Forests are reeling from climate change—but the future isn’t lost

Forests are reeling from climate change—but the future isn’t lost

The first thing you notice in this fire-scarred forest is the color. Not long ago this square of land south of Yellowstone National Park was a monochrome of ash and burned pines. But last summer, shin-high seedlings and aspen shoots painted the ground an electric green. Purple fireweed and blood-red buffalo berries sprouted around blackened logs. Yellow arnicas danced in the breeze. Five years after 2016’s Berry fire chewed through 33 square miles of Wyoming, this slice of scorched earth was responding to fire as Rocky Mountain forests have for millennia: It had entered a season of rebirth.

Monica Turner was cataloging that recovery. On a sweltering July day, Turner, a professor of ecology at the University of Wisconsin–Madison, shuffled along a line of tape she’d stretched 50 meters across the ground. She and a graduate student were counting every lodgepole pine seedling within a meter on either side. We were far enough from paved roads that there was no telling which forest inhabitants might be lurking—elk, deer, moose, wolves. The air was so hot I wondered fleetingly if the bear spray canister on Turner’s hip might explode.

THE BIGGEST AND OLDEST TREES ARE NOW THREATENED

Sequoia National Forest, California • Embers rain from the crown of a giant sequoia ignited by a windblown ember in September 2021. Dead twigs and leaf litter can collect in the crowns and cavities of older trees, providing fuel for fire. This tree in Long Meadow Grove survived. 

Fires in the past two years killed up to a fifth of the largest sequoias

Sequoia National Forest, California • On a burnt slope in the Sierra Nevada, their only native home, giant sequoias—some more than a thousand years old—stand like black daggers among the other dead: white firs, sugar pines, incense cedars. While sequoias can often survive ground fires because they have few low branches, this fire blew flames into the crowns. Climate change and fire suppression are fueling bigger wildfires.

So many tiny trunks crowded the researchers’ feet that covering a distance they normally would walk in seconds took almost an hour. In the end they counted 2,286 baby trees in an area half the size of a tennis court. This spot was producing 70,000 pines an acre. “This is what lodgepole pines do,” Turner said. “They come back gangbusters.”

Yet the previous day, in a neighboring patch of burned timber, Turner had documented something unsettling. Instead of a river of new pine seedlings, the ground was a mix of flowers, grasses, and caked earth. Aspens were there, but so were invasive grasses and sour weeds. Along one 50-meter tract, Turner had spotted just 16 baby pines; on another, only nine. All told, this patch was producing fewer than one-fiftieth as many young conifers as its neighbor.

The two patches of forest were almost identical. Before the Berry fire, both sites had burned around the time of the Civil War. But one distinction set them apart. The site with fewer pines had burned another time as well, in 2000. Trees that sprouted after that fire had not yet matured to produce enough seeds before being wiped out in 2016. In this place, rather than reseeding the pine forest, the Berry fire was resculpting the landscape into something new, perhaps for centuries or even millennia.

39 million mangroves died of thirst here

Northern Coast, Australia • Years of high heat and drought had stressed mangroves along hundreds of miles of the Gulf of Carpentaria coast. Then the intense El Niño of 2015-16 finished them off by causing a temporary 16-inch drop in sea level here, drying out the trees’ roots. This 2021 photo shows little recovery; the green belongs to a short mangrove species that survived the die-off.

In 2021, fires torched 21 million acres in a place known for cold

Eastern Siberia, Russia • Snow blankets a boreal forest that burned the previous summer in the Sakha Republic. Small fires occur regularly in this region about twice the size of Alaska. But in 2021, four times the average annual area ignited, potentially releasing ancient carbon from permafrost and transforming forest into grassland.

QUAKING ASPEN, NORTH AMERICA’S MOST WIDESPREAD TREE, IS DYING IN DROVES

Grand Prairie, Alberta, Canada • Forest scientists are struggling to figure out a response to “sudden aspen decline,” as they call the die-offs that have struck since the turn of the century. But they know drought and rising temperatures make the trees more susceptible to disease and insects—such as the tent caterpillars that defoliated this stand.

