Oscillation in the Moon’s orbit against the world’s largest mangrove die-off
During the summer of 2015, 40 million mangroves thirsty. This vast mass mortality – the largest ever recorded in the world – killed off rich mangrove forests along 1,000 kilometers of coastline in Australia’s Gulf of Carpentaria.
The question is, why? Last month, scientists found a culprit: a strong El Niño event, which caused a temporary drop in sea levels. This left the mangroves, which depend on tides covering their roots, high and dry during an unusually dry early monsoon season.
Case closed. Where is it? While the evidence clearly implicates El Niño, we discovered that this climate cycle had a very big accomplice: the Moon.
In our studypublished today, we mapped the expansion and contraction of mangrove forest cover over the past 40 years and found clear evidence that the Moon’s orbital wobble was having an effect.
Our mapping also shows that mangroves are expanding and their canopy thickening across the continent, which is most likely due to higher levels of carbon dioxide. Dramatic as it was, the decline of the Gulf of Carpentaria mangroves was entirely natural.
What clues revealed the role of the Moon?
During El Niño cycles like 2015, sea level fall around Australia and other Western Pacific countries.
But these climatic cycles affect the entire Indo-Australian region. If El Niño was the main cause, the mangroves elsewhere should also have been affected. But the death of these shrubs and trees living at low tide has been largely localized in the Gulf of Carpentaria. Mortality rates were highest along coasts that experience the full range of the tide. In contrast, mangroves have continued to thrive at the tidal limits of estuaries, far into the floodplains where climatic effects are expected to be felt most strongly.
This is where the Moon comes in – and in particular the “lunar wobble”. In 1728, astronomers noticed that the plane in which the Moon orbits the Earth is not fixed. Instead, it wobbles up and down, much like a spinning coin as it begins to slow down.
When we mapped the extent and distribution of Australia’s mangrove forests over the past 40 years, we found clear signs of the Moon’s wobble at work. This 18.6-year orbital cycle turns out to be the main reason why the mangrove canopy expands and contracts around most Australian coastlines – and explains the patterns of mangrove mortality in the Gulf of Carpentaria.
You might be wondering why the wobble has such an influence on the life or death of mangroves. It’s the tides. The wobble changes the way the Moon’s gravity pulls on the world’s oceans, so that periods of exceptionally high tides are followed by exceptionally low tides 9.3 years later.
Research by NASA scientists suggests that this cycle is likely to drive to major coastal flooding in the early 2030s, as extremely high tides meet accelerated sea level rise.
The lunar-mangrove cycle is clearly visible from above. When we mapped changes in the dense mangrove forest of northwest and western Australia, we observed sharp peaks in the closed canopy – where the leaves and branches of the mangrove thicken to cover more than 80% of the ground – coinciding with the highest tidal phase of the lunar cycle.
When the tides are at their highest, the water floods the mangroves and brings nutrients that accelerate growth. These periods potentially influence the amount of blue carbon stored by mangroves over thousands of square kilometres.
But when the tides are at their lowest, the mangroves cannot get the water they need. In 2015-2016, the lunar oscillation reduced the tidal range in the Gulf of Carpentaria – enough to reduce the tides by about 40 cm. Earlier mangrove dieback events in 1998 and 1982 also coincided with these lows.
In 2015, tides along Australia’s northern coast fell further under the influence of El Niño, which moves seawater eastward from the Pacific. The result of the overlapping of the lunar and climatic cycles in the Gulf of Carpentaria was the massive death of the mangroves.
One of the challenges we had was distinguishing between the effects of El Niño and the lunar oscillation, given that they tend to occur during the same period in the Western Pacific. Some scientists have even suggested the lunar oscillation may contribute to intense El Niño events.
To disentangle the two causes, we relied on an oddity in the lunar wobble – and an oddity in the coastline.
The timing of the lunar oscillation of high and low tide periods is reversed between coasts with two high tides each day (semi-diurnal tides) and those receiving a high tide each day (diurnal tides).
The Gulf of Carpentaria is one of the few coastlines in Australia with daytime tides. Most of the other coasts have two high tides each day. Together, this meant that in 2015, semidiurnal coasts had larger than usual tides, while rare diurnal coasts like those along the Gulf had smaller than usual tides.
This explains why the mangroves of the semi-diurnal coasts directly bordering the Gulf of Carpentaria were spared during the summer of 2015-16.
The northern coasts adjacent to the Gulf were in the high tide, high productivity phase of the 18.6-year cycle and were therefore protected from El Niño. In the diurnal Gulf of Carpentaria, the small tidal phase of the lunar oscillation cycle combined with El Niño. Falling sea levels and falling tidal range have pushed the mangroves overboard.
Interestingly, mangroves continued to grow near the tidal head of the Gulf rivers despite El Niño, as the effect of the lunar oscillation was less pronounced upstream.
This is good news for mangroves. We now know that short-term natural climate cycles like El Niño alone probably cannot cause widespread mangrove death. And we can anticipate times of danger when it coincides with the low tides brought by the lunar oscillation.
While mangroves still face an uncertain future to adapt to a world of higher seas, we can attribute the mass death of 2015 to “natural causes”.
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