Coastal national parks in the Park Service's South Atlantic-Gulf Region hold tidal wetlands and mangrove forests, and irreplaceable historic buildings and archeological sites on low ground. All are vulnerable to accelerating sea-level rise and more frequent high tide flooding. In a 2021–23 project funded by the Park Service's Natural Resources Preservation Program, the U.S. Geological Survey and the Park Service estimated the chance of flooding at several parks, among them San Juan National Historic Site in Puerto Rico.
The park
Puerto Rico is the easternmost island of the Greater Antilles. San Juan National Historic Site overlooks Bahía de San Juan and covers just 0.29 square kilometer of land. Within it are centuries of history: El Morro, Castillo San Cristóbal, Fortín San Juan de la Cruz (El Cañuelo) and the city walls. Coastal habitats face loss unless they can build up in place or move to higher ground; historic and recreational sites face damage or loss outright.
San Juan National Historic Site, on Puerto Rico's north coast at the mouth of Bahía de San Juan. Map: U.S. Geological Survey.
The eroded shoreline of Isla de Cabras, with Fortín San Juan de la Cruz (El Cañuelo) behind it, April 2021. Photo: National Park Service.
Two kinds of rising water
Relative sea-level rise. Global warming is raising the seas, but at different rates in different places, depending on ocean circulation and density, whether the land is rising or sinking, and where ice sheets and glaciers are losing mass. A multiagency report set out five global scenarios — low, intermediate-low, intermediate, intermediate-high and high — depending on greenhouse gas emissions and ice loss. This study used the regional and local projections tied to them, giving both a median value and a "likely" range (the 17th to 83rd percentile).
The five sea-level rise scenarios for the site to 2100: each line is the median projection, the shading its likely range. Chart: U.S. Geological Survey.
High tide flooding. It happens when spring tides (the highest tides, at full and new moon), strong winds or storms push water unusually high. NOAA sets local thresholds:
| Threshold | Typical effects | Height above mean higher high water at San Juan |
|---|---|---|
| Minor | Stormwater backups, road closures | 52 cm (±19 cm) |
| Moderate | Greater disruption, possible damage | 81 cm (±25 cm) |
| Major | Destruction, evacuations | 119 cm (±39 cm) |
High tide flooding is rare in the Caribbean today, but rising seas make it worse, and rapid increases are expected. By 2100, today's flood level could become an ordinary high tide. Repeated "minor" floods add up to serious damage for low-lying infrastructure, and tidal flooding today may be a sign of challenges to come.
What the maps show
Projections use 1992 as the base year.
| Scenario and year | Median rise (likely range) | Share of park land likely flooded |
|---|---|---|
| Intermediate-low, 2050 | 26 cm (20–31) | About 1% |
| Intermediate-low, 2100 | 58 cm (49–69) | About 1% |
| Intermediate-high, 2050 | 36 cm (27–50) | About 1% |
| Intermediate-high, 2100 | 156 cm (118–172) | About 4% |
"Likely flooded" means a probability above 0.66.
Where the site is likely to be inundated under the intermediate-low and intermediate-high scenarios in 2050 and 2100. Maps: U.S. Geological Survey.
Under all three high tide flooding thresholds, most of the park was unlikely to flood. The likely-flooded area grew from about 1% at the minor threshold to about 2% at the major threshold.
Flooding probability at the minor, moderate and major high tide flooding thresholds. Maps: U.S. Geological Survey.
Using the maps
The maps can help managers gauge present and future effects of rising water on habitats, historic and cultural resources, and infrastructure, and support decisions under the resist-accept-direct (RAD) framework. The same approach can map habitat change such as the migration of tidal saltwater wetlands. The data and methods are in a USGS data release.
Sources
Based on "Projected sea-level rise and high tide flooding at San Juan National Historic Site, Puerto Rico," U.S. Geological Survey Fact Sheet 2024–3021; a work of the United States government in the public domain. The maps, charts and photographs are reproduced from the fact sheet.
Licence: CC0 1.0 (public domain) · Adapted from pubs.usgs.gov
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