Australia's Great Barrier Reef will never be the same following the devastating marine heat wave that hit it between 2015 and 2016, according to a new study published Wednesday.

The new research found that the northern third of the reef -- which as a whole, is the largest living structure on the planet -- experienced a "catastrophic die-off" of fast-growing coral species, like staghorn and tabular corals.

These reefs have now shifted to a new state, with a different balance of coral species than were present prior to the marine heat wave. Scientists have tied that marine heat wave itself, and the increasing prevalence and severity of them, to human-caused global warming.

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The study, published in the journal Nature, shows that many coral species that comprise the Great Barrier Reef succumbed to ocean temperatures that were well above average. However, those corals died in water temperatures that scientists previously thought would still sustain the organisms, not kill them.

This raises the possibility that corals are more sensitive to ocean warming than previously thought, adding even more evidence that if global warming were to exceed about 2 degrees Celsius, or 3.6 degrees Fahrenheit, above pre-industrial levels, many coral reef ecosystems would cease to exist as we know them today.

These findings about the collapse of coral ecosystems could inform future decisions of whether to list unique ecosystems such as the Great Barrier Reef as "threatened" World Heritage Sites, something the Australian government has opposed for fear that it would hurt tourism.

The research team used satellites to map the pattern of heat exposure across the 3,863 coral reefs that make up the overall Great Barrier Reef.

According to the study, 30 percent of corals on the Great Barrier Reef died within just a nine-month period in 2016, as water temperatures exceeded a particular heat threshold.

Most of these losses occurred in the northern 434-mile section of the reef, which lost more than two-thirds of their corals, calling into question their ability to function as unique ecosystems.

The different colour morphs of Acropora millepora, each exhibiting a bleaching response during the mass coral bleaching event.Credit: ARC Centre of Excellence for Coral Reef Studies/Gergely Torda

"Our study shows that the transition of the GBR to a new system is already underway, due to global warming," said Terry Hughes, director of the ARC Center of Excellence for Coral Reef Studies in Australia, in an email. "It’s here and now, and it’s happening faster than we expected."

Reefs that were exposed to the warmest waters of this marine heat wave, which was tied to both a strong El Niño event in the tropical Pacific Ocean and human-caused global warming, suffered "an unprecedented ecological collapse," the study found, with species composition changing drastically, reducing the diversity of species present after the assault from the warmer than average seas.

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"Our study shows that coral reefs are already shifting radically in response to unprecedented heatwaves," Hughes said in a statement. He said focusing on protecting the more heat-tolerant coral species is key to ensuring the survival of the Great Barrier Reef.

Some scientists have even suggested using "assisted migration," or importing more heat tolerant species, to foster more resilient coral communities in places that suffer from coral bleaching-related mortality.

“Bleaching is not like a steamroller that just kills everything… there are winners and losers both between and within species,” said Mikhail Matz, who studies how corals adapt to climate change at the genetic level and was not involved in the new study.

Matz says the corals that remain after a major marine heat wave such as the one in 2015-16 might be genetically adapted to be more heat tolerant.

"We expect that if genetics works as we think it works then the next generation will be more heat tolerant, because this is natural selection going on," he said.

Although they don't look like it, corals are actually living animals, and they receive vital nutrients from symbiotic algae that live within them, providing them with their vibrant colors. When exposed to stress from high temperatures, corals can expel the algae, which causes the coral to expose its skeleton. These bleached coral are more susceptible to continued high ocean temperatures as well as damage from pollution and other threats.

While bleached coral can recover, a prolonged period of high temperatures can kill corals outright.

This is what happened during the longest global coral bleaching event on record, which lasted from 2014 to 2017, but was particularly pronounced in the northern third of the Great Barrier Reef during much of 2015 and 2016.

There are ‘winners’ and ‘losers’ among corals as they respond to the accumulating impacts of climate change.Credit: ARC Centre of Excellence for Coral Reef Studies/Mia Hoogenboom

Study co-author Mark Eakin, who directs a coral bleaching prediction program at the U.S. National Oceanic and Atmospheric Administration (NOAA), said part of the reason for the high rates of mortality in the northern Great Barrier Reef is that these areas had not been previously exposed to many bleaching events, and the less heat tolerant species succumbed almost immediately.

"You’re losing some of those more sensitive species," he said in an interview. "But what that also means is you’re losing a lot of diversity.”

The study paints a grim prognosis for the parts of the Great Barrier Reef hit hardest by the heat wave. The reef will likely lose some of the marine diversity that makes it such a valuable ecosystem.

However, that doesn't mean that all of the heat-sensitive coral will completely disappear.

Instead, picture a future in which heat resistant coral species dominate such reefs, playing host to a smaller number and variety of fish and other aquatic species. Such heat-tolerant corals may grow more slowly, since the more abundant, fast-growing species are less tolerant to heat stress.

The study concludes that the transition "has already begun on the northern, most-pristine region of the Great Barrier Reef, changing it forever as the intensity of global warming continues to escalate."

“The good news is that you’ll still have reefs, but they definitely won’t be as good of reefs as we have now,” Eakin said.

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