This story appears in the December 2016 issue of National Geographic magazine.

Assateague Island National Seashore, which sits on a 37-mile-long sliver of land just off the coast of Maryland and Virginia, is gradually shuffling west. Over centuries, as hurricanes and nor’easters drive sand from its Atlantic beaches across the island and into its bayside marshes, the entire island is scooting closer to the coast.

“It’s neat, isn’t it?” says Ishmael Ennis, hunching against a stiff spring wind. “Evolution!” He grins at the beach before him. It’s littered with tree stumps, gnarled branches, and chunks of peat the size of seat cushions—the remains of a marsh that once formed the western shore of the island. Later buried by storm-shifted sand, it’s now resurfacing to the east, as the island shuffles on.

Ennis, who recently retired after 34 years as maintenance chief at Assateague, has seen his share of storms here. This national seashore, in fact, owes its existence to a nor’easter: In March 1962, when the legendary Ash Wednesday storm plowed into Assateague, it obliterated the nascent resort of Ocean Beach, destroying its road, its first 30 buildings, and its developers’ dreams. (Street signs erected for nonexistent streets were left standing in a foot of seawater.) Taking advantage of that setback, conservationists persuaded Congress in 1965 to protect most of the island as part of the National Park System. Today it’s the longest undeveloped stretch of barrier island on the mid-Atlantic coast, beloved for its shaggy feral ponies, its unobstructed stargazing, and its quiet ocean vistas—which have always been punctuated, as they are on other barrier islands, by impressive storms.

View Images Surrounded by giant green anemones, a lone ocher sea star feeds on mussels and barnacles. Since 2013 sea stars along the Pacific coast have been dying in unprecedented numbers. Scientists suspect that warmer seas are weakening the species’ resistance to disease.

Scientists expect that as the climate changes, the storms will likely strengthen, sea levels will keep rising, and Assateague’s slow westward migration may accelerate. Ennis knows the island well enough to suspect that these changes are under way. Assateague’s maintenance crew is already confronting the consequences. On the south end of the island, storms destroyed the parking lots six times in 10 years. The visitors center was damaged three times. Repair was expensive, and after fist-size chunks of asphalt from old parking lots began to litter the beach, it began to seem worse than futile to Ennis.

View Images High in the Sierra Nevada, floodlit giant sequoias tower into the night sky. They can live 3,000 years, but California’s historic drought has tested them. “We’re treating this drought as a preview of the future,” says ecologist Nate Stephenson.

A tinkerer by nature—he grew up on a small farm on Maryland’s Eastern Shore—he realized the situation called for mechanical creativity. Working with the park’s architect, Ennis and his co-workers adapted the toilets, showers, and beach shelters so that they could be moved quickly, ahead of an approaching storm. They experimented with different parking lot surfaces, finally arriving at a porous surface of loose clamshells—the kind often used on local driveways—that could be repaired easily and, when necessary, bulldozed to a new location. “It was a lot of what we called ‘Eastern Shore engineering,’ ” Ennis says, laughing. “We weren’t thinking about climate change. We did it because we had to.” He lowers his voice, mock-conspiratorially. “It was all by accident.”

Accidental or not, these modest adaptations were the beginning of something broader. The seashore is now one of the first national parks in the country to explicitly address—and accept—the effects of climate change. Under its draft general management plan, the park will not try to fight the inevitable: It will continue to move as the island moves, shifting its structures with the sands. If rising seas and worsening storm surges make it impractical to maintain the state-owned bridge that connects Assateague to the mainland, the plan says, park visitors will just have to take a ferry.

When Congress passed the act creating the National Park Service in the summer of 1916, it instructed the agency to leave park scenery and wildlife “unimpaired for the enjoyment of future generations.” The law did not define “unimpaired.” To Stephen Mather, the charismatic borax magnate who served as the first director of the Park Service, it meant simply “undeveloped.” Early park managers followed his lead, striving both to protect and to promote sublime vistas.

