The Great Barrier Reef, the world’s largest living structure, is home to more than 400 species of hard corals, 1,500 species of fish, and dozens of other marine species. The vibrant underwater community is larger than Italy, spanning more than 340,000 square kilometers (130,000 square miles).

But all of these accolades are now at risk. In a new study, scientists found that the Great Barrier Reef has lost 50% of its corals in the last 25 years, primarily because of climate change.

Since the mid-1990s, virtually all coral populations along the Great Barrier Reef have declined, according to the study’s researchers from the ARC Centre of Excellence for Coral Reef Studies in Queensland, Australia. 

Larger species, such as branching and table-shaped corals, have been worst affected, nearly disappearing from the far northern areas of the reef. 

“They’re typically depleted by (up to) 80% or 90% compared to 25 years ago,” the study’s co-author Terry Hughes told AFP. “They make the nooks and crannies that fish and other creatures depend on, so losing big three-dimensional corals changes the broader ecosystem.”

The decline in the reef’s health is largely due to changes in ocean temperatures that stress healthy corals. Warmer temperatures cause corals to expel algae from their tissues, which leads to bleaching events. Without algae, the coral loses its characteristically vibrant colors, and also its major source of food, making it more susceptible to disease and death.

Reefs are capable of surviving bleaching events — that is, if the damage is not too great and if they are afforded sufficient time to recover. 

But because climate change has made such events more severe and more frequent, corals are finding it increasingly difficult to recover and maintain the reef’s incredible biodiversity.

It takes about 10 years for a “half-decent recovery for the fastest-growing species,” Hughes said. In the past five years, the Great Barrier Reef has suffered three mass bleaching events.

“No one climate event will kill the Great Barrier Reef, but each successive event creates more damage,” David Wachenfeld, chief scientist at the Great Barrier Reef Marine Park Authority, told the Guardian Australia. “Its resilience is not limitless and we need the strongest possible action on climate change.”

Mass bleaching was first seen on the Great Barrier Reef in 1998, which was the hottest year on record at the time. Since then, the reef has experienced five more mass bleaching events — in 2002, 2006, 2016, 2017, and 2020 — each one caused by unusually warm sea surface temperatures during the summer season.

The most recent event, which took place in the beginning of 2020, was also the most widespread ever witnessed. It caused severe bleaching in one-quarter of the Great Barrier Reef, according to aerial surveys of more than 1,000 individual reefs conducted in late March.

“My greatest fear is that people will lose hope for the reef,” Wachenfeld said. “People need to see these [bleaching] events not as depressing bits of news that add to other depressing bits of news. They are clear signals the Great Barrier Reef is calling for urgent help and for us to do everything we can.”

The survival of the world’s coral reefs, which host a greater variety of species than anywhere else in the world, is key to the health of the entire planet. Already, the world is suffering from a drastic loss of biodiversity, with animal populations declining by nearly 70% as a direct result of human activities. 

Humans, themselves, are not exempt from the suffering as a result of this loss, as biodiversity sustains people’s lives by providing water, air, and food.

In order to prevent coral reefs and the species they support from annihilation, scientists agree that countries must meet their Paris agreement commitments and keep global warming under 2 degrees Celsius above pre-industrial levels.