The celestial bodies’ complex, corkscrewing orbital paths through the cosmos are far from fixed, despite how they appear from Earth, and subtle changes in how our planet, our moon, and the Sun all move and interact, can have considerable consequences for life here.
One such process is how the moon orbits Earth. It doesn’t just whip smoothly round the Earth in an endless perfect spiral, but instead its revolutions fluctuate, or “wobble”, as Nasa puts it.
Over an 18-year cycle, this slow wobble either suppresses or amplifies tides on Earth.
During half of the cycle, high tides are higher, and low tides are lower. During the other half of the cycle, less extreme tides are recorded.
Meanwhile, the human-induced climate crisis means sea levels are only going one way – up.
This means that when the moon next wobbles its way into its amplification phase – beginning around 2030, higher tides, and higher sea levels could combine to impact global coastlines, Nasa has warned.
This is expected to “cause a leap in flood numbers on almost all US mainland coastlines, Hawaii, and Guam”, the agency said.
“Only far northern coastlines, including Alaska’s, will be spared for another decade or longer because these land areas are rising due to long-term geological processes.”
High-tide floods – also known in the US as nuisance floods or sunny day floods – are already a serious concern in many cities on the US Atlantic and Gulf coasts.
America’s National Oceanic and Atmospheric Administration recorded more than 600 such floods in 2019 alone.
But once the moon’s next amplification phase kicks in, the country will be in for “a decade of dramatic increases in flood numbers”, Nasa’s assessment has found.
Led by the members of the Nasa Sea Level Change Science Team from the University of Hawaii, the new study shows that high tides will exceed known flooding thresholds around the country more often.
Furthermore, these floods are expected to occur in clusters lasting a month or longer, depending on the relative positions of the moon, Earth, and the sun, which can supercharge the gravitational effect on the sea.
Nasa said that when the moon and Earth revolve into particular alignments with each other and the sun, the resulting gravitational pull on the ocean “may leave city dwellers coping with floods every day or two”.
“Low-lying areas near sea level are increasingly at risk and suffering due to the increased flooding, and it will only get worse,” said Nasa administrator Bill Nelson.
“The combination of the moon’s gravitational pull, rising sea levels, and climate change will continue to exacerbate coastal flooding on our coastlines and across the world.”
He added: “Nasa’s sea level change team is providing crucial information so that we can plan, protect, and prevent damage to the environment and people’s livelihoods affected by flooding.”
Phil Thompson, an assistant professor at the University of Hawaii and the lead author of the new study, said: “It’s the accumulated effect over time that will have an impact.”
He said because high-tide floods involve a small amount of water compared to hurricane storm surges, there was a tendency to view them as a less significant problem overall.
“But if it floods 10 or 15 times a month, a business can’t keep operating with its parking lot under water. People lose their jobs because they can’t get to work. Seeping cesspools become a public health issue,” he added.
The research is published in the journal Nature Climate Change.