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Published Study Identifies When Hundreds of Coastal Communities Will Face Inundation, Possible Retreat

Meeting Paris Agreement Goals Could Spare Many Communities from This Fate


More than 90 U.S. communities already face chronic inundation from rising seas caused by climate change, and the number could jump to nearly 170 communities in less than 20 years and as many as 670 by the end of the century, according to a study by analysts at the Union of Concerned Scientists (UCS) published in the peer-reviewed journal Elementa today. The analysis is the first to look at the entire coastline of the lower 48 states and identify communities that will experience flooding so extensive and disruptive that it will require either expensive investments to fortify against rising seas or residents and businesses to prepare to abandon areas they call home. The analysis projects when communities can expect to see this degree of flooding and which cities and towns might avoid such flooding if the long-term temperature goals of the Paris climate agreement are achieved.

The study was published on the same day a 2,200 square mile iceberg--one of the largest ever recorded and nearly the size of Delaware--broke off from an ice shelf in Antarctica, highlighting how quickly the planet is warming.

The analysis defines a threshold above which flooding becomes unmanageable for people's daily lives. The threshold--10 percent or more of a community's usable, non-wetland area is flooded at least 26 times per year or the equivalent of a flood every other week--was determined after consulting technical experts and residents of communities currently experiencing disruptive flooding. Once a community--delineated by the Census Bureau as county subdivisions--crosses this threshold, it is considered "chronically inundated." To put it in perspective, Miami Beach--widely considered a poster child for rising seas--has not yet reached the 10 percent threshold set in this analysis, but is already facing tough, costly choices.

"Some 90 communities, mostly in Louisiana and Maryland where the land is also sinking, are already facing chronic inundation from sea level rise," said Erika Spanger-Siegfried, senior analyst in the Climate and Energy Program at UCS and a report author. "As global temperature increases sea level rise, several hundred coastal communities are looking at the same kind of chronic flooding around the middle of the century--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. We hope this analysis provides a wake-up call to coastal communities--and us as a nation--so we can see this coming and have time to prepare."

The UCS study assessed three sea level rise scenarios: The "low scenario" assumes carbon emissions decline steeply, sea level rise is driven primarily by ocean warming with very little ice loss, and warming is limited to less than 2 degrees Celsius--in line with the primary goal of the Paris Agreement. The "intermediate scenario" projects carbon emissions peaking around mid-century and about 4 feet of sea level rise globally, with ice melting at a moderate rate that increases over time. In the "high scenario," emissions rise through the end of the century and ice melts faster to yield about 6.5 feet of sea level rise. Recent studies suggest the high scenario is increasingly plausible due to accelerating ice sheet loss. In fact, the National Oceanic and Atmospheric Administration recently released a substantially higher scenario.

The UCS analysis found:

  • By 2035, about 170 communities--roughly twice as many as today--will face chronic flooding and possible retreat from affected areas under the intermediate or high sea level rise scenarios, with more than 100 seeing at least a quarter of their land chronically flooded.
  • By 2060, about 270 communities will face chronic flooding and possible retreat from affected areas with intermediate sea level rise. This number jumps to 360 under the high scenario. About 40 percent of chronically inundated communities in either scenario would see at least half of their land flooded.
  • By 2100, about 490 communities--including roughly 40 percent of all oceanfront communities on the East and Gulf Coasts--will face chronic flooding and possible retreat with intermediate sea level rise, with nearly 300 seeing at least a quarter of their land chronically flooded. The number of communities jumps to about 670--including roughly 60 percent of all oceanfront communities on the East and Gulf Coasts--under the high scenario.
  • Many communities that never reach the 10 percent threshold of chronic inundation this century--such as Annapolis, Md. and Long Beach, Calif.--are nevertheless expected to see chronic flooding of important areas and infrastructure.

The chronically inundated communities in 2035--mainly on the Jersey Shore, mainland side of North Carolina's Pamlico Sound, southern Louisiana, and Maryland's Eastern Shore--are mostly clustered together in places already experiencing regular tidal flooding, or neighboring such places. By 2060, entirely new stretches of coastline become chronically inundated under both the intermediate and high sea level rise scenarios, including the greater Boston area and northern New Jersey, as well as additional communities along the northern coast of Texas, and Louisiana and Florida's Gulf Coasts.