The dome fire killed more than a million joshua trees in 2020

Mojave National Preserve, California • Yet these icons of the Mojave Desert already faced other threats. Seedlings appear less often as temperatures rise and a long drought persists; invasive grasses promote fire. This relatively cool pocket, where some trees survived the 2020 fire, is a potential refuge. Volunteers are planting seedlings to aid the recovery.

Yellowstone is part of a global trend. From the Amazon to the Arctic, wildfires are getting bigger, hotter, and more frequent as the climate changes. Australia’s forest fires in 2019 and 2020 burned an area as big as Florida. That’s devastating enough. But often overlooked amid the initial carnage is what happens after the trees die: Many forests now struggle to recover. That too is not limited to Yellowstone, nor is it always triggered by fire—but it is caused by climate change.

In many places, forests are no longer regenerating on their own. Some of the world’s most significant stands are instead transitioning to something new. Some will never be the same. Others may not come back at all.

THIS BURNED FOREST IS GROWING BACK, BUT OTHERS AREN’T

South of Yellowstone, Wyoming • Ecologist Monica Turner counts lodgepole pine seedlings sprouting (along with fireweed) among pines that burned in 2016. Fire opens seed cones, allowing lodgepoles to regenerate—but if another fire comes before trees mature, they may not grow back.

It’s a tough time to be a tree. Earth has lost a third of its forests over the past 10,000 years—half of that just since 1900. We logged them for timber. We cut them to make way for farms and cattle. We cleared land to build homes and roads. Globally, deforestation has decreased from its peak in the 1980s, but trends vary by region. In Indonesia, which had been mowing down forests for oil palm plantations, primary forest loss has declined since 2016. From August 2020 to July 2021, the Brazilian Amazon lost 5,000 square miles of rainforest, a 22 percent increase over the previous year. Since 1990, we’ve cut down more forest globally than there is forest in the United States.

Now fossil fuel emissions spewing from coal plants and tailpipes are rearranging forests in other consequential ways. As carbon dioxide and other greenhouse gases warm the planet, some of its estimated 73,000 tree species are pushing poleward and higher up slopes, dragging other life with them. Alders, willows, and dwarf birches are expanding across the Arctic, from Scandinavia to Canada, providing cover and food for snowshoe hares and moose. Trees are growing faster as they soak up excess CO2—a key ingredient for photosynthesis. That “greening” of the planet has so far helped slow climate change, protecting us from ourselves.

But climate change also is killing trees. And what has forest scientists increasingly uneasy is the quickening pulse of extreme events—fire, more powerful storms, insect infestations, and, most notably, severe heat and drought, which can worsen the effects of all the rest. These singular, frequently unprecedented episodes can swiftly inflict mass tree mortality, shifting forests that have been around since the last ice age to entirely new states.

“We have a whole set of mechanisms that are pushing Earth’s forests to grow more and suck up more CO2 ,” says University of Utah biologist William Anderegg. But those mechanisms “are fundamentally in tension with mechanisms that are pulling Earth’s forests toward a cliff—with more tree death and more loss of carbon.”

The forests that have plunged over that cliff already are only a small fraction of the three trillion trees and 10 billion acres of forest on this planet. Climate change still poses less of a threat to forests than logging and land clearing, but the threat is growing fast. “How big does that fraction get over time, and when does it overwhelm the other?” asks Matt Hansen of the University of Maryland, who monitors forests using satellites.

The problem is, we can’t yet quantify the planetwide scope of climate impacts. Satellite data show that Earth’s tree-covered area actually expanded from 1982 to 2016 by 7 percent, an area larger than Mexico. But that doesn’t mean forests are doing fine: The data don’t distinguish between natural forests and industrial tree farms, such as the millions of palm, eucalyptus, and pine trees planted as crops while rainforest is cleared. The data also don’t show which forests were lost to chain saws and which were killed by climate-related events. (An illustrated guide to how heat and drought are killing trees)

No computer model can yet project how climate will change forests globally—or how their carbon stores will feed back on climate. “Earth system models historically haven’t done a good job of capturing this,” says Charlie Koven, a climate scientist with the Lawrence Berkeley National Laboratory, who worked with the UN’s Intergovernmental Panel on Climate Change (IPCC). Only two of its 11 models include both fire and geographic shifts in plants.

