Ecosystems get hit with multiple threats and disturbances, and for the reef those include invasions by voracious coral-eating starfish, pollution running off from the land and destructive cyclones.
The overwhelming threat is the climate heating, which has caused corals to bleach en masse six times since 1998.
The Australian Institute of Marine Science (Aims), which runs the monitoring program, surveyed 87 reefs. The report counts hard coral – an important measure because their skeletons are what builds structure for reefs.
The increase in coral cover was thanks to a fast-growing acropora corals that are also the most susceptible to heat stress and are favoured by coral-eating starfish.
Resilience versus threats
Conditions in recent years have been relatively benign, with few cyclones, low numbers of starfish and two summers dominated by La Niña weather pattern that usually means cooler conditions.
But earlier this year was the first mass coral bleaching in a La Niña year – an event that shocked and surprised marine scientists who expect those cooler years will give corals a clear run to recover. Global heating now means even La Niña years are not safe for corals. The inevitable arrival of a warmer El Niño phase has many extremely worried.
The first ever mass bleaching was in 1998, followed by events in 2002, 2016, 2017, 2020 and 2022. One study found only 2% of all reefs have escaped bleaching since 1998.
For the most recent Aims monitoring report, about half the reefs were visited before this summer’s bleaching. While bleaching was widespread, Aims said the heat was likely not high enough to have killed many corals outright.
Depending on the severity of heat stress, corals can survive or die. If corals sit in hotter-than-usual water for too long, they lose the algae that gives them their colour and most of their food.
This means coral starvation, so the events have sub-lethal effects on the growth rate, the ability to reproduce and susceptibility to disease.
Reef scientists talk about the resilience of the reef – the ability to bounce back from disturbances.
“There’s no question this is good news,” says Dr David Wachenfeld, chief scientist at the Great Barrier Reef Marine Park Authority.
“But we would be in deep trouble if in 2022, at 1.1C of global heating, the reef had already lost that resilience. We would have no chance of keeping the reef in a healthy condition.
“According to last year’s [UN climate assessment], we are going to be at 1.5C of warming in the next decade. That’s an extremely confronting forecast. To a thermally sensitive ecosystem like the reef, that’s a lot and it’s only about a decade away.”
Global heating of 1.5C is considered a guardrail for reefs, after which the bleaching comes along too quickly for strong recovery.
“We’re on a trajectory to blast past 1.5C and get to 2.6C or 2.7C. So the resilience we see at 1.1C will not continue,” says Wachenfeld.
Unchartered territory
Dr Mike Emslie, who leads the Aims monitoring, says the rise in coral cover was expected, given the relatively benign conditions, but four bleaching events in seven years was uncharted territory.
“We have dodged a couple of bullets in the last couple of years and while this recovery is great, the predictions are the disturbances will get worse,” he says.
In some conservative media, the survey has been used to push arguments the reef is not under threat. “The naysayers can put their heads in the sand all they like, but the frequency of disturbances is going gangbusters,” says Emslie.
Wachenfeld points out that scientists have never said the reef is dead. “Scientists have been ringing an alarm bell, not a funeral dirge,” he says. “The notion scientists have been misleading people is a nonsense.”
He likens the reef’s resilience to a rubber band that can be stretched many times, but only so far before it snaps.
“It’s hard to predict when that will happen, but it’s a bit like that with the reef,” he says. “We have a limited amount of time to slow and stop the warming. There is no way this resilience can last forever.”