Half of the coral populations on Australia’s Great Barrier Reef — from “big mamas” to the little baby coral they spawn — have been wiped out in the warming ocean, a new study says.

Studying coral as if it were a residential demographic, and counting its abundance over 30 years starting in 1995, four Australian researchers determined that size didn’t matter when bleaching events, such as two that occurred in recent back-to-back years, strike the giant reef.

“The decline occurred in both shallow and deeper water, and across virtually all species — but especially in branching and table-shaped corals,” Terry Hughes, a professor at the ARC Center of Excellence for Coral Reef Studies in Queensland and a co-author of the research paper, said in a statement Tuesday. “These were the worst affected by record breaking temperatures that triggered mass bleaching in 2016 and 2017.”

increase on the southern slope, about 25 percent.

It’s a clear sign of rapid decline.

“We expect this decline to continue” because of warming caused by humans, Hughes said. “The only effective way to improve the outcome for coral reefs is global action on greenhouse gasses. If global temperatures rise to 3 or 4 [degrees Celsius], the reef will be unrecognizable, so there is no time to lose.”

“We have evidence from some parts of the reef that recruitment rates are only recovering very slowly … and are nowhere near levels prior to the bleaching events,” Andreas Dietzel, another professor at the ARC Center and a co-author of the paper, said in an email.

Coral that spawn the larvae that makes more coral “have declined dramatically over vast stretches of the Great Barrier Reef,” Dietzel said. “It will therefore take time for reproduction to recover. Corals are tremendously resilient because of their capacity to produce millions of babies but they/we desperately need a break from disturbances.”

The Great Barrier Reef is incredibly important to Australia. Before the coronavirus pandemic, more than 2 million tourists traveled to Queensland each year from all over the world to experience its color and biodiversity. Worldwide, reefs provide habitat for a quarter of marine animals and plants, coastal protection that limits flooding for 500 million residents, and fishing that provides protein and revenue, according to the Sydney Institute of Marine Science.

The institute and the University of Sydney recently partnered to develop an experimental technology called marine cloud brightening to offset bleaching events that kill coral. It’s a Hail Mary attempt to block sunlight and reflect it back to the sky to limit warming and higher rates of ocean acidification that cause mass mortality on the reef.

But that extreme measure, combined with other actions such as limiting reef fishing, might not be enough for a country that continues to lead the world in exporting coal, a fossil fuel that contributes to warming, which hastens the decline of coral, experts say.

“Our results show the ability of the Great Barrier Reef to recover — its resilience — is compromised compared to the past, because there are fewer babies, and fewer large breeding adults,” Dietzel said.

Bob Richmond, a research professor and director of the Kewalo Marine Laboratory at the University of Hawaii at Manoa, called the study “a really excellent piece of work,” with the most comprehensive research on Great Barrier Reef coral populations that he’s seen.

“What they’re showing is these demographic changes are occurring on a regional scale … on reef slopes that make it difficult for coral reefs to persist over time,” Richmond said. In his long experience of visiting and researching reefs around the world, “if I don’t see one-year-old, two-year-old, three-year-old corals, I know that reef is dead. It just doesn’t know it yet.”

As the population declines, the distance between coral formations increases, limiting their ability to reproduce. “These bleaching events are just hammering these reefs. This is just a really bad time and a really bad combination,” Richmond said.

Although half of the Great Barrier Reef’s loss of abundance happened over three decades, Richmond said the world doesn’t have three more decades before the rest potentially disappears.

“The problem is it’s an accelerated loss,” he said. “It’s hard to have a crystal ball and say a date. Scientists are always trying to be careful, but if we don’t act meaningfully in the next five years, we will not have vital and vibrant coral reefs as a legacy for future generations.”

There is hope in Australia and other reef formations around the world, such as in Indonesia, said Gabby Ahmadia, director of ocean science at the World Wildlife Fund. Some coral is developing a resistance to rising temperatures and even acidification.

“The reality of the situation is that coral reefs are declining around the world, but the hope is we can have better local solutions to overfishing, runoffs from land and farming practices,” Ahmadia said. “They are going to decline, no doubt, but we can conserve what’s left.”

Ahmadia looks at coral reef conservation as if it were a management portfolio, with high and low risks. Reducing pollution and fish harvests on reefs is a reliable, low-risk solution for governments to consider. At the high end are attempts to harden reefs in labs and desperate heaves such as blocking sunlight and cooling water.

“We work in Indonesia and reefs haven’t been hit as hard there,” Ahmadia said optimistically. The peril faced by coral “really varies,” she said. “There are a lot of studies predicting the future. I don’t think we know.”

But her optimism only goes so far if aggressive action isn’t taken.

“A lot of people say 90 percent of the coral loss will happen by 2050,” she said.