But the arguments began almost as soon as the agency was born. In September 1916 the prominent California zoologist Joseph Grinnell, writing in the journal Science, suggested that the Park Service should protect not just scenery but also the “original balance in plant and animal life.” Over the next few decades, wildlife biologists inside and outside the agency echoed Grinnell, calling for the parks to remain “unimpaired,” in ecological terms. But the public came to the parks for spectacles—volcanoes, waterfalls, trees you can drive a car through—and preserving them remained the agency’s primary concern.

Glacier National Park

MONTANA United States Glacial Retreat Glacier National Park In 1850 there were about 150 glaciers in what is now the park. Today only 25 remain. Grinnell Glacier has decreased in area by more than 75 percent. Mt. Gould 2015 1966 1850 1966 1850 Grinnell Glacier N Scale varies in this perspective. Matthew W. Chwastyk, NGM Staff Sources: National Park Service (NPS); Dan Fagre AND Mark Fahey, USGS

In the early 1960s, Secretary of the Interior Stewart Udall—who would oversee the addition of nearly 50 sites to the National Park System, including Assateague—became concerned about the agency’s management of wildlife in the parks. He recruited University of California wildlife biologist Starker Leopold, the son of famed conservationist Aldo Leopold, to chair an independent study.

The Leopold Report proved hugely influential. Like Grinnell, it called on the Park Service to maintain the original “biotic associations” that existed at the time of European settlement. In the decades that followed, the Park Service got more scientific. Park managers began setting controlled fires in forests where natural wildfires had long been suppressed; they reintroduced species that had vanished, such as wolves and bighorn sheep. The focus, though, was less on restoring ecological processes than on re-creating static scenes—on making each park, as the Leopold Report recommended, into a “vignette of primitive America.” In time that vision took on what Yellowstone historian Paul Schullery describes as an “almost scriptural aura.”

And yet, as Leopold himself later acknowledged, it was misleading. The notion of presettlement America as primitive ignored the long impact Native Americans had had on park landscapes, through hunting and setting fires of their own. It ignored the fact that nature itself, left to its own devices, does not tend toward a steady state—landscapes and ecosystems are always being changed by storms or droughts or fires or floods, or even by the interactions of living things. The ecological scenes the Park Service strove to maintain, from a largely imagined past, were in a way just a new version of the spectacles it had always felt bound to deliver to visitors.

“The Park Service has had a tacit agreement with the American public that it’s going to keep things looking as they’ve always looked,” says Nate Stephenson, an ecologist who studies forests at Sequoia, Kings Canyon, and Yosemite National Parks. “But time does not stop here.”

From the 1980s on, scientists gradually came to accept that a new sort of change was under way. The glaciers in Glacier National Park were shrinking, wildfires in Sequoia were getting larger, and coastal parks were losing ground to rising seas. Shortly after the turn of the century, researchers in Glacier announced that by 2030 even the park’s largest glaciers would likely disappear.

In 2003 a group of researchers at the University of California, Berkeley began to retrace the footsteps of Joseph Grinnell. In Yosemite and other California parks, the zoologist had conducted fanatically detailed wildlife surveys, predicting their value would not “be realized until the lapse of many years, possibly a century.” When the Berkeley researchers compared their own Yosemite surveys and other data with Grinnell’s 90-year-old snapshot, they noticed that the ranges of several small mammals had shifted significantly uphill, toward the ridgeline of the Sierra Nevada. Two other once common mammals, a chipmunk and a wood rat, were almost extinct in the park. The pattern was clear: Climate change had arrived in Yosemite too, and animals were migrating to escape the heat.

