A wobble in the moon’s orbit around Earth affects mangrove cover in Australia and likely contributed to the mass die-off of trees in the Gulf of Carpentaria, new research suggests.
A study published in the journal Progress of science 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 the Earth does not occur in a flat plane. “Since the 1720s, people have known to move up and down a few degrees,” said the study’s lead author, Professor Neil Saintilan of Macquarie University. He likened the motion to “when you’re spinning a coin, as it loses momentum, it wobbles.”
Changes in gravitational pull as a result of this lunar wobble are known to affect Earth’s tides. Previous investigation by NASA scientists has predicted that by the mid-2030s, the lunar wobble will amplify sea level rise caused by climate change, leading to high-tide flooding along coastlines.
Depending on the phase of the lunar nodal cycle, there can be “up to 40 cm difference in tidal range” in places like 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, the mangroves are flooded less frequently. “When they’re stressed, because they’re losing water through their leaves, they just drop their leaves.”
The scientists used historical satellite imagery to quantify the extent of mangrove cover in Australia each year between 1987 and 2020. The oscillation in canopy cover was “immediately obvious when plotting 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 the thickened mangrove canopy covered more than 80% of the ground area , coincided with the maximum tidal phases of the wobble of the moon.
They believe that the lunar wobble likely contributed to the mass die-off of mangroves in the Gulf of Carpentaria in 2015-16, an event in which an estimated 40 million trees died. At that time, a “low tide range” phase of the lunar wobble coincided with a severe El Niño.
“They had a combination of a 40cm drop in mean sea level associated with El Niño and, on top of that, a 40cm drop in tidal range. [due to the lunar wobble]Saintilan said. “There were mangroves in the streams [previously] being inundated every day that they might have been inundated only a handful of times in the entire dry season.”
One peculiarity of the lunar wobble is that it has opposite tidal effects along coastlines that have one daily high tide compared to those that have two daily high tides.
In a region with only one daily high tide, one phase of the lunar cycle can result in a lower tidal range and less frequent flooding. The same phase will have the reverse effect along the coasts with two daily high tides, resulting in more mangrove flooding.
The Gulf of Carpentaria is one of the few Australian coastlines to have a daily high tide. Mangroves in adjacent regions that survived the 2015-16 El Niño were in a “high tidal range” phase of the lunar cycle. El Niño was previously thought to be the cause of the massive dieback, but “the nodal cycle also appears to be a necessary condition for mangrove mortality,” Saintilan said.
“Until now, global warming has been good for mangroves. With higher sea levels, they have been expanding into areas where they couldn’t survive before,” she said. “But under high rates of sea level rise [greater than 7mm a year] … we know they cannot survive for long.”
Dr. Brad Tucker, an astrophysicist at the Australian National University, who was not involved in the study, compared the lunar wobble to the vertical rocking of an object in water. “It does this rocking up and down every 18.6 years,” he said. “If the moon is higher or lower relative to Earth, that will 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 on its perigee and apogee: how close and how far away it is.”
These gravitational effects were independent of the moon’s brightness phase, Tucker said.