Catastrophic climate change has upended the ecology of the Great Barrier Reef and there’s no going back, according to a groundbreaking study published on Wednesday in the journal Nature.
The coral bleaching event of March 2016, triggered by prolonged high ocean temperatures, dealt such a shock to the reef that millions of corals died almost immediately, researchers found. Altogether, a third of the Great Barrier Reef’s corals eventually succumbed, dying at lower heat levels than expected; especially hard hit was the 700km (435 mile)-long northern section of the world’s largest reef system. Staghorn corals, whose complex branching structures provide habitat to a variety of marine life, declined by a staggering 75 percent within eight months. Scientists say the destruction is already permanently transforming the Great Barrier Reef ecosystem, a process accelerated by an unprecedented second bleaching event that struck in February 2017 and killed 20 percent of corals in the middle section of the 2,300km-long reef. In the space of little more than a year, the Great Barrier lost half its corals.
“The die-off of corals drove a radical shift in the composition and functional traits of coral assemblages on hundreds of individual reefs, transforming large swaths of the Great Barrier Reef from mature and diverse assemblages to a highly altered, degraded system,” the researchers wrote about the 2016 bleaching event. “The abrupt, regional-scale shift in coral assemblages has also radically reduced the abundance and diversity of species traits that facilitate key ecological functions.”
Said Terry Hughes, the study’s lead author: “The northern Great Barrier Reef will never look quite the same again.
“The longer-term transition that we have been predicting now for many years as a consequence of global warming is well under way,” added Hughes, the director of the Australian Research Council Centre of Excellence for Coral Reef Studies at James Cook University in Townsville, Australia. “The take-home message is that climate change is not some future threat to coral reefs. It’s here and now. It’s been happening for a few decades and we’re well on the way to a very different-looking set of species on our reefs.”
That a pristine and remote reef system which is protected from fishing and pollution pressures suffered such a catastrophic die-off of corals underscores the threat of increasingly frequent bleaching events that do not allow a sufficient window for recovery. “It’s only a matter of time before we get a severe bleaching event in the southern third of the reef,” Hughes said.
C. Mark Eakin, a study coauthor and coordinator of Coral Reef Watch at the National Oceanic and Atmospheric Administration in the United States, said the conditions researchers discovered in the wake of the Great Barrier Reef bleaching events are likely to be replicated around the world as ocean temperatures continue to rise.
“What’s most important from an ecological perspective is that climate change is continuing to have huge impacts on coral reefs around the world,” he said. “If we don’t deal with the impact of climate change, we won’t see diverse coral reefs in the future. One of the big things we saw likely to happen elsewhere is this homogenization of coral ecosystems.”
In research published earlier this month, scientists discovered a loss of diversity among fish populations on 16 reef sites in one section of the Great Barrier Reef in the wake of the 2016 bleaching event.
For the new study, Hughes and his colleagues conducted aerial surveys of 1,156 individual reefs in the Great Barrier Reef system in March and April 2016, measuring the extent and severity of the bleaching as it unfolded. They confirmed the flyover findings with underwater surveys at 104 reefs. The researchers assessed mortality rates for different coral species during underwater surveys of 63 reefs. In October and November 2016, eight months after the bleaching event, the scientists returned to those 63 reefs to measure coral abundance at the same sites previously surveyed. They then used satellite data to calculate what are called “degree heating weeks” – a measurement of the intensity and duration of corals’ exposure to heat stress that was developed by Coral Reef Watch.
The scientists discovered that the northern section of the reef lost half of its coral cover between March and November 2016. Over the entire Great Barrier Reef, coral cover declined by 30 percent during that time.
Two findings surprised and unnerved the researchers.
First was the sudden death of corals exposed to the highest degree heating weeks. The existing scientific understanding was that bleached corals – which depend on symbiotic zooxanthellae algae for nutrition and their bright colors – die slowly. As the ocean warms, the heat makes the algae toxic to corals, which expel the organisms they shelter. Deprived of their colorful partners, corals turn bone-white and will starve and die unless water temperatures cool and the zooxanthellae return.
But in March 2016, as ocean temperatures in the northern section of the reef hit record highs, half of the corals killed during the bleaching event died almost immediately. “The heat exposure was so extreme they actually cooked and died very quickly,” said Hughes.
Second, scientists were able to establish for the first time the heat intensity thresholds at which corals bleach and die. “So, 2 degree heating weeks started the bleaching at a low level; by the time you get to 4 degree heating weeks, half the corals were bleached and they start to die; and by time you get to 6, we start to see a real transformation in the mix of coral species,” said Hughes. “Before the study we didn’t know where those tipping points were. They’re quite surprisingly low. That is useful to be able to predict future changes and how a given level of heat exposure is likely to have an impact on the corals.”
It can take staghorn corals at least a decade to fully recover; other coral species need more time, according to the study. The die-off of staghorn corals in the 2016 bleaching event affected 29 percent of the 3,863 reefs that comprise the Great Barrier Reef system. The reef is unlikely to avoid additional bleaching events before damaged coals can recover.
“On some of those northern reefs, depending on the heat exposure, we saw 60, 70, 80, even 90 percent mortality, which is obviously pretty extreme and we were surprised by the scale of it,” said Hughes. “To lose one in three corals in just nine months is an incredible shock to the system. The Great Barrier Reef is the world’s largest coral reef system and for two-thirds of it to be damaged in two years is heartbreaking really.”
Still, 1 billion corals have survived. Echoing other scientists, Hughes said the only hope of saving the Great Barrier Reef and other coral ecosystems from extinction is to achieve the Paris climate agreement target of holding global temperature rise to 1.5–2C (2.7–3.6F) by slashing fossil fuel emissions.
“I think if we can achieve that or close to it, we’ll still have a reef but it won’t look the same as the reef looked two years ago,” he said.