Over the years, part of his work with his forestry colleagues has involved providing information to help correct Sequoia National Park’s fire problem.

Many Western landscapes, including Muir’s beloved sequoia groves, are adapted to wildfires. But before the Leopold Report, firefighters had feverishly extinguished even small fires in the Sierras, and the results were sometimes disastrous. The sequoias, which need light and fire to germinate, languished in thick shade and stopped producing seedlings. In the absence of little fires, forests became dense and stockpiled with flammable bits of tree and leaf debris, and the risk of bigger, hotter, unstoppable infernos grew. In the late 1960s, Sequoia National Park began to fix the problem by lighting low, tame ground fires in the park—“prescribed burning,” as it’s known—a practice that has persisted in part because it works, but also because it is supposed to imitate a natural process, as Leopold instructed.

By the mid-1990s, though, it became clear to Stephenson that recreating the forests of centuries past this way was an unreachable goal. Two of his colleagues used scars on old trees to calculate how many fires burned through the forests of Sequoia before Europeans got there; it was far more than the number of blazes the park’s burn crew had deliberately set on their own. Stephenson realized that, given the vastness of the park and the small number of scientists and firefighters on staff, it would be nearly impossible to recreate the forests that once were. Meanwhile, Stephenson read early predictions of the Intergovernmental Panel on Climate Change, the international body that distills the best climate science from around the world. Already the IPCC was painting a dire picture: “many important aspects of climate change are effectively irreversible,” the group’s 1995 report said.

“I started to do some real hard visualization of possible futures,” Stephenson recalled. “In all of them—since I’m a forest guy—the forest looked pretty beat up.”

Stephenson first fell into despair. “I imagine if you’re a cancer patient, you go through something similar,” he says, “which is, it’s a complete upheaval of what you were thinking, where you thought you were going. And you probably go through all these emotional struggles and then you finally reach a point where you just say, ‘Okay, what am I going to do about it?’” In 2002, he found one outlet for his feelings: He began giving a series of talks to urge park service managers to consider the ways climate change might upset some of their long-held assumptions. Nature—if such a thing could even be defined—was never going to look like it had in the past, he told colleagues in the region, and they would ultimately have to rethink their goals.

It took a while for official park service policy heads to catch up with Stephenson, but there were others in the agency who had begun to think along these lines. Don Weeks, a park service hydrologist, had a climate change epiphany in 2002, while he and colleague Danny Rosenkrans, a geologist, were flying in a propeller plane over Wrangell-St. Elias National Park in southwest Alaska. The plane received a radio transmission about a flash flood roaring down the Tana River at the center of the park, and Rosenkrans “tells me to get ready to see something that’s going to blow my mind,” Weeks recounted.

As they approached the headwaters of the Tana, Weeks gaped at the sight of a 3-mile-wide glacial lake that had split open in one night and dumped its contents downstream. The lake had been stable for about 1,500 years until 1999, when it ruptured for the first time. When Weeks saw the lake collapse, its second occurrence at that point, it was “the most phenomenal thing I’ve ever seen in my life,” he said.

The whole tableau—the empty lakebed scattered with icebergs the size of houses and the engorged river below full of floating tree trunks ripped from the ground by flash flooding—stunned him. “I mean it was the apex of my field work as far as just seeing that level of change and the danger associated with that, the rawness of it,” he recalled recently. “To top that, I got to be standing at the edge of a volcano while it’s going off, I guess.” It was the most memorable event of his entire career. Suddenly, climate change was real to Weeks in a visceral way, and he was fascinated.

In 2010, he took a temporary post with the park service’s newly created Climate Change Response Program that eventually morphed into a full-time job. Here he encountered a group of scientists who were grappling with problems the park service had never before contemplated. For inspiration, they had turned to a strategy first hatched by the 20th-century futurist Herman Kahn, the man who inspired Stanley Kubrick’s dystopian comic film “Dr. Strangelove,” and who helped the U.S. Armed Services plan for the possible outcomes of global nuclear war. One of Kahn’s tools, “scenario planning,” has since become a popular means for business leaders to anticipate futures that are wildly different from the ones they always assumed lay down the road.

Scenario planning is like a role-playing game. You start with a scenario informed by both science and intelligent conjecture. Then you write speculative narratives about what could happen—akin to science fiction. In a national park, thinking the unthinkable sometimes means envisioning the demise of the very things you are devoted to protecting. It also means reckoning with national and local politics: What happens when the political tide turns away from both the science of climate change and the values of the National Park Service?

In a 2011 scenario planning workshop in Anchorage, Alaska, one group of scientists and park managers wrote a scenario that seemed part-warning, part-gallows-humor, in which a family of Alaska Natives tossed a faded park sign into a campfire and watched “the last letters of ‘Bering Land Bridge National Preserve’ turn black and disappear.”

The story implies a situation so dire that the park either is barely functioning or ceases to exist (though when I contacted Jeff Mow, one of the workshop’s participants and now the superintendent of Glacier National Park, he said that story was a reflection on how locals might regard the park and wasn’t intended to sound its death knell). Such bleakness may speak to the level of anxiety felt across parts of the park service. But the ultimate purpose of writing such scenarios is to avoid the worst case by considering options ahead of time.

