A flip in a natural moon cycle combined with rising seas from climate change will cause rapid and "dramatic" increases in flooding along U.S. coastlines by the mid-2030s, according to a NASA study.
The research, published last month in the journal Nature Climate Change, found that changes in the coming decades to the angle of the moon's orbit and its alignment with the Earth and sun will amplify the effects of sea-level rise, leading to a sharp rise in high-tide flooding in coastal communities.
The findings add new urgency to address rising sea levels in many Atlantic and Gulf Coast cities, where high-tide flooding is already an issue. But the study also highlights how flooding could become widespread for almost all coastal communities in the country by the middle of the next decade.
"This is the point in time when things will begin to rapidly change," said Philip Thompson, an assistant professor in the department of oceanography at the University of Hawaii at Manoa and lead author of the study. "When we think about when we need to improve and have things ready with our drainage systems and infrastructure, this gives a target point."
The surge in high-tide flooding will be driven in part by the moon's natural "wobbling" as it orbits the Earth. The moon's tilt changes over an 18.6-year cycle, and that motion affects the ebb and flow of Earth's tides.
For half of the cycle, the planet's regular, daily tides are suppressed, essentially counteracting the effect of rising seas. For the remainder of the cycle, the moon's wobble boosts the effects of sea-level rise.
"It doesn't change the strength of tidal forces or the gravitational forces of the moon, but it tends to increase the range between the highest and lowest tides of the day," Thompson said. "In other words, it tends to make the highest tides even higher and the lowest tides even lower."
By the mid-2030s, the lunar cycle will be set to amplify Earth's tides once again. When combined with rising sea levels due to climate change, the cumulative result is a significant increase in high-tide flooding, the researchers found.
Climate change contributes to sea-level rise because ocean water expands as it warms and higher temperatures cause land-based ice, such as glaciers and ice sheets, to melt. It's against this backdrop of rising seas that changes to Earth's tides could have the most damaging effect.
"If you have water in a bathtub and the height of that water is changing over time, that is sea-level rise," said study co-author Gary Mitchum, associate dean at the University of South Florida College of Marine Science. "If on top of that you put your hand in and slosh it back and forth so that waves spill over the side of the tub, those are the tides. The tides and sea-level rise are different, but they add up."
The researchers found that from the mid-2030s into the following decade, the changing lunar cycle will cause clusters of flooding in coastal communities. In St. Petersburg, Florida, for example, high-tide flooding is projected to increase from six or seven events a year to 10 times that amount in just over a decade.
This increase in flooding could have significant economic implications, in addition to disrupting day-to-day life, particularly because the high-tide flooding events could be clustered together based on seasonal cycles.
"In some cases, there might be flood events up to 20 times a month, or almost daily," said Benjamin Hamlington, a research scientist at NASA's Jet Propulsion Laboratory and a co-author of the study. "If you're dealing with that for a whole month, that starts to impact life in a different kind of way."
High-tide flooding is sometimes known as "nuisance flooding" or "sunny-day flooding," but Mitchum said those terms can be deceiving because they often downplay the severity.
"They sound like no big deal, and while each event may be minor in and of itself, it's death by a thousand cuts," he said.
Thomas Wahl, an assistant professor in the department of civil, environmental and construction engineering at the University of Central Florida, who was not involved with the research, said the report should be a wake-up call for coastal communities.
"What this shows us is that we're at a point where we need to make some decisions if we're going to be prepared in the 2030s," he said.
The findings also provide a realistic timeline for city planners to boost resilience into regions that will be increasingly affected by high-tide flooding.
"This helps illuminate the time scales that we're operating under," Wahl said. "Knowing how long it can take from having an idea of what to do to having something implemented, 10 years is not a very long time."