Climate Central’s and Zillow’s Surging Seas: Ocean at the Door map shows the vulnerability of old and new housing stock to rising seas plus chronic floods, helping homeowners, planners, renters, and real-estate investors understand the consequences of the changing climate for coastal property. Potential flood exposure maps are generated by comparing land elevation to the height of a typical once-a-year flood, plus local sea level rise projections over time. Areas below the resulting heights are classified as being in a “risk zone.” The shape and extent of the risk zone depends on the year selected, climate pollution scenario and other factors that can be tuned by clicking on Set Projections.
This map displays the number and value of homes located in the selected risk zone, including a special focus on recently built homes. The map incorporates the latest, high-resolution, high-accuracy lidar elevation data supplied by the National Oceanic and Atmospheric Administration (NOAA; for exceptions, see sources), and provides the ability to search by location name or postal code.
Climate Central's original Surging Seas has been featured on national network evening news, on the cover of The New York Times, in hundreds of other stories, and in testimony before the U.S. Senate. The Atlantic Cities named Surging Seas the most important map of 2012, and it was highlighted at the launch of the White House's Climate Data Initiative in March 2014. Climate Central’s sea level maps and tools are grounded in peer-reviewed science.
This map is paired with a companion report, brief, and interactive tool (scroll to the “Future Flood Risk to Homes” section of the tool after inputting a location name).The integrated project was executed in partnership between Climate Central and Zillow.
Methods and Qualifiers
Homes whose geographic coordinate position lie within the selected risk zone are counted and their Zestimates averaged on a high-resolution grid (roughly 100 meters by 100 meters).
Zillow provided location, value data (Zestimates), and build years for homes built through 2016 or 2017, depending upon location. (Address and build year data were provided by Zillow through the Zillow Transaction and Assessment Dataset, or ZTRAX. More information on accessing the data can be found at http://www.zillow.com/ztrax. Proprietary Zillow data, such as Zestimates, were provided under strict confidentiality; the results do not reflect the position of Zillow Group.)
The number and values of homes known to have been built after 2009 are also aggregated.
A full methodology is available in this report.
Elevation data used for the United States come almost entirely from ~5-meter horizontal resolution digital elevation models curated and distributed by NOAA in its Coastal Lidar collection, derived from high-accuracy laser rangefinding measurements. The same data are used in NOAA’s Sea Level Rise Viewer. (High-resolution elevation data for Louisiana, southeast Virginia, and a limited number of other areas comes from the U.S. Geological Survey, USGS, and elevation data for Alaska comes from the National Geospatial-Intelligence Agency’s digital surface model, ArcticDEM).
Flood control structures
Levees, walls, dams, or other features may protect some areas, especially at lower elevations. Data limitations, such as an incomplete inventory of levees and a lack of levee height data, make assessing the protection afforded by levees difficult. Levees are particularly prevalent in Louisiana and in the Bay Area and San Joaquin delta region of Northern California. Missing or mischaracterized levee data in these areas may have important effects on results, including known overestimates of exposure due to effects from missing levee data in Northern California.
We assume levees are always high and strong enough for flood protection. However, the American Society of Civil Engineers rated only 8% of levees as in “acceptable” condition. There is further no guarantee that existing levees will be maintained through 2050 or 2100. On the other hand, new defenses could also be built within these timeframes. Some areas and assets that appear to be protected by levees, ridges, or other features may not actually be protected (see Error section, below).
We use data from the FEMA/USACE Midterm Levee Inventory for our national flood control structure dataset, and supplement this with local data from Louisiana (see details here) and Massachusetts (provided by Chris Watson at the University of Massachusetts Boston; for more information, view the Surging Seas column of the web tools comparison matrix for Massachusetts).