For a while the Park Service avoided talking about the subject. To acknowledge the reality of human-caused climate change was a political act, and the Park Service doesn’t discuss politics with its visitors. At Glacier the interpretive signs made only a passing reference to rising temperatures. Rangers avoided talk of causes. “We were very constrained,” remembers William Tweed, former chief of interpretation at Sequoia and Kings Canyon. “The message we got from above was basically, ‘Don’t go into it if you can help it.’ ”

Everglades National Park

United States FLORIDA SEA-LEVEL RISE Everglades National Park With 84 percent of the park already at an altitude of less than a meter, sea-level rise is increasing the extent of saltwater intrusion during storms and high tides. The park’s freshwater inland marshes are giving way to brackish and marine habitats. Typical conditions Saw grass marsh habitats flourish in groundwater, while mangroves thrive in saline and brackish environments. SAW GRASS MARSH MANGROVES PEAT SALT WATER FRESH- WATER SALT- WATER Sea-level rise As encroaching salt water degrades peat, marshes recede and collapse, and mangroves advance inland. SALTWATER INTRUSION SALTWATER INTRUSION SEA- LEVEL RISE SEA- LEVEL RISE Sources: NPS; EVERGLADES FOUNDATION; Florida INternational University; South Florida Water MAnagement District

The problem, though, ran deeper than transient politics. People had long come to national parks to experience the eternal—to get a glimpse, however deceptive, of nature in its stable, “unimpaired” state. The inconvenient truth of climate change made it more and more difficult for the Park Service to offer that illusion. But no one knew what the national parks should offer instead.

When Nate Stephenson was six years old, his parents fitted him with boots and a hand-built wooden pack frame and took him backpacking in Kings Canyon National Park. For most of the 53 years since, Stephenson has been hiking the ancient forests of the Sierra Nevada. “They’re the center of my universe,” he says. Soon after he graduated from UC Irvine, he packed up his Dodge Dart and fled Southern California for a summer job at Sequoia National Park. Now he’s a research ecologist there, studying how the park’s forests are changing.

While park managers are often consumed by immediate crises, researchers like Stephenson have the flexibility—and the responsibility—to contemplate the more distant future. In the 1990s this long view became deeply disturbing to him. He had always assumed that the sequoia and foxtail pine stands surrounding him would last far longer than he would, but when he considered the possible effects of rising temperatures and extended drought, he wasn’t so sure—he could see the “vignette of primitive America” dissolving into an inaccessible past. The realization threw him into a funk that lasted years.

Joshua Tree National Park

CALIFORNIA United States DROUGHT Joshua Tree National Park Joshua tree seedlings need rain to endure summer heat, but droughts have become more frequent and longer lasting. Scientists predict that by 2100 only isolated pockets of these now widespread trees will be found within park boundaries. Months of severe drought Palmer Drought Severity Index 12 10 8 6 4 2 0 1895 1955 2015 Monica Serrano, NGM Staff Sources: Cameron Barrows, UC RIVERSIDE; NOAA

“I was a firm believer in the mission of the Park Service,” Stephenson remembers, “and suddenly I saw that the mission we had was not going to be the same as the mission of the future. We could no longer use the past as a target for restoration—we were entering an era where that was not only impossible, but might even be undesirable.”

Stephenson began what he calls a “road show,” giving presentations to Park Service colleagues about the need for a new mission. Somewhat mischievously, he proposed a thought experiment: What if Sequoia National Park became too hot and dry for its eponymous trees? Should park managers, who are supposed to leave wild nature alone, irrigate sequoias to save them? Should they start planting sequoia seedlings in cooler, wetter climes, even outside park boundaries? Should they do both—or neither?

His audiences squirmed. Leopold had left them no answers.

On a late September day in Sequoia National Park, the sky is clear, blue, and thanks to a brisk wind, free of smoke from the wildfire burning just over the crest of the Sierra Nevada. Stephenson and his field crew are finishing a season of forest surveys, adding to a decades-long record of forest health. In their lowest-elevation study sites, below the sequoia zone, 16 percent of the trees have died this year, approximately 10-fold the usual rate. “It’s about what you’d see after a low-grade wildfire,” says Stephenson. Weakened by years of drought, many of the low-elevation trees are dying from insect attacks. At higher elevations, in the sequoia stands, several old giants have dropped some of their needles to combat drought stress; a few that were already damaged by fire have died. “It’s not ‘The sequoias are dying,’ ” Stephenson emphasizes. “The sequoias are doing relatively well. It’s the pines, the firs, the incense cedars—the whole forest is affected.”

