A wobble in the moon’s orbit around Earth affects mangrove cover across Australia and likely contributed to mass tree deaths in the Gulf of Carpentaria, new research suggests.
A study published in the journal Science Advances has found that an 18.61-year cycle known as the lunar nodal cycle shapes the condition of tidal wetlands.
The moon’s orbit around Earth does not occur in a flat plane. “Since the 1720s, people have known that it moves up and down by a few degrees,” said the study’s lead author, Prof Neil Saintilan of Macquarie University. He likened the motion to “when you’re spinning a coin – as it loses momentum, it kind of wobbles”.
Changes in gravitational pull as a result of this lunar wobble are known to affect the Earth’s tides. Previous research conducted by Nasa scientists has predicted that in the mid-2030s, the lunar wobble will amplify rising sea levels caused by climate change, resulting in high-tide floods along coastlines.
Depending on the phase of the lunar nodal cycle, there can be “as much as 40cm of difference in the tide range” in places such as the Gulf of Carpentaria, Saintilan said.
Mangroves “grow between the average high-tide level and the highest high-tide levels”, he said. At lower tidal ranges, mangroves are inundated less frequently. “When they’re stressed, because they lose water through their leaves, they just drop their leaves.”
The scientists used historical satellite imaging to quantify the extent of mangrove cover across Australia every year between 1987 and 2020. The oscillation in canopy cover was “immediately obvious when you graph the data”, Saintilan said.
Along the Arnhem coast in the Northern Territory and the Carnarvon coast in Western Australia, the researchers found that peaks in closed canopy cover – where thickened mangrove canopy covered more than 80% of ground area – coincided with the peak tidal phases of the moon’s wobble.
They believe the lunar wobble likely contributed to mass mangrove dieback in the Gulf of Carpentaria in 2015-16, an event in which an estimated 40m trees died. At the time, a “low tidal range” phase of the lunar wobble coincided with a severe El Niño.
“They had a combination of a 40cm drop in the mean sea level associated with the El Niño and, on top of that, a 40cm drop in tide range [due to the lunar wobble],” Saintilan said. “There were mangroves in creeks [previously] being inundated every day that might have been inundated just a handful of times in the whole of the dry season.”
A quirk of the lunar wobble is that it has the opposite tidal effects along coastlines which have one high tide daily compared to those that have two high tides daily.
In a region with only one daily high tide, a phase of the lunar cycle may result in a lower tidal range and less frequent water inundations. The same phase will have the inverse effect along coastlines with two daily high tides, resulting in more mangrove inundation.
The Gulf of Carpentaria is one of few Australian coastlines that has one high tide daily. Mangroves in adjacent regions that survived the 2015-16 El Niño were in a “high tidal range” phase of the lunar cycle. The El Niño was previously thought to be the cause of the mass dieback, but “the nodal cycle also seems like a necessary condition for mangrove mortality”, Saintilan said.
“So far, global warming has been good for mangroves. With higher sea levels they’ve been expanding into areas that they could not survive before,” he said. “But under high rates of sea level rise [greater than 7mm a year] … we know that they can’t survive for too long.”
Dr Brad Tucker, an astrophysicist at the Australian National University, who was not associated with the study, likened the lunar wobble to the vertical bobbing of an object in water. “It does this bobbing up and down every 18.6 years,” he said. “If the moon is further up or down in relation to Earth, that’s going to change the gravitational pull.”
Another factor affecting tidal activity on Earth is that “the moon is not a perfect circle when it orbits,” Tucker said. “It varies in its perigee and apogee – how close and far away it is.”
These gravitational effects were independent of the brightness phase of the moon, Tucker said.
