Swift on the heels of an alarming federal report assessing high-tide flood risk, a new and independent analysis projects loss figures for this slow-moving catastrophe in hundreds of America’s oceanfront communities.

Here are a few of the stunners published Monday in the Union of Concerned Scientists’ report, “Underwater: Rising Seas, Chronic Floods, and the Implications for U.S. Coastal Real Estate”:

• By the year 2045, owners of some 311,000 homes are likely to experience intolerably frequent inundations from steadily rising sea levels, essentially independent of storm events.
• As of today, those properties have a collective market value of about $117.5 billion, house about 550,000 people, and contribute $1.5 billion annually to local tax bases. Of course, all of those numbers will increase steadily over the 27 years between now and the end date for the study’s near-term projections (which, UCS notes, is within the lifespan of a typical new mortgage).
• Commercial properties (and the jobs they support) are also imperiled — about 14,000 of them, with an assessed valuation of roughly $18.5 billion.
• Extend the analysis to century’s end and the rising tides lift all stats: 2.4 million homes chronically inundated; 4.7 million people deciding whether to leave them; $912 billion in market value and $12 billion in property taxes teetering on the brink. (Also, 107,000 commercial properties, worth $152 billion.)

Although UCS is of course an advocacy group, it is guided by serious expertise throughout its staff and board and its research is widely respected, not to mention published from time to time in peer-reviewed journals. Such was the case a year ago, when the organization introduced its concept of “chronic inundation” and a methodology for estimating when relatively minor but persistent flooding begins to drive abandonment of coastal communities.

A wet basement every other week

Having consulted a wide range of experts, UCS settled on a benchmark of 26 flood events per year, afflicting at least 10 percent of a community’s land area, as the point of “effective inundation” so sufficiently annoying that “current use is no longer feasible.”

As it happens, such flooding is far easier to track and forecast than storm-driven surges, thanks to the National Oceanic and Atmospheric Administration’s tide-gauge data. NOAA monitors sea levels at some 200 points on the coastlines of the lower 48 states, and in most cases has been doing so for 100 years or more.

These measurements yielded the agency’s recent report of a record-setting surge in high-tide flooding, which has increased by 50 percent in the last eight years — and 100 percent in the last 30.

Last year in particular was a record-breaker, NOAA said, and this year is likely to be one, too, as we look to a future of increasingly bothersome tidal flooding with clear implications for infrastructure. But it didn’t really address the impact on homes, their inhabitants and the communities they comprise.

That’s where UCS has stepped in. Using NOAA’s current identifications of zones where high-tide flooding is occurring, and an agency projection of sea-level rise, it “determined how many residential and commercial properties along the entire lower 48 coastline are at risk of becoming chronically inundated from high tides… in the coming decades, even in the absence of major storms.”

Then it identified properties within the wet zones and — in what I’ll call a brilliant stroke of efficiency — used the online real-estate company Zillow’s tirelessly updated sales prices and related data to quantify the dollar value of what’s at stake.

Now, you may have qualms about how Zillow values your own home, or your neighbor’s, but it seems reasonable to assume that its over- and underestimates might average out well enough over a third of a million properties.  

You might also quibble with UCS’s decision, in looking at three NOAA sea-level scenarios, to pick the one that assumes the worst between now and the end of the century.

Fine. Do your own study. As for me, I accept UCS’s reasoning that this is a prudent approach when you consider that these properties, in the vast majority of cases, represent nearly all the wealth their owners have been able to accumulate.

Besides, there are few visible pathways to a dramatic slowing of sea-level rise — and far more indications of unanticipated acceleration, like last week’s finding that Antarctica is losing ice to the sea at a rate three times higher than a decade ago, and climbing.

Where the risk is highest

Where is the flooding likely to be worst? On a regional basis, UCS says, the picture looks like this:

States with the most homes at risk by the end of the century are Florida, with about 1 million homes or more than 10 percent of the state’s current residential properties, New Jersey with 250,000 homes, and New York with 143,000 homes. States that could lose the most in home property values by 2100 are Florida at $351 billion, New Jersey at $108 billion, and New York at nearly $100 billion.

Decreases in property values also mean a lower property tax base. Florida, New York and New Jersey will see the biggest hits to their annual property tax revenue with municipalities losing about $5 billion, $1.9 billion and $1.7 billion total respectively.

But there’s another way of looking at the impact, based on community size and character, says UCS senior climate scientist Kristina Dahl:

Some smaller, more rural communities may see 30, 50, or even 70 percent of their property tax revenue at risk due to the number of chronically inundated homes. Tax base erosion could create particular challenges for communities already struggling with high poverty rates.

UCS has published these findings with an impressive array of maps, many of them interactive, and a set of state-by-state breakouts, which may explain why most of the mainstream media coverage so far has appeared in smaller-city newspapers or TV broadcasts — it’s a great local story.

One exception is CNN’s fine national story, focused more on the end-of-the-century impacts. As often, the UK Guardian offered top-notch coverage of an environmental threat outside its borders, including this interesting bit of context:

The report does not factor in future technological advances that could ameliorate the impact of rising seas, although the U.S. would be starting from a relatively low base compared with some countries given that it does not have a national sea level rise plan. And the current Trump administration has moved to erase the looming issue from consideration for federally funded infrastructure [an issue discussed here previously].

And Anne C. Mulkern of  E&E News took a close look, reprinted in Scientific American, at the especially dire implications for the low-lying San Francisco Bay area, where tides are amplified by ocean currents, gravitational effects and Earth’s rotation. The upshot is that 3 feet of sea-level rise around the world shows up as 4 feet in coastal California:

San Mateo County is one of the most affected places in the Bay Area. By the turn of the century, flooding could affect more than 9,700 homes under an “intermediate scenario,” which includes moderate ice sheet melt and global sea-level rise of 4 feet by 2100, the analysis said. If there’s more rapid ice sheet loss and global average sea-level rise of 6.6 feet by that year, water inundation would affect nearly 14,000 homes, the analysis said.

In the county, several low-lying wastewater treatment plants that serve multiple cities are at risk with 3 feet of sea-level rise, said Hilary Papendick, climate change program manager for San Mateo County. In addition, San Francisco International Airport, one of the biggest airports in the country, is “very vulnerable,” she said.

Other areas at high risk by the end of the century include Long Beach, Seal Beach, Huntington Beach and Newport Beach, south of Los Angeles. Those areas have wetlands, channels and storm drains that will allow rising water to flow onto land.