Our warming planet is melting lots of ice and heating the waters of the oceans, creating a clear trend of rising oceans. In some areas of the US, this is starting to cause what’s called nuisance flooding, where high tides cause coastal flooding even in the absence of storms. As the oceans continue to rise, figuring out what areas are likely to become vulnerable to coastal flooding and when is going to be critical to understanding how to manage coastal development.

Figuring out coastal development is complicated. The rate of sea level rise can vary from year to year, the local ocean levels can vary as the land settles or rises, and the pace of sea level rise is increasing. And now, a team of researchers has quantified an additional factor: regular variations in the Moon’s orbit, which influence the levels reached by high tide. The team goes on to show that these changes can suppress the impact of rising seas for a time but can then contribute to a rapid increase in floods.

Cycling the Moon

The plane of the Moon’s orbit isn’t located exactly at the equator; instead, it’s tilted slightly. That means, for part of its orbit, the Moon is orbiting above the Northern Hemisphere, and for the other part, it’s over the Southern Hemisphere. The locations in its orbit where the Moon crosses between the two hemispheres are called nodes, and these shift over time. It takes a bit over 18 years for a node to complete an orbit around the Earth.

This cycle has consequences for the tides, although the impacts vary in time and based on geographic location. But in general, a high-tide mark at one point in the 18-year cycle can differ in elevation from the high-tide mark at a different point. Put in concrete terms, in St. Petersburg, Florida, the peak high tide at one point of the lunar nodal cycle is 4.7 centimeters higher than it is at the low point.

That complexity is layered on top of all the additional factors that cause variations in sea level rise. To figure out what this might mean for flooding, the researchers turned to data and models from NOAA. NOAA has determined the level of high tide that will trigger flooding at locations throughout the US, including minor and moderate levels of flooding. The agency also maintains different projections for sea level rise to the end of the century, based on whether we expect the total rise to be at the low, middle, or high end of the range of what’s expected by the century’s end.

All of this was combined with the Moon’s influence to create an ensemble of models that project daily flood risk out through the next few decades. The team made estimates for 89 different locations throughout the US, including some of its island territories.

The Moon and more

Given that sea levels are expected to rise through the remainder of the century, it’s no surprise that these projections show an increased rate of flooding. But the Moon’s influence often had a surprising impact on the rate. In many locations, the Moon acted to suppress flooding for a while, blocking what would normally have been a gradual increase. As the orbit shifted, it would then act to accelerate the rate of flooding.

This creates what’s termed an inflection point, where the behavior of the system changes relatively suddenly. Specific locations go from a very gradual increase in flooding days to a significantly more rapid rise. In most of the US, that inflection point occurs in the 2030s to 2040s—not very far from the present. For example, Boston is expected to see about six additional days of moderate flooding per year by the end of the decade from 2031 to 2041. But by the end of the decade following, the city will see an extra 46 days of moderate flooding. Similar trends were seen in many other cities.

While that’s a significant influence, it’s not the only one. There’s a nonlinear relationship between sea level rise and flooding days, since a smaller margin between flooding and not makes it much easier for high tides to cause flooding.

One factor the ensemble can’t take into account is the fact that sea level rise tends to vary over time, in part due to short-term influences like El Niño. If you look at global sea level data, for example, you’ll see that there are a number of short-term drops in sea level (such as around 2010), and they’re often interspersed with periods when the ocean’s rise is faster than its average pace (see 2015-2017). These fluctuations can’t be predicted in advance, but they could easily slow or boost the rate of flooding.

Finally, many areas of the US are gradually subsiding as the sediment upon which they’re built compacts. Other areas that were buried in ice during the last glacial period are still rebounding from the elimination of that added mass. These also create differences between locations that influence the rate at which flooding becomes a problem.

Flooding clusters

The data also makes apparent that the rising waters aren’t evenly distributed around the year. For example, by 2050, Honolulu is expected to experience about 63 days of flooding a year. But nearly half of those days are likely to occur within a three-month period. This occurs both because factors that tend to cause flooding don’t necessarily go away the next day and partly just because the average rate of occurrence tends to include lots of variation over the course of a year.

Overall, a number of conclusions can be drawn from this work. Right now, flooding from sea level rise is something that, unless you live in a handful of locations, you can ignore. But the authors estimate that, in a little over a decade, sea level rise will stop being a regional issue and become a widespread national problem. And within a decade of that, a lot of places will have passed the inflection point and will be on a path of rapidly increasing flooding events.

Finally, all of this is based on low-to-moderate sea level rise scenarios. Should things happen faster than that, the time window we’ll have before needing to deal with these problems is going to be considerably narrower.

All of that makes it essential that these floods are taken into consideration immediately. Infrastructure we are building at present is unlikely to reach its end of life before flooding becomes far more frequent. Housing, roads, and other facilities that we are currently using will likely need to be protected or abandoned. And coastal states may want to start setting aside money to deal with the flooding that will invariably occur.

Nature Climate Change, 2021. DOI: 10.1038/s41558-021-01077-8  (About DOIs).

https://arstechnica.com/?p=1775463