The current drought may be a preview of the future, but the trouble with climate change—at Sequoia and elsewhere—is that many of its effects are hard to predict. Average temperatures at Sequoia will rise, and snow will give way to rain, but it’s not clear whether total precipitation will increase or decrease, or whether the changes will be gradual or abrupt. “We don’t know which scenario is going to play out,” says Sequoia and Kings Canyon Superintendent Woody Smeck. The Park Service can no longer re-create the past, and it can’t count on the future. Instead, it must prepare for multiple, wildly different futures.

WASHINGTON United States OCEAN ACIDIFICATION Olympic National Park As the ocean absorbs some of the CO₂ emitted by humans, its pH falls, making it more acidic. That can harm marine life—making it harder for species like mussels to form shells. Ocean pH pH measurement Trend line 9.0 More acidic 7.0 2000 2008 2016 Mussel-shell thickness Cross section 1970s 6.7mm 2000s 4.8mm California mussel (Mytilus californianus) Sources: Tim Wootton and Cathy Pfister, University of Chicago

In 2009 Park Service Director Jonathan Jarvis assembled a committee of outside experts to reexamine the Leopold Report. The resulting document, “Revisiting Leopold,” proposed a new set of goals for the agency. Instead of primitive vignettes, the Park Service would manage for “continuous change that is not yet fully understood.” Instead of “ecologic scenes,” it would strive to preserve “ecological integrity and cultural and historical authenticity.” Instead of static vistas, visitors would get “transformative experiences.” Perhaps most important, parks would “form the core of a national conservation land- and seascape.” They’d be managed not as islands but as part of a network of protected lands.

The report is not yet official policy. But it’s the agency’s clearest acknowledgment yet of the changes afoot and the need to manage for them. Exactly what that management looks like isn’t certain, and much of it will be worked out park by park, determined by science, politics, and money. Some parks have already gone to great lengths to resist change: Cape Hatteras National Seashore, for instance, spent almost $12 million to move a famous lighthouse a half mile inland. But such dramatic measures are rare and likely to remain so; the Park Service budget today is about what it was in 2008.

Instead, many parks are looking to boost their tolerance for change, adapting their own infrastructure and helping their flora and fauna do the same. At Indiana Dunes National Lakeshore, scientists are searching the oak savannas for cooler microclimates into which the Park Service might transport the endangered Karner blue butterfly, which has been all but driven from the park. In Glacier, biologists have already captured bull trout and carried them in backpacks to a higher, cooler lake outside their historic range. The idea is to give the fish a refuge both from climate change and from invasive lake trout.

At Sequoia, Stephenson wants park managers to consider planting sequoia seedlings in a higher, cooler part of the park—to see how the seedlings fare, and also how the public would respond to experimenting with the icons. “We have to start trying things,” he says.

At Assateague, while Ennis’s successors prepare the parking lots and toilets for change, Liz Davis, the chief of education, is preparing the park’s younger visitors. In 25 years at Assateague she has introduced countless school groups to the seashore. When elementary students visit, she takes them to the beach, shapes a model of the island out of sand, and throws a bucket of seawater across it to show how the island shifts. Then she turns the model over to the kids: Where would they put the parking lots and campgrounds? How about the visitors center? “They get really into it,” she says, laughing. “They’ll say, No, no, don’t put the new ranger station there, it’ll get washed away!”

Like the Park Service, visitors must learn to accept that their favorite park might change. “People ask, ‘Will I still be able to enjoy it? Will my kids and grandkids be able to enjoy it?’ ” Davis says. “The answer is yes, they will. They might not enjoy it in the same way, and they might not get here the same way. But they will still be able to enjoy it.”