Will sea walls replace accessible beaches in our lifetimes?

— Emily Hardy, Oceanside, Calif.

For almost as long as people have been building near beaches, they have been protecting those buildings with sea walls. These barriers, which deflect strong waves and prevent coastal erosion, were constructed around ancient Roman harbors and medieval British cities. Archaeologists in Israel have even uncovered evidence of a 7,000-year-old rock wall constructed in the aftermath of the Ice Age to hold back rising oceans as the world defrosted.

But modern, human-caused climate change is escalating the threats that make sea walls necessary. Sea levels are rising, putting communities at greater risk from floods. Storms are intensifying, causing more and more sand to erode from beaches. Cities such as New York, Boston and Miami — as well as smaller, beachfront communities from California to Cape Cod — could lose thousands of homes and suffer billions of dollars in damage if they don’t find a way to hold back the water.

Sea walls have been credited with saving lives in extreme events. When the 2004 Christmas Day tsunami bore down on the Indian coastal city Pondicherry, a 300-year-old sea wall prevented powerful waves from flooding the city center.

Yet these barriers can also transform the very places they aim to protect. A hard structure might keep the land behind it safe from the intense energy of ocean waves. But all that energy will instead erode the sandy beaches in front of the sea wall, erasing the places where people like to lounge and play. Shoreline habitats that have evolved with the give and take of the tides — salt marshes, sea grass meadows, rolling expanses of sand dunes — will also be disrupted. And, by deflecting the force of oncoming waves, “hard defenses” in front of one community may contribute to a greater deluge in another.

There are alternatives to sea walls. Some communities use barge loads of sand to replenish their depleted beaches, a practice called “beach nourishment.” This can help counter storm surge, but it requires constant upkeep. Piles of boulders known as “rip rap” or “rock armor” can absorb the force of powerful waves. Jetties — breakwaters constructed perpendicular to the shore — can also protect a coastline, although they may interfere with the natural movement of sand and rocks.

Nature has its own mechanisms for protecting against storms. The dense, anchored root systems of mangrove forests help dissipate storm energy, prevent erosion and filter water as it drains from the land into the sea. As an added bonus, the world’s mangroves are a potent tool for removing greenhouse gases from the atmosphere; one study found that 75 billion pounds of carbon are sequestered in these rich habitats each year.

Oyster beds provide a natural breakwater in front of coastal communities. One study of sand deposits left by hundreds of years of storms in New York City found that wave energy in the city’s harbor increased 200 percent when people dredged up the shellfish for food and other purposes. After seeing the destruction wrought by Hurricane Sandy in 2013, New York is attempting to restore that natural protection by resurrecting reefs throughout the harbor. A similar effort in the Chesapeake Bay, where the oyster population now numbers a billion strong, has been credited with cleaning the water and providing protection against erosion.

A 2014 study of salt marshes — where hardy grasses grow in spongy peat that gets flooded by the tides — found that they are more durable and better at preventing erosion than human-built bulkheads.

Meanwhile, some communities are considering what to some might seem unthinkable: managed retreat.

The transformation of the world’s waterfronts is already underway. In North Carolina’s Outer Banks, a beloved East Coast vacation spot, average sea levels are about 8 inches higher than they were 40 years ago. Around the globe, the pace of sea level rise over the past 10 years was about twice the rate recorded during most of the 20th century. Even if humanity acts to halt climate change, the world’s oceans are projected to be about one foot higher in 2100 than in 2000. If we do nothing, that increase could be as much as eight feet.

The evidence is undeniable: There is no going back to the beaches we once knew. The only question is, how do we want them to change? Sea walls could protect homes but not necessarily the sand in front of them. Nourishment could help sustain beaches if communities commit to the upkeep. Many environmental experts and coastal researchers say natural solutions are the way to go — after all, they’ve been working for the planet longer than humans have been alive. All of these options come at a cost, and communities will have to evaluate whether it’s better to pay or move away from vulnerable coasts.

So Emily, your question remains to be answered. Scientists have collected the data, engineers have examined the options. It’s up to us as a society to decide what happens next.