New research has found that a “wobble” in the moon’s orbit likely contributed to the mass death of trees in northern Australia.
According to a study published in the journal Science Advances, an 18.61-year cycle, known as the lunar nodal cycle, likely caused the mass death of mangroves.
The moon’s orbit around the earth does not occur on a flat plane. It was compared to a spinning coin by the study’s lead author, Professor Neil Santilan.
He said like a coin that turns when it loses momentum, the moon “wobbles a little”.
Changes in gravitational pull following the moon’s wobble affect tides and, as this study suggests, mangrove growth in Australia.
Scientists used historical satellite imaging to quantify the extent of mangrove coverage across Australia between 1987 and 2020. The wobble in canopy coverage was “immediately obvious when graphing the data,” Saintilan said.
Depending on the phase of the lunar nodal cycle, there can be “up to 40 cm of difference in the tidal range” in places like the Gulf of Carpentaria in northern Australia, Saintilan said.
Along the Arnhem coast in the Northern Territory and the Carnarvon coast in Western Australia, the researchers found that the peaks in the closed canopy cover – where thickened canopy of mangroves covered more than 80% of the soil area – it coincided with the peak phases of the moon’s oscillating tides.
They believe the lunar wobble likely contributed to the mass death of mangroves in the Gulf of Carpentaria in 2015-16, an event in which about 40 meters of trees died. At that time, a “low tide” phase of the lunar wobble coincided with a severe El Niño.
“They had a combination of a 40 cm drop in mean sea level associated with El Niño and, in addition, a 40 cm drop in the tidal range. [due to the lunar wobble]”Said Mr Saintilan.
“There were mangroves in the streams [previously] being flooded every day it could have only been flooded a handful of times throughout the dry season.
A quirk of the lunar wobble is that it has opposite tidal effects along coasts that have one high tide per day compared to those that have two high tides per day.
“So far, global warming has been good for mangroves. With higher sea levels they have expanded into areas where they couldn’t survive before, “Saintilan said.” But with high rates of sea level rise [greater than 7mm a year] … We know they cannot survive for long ”.