The global number of trees isn’t the only thing that matters. Climate change is reshaping forests locally almost overnight, transforming them even where there are policies to protect them. It’s happening so fast we can’t discern the consequences. While we’re losing trees of all types and sizes, the biggest and oldest harbor the most carbon, are important for biodiversity, and will be the hardest to get back. “Big trees are disproportionately important and cannot be replaced quickly—if ever,” says Nate Stephenson, a scientist emeritus with the U.S. Geological Survey.

That will matter to us all. Humans are bound to the woods. Our history is linked to trees. We climbed down from their canopies and used them to make fire. The advent of paper—and the printing press—let literature and science flourish. Trees feed us, shelter us, give us medicine. We lean on them in ways we scarcely acknowledge, as sources of wonder and inspiration or to decompress in a noisy world.

One of my favorite escapes is the Hoh Rainforest on the Olympic Peninsula, four hours from my home in Washington State. It’s a place where glistening ferns tall enough to hide elk crowd the ground while ancient spruces and big-leaf maples draped in emerald moss block the sky. What you can see in such places is complex enough, but humans also are beginning to appreciate how much is going on out of sight. Trees in a forest are not isolated individuals; they share nutrients and data across species in underground fungal networks. They talk to one another, passing chemical messages, warning of pest invasions and other dangers.

Since 1990, we’ve cut down more forest globally than there is forest in the United States.

Old-growth forests are collaborative, Korena Mafune, a postdoctoral research fellow at the University of Washington, told me as we walked through the Hoh recently. She suspects a diminutive version of this fungal network may even exist on high branches. She’s found soil beneath moss growing in the canopy, with tiny trees sprouting from the living branches of big old ones—“a mini-forest within a forest,” she says. She worries that even this ancient place, so much richer than a tree plantation, could change rapidly if a hot enough dry spell lasted too long.

Already, snow melting early in Alaska is depriving yellow cedars of their warming blanket, letting cold snaps freeze their roots and killing them by the thousands. Heat and drought sparked by climate change have killed up to 20 percent of trees in Africa’s Sahel, in southwest Morocco, and in the western U.S. since 1945, according to the latest IPCC report. Five of the eight most abundant tree species in the American West have declined significantly just since 2000, mostly from fire and insect infestations. Lodgepole pines top the list.

“Forests are far more vulnerable in the climate change era than people think,” says Craig Allen, a landscape ecologist and collaborator of Monica Turner’s who retired last year from the U.S. Geological Survey. He’s been trying to alert people to that danger for two decades now.

FIRST CAME THE
DROUGHT, THEN THE
DEVASTATING HAIL

Southeast Coast, Brazil • Six months after Australia’s mangrove die-off in 2015, the same El Niño caused a storm that hit mangroves in the estuary of the Piraquê-Mirím River. Drought had stressed the trees, partly by boosting the water’s salinity. Hail and wind killed nearly a third of them. Globally, the main threat to mangroves—clearing for timber or farming—has declined. But climate change is a rising concern.

RISING SEAS
ARE CREATING
‘GHOST FORESTS’

Along the Cape Fear River, North Carolina • Seawater seeps into
aquifers and freshwater wetlands, killing
vegetation such as
these bald cypresses near Eagles Island.
Dredging encouraged
the intrusion and killed
the trees long ago.
Cypress stands all over
the Southeast have
been decimated since
the 19th century by
logging and draining
of wetlands.

Turner has a quick smile, bobbed sandy hair, and, at 62, a college student’s capacity to stay upbeat while working nonstop. I spent several days with her last summer in the John D. Rockefeller Jr. Memorial Parkway. The parkway is not a highway but a parcel of sagebrush and pine larger than Manhattan. It links Yellowstone and Grand Teton National Parks. Turner seemed so at home on this forested plateau that her Long Island accent kept catching me off guard.

