Fig 1.
This analysis covers sea level rise zones (3 feet / 0.91 meters and 6 feet / 1.83 meters) across the conterminous United States [33] and by Census Division [34], with the 6 feet SLR zones displayed in blue (a). The spatial distribution of the year the first structure was built, provided by the HISDAC-US database [22], illustrates the historical development of cities, such as Houston, displayed in the inset (b). All data used in figure available through an open license for U.S. government datasets [33, 34] or through a CC0 license [22].
Fig 2.
The two major drivers of urban development, measured in this paper are: 1) expansion (panel a to b), or the aerial growth of developed land, and 2) structure density (panel f to g), or the addition of more or bigger structures to already developed areas. An example of high expansion within SLR zones [30] is in Fort Meyers, FL where developed land rapidly expanded after 1950 (panels c-e). Los Angeles, CA is an example of high growth of structure density within SLR zones, particularly after 1950 (panels h-j). Both examples are measured using the built up properties (BUPL) data layer from HISDAC [39]. All data used in figure available through an open license for U.S. government datasets [33, 34] or through a CC0 license[39].
Fig 3.
The 6 feet SLR zone, the inland part of coastal Micro- and Metropolitan Statistical Areas (MSAs; without SLR zone), and all MSAs of the conterminous United States (CONUS) show over time: a) increasing density (structures per grid cell) of developed area; b) recent increases in densification (structures added relative to expansion; values greater than 1 mean that density is increasing faster than the addition of developed land grid cells; c) a steady increase in built-up percentage (percentage of grid cells developed within the MSA or SLR zone); and d) changing rates of expansion of developed area (increase in developed grid cells) with steep increases between 1950 and 1970, and dramatic decreases after 1980. Densification (b) and expansion (d) were calculated over 10-year time increments. Compared to all MSAs across CONUS, SLR zones and coastal MSAs are denser, more developed, and consistently have higher rates of expansion than MSAs across the country as a whole.
Fig 4.
Regional trends in the 6 feet SLR zone tell a varied story about the timing and intensity of historical coastal development. Built environment characteristics are based on structure density (structures per developed grid cell); a); densification (structures added relative to expansion; values greater than 1 mean that density is increasing faster than the addition of developed land grid cells); b); proportion of the SLR zone developed (proportion of grid cells developed within the unit of measurement); c); and expansion (increase in developed grid cells); d). Colors denote coastal MSAs aggregated to Census Divisions. Densification (b) and expansion (d) were calculated over 10-year time increments.
Fig 5.
The number of structures in the 6 feet SLR zone is used as an indicator of built environment exposure, illustrating high exposure, particularly in Florida and the South Atlantic division.
Relative built environment exposure (the percentage of total structures in the MSA contained in SLR zone) is depicted in MSAs in 2015 (a) and over time by Census Division (b). Absolute built environment exposure (absolute number of structures in SLR zone) is illustrated by MSA in 2015 (c) and over time by Census Division (d). All data used in figure available through an open license for U.S. government datasets [34] or through a CC0 license [39].
Fig 6.
Results from six regressions (a-e) using 3 dependent variables (share of built-up land, structure density, total structures) x 2 independent variables (wind speed, total historical hits) across five units of analysis (MSA zones within and out of the SLR and no, low, or high impact from hurricanes). The dependent variables were measured in 2015 and the models include a census division fixed effect. The coefficient estimates (y-axis) from the regressions are all relative to a SLR zone with no storms in the database (the zero line in the graphs). In models that use wind speed, MSAs classified as low severity were hit by less severe storms that have average max wind speeds < 100 mph; high severity MSAs were hit by severe storms, those that have average max wind speeds > 100 mph (b, d, f). In the regression models using hurricane hits (a, c, e); severe levels of storm hits are defined as places hit by 5 or more storms since 1900, less severe levels are those with 1–4 storm hits since 1900. Higher coefficient estimates illustrate higher rates of structure density or total structures relative to MSAs not affected by hurricanes.
Table 1.
Built environment and cumulative storm hits panel regression analysis from 1900–2015.
Fig 7.
The year a building is constructed can give an indication of the building codes and practices of the time, and thus, built environment vulnerability.
Here we show the percent of structures older than 1975 across MSAs (a). We illustrate the fine scale detail provided by HISDAC-US [22] to understand the development and age of neighborhoods in Tampa, FL (b). All data used in figure available through an open license for U.S. government datasets [34] or through a CC0 license [37].