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A new way of seeing sea-level rise looks at chronic flooding of U.S. cities

The authors ask: How much flooding must occur in a community in, say, coastal Mississippi before people decide to pull up stakes and move to someplace like Minnesota?

How much flooding must occur in a community in, say, coastal Mississippi before people decide to pull up stakes and move to someplace like Minnesota?
REUTERS/Randall Hill

Perhaps the most common way we think about sea-level rise employs the analogy of water rising in a bathtub until it covers the toes, then the knees, then the navel ….

This is the scenario of permanent inundation — the water comes up and the land goes under, there being no drain plug to pull — and it makes sense when we think of island places like the Marshall Islands or the Maldives, where the land is low-lying and surrounded by ocean. Even if seawalls or other barriers were feasible, which they usually are not, the protection they brought could be highly impermanent.

But bathtub imagery isn’t so good a fit for, say, low-lying coastal portions of the United States, with all that interior behind them. And while the satellite-derived maps are terrific for indicating where the new coastline might be, they’re not so good at conveying its impact.

What will it mean, really, to have the median sea level rise by a few inches or more at Miami, or Manhattan? And given the fairly slow rate of this change, and the value of real estate at stake, can’t an infrastructure solution be found?

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A different and rather disturbing way of seeing this problem is at the heart of an analysis published last week by the Union of Concerned Scientists. It begins by taking into account the action of tides in creating a sort of fluctuating waterline, which brings a pattern of impermanent inundations, or what we might call saltwater flooding.

Reasoning that frequent flooding will drive people from their homes long before the inundation becomes permanent, the authors ask: How much flooding must occur in a community in, say, coastal Mississippi before people decide to pull up stakes and move to someplace like Minnesota?

After consulting a range of experts in various disciplines, from hydrology to real estate to municipal governance, the authors settled on this plausible benchmark:

When coastal flooding occurs at least 26 times per year, or once every two weeks, and covers at least 10 percent of its land area, a community has probably reached the point of “effective inundation” in which “current use is no longer feasible.”

The choice then becomes one of building seawalls or other infrastructure  — if possible and affordable  — or simple abandonment.

And in many locales that dilemma is not so far off in the future.

90 communities in trouble now

More than 90 communities in the lower 48 states, mostly on the Atlantic and Gulf coasts, already face this level of chronic flooding, according to UCS calculations; in Louisiana and Maryland, where the bulk of the present threat is located, the problem is worsened by subsidence, or sinking land, that has coincided with sea-level rise.

Using a middle-of-the-road forecast from the National Climate Assessment, the analysis finds that this number could nearly double in the next 20 years, while expanding to many more regions, and grow by the end of the century to some 490 communities, “from beach vacation destinations like the Jersey Shore and the Gulf Coast of Florida to larger cities, including Boston, Galveston, Savannah and Fort Lauderdale. By late century, four of the five boroughs of New York City (excluding the Bronx) would be chronically inundated.”

The assessment also includes a forecast based on higher emissions of globe-warming gases and faster sea-level rise; UCS calculates that this path would result in 670 communities facing chronic inundation.

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And it considers a more optimistic path as well, more or less in line with the  trends that could result from full realization of the Paris climate accord. In that happy scenario, the frequent-flooder list is shortened by 199 communities, or less than one-third.

UCS is of course an advocacy organization  — it advocates for science and humanitarian concern in the shaping of public policy on global warming, energy systems and nuclear weapons  — so I hasten to point out that a paper based on this work passed muster for publication in Elementa, a peer-reviewed journal devoted to “science of the Anthropocene” and published by the University of California Press.

As a matter of policy, the paper points out, chronic inundation leaves communities with three basic coping strategies  — to defend with seawalls and other infrastructure, to accommodate with large-scale pumping and other dewatering systems, and to retreat by abandoning flooded land to the sea.

All of these are costly. Miami Beach, for example, has already spent $400 million on accommodation  — enlarging the capacity of its storm sewers  — and although it sure is heading for the chronic inundation list, it isn’t there yet, because it hasn’t crossed the 10-percent-of-land-area line.

Neither has Annapolis, another frequent flooder at some 40 times per year, with parts of the U.S. Naval Academy in its increasingly wet zone  — just one of the scores of U.S. military installations that face increasing interference from a rising ocean.

Afflicting the disadvantaged

And, as is often the case, the nonmilitary communities most at risk are not necessarily those most able to afford appropriate responses. From a UCS summary of the analysis:

By using a previously published index of socioeconomic vulnerability, the study identified that nearly 60 communities facing chronic inundation in the next 20 years are also contending with social and economic challenges that may leave them with fewer resources to plan or adapt, and thus exposed to disproportionate harms. While equitable solutions to chronic inundation will require inclusion of all voices, people of color and low-income people are too often excluded from decisions affecting their neighborhoods and communities, and face significant hurdles accessing federal and state programs, as well as funding.

Meanwhile, the underlying forces continue to intensify. From the Elementa paper, with citations omitted:

[R]ecurrent tidal flooding is already emerging as one of the most visible and quantifiable present-day signs of climate change. The East and Gulf Coasts of the U.S. experienced some of the world’s fastest rates of sea level rise during the 20th century. …

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Whereas minor coastal flooding along the East, Gulf, and West coasts of the U.S. occurred just once every one to five years in the 1950s, it was occurring about once every three months by 2012.

And, by the way, this does not include flooding driven by storm surge. That’s a completely separate factor but not, as we remember from hurricanes Sandy and Katrina, an unimportant one.

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The paper as published in Elementa can be read and downloaded here [PDF] without charge; the full UCS report is here; and some nifty interactive tools for looking at chronic inundation in specific communities are here.