Turner showed up in Yellowstone in 1978 to work as a summer ranger, giving guided nature talks at twilight. Yellowstone, with its golden meadows and kaleidoscopic thermal pools, transfixed her. She eventually would return and spend decades studying its trees.

In 1988 Turner and a colleague, ecologist Bill Romme, crisscrossed its wildlands in a helicopter, scanning the aftermath of the park’s worst fire season in a century. A third of Yellowstone—793,880 acres—had gone up in smoke in a few months. Turner feared it would never recover. But during that flight she began to believe what Romme had recently suggested: This was what Yellowstone was supposed to do.

Many people had assumed Yellowstone’s fires blew up because firefighters more than a century earlier had begun suppressing wildfires, allowing excess trees to pack forests like kindling. This is true in parts of the West. But while traversing game trails to map the park’s fire history, Romme discovered that Yellowstone historically burned very severely once in a great while. “There had not been very many fires even in the days before fire suppression,” he told me one morning in the park. “It was really kind of shocking.”

Yellowstone is lodgepole country. Their thick, slender trunks occupy 80 percent of the park’s woods. Some are serotinous, meaning they need fire to unlock cones that hold their seeds. Romme had shown that these forests had seen monster stand-clearing blazes in the 1700s and 1800s. Such fires were rare because the park was “too moist, and it was too cool,” he said. But every 100 to 300 years, in an exceptionally hot, dry summer, enormous patches would ignite in one great conflagration, allowing the woods to be reborn.

Forests, Turner realized, were resilient. It would take time to accept how that could change.

A TREE’S RINGS
REVEAL A LONG HISTORY
OF SURVIVING FIRE

Jemez Mountains, New Mexico • From 1650 on, this ponderosa pine survived 15 fires—but in the 20th century most fires were suppressed. Fuel built up in the forest; a long, hot drought settled in. A monster blaze in 2011 ravaged 45 square miles in its first night. The result? “An extinguished ecosystem that will never be seen again here,” says scientist Craig Allen.

An early warning came in 2002, during the Southwest’s worst drought in five decades. Weeks before meeting Turner, I scrambled up a dusty embankment near New Mexico’s Bandelier National Monument. Beside me, Craig Allen and Nate McDowell, an earth scientist at the Pacific Northwest National Laboratory, examined a picture Allen had taken in 2002. It showed dense throngs of piñon pines, their needles tinged orange because they were dying.

Allen swept an arm toward a nearby mesa. He’d studied forests in this scratch of arid woodland near the Jemez Mountains since the 1980s. Now the adult pines from his picture were gone. What remained was cracked earth, hardy junipers, and an occasional seedling.

A drought in the 1950s had brought even less rain, and yet between 2002 and 2004 the impact on trees was worse: In some areas, more than 90 percent perished, many falling victim to bark beetles, natural predators that spread as never before. All told, some 350 million piñons, New Mexico’s state tree, died across the Southwest. Unprecedented fires eviscerated hundreds of thousands of acres of ponderosa pines.

Allen was taken aback by the severity. But bit by bit, he, McDowell, and their colleagues came to understand: This drought was hotter. The slight increase in temperature attributable to greenhouse gas emissions was already enough to set the death of New Mexico’s trees in motion.

And what’s become ever more clear to Allen, through his own work and that of many others, is that trees the world over are vulnerable to the added heat. The warmer atmosphere sucks more moisture from plants and soil. To cut their losses during droughts, trees close pores in their leaves, called stomata, or shed leaves entirely. But that limits the CO2 they take in, leaving them both hungry and parched all at once. When it’s especially hot, they even leak some of the water they’re desperate to retain.

When soil gets dry enough, trees can no longer maintain pressure in the internal conduits that carry water up to their leaves. Air bubbles interrupt the flow, causing fatal embolisms. Some trees protect themselves with deeper roots, for example, or by storing more water—but those investments come at the expense of growing taller to compete for light and space with other trees.

The upshot, scientists figured out in just the past decade, is that many trees in most landscapes, from the hot, rainy Amazon to cold, dry Alberta, are operating at the limits of their hydraulic systems, even under normal conditions, with little safety margin. That means a hot drought can push them over the threshold. The 2002 drought in the Southwest did exactly that: Tree-ring records would later show it was the driest and worst year for growth in a millennium. No other year even came close.