Errors or omissions in elevation or levee data may lead to areas being misclassified. Furthermore, this analysis does not account for future erosion, marsh migration, or construction. As is general best practice, local detail should be verified with a site visit. Sites located in zones below a given water level may or may not be subject to flooding at that level, and sites shown as isolated may or may not be ultimately disconnected from the ocean. Areas may be connected to water via porous bedrock geology, as is common in South Florida, another area with plentiful levees that line drainage channels and canals. Low-lying areas may also be connected by channels, breaks in levees or seawalls, or drainage passages, such as sewers, that the elevation data fail to or cannot pick up. In addition, sea level rise may lead to flooding even in isolated low zones during rainstorms by inhibiting drainage.
At any water height, there will be isolated, low-lying areas whose elevations fall below the water level, but are protected from coastal flooding by either man-made flood control structures (such as levees) or by the natural topography of the surrounding land. Elevation data is accurate enough that non-connected areas can be clearly identified and treated separately in analysis; these areas are colored green on the map. Levee data are complete enough to factor levees into determining connectivity, as well.
Data LayersSet Projections | Risk Areas | Homes | Recently Built | Zestimates | For Recently Built
Scientists agree that climate change has been driving a rise in global sea level and that the rise will continue to accelerate, leading to ocean intrusion on land and aggravated coastal flood risk.
Users may select from among different carbon pollution scenarios, including “unchecked pollution” (technically, Representative Concentration Pathway 8.5, or RCP 8.5), “moderate carbon cuts” (RCP 4.5), and “deep carbon cuts” (RCP 2.6), this last choice meaning a peak in emissions near the year 2020 followed by a sharp decline to zero near 2070 and then by net negative emissions. Our current path appears to lie between unchecked pollution and moderate cuts.
Users may also select from two different sea level rise models and time periods (2050 and 2100). Local sea level rise projections used here are based on two recent peer-reviewed research papers (Kopp et al. 2014; Kopp et al. 2017) building off of global projections from the Intergovernmental Panel on Climate Change and, in the more recent paper, emerging research on the potential instability of Antarctic ice sheets (DeConto and Pollard 2016), which could lead to significantly more sea level rise in the second half of the century.
Findings are based on exposure to local sea level rise plus local annual flood levels. Annual average flood levels were derived using methods from Tebaldi et al. 2012 with water level station data updated through the end of 2015. We assess risk zones based on projected sea level plus the height above sea level that floods exceed on average once per year. The 50th percentile of sea level rise projections are shown for each scenario and model combination, reflecting mid-range sensitivity of sea level to climate pollution.
- Below projected annual flood level: Areas below the level corresponding to the user selected sea level scenario and year, and connected to the ocean.
- Below but isolated: Areas below water level corresponding to user selection, but not connected to the ocean, due to natural or built breaks such as levees.
- Levees: Built flood control structures including dams, gates, and surge barriers, as well as levees.
Homes per acre, based on address-level data and displayed on a ~100x100 meter grid. Data source: Zillow (home locations)
Same as “Homes,” but restricted to homes with known build years from 2010 through 2016 (for Alabama, Florida, Mississippi, and New York) or 2017 (all other coastal states) inclusive. Data sources: Zillow (home locations) and the Zillow Transaction and Assessment Dataset (ZTRAX; build years).
Average Zestimate per acre for all rated homes, based on address-level data, and displayed on a ~100x100 meter grid. Data source: Zillow (home locations and Zestimates)
For Recently Built
Same as “Zestimates,” but restricted to homes with known build years from 2010 through 2016 (for Alabama, Florida, Mississippi, and New York) or 2017 (all other coastal states) inclusive. Data sources: Zillow (home locations and Zestimates) and ZTRAX (build years)
SourcesSee Data Layers, above, for most sources. Others follow.
- U.S. elevation data: National Oceanic and Atmospheric Administration (NOAA), ArcticDEM, U.S. Geological Survey (USGS) (for Louisiana, southeast Virginia, Alaska, and limited other areas).
- U.S. tidal elevation data: NOAA.
- Map tiles by Stamen Design, under CC BY 3.0. Data by OpenStreetMap, under CC BY SA.
Climate Central offers coastal area flood and sea level risk assessment services in the United States and globally, including infrastructure and portfolio analytics, digital tool-building, and other tailored services and consulting for governments, NGOs, businesses, and investors. Learn more.
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