What caused the world’s largest die-off of mangroves? A wobble in the moon’s orbit is partly to blame

2015 Gulf of Carpentaria mangrove die-off, from space [image credit: NASA]

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2015 Gulf of Carpentaria mangrove die-off, from space [image credit: NASA]

Even the type of local tides was involved. Researchers conclude: we can chalk the 2015 mass death up to “natural causes.”
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Over the summer of 2015, 40 million mangroves died of thirst, says Phys.org.

This vast die-off—the world’s largest ever recorded—killed off rich mangrove forests along fully 1,000 kilometers of coastline on Australia’s Gulf of Carpentaria.

The question is, why? Last month, scientists found a culprit: a strong El Niño event, which led to a temporary fall in sea level.

That left mangroves, which rely on tides covering their roots, high and dry during an unusually dry early monsoon season.

Case closed. Or is it?

While evidence clearly implicates El Niño, we found this climate cycle had a very large accomplice: the moon.

In our study, published in Science Advances 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 had an effect.

Our mapping also shows mangroves are expanding and their canopy thickening across the entire continent, which is most likely due to higher carbon dioxide levels. Spectacular though it was, the Gulf of Carpentaria mangrove dieback event was entirely natural.

What clues gave away the moon’s role?

During El Niño cycles like the one in 2015, sea levels fall around Australia and other countries in the western Pacific.

But these climate cycles affect the whole Indo-Australian region. If El Niño was the main cause, mangroves elsewhere should have been hit too. But the deaths of these tidal-flat dwelling shrubs and trees were largely localized to the Gulf of Carpentaria.

Death rates were highest along shorelines that experience the full range of the tide. By contrast, mangroves continued to thrive at the tidal limits of the estuaries, far into the floodplains where climatic effects ought to be most strongly felt.

That’s where the moon comes in—and particularly the “lunar wobble.” Back in 1728, astronomers noticed the plane in which the moon orbits Earth isn’t fixed. Instead, it wobbles up and down, a bit like a spinning coin as it begins to slow.

When we mapped the extent and distribution of Australian 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 mangrove canopy expands and contracts around most of Australia’s coastlines—and explains the patterns of mangrove mortality in the Gulf of Carpentaria.

You might be wondering why the wobble has such influence over whether mangroves live or die. It’s the tides. The wobble changes how the moon’s gravity pulls on the world’s oceans, so periods of exceptionally high tides are followed by exceptionally low tides 9.3 years later.
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We now know short-term natural climate cycles like El Niño likely cannot cause widespread mangrove deaths by themselves. And we can anticipate the danger times when it coincides with the low tides brought by the lunar wobble.

While mangroves still face an uncertain future adapting to a world of higher seas, we can chalk the 2015 mass death up to “natural causes.”

Full article here.