All this awakened Allen to what he now sees as a grave global threat. “Seeing the transformation of this landscape that I’d studied my whole adult life … climate change wasn’t theoretical anymore,” he told me. He started tracking the mass mortality events elsewhere. Over the next two decades, heat and drought would kill billions of trees directly and indirectly—in Spain, in South Korea, throughout Australia. In central Siberia, Russia lost two million acres of firs. In Texas in 2011, drought killed more than 300 million trees—one out of every 16 in the state.

Increasing warmth helped deadly forest pests spread, weakening trees and letting beetles and moths live through the winters or reproduce more often. Such invasions wiped out trees in Honduras, Turkey, and Algeria. In central Europe they arrived as a shocking new plague.

WHICH TREES WILL
SURVIVE A HOTTER,
DRIER FUTURE?

Hölstein, near Basel, Switzerland • How exactly do trees die of thirst? As part of a 20-year project to answer such questions, University of Basel plant ecologist Ansgar Kahmen and technician Lucio Rizzelli regularly ride into the canopy of a research forest. Here they’re measuring the water vapor that a Norway spruce sheds through its needles.

WHAT FORESTS DO
FOR US TRANSCENDS
CALCULATION

Southwest of Copenhagen, Denmark • The tangible benefits—food, wood, carbon storage—are not the only ones. Immersing oneself in woods, or “forest bathing,” has been shown by scientists to reduce mental and physical stress. At Camp Adventure, visitors ascend a spiraling 150-foot-high boardwalk to get a fresh perspective on trees—and perhaps on life.

On a chilly day last fall, I struggled up 227 steps inside a former Cold War surveillance station on a 4,300-foot peak outside Prášily, a Czech village near the border with Germany. I huffed to keep pace with Petr Kahuda, a ranger at Šumava National Park, and Zdeněk Patočka, a forest scientist at Mendel University. The tower was built in the 1960s to listen in on NATO radio transmissions, but after the Iron Curtain fell, the Czech government opened it and this 170,000-acre park to the world. At the top, a circular balcony overlooks rolling forests that once fueled the region’s glass industry. Now, huge portions of its trees are dying, victims of bark beetle attacks.

In 2018 central Europe experienced its worst drought in five centuries. Summer temperatures hit nearly six degrees Fahrenheit above average. Tree deaths skyrocketed, and weakened survivors attracted beetles. Worst hit was Czechia. Loggers raced to salvage what they could. People were so desperate, Kahuda said, that one man offered Šumava National Park his sheep, hoping their smell might drive away the insects.

In Germany, 750,000 acres of forest died from 2018 to 2020. No one knew quite how to respond. History aggravated the crisis: Almost no native forests remain in central Europe. Humans have thoroughly transformed the landscape. Originally dominated by beech and oak, many forests had been replanted with Norway spruce and pine. After World War II, clear-cuts were made to ship timber and pay reparations to the Allies.

But while spruce grows naturally at higher, cooler elevations, foresters also planted it down low. It did fine there for 70 years. Then, says Henrik Hartmann, a forest expert at the Max Planck Institute for Biogeochemistry, “climate change made this formerly suitable habitat inadequate.”

For a while, Turner kept her faith in Yellowstone’s cycle of fire and rebirth. Trees die; it’s part of the equation. But at a 2008 conference in Jackson Hole, Wyoming, she was confronted with the possibility that the equation had changed. A colleague presented maps suggesting that Yellowstone in coming decades could see fire seasons like 1988’s nearly every summer. That year “would no longer be exceptional—and the exceptional years would be out of control,” Turner recalls.

She didn’t buy it at first. For thousands of years Yellowstone’s monster blazes had burned erratically at different intensities, scorching some spots and skipping others. The mosaic let animals and trees recolonize easily. Her own work, influenced by that long-ago chopper ride, had thoroughly documented that pattern. But what if the system no longer worked that way?

