Before Terry Lim handed me an aluminum flask filled with a blend of gasoline and diesel and asked me to set fire to the Tahoe National Forest, he gave me a hard hat, a pair of flame-resistant gloves, and a few words of instruction. “You want to dab the ground,” he said. “Just try to even out the line.”

The line was a low ridge of flame, no more than a foot high, creeping toward us through the forest. In front of it, the ground was springy, carpeted with a dense layer of pine needles and studded with tufts of grass. Specks of sunlight shimmered in the deep, almost kaleidoscopic green, bouncing off lime-colored ferns and conifer boughs. A foot-long alligator lizard skittered in front of me, pausing to pump out a couple of quick pushups before vanishing into the brush. Beyond the line, the ground was black and silent. Silhouettes of large trees loomed out of a sallow gray haze.

The lit cannister of fuel I was holding, known as a drip torch, had a long, looped neck that emitted a jaunty quiff of flame. I took a deep breath, and ducked my way through the scrub to the far end of the line. Then I walked back, dotting the tip of the torch’s neck to the forest floor a few feet in front of the flames, as if I were tapping out a message in Morse code. The dots and dashes ignited small fires, which joined up so rapidly that at one point I set fire to my boots. A swift, panicky battering with my gloved hands smothered the flames before any damage was done.

The main fire was advancing into the wind, so it moved slowly and stayed close to the ground. But my new flames had the wind at their back and quickly jumped across the gap separating them from the original front, transforming the line’s ragged edge into a wall of flame. It was mesmerizing and thrilling, and I couldn’t wait to do it again. As the afternoon wore on, I began setting my ignitions farther away from the line, in order to consume the forest faster. I started to anticipate how terrain would affect the pace of fire: open stretches of pine needles caught instantly, but I learned to place my dabs in tight clusters near saplings and denser shrubbery.

I wasn’t really supposed to be setting the forest on fire. That was the job of the United States Forest Service crew whose work I was there to observe. Their task was to carry out a prescribed burn—a carefully controlled, low-intensity fire that clears duff and deadwood, reducing the risk of a catastrophic wildfire. But the crew were temporarily occupied by what they called “a slop-over event”: a rogue ember had leaped across a trail that acted as a firebreak at one edge of the burn, sparking a half-acre blaze so hot that standing within a few feet of it made my chest hurt. While the crew used chainsaws and hoes to create a new firebreak, it fell to me to insure that no part of the line got ahead of the rest. If flames are allowed to break ranks and surge forward, they can whirl around and start running with the wind, burning more intensely and smokily than the prescription allows.

It took the team more than an hour to fully contain the slop-over. Then they returned to the line with their drip torches. By the end of the day, they had set fire to a hundred and twenty acres of forest. As Lim walked me out of the woods, through the gray-gold twilight of the burn zone, he gave a satisfied sigh. “See, now that’s nice,” he said. “The trees have breathing room.”

The contrast between that day’s prescribed burn and the uncontrolled blaze that the crew had rushed to extinguish epitomizes California’s spiralling problem with fire. Throughout the twentieth century, federal policy focussed on putting out fires as quickly as possible. An unintended consequence of this strategy has been a disastrous buildup in forest density, which has provided the fuel for so-called “megafires.” The term was coined by the Forest Service in 2011, following a series of conflagrations that each consumed more than a hundred thousand acres of woodland.

Megafires are huge, hot, and fast—they can engulf an entire town within minutes. These fires are almost unstoppable and behave in ways that shock fire scientists—hurling firebrands up to fifteen miles away, forming vortices of superheated air that melt cars into puddles within seconds, and generating smoke plumes that shroud distant cities in apocalyptic haze. Centuries-old trees, whose thick bark can withstand lesser blazes, are incinerated and seed banks beneath the forest floor are destroyed. Without intervention, the cinder-strewn moonscape that megafires leave behind is unlikely to grow back as forest.

Six of the ten worst fires in California’s history have occurred in the past eighteen months, and last year’s fire season was the deadliest and most destructive on record. More than a hundred people were killed, and more than seventeen thousand homes destroyed. Experts have warned that this year’s fire season could be even worse, in part because record-breaking rains early this year spurred the growth of brush and grasses, which have since dried out, creating more fuel. Governor Gavin Newsom proclaimed a wildfire state of emergency in March, months before fire season would normally begin.

The tools and techniques capable of stopping megafires remain elusive, but in the past few decades a scientific consensus has emerged on how to prevent them: prescribed burns. When flames are kept small and close to the ground, they clear the leaf litter, pine needles, and scrub that fuel wildfire, and consume saplings and low-level branches that would otherwise act as a ladder conveying fire to the canopy. With the competing vegetation cleared out, the remaining trees grow larger, developing a layer of bark thick enough to shield them from all but the hottest blazes. California’s state legislature recently passed a bill earmarking thirty-five million dollars a year for fuel-reduction projects.

“And yet no one is actually burning,” Jeff Brown, the manager of a field station in the Tahoe National Forest, told me when I visited him there recently. Although prescribed burns have been part of federal fire policy since 1995, last year the Forest Service performed them on just one per cent—some sixty thousand acres—of its land in the Sierra Nevada. “We need to be burning close to a million acres each year, just in the Sierras, or it’s over,” Brown said. The shortfall has several causes, but, some fifteen years ago, Brown set himself the almost impossible task of devising a plan for the forest he helps maintain that would be sophisticated enough to overcome all obstacles. Now he is coördinating an urgent effort to replicate his template across the Sierra Nevada.

The Sagehen Creek Field Station, where Brown is the manager, lies twenty miles north of Lake Tahoe, in the eastern Sierra Nevada. It was established in 1951 to conduct fishery and wildlife research, and is part of the University of California, Berkeley. Its amenities include a dozen radio-linked meteorological towers, snowpack sensors, tree-sap monitors, and a stream-depth gauge. It is not open to the public, but some twenty small red cabins are occupied by an ever-changing assortment of visiting researchers, student field-trippers, and even artists-in-residence.

In pre-Colonial times, California’s forests burned regularly, thanks to lightning strikes and fires deliberately set by Native Americans. Photograph by Kevin Cooley for The New Yorker

When I drove there, in May, there were still patches of snow in the shade, but the banks of Sagehen Creek were dotted with the first buttercups of spring. I followed a rutted dirt road for a couple of miles through the forest, arriving at a simple shingled cottage, where Brown lives with Faerthen Felix, the station’s assistant manager. From here, they help oversee the Sagehen Experimental Forest, nine thousand acres of mountain meadows, alkaline fens, and pristine streams surrounded by dense stands of Jeffrey and lodgepole pine.