High-tide flooding in Honolulu. (Credit: Hawaii Sea Grant King Tides Project)
In the mid-2030s, every U.S. coast will experience rapidly increasing high-tide floods, when a lunar cycle will amplify rising sea levels caused by climate change.
High-tide floods—also called nuisance floods or sunny-day floods—are already a familiar problem in many cities on the U.S. Atlantic and Gulf coasts. The National Oceanic and Atmospheric Administration (NOAA) reported a total of more than 600 such floods in 2019. Starting in the mid-2030s, however, the alignment of rising sea levels with a lunar cycle will cause coastal cities all around the U.S. to begin a decade of dramatic increases in flood numbers, according to the first study that takes into account all known oceanic and astronomical causes for floods.
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. What’s more, the floods will sometimes occur in clusters lasting a month or longer, depending on the positions of the Moon, Earth and the Sun. When the Moon and Earth line up in specific ways with each other and the Sun, the resulting gravitational pull and the ocean’s corresponding response may leave city dwellers coping with floods every day or two.
Why will cities on such widely separated coastlines begin to experience these higher rates of flooding at almost the same time? The main reason is a regular wobble in the Moon’s orbit that takes 18.6 years to complete. There’s nothing new or dangerous about the wobble; it was first reported in 1728. What’s new is how one of the wobble’s effects on the Moon’s gravitational pull—the main cause of Earth’s tides—will combine with rising sea levels resulting from the planet’s warming.
In half of the Moon’s 18.6-year cycle, Earth’s regular daily tides are suppressed: High tides are lower than normal, and low tides are higher than normal. In the other half of the cycle, tides are amplified: High tides get higher, and low tides get lower. Global sea level rise pushes high tides in only one direction—higher. So half of the 18.6-year lunar cycle counteracts the effect of sea level rise on high tides, and the other half increases the effect.
The Moon is in the tide-amplifying part of its cycle now. However, along most U.S. coastlines, sea levels have not risen so much that even with this lunar assist, high tides regularly top flooding thresholds. It will be a different story the next time the cycle comes around to amplify tides again, in the mid-2030s. Global sea level rise will have been at work for another decade. The higher seas, amplified by the lunar cycle, will cause a leap in flood numbers on almost all U.S. mainland coastlines, Hawaii, and Guam. 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.
