If you go to a beach this summer, you might end up sunbathing in disputed territory. That’s partly because of climate change and partly because of a legal principle from the Roman Empire.

Most beaches have a natural defense against rising seas: The sandy area simply moves landward. But when property owners install sea walls or other barriers to protect beachfront homes and other buildings, the beach has nowhere to go. So it vanishes underwater.

Geologists call it coastal squeeze. It’s not a new problem, but it’s been accelerating recently as climate change causes sea levels to rise. And that’s prompting urgent questions about how coastal landscapes should be managed.

Richard K. Norton, a professor at the University of Michigan School of Law, described the situation with a question: “Are you going to save the beach house, or do you want to save the beach?” he said at a recent conference in New York City organized by Columbia University. “Because you cannot save them both.”

At issue is a legal concept from the sixth century A.D., when Emperor Justinian ordered the codification of Roman laws. The resulting code declared that features of nature like the air, running water, the sea and “the shores of the sea” must be held in trust for the use of the public. That idea passed into English common law, and then to the United States.

Today, most states define the beach below the high-tide line as public trust property, meaning members of the public have free access.