It seems that the way the Moon moves in its orbit affects mangrove tree cover across Australia and new research suggests the it likely contributed to mass tree deaths in the Gulf of Carpentaria.
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.
If you want to know more about the Lunar Nodal Cycle, you’ll find a range of scientific papers here.
The Moon’s orbit is inclined by about 5 degrees to the Earth’s equator so, as the Earth rotates, the Moon will be above or below the Earth’s equatorial plane at different times.
Astronomers have known about this, the Moon’s inclination, since the 1720s.
If you’re having a hard time visualizing it, think about when you spin a coin and its at that point where it’s near the end of the spin and the coin is rotating on its rim – it’s just kind of wobbling as the spin winds down (loses momentum).
The changes in the Moon’s gravitational pull on the Earth affect the tides.
How big a given tide will be is based on where The Moon is in its orbit and how far away from Earth it is.
Research conducted by NASA 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 (15.75 inches) of difference in the tide range in places such as the Gulf of Carpentaria.
Mangroves grow between the average high-tide level and the highest high-tide levels.
But at lower tidal ranges, mangroves are inundated by water less frequently and when they’re stressed, because they lose water through their leaves, they end up dropping their leaves.
Scientists used historical satellite imaging to determine the extent of mangrove cover across Australia every year between 1987 and 2020.
They found that the oscillation in canopy cover was immediately obvious when the data was put into you graphs.
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 40 million trees died.
At the time, a “low tidal range” phase of the lunar wobble coincided with a severe El Niño.
There was a combination of a 40cm (15.75 inch) drop in the mean sea level associated with the El Niño and, on top of that, a 40cm (15.75 inch) drop in tide range [due to the lunar wobble].
There were mangroves in creeks that had previously been 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 a 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 the death of mangrove trees.
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 but, under high rates of sea level rise [greater than 7mm a year], 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 are independent of the phase of the moon.
The Moon’s wobble is also known as Lunar Libration, which is a combination of all the effects of the Moon’s orbit around Earth.
Due to this, we are actually able to see about 59% of the Moon’s surface over time rather than the 50% you might expect.
You can see the effect of lunar libration in this video:
The effect is different from day to day so animations like this over a month will also be different from month to month.
And because no two locations get the same view of the Moon since the Earth and Moon are moving all the time, the libration (and resulting animations) will be slightly different for each observer.
