One-fourth of the buildings within 500 feet of the nation’s coastlines are threatened by erosion in the next 60 years.

A study released by the Federal Emergency Management Agency concluded that close to 87,000 homes and other buildings stand on land likely to wash away into the oceans or the Great Lakes.

“The findings are sobering,” FEMA Director James Lee Witt said in a statement. “If coastal development continues unabated, and if the sea levels rise as some scientists are predicting, the impact will be even worse.”

The study estimated that 338,000 structures are within 500 feet of the coasts in the 48 contiguous states and Hawaii. It did not include Alaska or such large coastal cities as New York, Chicago, Los Angeles and Miami, which it said are heavily protected against erosion.

Costs of erosion damage to the owners of homes and other buildings are expected to average more than half a billion dollars per year, with the Atlantic and Gulf of Mexico coasts the most threatened. Flood insurance does not cover damage caused by gradual erosion.

Climatologists report that the nation appears to be entering a period of increased hurricane activity. The additional storms can be expected to increase the coastal erosion threat.

In many areas, improved construction and raising homes on posts has improved the ability to survive storms, creating an unexpected result after storms that cause severe erosion, said William Seitz of Texas A&M University at Galveston.

Now, houses that would simply have tumbled in the past remain standing, but out in the water, he said. They have to be condemned anyway because their septic systems can no longer function.

Outside of storms, erosion is often a gradual process, but it can be relentless. Its impact is easily illustrated by the problem of the Cape Hatteras Lighthouse, which was 1,500 feet from the shore when erected in 1870. By 1987 the lighthouse was in danger of falling victim to erosion. Finally, last year, the National Park Service moved it back 2,900 feet at a cost of $9.8 million.

The cost of moving homes can be beyond the means of most people.

The study was conducted by the H. John Heinz III Center for Science, Economics and the Environment, a Washington research organization.

Of 86,600 structures estimated to be threatened by erosion in the next 60 years, 53,000 are on the Atlantic Coast, 16,000 on the Great Lakes, 13,000 along the Gulf of Mexico and 4,600 on the Pacific Coast.