Sea-Level Rise and High-Tide Flooding Projected for Dry Tortugas National Park

Dry Tortugas National Park lies almost 113 kilometers west of Key West, Florida, and can be reached only by boat or seaplane. Its land covers just 1.03 square kilometers, and at its center stands one of the largest nineteenth-century masonry forts in the United States. Like other national parks and preserves along the South Atlantic and Gulf coasts — with their tidal wetlands, mangrove forests, historic buildings and archeological sites in low-lying ground — it is vulnerable to accelerating sea-level rise and increasingly frequent high-tide flooding.

A collapsed section of the moat wall after Hurricane Irma, which made landfall in southern Florida in 2017. Photo: D. Diaz, National Park Service, October 2020.

In a project funded by the Natural Resources Preservation Program in 2021–2023, the U.S. Geological Survey, working with the National Park Service, estimated how likely different parts of the park are to be flooded. The maps help show how habitats may change and how the park’s infrastructure and cultural resources may be exposed.

Why local sea-level rise differs

Sea level is rising around the world with climate change, but not evenly. Ocean circulation and water density, the rise or sinking of the land, and where ice sheets and glaciers are losing mass all make local rates — relative sea-level rise — differ from the global average.

A multiagency report set out five global scenarios — low, intermediate-low, intermediate, intermediate-high and high — depending on greenhouse gas emissions and ice loss. The study used the regional and local projections for each. Because natural processes are uncertain, each scenario has a median projection and a likely range (the 17th to 83rd percentiles).

The five sea-level rise scenarios for the park to 2100: each line is the median projection, the shading its likely range. Chart: U.S. Geological Survey.

High-tide flooding

High-tide flooding happens when spring tides — the highest tides, at full or new moon — or high winds or storms push water to extreme levels. NOAA defines local thresholds: minor floods bring disruptions such as stormwater backups and road closures; moderate floods cause greater disruption and possibly damage; major floods can destroy property and force evacuations.

Along the eastern Gulf coast today, minor high-tide floods happen only a few days a year, and moderate and major ones less than once a year. Rising seas will change that fast: by 2100, under the intermediate-low scenario, minor flooding is projected more often than every other day — today’s flood level becoming tomorrow’s ordinary high tide. Repeated flooding, even minor, adds up to real damage.

Key findings

Projected sea-level rise at the park, measured from 1992:

Median, with likely range in parentheses.

The share of the park’s land classified as likely to be inundated (probability above 0.66):

Where the park is likely to be inundated under the intermediate-low and intermediate-high scenarios in 2050 and 2100. Maps: U.S. Geological Survey.

Under today’s high-tide flood thresholds — 52 cm (minor), 82 cm (moderate) and 119 cm (major) above mean higher high water — most of the park’s land is likely to flood, from about 58% at the minor threshold to about 83% at the major one.

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 assess 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 estimate future habitat change, such as the landward migration of tidal saltwater wetlands. The underlying data and methods are published in a USGS data release (Thurman and others, 2024).

Sources

Based on "Projected sea-level rise and high tide flooding at Dry Tortugas National Park, Florida," U.S. Geological Survey Fact Sheet 2024–3023, produced with the National Park Service; a work of the United States government in the public domain. The maps, charts and photographs are reproduced from the fact sheet.