SHINNECOCK NATION, Southampton, N.Y. — A maritime people who once spanned a large swath of the eastern Long Island shore, the Shinnecock Indians have been hemmed into a 1.5-square-mile patch of land on the edge of a brackish bay. Now, because of climate change, they’re battling to hold on to what they have left.

Rising seas are threatening to eat away at the Shinnecock lands. But the tribe is using everything at its disposal to calm the waves and restore a long, slim beach at the edge of Shinnecock Bay: dredged sand, sea grasses, beach grasses, boulders, oyster shells.

It’s a forever battle. Climate change is swelling and heating the world’s oceans at an accelerating pace. Inevitably, the Shinnecock will have to bring more sand to replenish what the rising tide keeps washing away. More grass will have to be planted. This spring, Shavonne Smith, director of the tribe’s environmental department, wants to expand the oyster reef designed to dissipate the energy of the waves.

“We have an inherent responsibility to protect the homeland,” Ms. Smith said on a recent Monday morning walk along the shore. “It’s not the type of thing where you can work against nature. You work with it.”

The project is using wind and water to sculpt the sand into gently rising dunes. It has taken nearly four years to restore a modest 3,250-foot stretch of beach and buffer the burial grounds that lie just beyond.

What the Shinnecock are doing on their land represents what climate adaptation experts call nature-based solutions. Several such efforts are underway elsewhere. New York City’s oyster reefs are being restored to protect Manhattan from storm surges. Marsh grasses have been planted to control erosion in parts of the Florida panhandle. Mangroves have been restored in Vietnam to protect coastal communities from sea level rise and storm surges.

To what extent these natural defenses will succeed in slowing down climate hazards remains uncertain. Ultimately, it depends not on nature, but on how quickly the world as a whole reduces the emission of planet-warming gases and stems the rate of sea level rise.

The global outlook for beaches is bleak. One study, published in Nature Climate Change this week, found that more than half of the world’s sandy beaches could disappear by the end of this century.

For now, the Shinnecock experiment has succeeded in saving their land from being devoured by the sea, Dorothy Peteet, a paleoclimatologist at the Lamont-Doherty Earth Observatory at Columbia University, observed on a recent visit.

“The question is how long will it work,” Dr. Peteet said. “It all depends on how fast the sea comes in, basically.”

Roughly 500 members of the Shinnecock Nation, which won federal recognition a decade ago, live on their traditional land, encircled by the summer homes and yacht clubs of the Hamptons. Across the water, sea walls try to hold back the water from the shoreline homes of Southampton, but they ultimately cause the beach to be washed away, which is why, said Eric Shultz, head of Southampton’s board of town trustees, officials are trying to dissuade residents from building new sea walls.

On the Shinnecock’s beach, facing Shinnecock Bay and the barrier island out in front, the sands have shifted over the years, as sands naturally do. A hurricane in 1938 knocked out a small chunk of the barrier island, bringing the force of the ocean into the calmer bay.

People in their 70s and 80s remember when the beach was wide. By the time Ms. Smith, 42, came here as a child, it was mostly pebbles. The trees along the shore, saltwater licking at their roots, were beginning to die off. Several times, recalled the Rev. Mike Smith, the 71-year-old former pastor of the Shinnecock Presbyterian Church, the road that encircled the cemetery flooded.

Then came Hurricane Sandy in 2012. The water swept over the pebbles and swelled up into the cemetery. It flooded the basement of the pastor’s house, jumping over the concrete berm he had built. It raced up Seneca Bowen’s yard.

Gradually, the beach that was part of Mr. Bowen’s yard was replaced by a marsh. The water just kept coming in.

To Chris Pickerell, marine program director of the Cornell Cooperative Extension program, Sandy was a foretaste of what is to come in the era of climate change. “The extent and duration of flooding on the reservation showed what unabated sea level rise will look like in the future,” he said. “Sea level rise, combined with storms like Sandy, alter these natural rhythms and can result in catastrophic damage.”

Across New York state, between 1880 and 2015, wetlands shielded by barrier islands have eroded by about 20 inches per year, according to the United States Geological Survey. The ocean-facing coast has eroded much faster. On the Shinnecock land, by the end of this century, sea level rise, relative to the land, is estimated to be between 2.1 and 4.4 feet, according to a climate adaptation report prepared by the tribe. In the coming decades, depending on how fast the sea rises, climate models project rising risks of chronic floods of 6 feet or more in the area.

“Something that was just slowly happening over time has picked up speed,” Ms. Smith said.

Hurricane Sandy also brought an opportunity: federal reconstruction money that the Shinnecock could use to save their territory.

In 2016, they poured 30,000 cubic yards of dredged sand on what they call Cuffey’s Beach. They filled biodegradable bags with oyster shells and then laid them in rows to calm the waves and clean the water in the process. Mr. Pickerell and his colleagues propagated local grass seeds to plant: sea grass in the bay, cord grass for the intertidal marsh area, and a native beach grass called Ammophila breviligulata to hold down the sand. A row of rocks was planted near the high tide line to protect the cord grasses from the southwestern winds that blow across the bay in summer.

But nature does what nature will. Ducks have eaten much of the cordgrass. More will have to be planted. At some point, more sand will have to be poured on the beach, too. Ms. Smith aspires to expand the oyster reef in the coming months.

At high tide, as the sun shined brightly on the bay, Ms. Cause stood on Cuffey’s Beach. Before the coastal restoration project, the water would come up to her knees. “We’ve been able to push back the water some, slow the water down, allow the plants and things to do what they need to do to revive themselves,” Ms. Cause said. “Just trying to use the natural things that were given to us.”

The beach was dotted with signs of life: sea lettuce, oyster shells, horseshoe crab remains, a dark green seaweed known as dead man’s fingers. The occasional great blue heron passed over the marsh. When the tide ebbed in midafternoon, a man dug for clams on the far end of a sand bank.

“This is what we have left. So then what?” Ms. Cause said. “We would like to just have it still be preserved here for our future, our future generations. The beach. The land. The clean water.”