The United States could see nuisance flooding in the 2030s more often than ever before, according to a new study published on June 21 in the journal Nature Climate Change.
The paper takes into account all known factors that can influence floods—both oceanic, like climate change-related sea level rise, and astronomical, like the moon’s influence on tides, reports Rachel Trent for CNN. The researchers predict that in the mid-2030s, the effects of the lunar cycle combined with sea level rise will cause coastal areas of the U.S. to see near-daily tidal flooding for a month or more, George Dvorsky reports for Gizmodo.
High tides don’t reach the same peak every year. The peak depends on the alignment of the moon’s orbit with the Earth and the sun, which changes gradually along an 18.6-year cycle. Nuisance floods are minor floods that can occur at high tide, often causing water to pool in low-lying roads, parking lots and subway stations. As the name suggests, nuisance floods don’t cause immediate catastrophic damage. But they can be inconvenient to work around and strain infrastructure over time.
“It’s the accumulated effect over time that will have an impact,” says University of Hawaii oceanographer Phil Thompson, the lead author of the study, in a statement by NASA. “If it floods ten 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.”
The last time that the moon’s orbit exacerbated tidal flooding was in 2015, Deborah Byrd reported for EarthSky at the time. The effect is strongest at the minor lunar standstill, which is the point in the 18.6-year cycle when the moon’s orbit comes closest to the plane of Earth’s equator. At that point, several astronomical forces align to amplify the lows of low tide and highs of high tide, and the latter can cause nuisance floods.
The National Oceanic and Atmospheric Administration recorded more than 600 tidal floods in the U.S. in 2019, when the amplification effect from the moon’s orbit was less than it was in 2015. The next time the effect is due to be at its strongest is in 2033 and 2034.
To make specific predictions about the combined effects of astronomical and oceanic phenomena, researchers studied 89 tide gauge locations in every coastal U.S. state and territory except for Alaska. The new computational model combines NOAA’s sea level rise predictions, flooding thresholds, astronomical cycles, and other tide-influencing events like El Niño, per the NASA statement.
The researchers predict that the effect will be worst on the Pacific coast, the Gulf of Mexico, and the Pacific Islands, according to a statement by the University of Hawaii at Manoa. The predictions extend to the 2080s; for example, the study found that Honolulu is expected to experience 63 days of flooding by 2050. Most of those floods will be concentrated during a three-month period, reports John Timmer for Ars Technica.
"Scientists, engineers and decision-makers are accustomed to thinking about rare high-impact events, for example, a 100-year storm,” says Thompson in the University of Hawaii statement. “But we demonstrate that it is important to plan for extreme months or seasons during which the number of flooding episodes, rather than the magnitude, is exceptional.”