Coastal cities across the U.S. are likely to see a dramatic surge in high-tide floods starting in the mid 2030s as rising sea levels due to climate change align with natural shifts in the moon's orbit, a new NASA study warns.
The study, led by members of NASA's Sea Level Chance Science Team from the University of Hawaii and published in the July 2021 issue of the journal Nature Climate Change, is the first to take into account all known oceanic and astronomical causes for floods, NASA said in a news release.
High-tide floods, also known as nuisance floods or sunny day floods, occur not because of storm surges from extreme weather or excessive precipitation, but instead when the tide rises into populated areas. The floods can overwhelm storm drains, close roads and compromise infrastructure over time, according to the National Oceanic and Atmospheric Administration.
“It’s the accumulated effect over time that will have an impact,” said Phil Thompson, an assistant professor at the University of Hawaii and the lead author of the new study.
High tide floods are less dramatic and involve less water than hurricane storm surges, so they are often seen as a less pressing problem.
“But if it floods 10 or 15 times a month, a business can’t keep operating with its parking lot underwater," Thompson said. "People lose their jobs because they can’t get to work. Seeping cesspools become a public health issue.”
The extreme tide flooding, which will occur on both U.S. coasts, is due to a so-called "wobble" in the moon's orbit. The wobble occurs on an 18.6-year cycle, and is entirely natural, first discovered in 1728.
But the new development of rising sea levels, as the global temperature warms due to human-induced climate change, amplifies one of the effects of the wobble.
The moon's gravitational pull is the cause of everyday high and low tides. But the Earth's regular high and low tides change with the moon's wobble.
For the first half of the 18.6-year cycle, the tides are suppressed: high tides are lower, and low tides are higher. In the other half of the cycle, tides are amplified, with higher high tides and lower low tides.
High sea levels push all high tides higher, so when the next 18.6-year moon wobble cycle amplifies high tides, they will regularly top flooding thresholds.
As a result, high tides will exceed known flooding thresholds around the country more often.
The floods will also occur in clusters, which could last a month or longer at a time depending on the position of the moon, sun, and Earth, NASA said.
As the moon and Earth line up with each other and the sun in specific ways, some city dwellers could see flooding every day or two.
Ben Hamlington of NASA’s Jet Propulsion Laboratory in Southern California co-authored the paper and also leads NASA’s Sea Level Change Team. He said the study is vital for coastal urban planners, who may tend to focus on preparing for extreme weather events over chronic flooding.
“From a planning perspective, it’s important to know when we’ll see an increase,” Hamlington said. “Understanding that all your events are clustered in a particular month, or you might have more severe flooding in the second half of a year than the first – that’s useful information.”
The moon is currently in the tide-amplifying part of its 18.6-year wobble, in 2021. But most U.S. coastlines have not yet seen enough sea-level rise to notice the flooding effects.
By the mid-2030s, the next time the wobble enters its tide-amplifying phase, global sea level rise due to climate change will have had another decade to advance.
The problem will occur around the U.S., not just on one coast or the other, with "a leap in flood numbers on almost all U.S. mainland coastlines, Hawaii, and Guam."
Far northern coastlines, such as Alaska's, "will be spared for another decade or longer because these land areas are rising due to long-term geological processes," NASA said.
The study projected results out to 2080 by mapping "NOAA’s widely used sea level rise scenarios and flooding thresholds, the number of times those thresholds have been exceeded annually, astronomical cycles, and statistical representations of other processes, such as El Niño events, that are known to affect tides."
NASA's sea level portal will be updated with the results of the study as a resource for decision-makers and the public.