In 2015, there were 2 billion corals spanning more than 1,400 miles throughout the Great Barrier Reef.

Two years and two major heat waves later, that population nearly halved, sending the planet’s largest living structure into a death spiral that it will likely never recover from, according to a new paper published in Nature.

“You could say [the ecosystem] has collapsed. You could say it has degraded,” Terry Hughes, an author of the paper, told the Atlantic. “I wouldn’t say that’s wrong.”

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“A more neutral way of putting it is that it has transformed into a completely new system that looks differently, and behaves differently, and functions differently, than how it was three years ago,” he added.

Coral is highly sensitive to temperature increases and a 2 or 3 degree Celsius shift can trigger massive bleaching events that take more than a decade to recover from, according to the New York Times.

If reefs are hit with successive bleaching events, when corals expel color- and food-giving algae under extreme stress and turn a bone-white, then the damage sustained can be irrevocable.

And that’s what seems to be happening around the world as ocean temperatures rise from climate change.

Read More: 5 Coral Reefs That Are Dying Around the World

The consequences are especially stark throughout the Great Barrier Reef.

In 2016, a powerful El Niño sharply raised ocean temperatures, killing more than half of the coral in the northern section of the reef. Then, a heat wave in 2017 catastrophically harmed the middle section, according to the authors of the report.

Overall, the reef sustained irreversible damage, according to the Times.

“We’re at a point where we’ve lost close to half of the corals in shallow-water habitats across the northern two-thirds of the Great Barrier Reef due to back-to-back bleaching over two consecutive years,” said Sean Connolly, also with the center for coral reef studies at James Cook University.

And now heat-resistant strains of coral are taking over, potentially disrupting the dynamics that have fostered tens of thousands of animal and plant species and changing the vivacity that made the reef a visual marvel, the Atlantic reports.

“The most likely scenario is that coral reefs throughout the tropics will continue to degrade over the current century until climate change stabilizes, allowing remnant populations to reorganize into novel, heat-tolerant reef assemblages,” write the authors of the paper.

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The report says that coral reefs could be partly protected if action is taken to mitigate climate change by upholding the Paris Climate Agreement, which seeks to prevent the global temperature from rising more than 2 degrees Celsius from pre-industrial levels.

“We’re in uncharted territory,” Hughes told the Times. “Where we end up depends completely on how well or how badly we deal with climate change.”

Coral reefs are also threatened by industrial pollution, invasive species, overfishing, plastic pollution, and ocean acidification, which is when the oceans absorb too much carbon dioxide from fossil fuel emissions and acidify.

Losing coral reefs would have a profound impact on the global environment and human society, according to UNESCO.

Reefs foster vast ecosystems, providing food, shelter, and breeding grounds for thousands of marine creatures.

They provide natural buffers for coastal communities, preventing waves from destroying shorelines. Meanwhile, the many types of fish swimming throughout their lattices, offer a ready supply of food for people.

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