An 18.61-year cycle, known as the lunar nodal cycle, likely caused mass mangrove dieback, a study published in the Science Advances journal said.
The moon’s orbit around earth does not occur in a flat plane. It has been compared to a spinning coin by the study’s lead author Professor Neil Santilan.
He said like a spinning coin when it loses momentum, the moon “kind of wobbles”.
Changes in gravitational pull as a result of the lunar wobble affect tides and, as this study suggests, the growth of mangroves in Australia.
Scientists used historical satellite imaging to quantify the extent of mangrove cover across Australia between 1987 and 2020. The oscillation in canopy cover was “immediately obvious when you graph the data”, Mr Saintilan said.
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 in northern Australia, Mr Saintilan said.
Along the Arnhem coast in the Northern Territory and the Carnarvon coast in Western Australia, researchers found that peaks in closed canopy cover – where thickened mangrove canopy covered more than 80 per cent 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],” Mr 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.
“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,” Mr Saintilan said. “But under high rates of sea level rise [greater than 7mm a year] … we know that they can’t survive for too long.”