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Coral Bleaching Is the New Normal

A true representation of the Anthropocene.
Image: Stop Adani/Flickr

For those of us living in the Anthropocene, evidence of climate change slaps us in the face on a daily basis (like snow literally hitting me in the face last week during the blizzard). But coral reefs are often far from our minds.

Corals come in a wide range of colors for your viewing pleasure, and this color comes from the algae that live within their tissues in a mutually beneficial, or symbiotic, relationship. The algae gives corals energy, and in return the corals give shelter and nutrients to the algae. When corals get stressed out, though, like when it’s just too damn hot in the water, they expel the algae in a phenomenon called coral bleaching, because the lack of algae turns the corals white. Corals can live for a time without the algae, but they will eventually begin to starve and die.


Coral reefs offer far more to the human race than colorful dives and awesome fish. “Food security, shoreline protection and livelihoods from reef tourism provide the social and economic motivation for saving reefs,” Terry Hughes, the lead author of a paper on coral bleaching that was published last week in Science and one of the world’s leading researchers on coral bleaching, told me in an email.

In the paper published yesterday, “Spatial and temporal patterns of mass bleaching of corals in the Anthropocene,” a group of researchers including Hughes investigated the global frequency of severe coral bleaching events. Severe bleaching is when more than 30 percent of corals are bleached in an area of tens to hundreds of kilometers. As Hughes, one of the world’s leading researchers on coral bleaching, said, “Coral bleaching is the new normal.”

The researchers compiled a four-decade history of recurrent bleaching from 100 globally-distributed coral reef locations in 54 countries, examining timing, recurrence, and intensity of bleaching episodes. They began with 1980 because, according to coral growth bands, mass bleaching did not occur before this date, Hughes said.

The researchers found that, since 1980, the frequency of severe coral bleaching events has increased from once every 25 to 30 years to once every 5.9 years. They also found that the risk of such events has increased by 4 percent every year since 1980. This makes sense, considering their finding that the global temperature conditions for bleaching are becoming more prevalent according to climate modeling.


Figure 3 from Hughes et al. shows the global extent of coral bleaching from 2015 to 2016, with red circles representing severe bleaching (>30% of corals), orange moderate (<30% of corals), and blue no substantial bleaching.

This is the first global study tracking the trend in coral bleaching over the past four decades, Hughes told me. It comes at an opportune time in the wake of the 2015-2016 bleaching event, that impacted 75 percent of the reefs examined in the study.

“The evidence for global warming and its impacts on people and ecosystems is undeniable,” Hughes said.

The number of coral bleaching events are clearly increasing in concordance with the global temperature rise, despite geographic variation. Hughes said only halting climate change will permanently end the problem of global coral bleaching.

“Local action can help speed up recovery, and are important,” Hughes wrote. “But the future of coral reefs hinges on rapid action on greenhouse gas emissions.