Our burning of fossil fuels is warming the planet and melting its ice sheets.
Seawater also expands as it gets hotter, which contributes to sea level rise around the world.
By 2050, an estimated 2.5 million Americans, and many of the nation's most treasured historic sites, could face severe coastal flooding, according to the nonprofit news and science organization Climate Central.
"The map is changing," says Rob Young, a coastal geologist at Western Carolina University whose team is assessing climate risks at all 107 coastal units of the national park system.
"And climate change and rising sea levels are making that map change more quickly than it was 100 years ago."
The consequences extend far beyond archaeological sites. Flooded roads and parking lots increasingly restrict visitor access to national parks.
Historic forts experience more frequent inundation. Barrier islands along the south-east are eroding so rapidly that some landscapes are becoming almost unrecognizable.
Young points to North Carolina's Outer Banks, where homes continue to collapse into the Atlantic as the shoreline retreats.
In 1999, engineers moved the Cape Hatteras Lighthouse nearly a quarter mile inland to protect it from the encroaching sea.
Some landmarks, however, cannot simply be relocated. "We're not gonna move Fort Sumter," Young says.
Instead, he argues, the country will have to decide which places can realistically be defended and which may ultimately have to be surrendered to a changing coastline.
"So no, it's not gonna look like what it looks like now for your kids or for your grandkids," Young says.
