In the summer of 2015, some 18,000 acres of lush mangrove forest turned brown and perished in northern Australia’s Gulf of Carpentaria. With 40 million dead trees, this event became the largest mangrove die-off in recorded history.
Scientists thought they knew the reason. At that time, a massive El Niño event had led to a drop in sea level, depriving mangrove roots of water. But a new study published this month in Science Advances suggests another culprit: the moon.
Researchers have known about a phenomenon called the moon wobble since 1728. When the moon orbits Earth, it doesn’t revolve on a flat plane. Instead, the orbit is tilted, and it moves along a path like the one a spinning coin follows before falling flat. This results in an 18.6-year cycle. For half of that time, the moon suppresses the tides, and during the rest, it elevates them.
The scientists examined satellite images from 1987 to 2020 and realized that mangroves experience a growth and dieback cycle on the same time scale as the moon's wobble. They traced how the tree cover expanded and contracted over time in correlation with this lunar effect.
“When we looked in detail at the timing of the peaks and troughs of the lunar cycle, it matched perfectly with changes in mangrove canopy cover—one of those 'Eureka!' moments you get a few times in your career,” lead author Neil Saintilan, a professor of environmental sciences at Macquarie University in Australia, tells Science Alert’s Clare Watson.
During periods of amplified tides, mangroves flourished. But in periods where the tide was suppressed, they began to die.
“We found clear signs of the moon’s wobble at work,” writes Saintilan in The Conversation. “Over 2015-2016, the lunar wobble reduced tide range in the Gulf of Carpentaria—enough to slash tides by an estimated 40 cm. Earlier mangrove dieback events in 1998 and 1982 also coincided with these troughs.”
El Niño certainly contributed to the tree die-off, but the weather phenomenon alone couldn’t explain why mangroves in non-tidal regions stayed alive that year. This helped point the team toward looking at the moon’s impact.
But Norman Duke, an ecologist from James Cook University in Australia who led a previous study linking mangrove die-offs to El Niño, says he's not convinced the moon wobble is the main reason for mangrove changes, writes Genelle Weule of the Australian Broadcasting Corporation (ABC).
“Undoubtedly, tides are right there, but they're not the only thing,” Duke tells the publication. “My hunch is that it is El Niño, with rainfall and tide influences after that.”
Proportionally, mangroves store a huge amount of carbon—more, even, than tropical rainforests—by sequestering it away in their soil and trunks. Because of that, these trees play a significant role in fighting climate change, and scientists say it’s important to understand what factors affect how they’ll fare.
Octavio Aburto-Oropeza, a marine ecologist at the Scripps Institution of Oceanography who was not involved in the research, tells Katherine Kornei of Science News that understanding the moon’s effect on mangroves is vital.
He adds that this knowledge could help guide proactive policies during periods of higher tides and more mangrove growth. Limiting harmful human activities near the trees at those times could give the crucial carbon-sequestering plants a better chance to thrive, he tells the publication.