In 2012, a group of staff from Sequoia and Kings Canyon National Parks, including Stephenson, gathered at a conference center in the Sierra Nevada foothills with scientists and experts from the U.S. Forest Service, the Bureau of Land Management, state agencies, and academia. Armed with maps, large sheets of tracing paper, and a set of colorful markers, they sat down to play the game.

They considered different ecological and social-political scenarios—in which, say, there was more or less rain and snow, the public was on board with their work or illegally stealing water from the park, and federal policymakers either offered little or a great deal of support. The players fleshed out the details of their scenarios—tree die-offs, insect infestations, cuts and boosts to the park budget—then made their moves. Over the course of the game, an imaginary fire rose up from the dry forest below the park and raged through the sequoia groves. The players envisioned what would happen next. What had they won and lost because of climate change, fire, and drought?

It was still early in the life of the drought, and “we didn’t know it was going to be the most severe drought in at least 120 years,” said Koren Nydick, science coordinator for the two parks. “We did not expect some of the things in our scenarios to actually happen so fast.”

As the drought wore on, Stephenson became especially concerned about what would happen to the young sequoias. He periodically patrolled Giant Forest, 1,000 feet above his research plot, looking for signs of damage. He had long thought climate change would hit the sequoia seedlings first, and in the fall of 2014, he crept through the forest on his knees, his hands covered with dust, eye-level with the dainty, baby sequoias sprouting like small Christmas trees at the feet of their behemoth parents. He paused at the base of a massive sinewy trunk, took a breath, and turned his gaze skyward. There in the crown of a full-grown sequoia he saw tufts of brown, dying leaves. “I looked up and went, ‘What the hell is going on?’” he says.

That same season, Stephenson and a field crew from the USGS surveyed the sequoias in several groves, looking for more signs of dead leaves. Park managers braced for bad news. While a number of media outlets ran stories speculating whether the old trees might ultimately keel over, in the end, only about 1 percent of old sequoias lost more than half their leaves. Most of those dropped their brown leaves that season and then greened up the next as if nothing had ever happened.

The next year, after an exceptionally snow-deprived winter, a blaze named the Rough Fire ignited in the desiccated slopes of Sierra National Forest, just west of Kings Canyon National Park. It devoured Kings Canyon Lodge, a rustic wood-frame building that hosted a burger-and-ice-cream restaurant, and ascended into Grant Grove, the dwelling place of another famous assemblage of sequoia trees.

In parts of the grove, the flames burned hot and high, seared the crowns of trees and killed off most of them, including some old sequoias. But when the Rough Fire reached the part of the forest where the park service had carried out prescribed burning over the decades, it quieted, and many of the big trees there were spared. Just as they predicted, drought and wildfire had taken a toll, but their work in the forest had saved some of the trees—and that offered some hope.

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In the past three years, the Climate Change Response Program has surveyed scientists and managers in the parks about climate change. All over the country, hundreds of units in the National Park Service are facing unusual situations stirred up by climate change—and in some cases, the need to act on these directly contradicts park policy on what is “natural.”

Some parks are even discussing radical interventions in the wild that the agency would never have tried in the past. Glacier National Park, for instance, has experimented with loading bull trout into water containers and carrying them by backpack to lakes at high-elevations, where they might survive if the heat becomes unbearable for them elsewhere in the park—a strategy called “assisted migration.” In-house, the agency jokingly came up with the name “gnarly issues,” from surfer jargon, to describe these situations.

One of the gnarliest issues came up a year later in the Pacific Northwest. In May 2015, during one of the driest springs on record in Olympic National Park, a lightning strike lit a fire in the remote old-growth Queets rainforest. It kept burning through a record-breaking hot summer until September, scorching 2,800 acres. In August, lightning set another 7,000 acres ablaze on the west side of North Cascades National Park. The fire leaped across the Skagit River, jumped a highway, and charged up the mountainsides. It rushed toward the park visitor center, forcing tourists to flee.

Though large fires are common in dry regions like the Sierra Nevada, they rarely occur in wet forests like these. Some trees don’t deal well with fire, and in places like rainforests and alpine forests, pervasive dampness keeps blazes from traveling far. Only when the air is uncommonly dry and hot and the wind steady can a fire grow in size here. It then often kills nearly everything in its path. Fires like this tend to come only every few centuries to patches of forest on the wet, west side of the Cascade Range or Olympic Mountains. But these two fires, the largest west-side burns in either park’s history, had blazed up in the same season. Were they a warning sign of hotter, more fire-prone seasons to come?

On a hot day in August of last year, I donned a heavy black hard hat and followed Karen Kopper, her lead field technician, aptly named Cedar Drake, and a crew of four field researchers into a dusty, blackened section of forest in North Cascades National Park. Kopper, a petite, sandy-haired woman with a serious demeanor, works for North Cascades as a fire ecologist. She’s also writing a history of forest fires of the Pacific Northwest. But until 2015, she’d never seen a blaze burn so large on this side of the park.

We walked into what used to be a lush, dense, old-growth forest: home to centuries-old stringy-barked cedars with sinuous roots, towering Douglas firs, and hemlocks. Before the fire, the ground was a carpet of moss, huckleberry bushes, and sword and bracken ferns, and was usually sodden with rain for about nine months of the year or more.