Half of the Great Barrier Reef's coral has been wiped out in the last 27 years, a new study says.

If the mass die-off continues, the study warns, less than 25 percent of the coral cover would exist in 2022.

Published Tuesday in the Proceedings of the National Academy of Sciences, the study was conducted by researchers from the Australian Institute of Marine Science and the University of Wollongong.

The Great Barrier Reef is the world's largest coral reef, stretching more 1,615 miles off Australia.

"Coral cover is the simplest index of reef health and the health of the Great Barrier Reef has gone down dramatically," institute senior scientist Hugh Sweatman told The Sydney Morning Herald.

"The coral provides shelter and food for thousands of organisms so you don't just lose the corals themselves you lose the species that depend on them," Sweatman said.

Two-thirds of the coral loss has occurred since 1998.

The researchers said tropical cyclones were responsible for 48 percent of the coral disappearance and outbreaks of the crown of thorns starfish -- a natural predator of coral -- accounted for 42 percent of the die-off.

"We can't stop the storms but perhaps we can stop the starfish. If we can, then the reef will have more opportunity to adapt to the challenges of rising sea temperatures and ocean acidification," John Gunn, chief executive officer of the institute said in a statement.

Bleaching, which was responsible for 10 percent of the coral loss, is attributed to rising atmospheric greenhouse gases, the authors say.

Flood waters carrying fertilizers and other agricultural runoff are thought to boost the survival of crown of thorns larvae, exacerbating the coral die-off.

"About half of what's put on never makes it to the crop, and if it doesn't make it to the crop, it makes it to the reef," Nick Heath of the World Wildlife Fund told the Australian Broadcasting Corp. "So it's very sad that we're wasting this precious resource that costs farmers a lot of money, but then it goes out to the reef and is turning the reef into rubble as well."

While the Great Barrier Reef Marine Park Authority has a program called Reef Guardian Farmers which recognizes farmers who don't have significant run-off from their farms, Heath says more federal government funding is needed to ramp up conservation efforts.

Noting that 60,000 jobs are "dependent on a healthy reef, it's worth it for us to put more in to save the reef," Heath said.