How the Moon's wobble might have murdered 40 million mangrove trees

Werewolves really hit the mainstream after the release of The Wolf Man, but they’ve been around since at least the Epic of Gilgamesh and remain popular today. Stories of humans turning deadly under the glowing gaze of the full Moon mirror the real world behaviors of countless animals who turn to the Moon for guidance to drive their behaviors. In stories, the Moon drives people to madness, but in the real world the Moon is getting its own hands dirty. At least, those are the findings from a recent study published in the journal Science Advances.

Between 2015 and 2016, the mangrove forest in Australia’s Gulf of Carpentaria suffered considerable losses. Roughly 18,000 acres comprised of 40 million mangrove trees turned brown, dropped their leaves, and seemingly died. The dieback was laid at the feet of El Niño, which had moved through the area at the time, lowering water levels and drying up large swathes of mangrove forest.

For Neil Saintilan from the School of Natural Sciences at Macquarie University, however, something didn’t add up. To Saintilan’s mind, the effects of El Niño weren’t enough to explain away the level of dieback the mangroves had experienced. Moreover, certain regions were heavily impacted while others were doing just fine. It wasn’t what we might have expected if the dieback were the result of El Niño’s sweeping hand. Saintilan believed there was something else at work.

“I felt there had to be more going on because El Niño is a continental scale and even broader phenomenon, and it was a fairly local dieback event. Coincident with that, we had the release of a really unique data set pulled together by geoscientists in Australia. It showed annual mapping of mangrove canopy cover for the whole of the Australian continent in really high resolution, going back about 40 years,” Saintilan told SYFY WIRE.

When scientists looked at the canopy cover data going back for decades, they found a regular oscillation through two complete cycles. The mangroves would grow and increase their territory followed by a retreat. Then the cycle repeated itself. When Saintilan saw that, he turned his eyes to the sky.

“Whenever you see something as predictable as that in nature, you tend to think it’s orbital. Rising and setting of the Sun, the planets, the tides, anything that’s fairly regular probably has some kind of orbital link,” Saintilan said.

Around the time Saintilan got to thinking about this problem, NASA published a paper looking at the way the Moon wobbles over an 18.6 year cycle and how that might impact tides in the future as sea levels continue to rise. During one half of this decades-spanning cycle the tides are suppressed. In the other half, they are amplified. Saintilan had that study in the back of his mind as he was looking at the trends in mangrove productivity and the two pieces clicked together.

“We think about daily tides and monthly tides, but it also happens over much longer time periods and the 18.6 year cycle is probably the most important of those. When we lined it up, it was pretty much 18.6 years. We had a regular pattern spaced out between 18 and 19 years, in the density of the crown cover,” Saintilan said.

It was this lunar wobble cycle which turned out to be the missing piece. El Niño conspired to lower sea levels across the continent and the Moon’s position within the 18.6 year cycle made water levels even worse. Those two factors together were enough to result in the largest mangrove dieback event in recorded history. A werewolf might kill a handful of people or animals during its transformation, but when the Moon hits just right, it can kill millions. Millions of trees, that is.

The question then became why the Gulf of Carpentaria was so heavily affected while other regions were fine. Saintilan explained that came down largely to where the mangroves were located and how the tides were impacted in different places.

Coastlines are broken up into two groups with relation to their tides. Diurnal coastlines have one tide a day while semidiurnal coastlines have two tides. The effect of the lunar wobble is opposite, depending on the type of coastline and its associated tides.

“At the point where you have really big tides in semidiurnal coastlines, you have smaller tides in diurnal coastlines. The Gulf of Carpentaria is pretty much the only coastline in Australia that has big mangrove areas and is diurnal,” Saintilan said.

That meant that even while El Niño was in full swing, many mangrove forests in Australia were doing just fine. It’s estimated that El Niño lowered the water levels by about 40 centimeters and the lunar wobble lowered the level an additional 40 centimeters along the diurnal coastline. In the game of tides, the Gulf of Carpentaria rolled snake eyes.

It’s this combination of factors which pushed the mangroves beyond their limits. In other areas and at other times, dieback of mangrove forests is normal. Parts of the forest drop their leaves and wait out the hard times. Then, when the waters return, they come back stronger than ever. In the Gulf of Carpentaria, however, the one-two punch was too much to handle.

“Generally speaking, you have dieback but not mortality and the mangroves recover within a year. The only place you saw widespread mortality was the Gulf of Carpentaria. Approximately 6% of the entire mangrove area actually died and still hasn’t come back,” Saintilan said.

Luckily for the mangroves, this combination of factors is only expected to happen once or twice in a century and the overall conditions for mangroves are favorable. Tucked into the data was a strong climate change signal driving an increase in mangrove production over time. Sea level rise and elevated carbon dioxide concentrations are both things that mangroves like, and they’re likely to thrive as climate change progresses.

Humans aren’t quite so lucky. That same NASA paper looking at the lunar wobble cycle predicts widespread flooding around 2030 when rising sea levels line up with amplified tides. First the Moon came for the mangroves… next it’s coming for us.