The Great Barrier Reef is one of the planet’s natural gems, stretching over 2,300km along northeastern Australia.
But as well as being a bucket list favorite and a huge mass of biodiversity on 3,000 individual reefs, the World Heritage-listed body is at the heart of the climate crisis.
Yet this week a report on the amount of coral across the reef showed the highest level in 36 years of monitoring in the northern and central parts.
But that doesn’t mean the crisis is over.
Ecosystems are hit with multiple threats and disruptions, and for the reef, these include invasions of voracious coral-eating starfish, pollution that drains from land, and destructive cyclones.
The overwhelming threat is global warming, which has bleached corals 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 corals – an important metric because their skeletons are what build reef structure.
The increase in coral cover is due to fast-growing acropora corals which are also the most susceptible to thermal stress and are favored by coral-eating starfish.
Resilience against threats
Conditions in recent years have been relatively benign, with few cyclones, low starfish counts and two summers dominated by the La Niña weather pattern, which generally means cooler conditions.
But earlier this year saw the first massive coral bleaching in a La Niña year – an event that has shocked and surprised marine scientists who expect these cooler years to give corals a clear run. to recover. Global warming now means that even La Niña years are not safe for corals. The inevitable arrival of a warmer phase of El Niño worries many people.
The first-ever mass bleaching took place in 1998, followed by events in 2002, 2016, 2017, 2020 and 2022. A study found that only 2% of all reefs have escaped bleaching since 1998.
For the latest Aims monitoring report, about half of the reefs were visited before this summer’s bleaching. While bleaching was widespread, Aims said the heat probably wasn’t high enough to kill many corals instantly.
Depending on the severity of heat stress, corals may survive or die. If corals stay too long in warmer water than usual, they lose the algae that give them their color and most of their food.
This means coral starvation, so events have sublethal effects on growth rate, reproductive capacity, and disease susceptibility.
Reef scientists speak of reef resilience – the ability to bounce back from disturbances.
“There is no doubt that this is good news,” says Dr David Wachenfeld, chief scientist at the Great Barrier Reef Marine Park Authority.
“But we would be in big trouble if in 2022, at 1.1°C global warming, the reef had already lost that resilience. We would have no chance of keeping the reef in good condition.
“According to last year [UN climate assessment], we are going to be 1.5 C warming over the next decade. This is an extremely confronting forecast. For a heat-sensitive ecosystem like the reef, that’s a lot and only a decade or so left. »
A global warming of 1.5°C is seen as a safeguard for reefs, after which bleaching happens too quickly for strong recovery.
“We are on a trajectory to exceed 1.5C and reach 2.6C or 2.7C. So the resilience we see at 1.1°C will not continue,” says Wachenfeld.
Dr Mike Emslie, who leads Aims monitoring, says the increase in coral cover was expected, given the relatively benign conditions, but four bleaching episodes in seven years was uncharted territory.
“We’ve dodged a few bullets over the past two years and while this recovery is great, the forecast is for the disruption to get worse,” he says.
In some conservative media, the survey has been used to argue that the reef is not under threat. “Opponents can stick their heads in the sand all they want, but the frequency of disruption is becoming a gangbuster,” Emslie says.
Wachenfeld points out that scientists never said the reef was dead. “The scientists sounded the alarm, not a dirge,” he says. “The idea that scientists have misled people is nonsense.”
He compares the resilience of the reef to a rubber band that can be stretched many times, but only until it breaks.
“It’s hard to predict when that will happen, but it’s kind of like that with the reef,” he says. “We have a limited time to slow down and stop the warming. There’s no way this resilience can last forever.