Turner started investigating. She learned that baby pines grew poorly in hot, dry seasons. She’d been taught that young lodgepoles were too green to burn, but she found them supporting explosive fires. She watched areas of the park burned in 1988 catch fire again. She saw fires crashing through before young trees produced mature seed cones. Some burned so big and hot that no seed trees survived to regrow the forest.

In five spots around Grand Teton and Yellowstone, Turner found forests coming back sparsely or not at all. Climate change was reshaping some of the most storied scenery. Simulating a future in which we don’t curtail emissions, she caught glimpses of some of her favorite places as her children might one day see them: At Oxbow Bend, where Mount Moran is reflected in the Snake River, the thick stand of conifers could be replaced by sagebrush, grasses, and aspens; along Firehole Canyon Drive or the Madison River, the pine forests could become meadows.

Turner had thought of Yellowstone as “the most resilient place in the world.” Now her research showed its forests transitioning to a new state. Other scientists were reaching similar conclusions elsewhere. Camille Stevens-Rumann, a forest ecologist at Colorado State University, examined 1,485 sites from 52 fires in Colorado, Idaho, Montana, and Washington. The number of burned sites that didn’t recover jumped from 19 percent before 2000 to 32 percent in the years after. “And by ‘not recovering,’ I mean not a single tree—not one,” she says.

Fires are reshaping storied scenery: some forests are coming back sparsely— or not at all.

Not long ago, the U.S. Forest Service mostly planted trees only after forests had been logged—it counted on burned areas regenerating naturally. Now, “over 80 percent of our reforestation needs are being driven by catastrophic wildfire,” says David Lytle, the agency’s forest and rangeland management director. More than half of the millions of acres burned recently in 154 national forests won’t grow back without replanting. Even then, on tens of thousands of acres, seeds may never take root, Lytle says.

But around the world, more than just drought and fire are at play. After extreme heat and drought had weakened mangroves across hundreds of miles of northern Australian coast, an El Niño event in 2015-16, likely worsened by climate change, caused a temporary regional drop in sea level. Eighteen thousand acres of mangroves died of thirst. In southeastern Brazil, the same El Niño drove down precipitation, stressing mangroves along the flat, brown Piraquê-Mirím River. Then, one June day in 2016, plum-size hail pummeled this hot landscape for the first time on record, as 60-mile-an-hour gusts blew foliage off trees and drove trunks sideways across 1,200 acres.

Five years later I visited with Angelo Bernardino, an oceanographer with Federal University of Espírito Santo. From a boat on the river, we watched soil around the dead trees sloughing into the water, ensuring that few if any mangroves would ever sprout here again.

HOW BURNED TREES
CAN NURTURE THE
LANDSCAPE

Near Boulder, Colorado • With the help of a
helicopter, charred
trees ground to mulch
are showered like
cremated remains over a forest that burned
in 2020. The mulch
will help stabilize the
slopes in these foothills,
letting new vegetation
take root and preventing soil erosion,
which otherwise
could harm nearby
lakes and streams.

HOW THESE ANCIENT
TREES COULD WEATHER
CLIMATE CHANGE

Great Basin National Park, Nevada • A pale moon shines through the skeletons of bristlecone pines 21 years after a wildfire—made rare and more intense by years of fire suppression—ripped through 1,650 acres here in 2000. Some bristlecones are about 5,000 years old, making them the longest-lived individual organisms on Earth. Seedlings have sprouted among the dead, offering hope that this species might be one of the best equipped to endure a warming climate

If any species could withstand climate shifts, you might think it’d be giant sequoias, many of which have stood since the reign of Julius Caesar. Instead, change has come frighteningly fast.

In 2012 the cover story in National Geographic’s December issue profiled one stunning specimen in Sequoia National Park. At 247 feet in height, nearly half that of the Washington Monument, the President, as the behemoth is called, was thought to have been a seedling when fewer people walked Earth than live in modern France. It held more leaves than there are people in China. Our story told of sequoias’ remarkable resilience: the way tannins supposedly made them impervious to wood-boring beetles; how their thick bark was nearly flame resistant. Researchers were wary about the future but not alarmed.