"By 2060, hundreds of U.S. coastal communities--cities and towns of all kinds--face chronic inundation," said Kristy Dahl, a report author and climate scientist consultant to UCS. "In Texas, for example, bigger cities and industrial centers like Galveston and Sabine Pass become chronically inundated by mid-century. So do many tourist destinations, such as Sanibel and Captiva Islands in Florida, Hilton Head in South Carolina, Ocean City in Maryland, and more than a dozen towns along the Jersey Shore."

Chronic flooding becomes a problem for many major cities in the coming decades, but at the end of the century with the high scenario, that number tops 50 communities--both big cities and large county subdivisions--with populations over 100,000, including Boston, Newark, Fort Lauderdale, and four of the five boroughs of New York City. Residents in these cities will need to grapple with the question of whether to adapt or relocate. And while the West Coast was previously able to escape mostly unscathed, by 2100 the San Francisco Bay and greater Los Angeles areas join the ranks of the chronically inundated.

The communities that will be affected have three basic strategies to cope with rising seas: defend, accommodate and retreat. Some East and Gulf Coast communities are already employing defensive and accommodation measures such as seawalls, tide gates, levees, elevated homes and large-scale pumping systems. Efforts to accommodate or keep out water may stall the inundation projected by this analysis, but often at great cost and for a limited time. That means hundreds of communities along the coasts, from Maine to the state of Washington, will be forced to make difficult choices about how much to invest in flooded areas versus when to retreat from them. Likewise, residents in affected areas will be forced to decide their tipping point for remaining at home or relocating.

"By making sound decisions soon, communities can prepare for chronic inundation in the time they have," said Shana Udvardy, report author and climate preparedness specialist at UCS. "This could help them avoid serious losses not only of homes, schools, businesses, and other infrastructure, but also of regional history, sense of place, local culture, and the community's way of life."

The analysis also highlights that some Americans will be harder hit than others. 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.

The analysis makes a number of policy recommendations to help coastal communities at risk of chronic inundation, including phasing out policies that encourage risky coastal development, and bolstering existing policies or enacting new ones that would bring about investments to make communities more resilient to sea level rise. But it's achieving the long- term temperature goals established in the Paris Agreement and limiting global ice loss that could have the greatest effect. Holding warming to between 1.5 and 2 degrees Celsius by the end of the century could spare between roughly 200 and 380 U.S. coastal communities, including nearly 50 major U.S. cities and many more cities worldwide, from chronic flooding and possible retreat, depending on the amount of sea level rise.

"Meeting the long term goals of the Paris Agreement would offer coastal communities facing chronic flooding their best chance to limit the harms of sea level rise," said Rachel Cleetus, lead economist and climate policy manager at UCS. "Despite President Trump's attempts to undermine near-term federal action on climate change, other countries as well as U.S. states, cities, businesses and citizens are showing firm resolve to fulfill the promise of Paris. They understand that if we fail to limit warming, we're committing a great many people to a future of flooding and inundation, and the hard choices and significant costs that come with it."

To view the report PDF, click here.

The Elementa journal article can be found by clicking here.

To view a spreadsheet that sorts the chronically inundated communities by state, click here. To see the communities sorted by year, click here.

To use the interactive mapping tool, click here. The various tabs allow you to explore the amount of land area flooded, and the communities that are affected by the rising seas--including the ones that may have fewer resources to cope with chronic inundation, and ones that could avoid such flooding if the Paris Agreement's temperature goals were achieved. By scrolling, you will see buttons for each time frame examined in the report for both the intermediate and high sea level rise scenarios. As you zoom in, the maps become more detailed. You can also click on a specific community for more details about it.

For state-specific fact sheets, community case studies, Spanish language materials, blogs and a video, click here.

The Union of Concerned Scientists is the leading science-based nonprofit working for a healthy environment and a safer world. UCS combines independent scientific research and citizen action to develop innovative, practical solutions and to secure responsible changes in government policy, corporate practices, and consumer choices.