When governments find themselves fighting the threat of coastal erosion, their default response tends to be pretty simple: If sand is disappearing from a beach, they pump in more sand to replace it. This strategy, known as “beach nourishment,” has become a cornerstone of coastal defenses around the world, complementing hard structures like sea walls. North Carolina, for instance, has dumped more than 100 million tons of sand onto its beaches over the past 30 years, at a cost of more than $1 billion.

The problem with beach nourishment is obvious. If you dump sand on an eroding beach, it’s only a matter of time before that new sand erodes. Then you have to do it all over again.