KEYHAVEN, England — Fearful of a French invasion after breaking with Rome, Henry VIII erected a line of massive coastal forts along the English Channel, and one of the most imposing is called Hurst Castle. It has stood on its sandy spit since 1544, through the Napoleonic Wars and World War II. Its garrison protected the Allied forces on D-Day.

But earlier this year, a large section of the castle — a wing constructed in the mid-19th century by the best military engineers in the world — tumbled into the fast currents of the Solent strait.

Hurst Castle has done its duty, but it is hard to fight the sea — specifically, its caretakers say, the steadily rising waters and more intense winter storms of a warming world.

All nations stand to lose cultural monuments to climate change, including the United States. But Britain is especially vulnerable. The country is stuffed to the attic with heritage properties.

Whereas animals might migrate, seeking more hospitable habitats, a Norman church, Roman villa or neolithic stone circle cannot move. They’re stuck where they are, built for preindustrial climates, centuries ago.

Britain’s preservationists find themselves trying, sometimes struggling, “to protect the past for the future.” They know not everything can or will be saved. There will be triage — and loss.

“It looks impregnable, like it’s been there forever, doesn’t it?” said Roy Porter, senior curator at Hurst Castle, as a small ferry puttered around the point.

As the boat turned, suddenly it came into view: the catastrophic collapse of the east wing. It looked as if the castle wall had been struck by a broadside of cannon fire from a ship of war.

Equally bad, but less visible, are deep cracks in the fortified casemates of the Victorian-era west wing, now ringed by fencing and off-limits to visitors for the foreseeable future.

“This place was designed to withstand the worst of the worst,” said Rob Woodside, estates director for English Heritage, which owns and cares for Hurst Castle along with another 400 cultural sites.

By “worst of the worst,” Woodside meant siege and shelling. “These historic structures were built for the climate of their times, not the changing climate we are seeing today,” he said.

In recent years, Hurst Castle lost its seaside beach, likely as a result of decades of coastal development. Then the waters rose, the storms came, and the sea exposed and undercut the castle’s foundations.

To repair the collapse, and keep the Tudor heart of the castle open, English Heritage has imported 5,000 tons of Cornish granite and 6,000 tons of pebble beach to create new sea defenses, at a cost of more than $2 million.

Much more will be spent. And then what? The past 100 years have seen five inches of sea-level rise on this very spot, Woodside said. By the end of the century, it could be more than three feet. The castle sits on a sand bar.

“Hurst Castle is one of the canaries in the coal mine, but it is just one of many,” said Keith Jones, climate change adviser for the National Trust, which cares for 28,000 historic buildings, including castles, mansions, barns, lighthouses, mills, pubs and villages, as well as holiday cottages.

The National Trust warned in March that while 5 percent of its 67,426 sites — natural and constructed — already face the “highest level” of threat from climate change, that portion could rise to 17 percent over the next 40 years, depending on what actions the world takes to limit future warming.

Although it is not possible to attribute one event to climate change, already winters are warmer and wetter in Britain, which means more severe and frequent flooding.

The summers are hotter and drier, too, which contributes to a phenomenon called “ground heave” — the expansion and contraction of soil. The National Trust recently cited that as the cause of a wall collapse at an 18th-century barn at Malham Tarn, a glacial lake in the Yorkshire Dales in northern England.

Woodside of English Heritage said his organization “is seeing more masonry falls this year than any time in recent memory.”

Historically, the British weather is gray but relatively benign. Now there are frequent violent downpours — real gullywashers, a month’s worth of precipitation in a day.

The London Underground flooded last week from such rain — imagine what a deluge can do to medieval abbey gutters.

At Lyme Park in Cheshire, one of the grandest estates in England, the grounds — the gardens and deer park — were inundated not once, but twice, in 2019.

“The tapestries really do not like it,” Jones said, recalling that staff stayed up through the night, prepared to protect the paintings and antiques if the water levels kept rising.

Landscapers working at the 400-year-old gardens at Ham House, on the River Thames west of London, have had to contend with soaring heat and humidity — and the possibility of summer days with temperatures above 100 degrees Fahrenheit.

To adapt, the gardeners there are now cultivating Mediterranean plants, including agaves. In the walled kitchen garden, they’re trying eggplants and chile peppers.

The 17th-century mansion has had to close on days when the interior temperatures soared, Jones said. It was once also shut down when high winds were forecast to protect visitors from the possibility that its ancient trees could tip over.

“We are not used to gust speeds reaching almost 50 mph in August” and neither are the trees, which are top-heavy in full leaf, observed Ham House’s head gardener, Rosie Fyles. “I’m now trying to apply a climate change perspective to every single action. It’s a new, vital future view that makes my head spin.”

Conservationists say that in the past it might have been enough to replace gutters and roofs, the endless maintenance necessary to keep old houses standing.

Now, they say, they will have to work at a much larger scale.

The National Trust plans to plant 20 million trees over the next few years to slow water runoff and provide cooling shade. It is considering reintroducing beavers to control the rivers running through its monuments.

Mairi Davies is the climate change policy manager for Historic Environment Scotland, which helps protect 336 properties in state care. Because so many of Scotland’s cultural sites are on the coast, conservationists are investigating nature-based solutions, too, like building new saltwater marshes and recharging beaches.

There are limits.

On Sanday in the Orkney Islands, the 3,000-year-old Meur burnt mound was imperiled by rising seas. Structures of that sort are some of “the most enigmatic, problematic and exciting archaeological remains of the Bronze Age,” according to archaeologists at SCAPE, a group of researchers who work with the public to investigate and promote ancient sites on Scotland’s coast. 

The beachside Meur mound, which includes a small tank that would have been filled with fresh water and heated with hot stones, may have been a prehistoric sauna or used for brewing or boat-making. It is a mystery to be solved in the future.

As the waters rose, the archaeologists and volunteers decided that rather than sacrifice the Bronze Age hot tub to the sea, they would excavate the mound and move the whole thing to an interpretive center on drier land.

That was one solution. But as Davies observed, that’s not really possible with a castle.