The wobbly orbit of the Moon, combined with the effects of human-driven climate change, will substantially increase coastal flooding across the United States starting in the 2030s, according to a new study co-authored by NASA scientists that raises the alarm about this impending deluge.
We haven’t yet felt the full brunt of this hazardous brew of astronomical and anthropogenic influences because rising sea levels, a major byproduct of climate change, are still below critical tipping points. By the 2030s, however, sea levels will be high enough to magnify the effects of the Moon’s normal orbital rhythms, resulting in a tripling or a quadrupling of floods related to high tides across the majority of America’s coastline, excepting Alaska.
Scientists led by Phil Thompson, an assistant professor at the University of Hawaii, note that these flood surges are likely to occur in devastating clusters that will require different adaptation strategies compared to short-term extreme events, like heat-waves or hurricanes, according to a recent study published in Nature Climate Change.
“Around the mid-2030s, locations along the Pacific and Gulf of Mexico coastlines will experience rapid increases in high-tide flooding” that will “lead to extreme seasons or months during which many days of high-tide flooding cluster together,” Thompson and his colleagues said in the study.
“Scientists, engineers, and decision-makers are accustomed to the statistics and impacts of isolated extreme events, but given the cumulative nature of high-tide flooding impacts, we describe extreme months or seasons during which the number of flooding episodes, rather than the magnitude, is exceptional,” the team added.
Many previous studies anticipate the general risks of anthropogenic sea level rise to coastal communities as well as the extreme weather events linked to human-driven climate change, such as intensifying Atlantic hurricane seasons. Thompson and his colleagues now offer the first study that accounts for all oceanic and astronomical drivers of floods, and projects their intertwined effects into the future.
While most people are familiar with the regular cycles of lunar phases, which last roughly one month, the Moon’s nodal cycle is a bit more esoteric. Over the course of about 18 years, the path of the Moon’s orbit around Earth precesses, or wobbles, an effect that has been known about for centuries and is accounted for in tidal predictions.
During one half of the cycle, this wobbling effect takes the edge off of both tidal extremes by making high tides lower and low tides higher. In the other half of the cycle, the Moon’s precession amplifies both extremes, making high tides higher and low tides lower.
We’re currently living through this amplified half of the cycle and will soon enter the nodal half in which the tidal extremes get toned down. Thompson and his colleagues warn this will also mute the effects of sea level rise at high tides, which they worry will leave coastal communities ill-prepared for extreme flooding projected for the following decade.
“Nodal cycle modulations of tidal amplitude will suppress sea-level-rise-induced increases in high-tide flooding during certain periods and may delay the onset of environmental adaptation triggers,” the team said. “Such delays could produce complacency and inaction through false confidence in benign pathways” even though “periods of little perceptible change are expected in many locations—only to be followed by periods of exponential high-tide flooding increase.”
The team modeled the potential duration of these sudden exponential floods in Boston, Massachusetts, La Jolla, California, Honolulu, Hawai’i, and Saint Petersburg, Florida. The results revealed that Saint Petersburg could experience 67 days of additional flooding in the 2030s, in part because sea levels there are expected to rise by about nine centimeters over the next decade. Boston may experience only one additional day of flooding in the 2030s, but that number could rise to 46 days in the 2040s.
Those coastal changes may vary locally, but overall they will spark “the onset of an expected transition in high-tide flooding from a regional issue to a national issue with a majority of US coastlines being affected,” according to the study.
Moreover, as sea levels continue to rise over the course of the 21st century, the Moon’s nodal cycle will create more frequent, severe, and sustained floods, especially when they coincide with other events such as El Niño. While the study focused on American coastlines, these combined influences will affect seashores around the world, and require the development of new projections and survival plans.
“Just as engineers and coastal planners are accustomed to planning for rare, large-amplitude extreme events, adaptation and mitigation strategies focused on high-tide flooding should account for brief periods experiencing an extreme number of high-tide flooding days,” the team concluded.