According to Jack Kittinger, senior director of the Global Fisheries and Aquaculture program at CI, the study represents “a tour-de-force analysis on bleaching stress for coral reef ecosystems.”

Most corals derive the bulk of their energy from photosynthetic algae living symbiotic within their tissues. The algae, in turn, benefit from the protection offered by the coral’s structure and the carbon-dioxide and nitrogen waste they produce.

Bleaching occurs when corals expel the algae due to high temperature or other environmental stresses, causing the corals to lose the characteristic bright hues imparted by the algae. Not all bleaching events are caused by warm water, but it’s a major and growing cause. Because bleaching events have been relatively rare in the past, corals have traditionally had a good track record of recovery following a bleaching event. But the increased frequency of bleaching caused by climate change has thrown into question previous assumptions of resiliency.

As the geographic footprint of recurrent bleaching spreads, fewer coral reefs remain untouched. Only six of the 100 locations that the scientists examined have escaped severe bleaching, and this number is expected to decline.

“If I’m being realistic here, we will certainly lose reefs as we know them in most places,” said Kittinger. “What will emerge from this mass extinction of reefs will be unique new ecosystems the world has never seen before, composed of new groups of species that are able to tolerate our hot world.”

“Will these systems provide the same benefits? It’s doubtful.”

Morgan Lynch is a staff writer for Conservation International.

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