A bleached staghorn coral sits on the Great Barrier Reef in March 2017. (Arc Centre of Excellence for Coral Reef Studies/Bette Willis)

At a Glance

A massive bleaching event in 2016 has already killed large sections of the Great Barrier Reef.Stress from the bleaching is causing the reef's resilience to wane.A fourth bleaching event is under way, which may kill off corals that are still recovering.

Large portions of Australia'sGreat Barrier Reef have died because of another year of coral bleaching, researchers say, and the only way to save the rest of it is to reduceglobal warming.

This is the second year in a row that in the vital underwater ecosystem, killing large portions of the shoal, according to a study published Thursday in the journal Nature study. The phenomenonhas also caused the reef’s resilience to wane rapidly.

"," saidTerry Hughes, lead author of the study and director ofAustralian Research Council Center of Excellence for Coral Reef Studies,in a release on the study. "That's a bit sobering. We can't stop bleaching locally. We actually have to do something about climate change."

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The scientists say global warming is the number one threat to the reef, states the release. The 2016 bleaching event reinforces the need to reduce climate change and fully implement the Reef 2050 Plan, which was designed to boost the reef’s resilience.

The plan was released by the Australian and Queensland governments in March 2015 and . It responds to the pressures facing the reef and seeks to boost its resilience to longer term threats like global warming.

Coral bleaching occurs when , according to a previous report. The corals can't cool down and find new algae fast enough, which causes them to die out and become a milky shade of white before they begin to decompose and attract turf algae.

With the use of aerial and underwater surveys of the reefs, scientists discovered that efforts to reduce pollution and overfishing had little effect on the 2016 bleaching event, according to the study.This suggests that local protection of the reefs provides little to no resistance when it comes to extreme heat and that bleaching in 1998 and 2002 did not make the corals stronger.

The team also discovered that 91 percent of the reef has been bleached at least once during the three bleaching events. Even more alarming, Hughes told the Associated Press, is that a fourth bleaching event is already underway. Corals need years to recover from bleaching, so back-to-back events increase the possibility that the bleached coral will die.

“It broke my heart to see so many corals dying on northern reefs on the Great Barrier Reef in 2016,” Hughes said. “A fourth event after only one year is a major blow to the reef."

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In addition to being a thriving underwater ecosystem, the coral reefs are crucial to surrounding communities: People rely on them for everything from food to medical research to protection from damaging coastal storms.

Given enough time, bleached coral can recover if the water cools, but if the temperature stays too high for too long, they will die.

Hughes and his team of researchers returned to the reef a year after the worst bleaching event on record with the hope that temperatures over the weeks to come will cool off quickly and that this year’s bleaching won’t be anything like 2016’s, also according to the release.

According to coral reef scientist Julia Baum, intense bleaching events are no longer isolated and are occurring more regularly.

"None of us were expecting the water to be heating up again right now," Baum said. "I think it's beyond what any of us could have imagined. It's our worst nightmare.

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