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Bleaching Poses a Threat to Hawaii’s Coral Reefs, Scientists Say

Image: Bleached coral

Bleached coral at Lisianski Island in the Papahanaumokuakea Marine National Monument, in 2014. The pale coral is bleached due to thermal stress, while the lavender-colored coral is healthy. Courtney Couch / NOAA - HIMB via AP

Coral reefs, already under threat around the globe, may be in particularly acute danger in the waters of Hawaii because of a phenomenon known as bleaching.

Coral makes up less than 1 percent of the underwater ecosystem yet helps to protect 25 percent of marine species, generates tourism revenue and boosts fishing, according to data from The Nature Conservatory.

Many scientists say coral bleaching, which causes a change in color and can result in the eventual death of algae, is but one of the effects of warmer ocean temperatures.

‘We have to make sure we keep reefs healthy’

The warning came from a report this month by the National Oceanic and Atmospheric Administration, which finds that a global coral bleaching event that began last year in the Pacific Ocean may be spreading to the western Atlantic Ocean—and to Hawaii.

The potential result could be widespread death of corals and an effect on the "long-term supply" of fish and shellfish, according to the report.

"We are worried," wrote NOAA's Coral Reef Watch lead scientist Mark Eakin in the report. He told CNBC in an interview that bleaching was undermining "a very important source of tourism income" in places like Hawaii.

In fact, coral bleaching could mean putting a big cash cow out to pasture. One NOAA study quantified the economic value of the state's coral reefs, putting the figure close to $34 billion.

Hawaii's "economy is so fundamentally tied to the corals," says Ruth Gates, a research professor at the Hawaii Institute of Marine Biology. Bleaching has affected as much as 60 percent of the state's coral since last fall, she said.

The news isn't entirely bleak, as experts like The Nature Conservatory say reefs are in fact pretty resilient and bleaching needn't always be a death sentence.

"What's been astonishing is that 90 percent of the coral here recovered," Gates told CNBC. Some of the bleached corals have even managed to reproduce already, she adds, attributing the recovery to the "pretty good health" the corals were in.

That said, the idea of a second bleaching event—coming so quickly on the heels of the first—is "really not going to be great for corals here," Gates says.

Since 1998, there have been only two previous global coral bleaching events, but the last time Hawaii experienced it was in 1996, Eakin said. Hawaii has held up well, but a second round of bleaching would be the first time it's ever occurred in back-to-back years, he added.

Underwater nurseries save coral reefs

"These events are coming so quickly that corals aren't getting a chance to recover," said Eakin, adding that the coral may take decades to bounce back from the one-two punch.

Coral nurseries in the Florida Keys, for example, were already hit last year by warming ocean waters, according to NOAA. Protected national marine sanctuaries, like those in California that one federal study found made more than $200 million in annual economic output from recreational fishing alone, are also at risk from this year's expected bleaching, Eakin said.

Tourism and fishing activity are themselves threats to coral reefs, according to the United Nations Environment Program, but they cite climate-related bleaching as "the largest single cause" of a 27 percent loss of coral reefs since 2000.

Threatening to spread coral bleaching are "unusually warm" and rising temperatures in the north and equatorial Pacific and western Atlantic oceans, according to NOAA.

Eakin explains that acidification -- when the ocean's chemistry is altered through its absorption of carbon dioxide from the air -- exacerbates the effect. "For coral reefs it's essential we reduce the amount of carbon dioxide in the atmosphere," he said, adding that an upcoming U.N. convention on the subject would be "hugely important" to helping arrest the phenomenon.

Researchers, however, have seen instances in which coral can bounce back from climate change and rising ocean temperatures, when that coral is not in direct contact with human activity. This means protecting the reefs from pollution, water runoff and the overfishing of certain species.

Additionally, bleached corals in polluted waters are at a higher risk of disease and death, adds the Hawaii Institute's Gates, who says it's become a point of discussion among the public and business community in Hawaii since last year.

"It gave us the opportunity," she said, "to talk about, how do you develop an island in a way that maintains your resources?"