Last summer, less than a decade later, I sat in the canopy of a nearby sequoia and stared over at the President. My throat itched from the smoke of a nearby wildfire. My legs ached from hauling myself 200 feet up a climbing rope to join forest ecologist Anthony Ambrose. I’d come because he and other scientists were suddenly rattled.

In 2014, two summers after that story was published, sequoias began shedding needles, a severe move to curb water demand during a horrendous drought. Then scientists noticed 33 trees succumbing to fatal beetle attacks. Ambrose saw tunnels carved through bark. He saw branches trying to push insects out by oozing pitch. He worried other trees might be next.

Before then, sequoias were considered “freaks” of the conifer world because “nobody had ever seen one killed by insects,” Nate Stephenson had told me the day before I met Ambrose. Stephenson would know. After studying these monarchs for more than 40 years, he probably understands them better than anyone else.

In 2015, shortly after the needles fell and the bugs arrived, Stephenson met with Christy Brigham, who’d recently arrived as the park’s chief of resources. “How bad is it?” she asked. Stephenson saw no reason for panic.

Drought and fire threats to sequoias had been predicted by climate modelers, but most didn’t expect serious danger for decades. Sequoia and Kings Canyon National Parks had pioneered the setting of prescribed burns to clear brush and logs from the understory so that wildfires didn’t explode. The parks would now light even more controlled blazes, Brigham decided. She hired Ambrose and forest ecologist Wendy Baxter to track how sequoias were managing water stress.

Ambrose has climbed enough sequoias to know they are tough old beasts. He’s seen them struck by lightning only to grow new canopy branches. He’s watched them slow their photosynthetic machinery in dry times. Trees that can drink 800 gallons of water a day don’t survive thousands of years without learning to “hunker down,” he says. But by 2021, as we sat together staring at the President after the most shocking fire season on record, Ambrose was wondering how much more these trees could take.

Sequoias need low-intensity ground fires to release seeds from their cones and clear soil, so seeds can take root. Their high branches make them unlikely hosts for canopy fires. But in 2020 our history of suppressing fire collided with a rapidly changing climate. The same dry spell that cost sequoias foliage had killed tens of millions of trees—sugar pines, incense cedars, and white firs—in densely packed forests nearby. That’s where the Castle fire began.

Soon it jumped ridges and spotted into the sequoias. Long flames ignited their crowns. Heat and wind shot smoke tens of thousands of feet high. Embers exploded. High branches collapsed, plunging seed cones into flames, incinerating future generations.

In one grove Brigham found hardly any seeds. “There was nothing on the ground except ash. We have never seen that before. Never.” After the fire, Brigham took stock. Up to 14 percent of all the large sequoias in the Sierra Nevada, their native habitat, were dead or mortally wounded.

Months after I left Ambrose, it happened again. Fires in September 2021 charred sequoia bark and sent twigs raining for miles. Ambrose’s study trees lost water 24 hours a day. Flames came so close to the General Sherman—the biggest tree on Earth—that firefighters wrapped it in flame-resistant material.

The 2021 fires claimed another 3 to 5 percent of large sequoias. Up to 19 percent of these magnificent trees—trees that had weathered everything for a millennium or more—had been lost in just two years.

Losing forests to climate change isn’t just about such heartbreak. There are other consequences for people and wildlife. Wildfire smoke increasingly fouls the air of major cities such as San Francisco and Seattle. Australia’s 2020 megafires killed 33 people—and a billion animals, including 60,000 koalas. The fires may have expanded the country’s list of endangered animal species by 14 percent.

Losing forests also releases carbon that amplifies the climate threat. The future on that score looks uncertain but worrisome.

In North America’s boreal forest, from Alaska to Newfoundland, massive fires now release incredible amounts of carbon—not only from the trees themselves but also from the moist peat soils in which they grow. Jennifer Baltzer, a forest ecologist at Wilfrid Laurier University in Ontario, has found that in many burnt patches, the dominant species, black spruce, is being replaced by other species such as aspen—which in principle might soak up more carbon than spruce over time and be less likely to burn. But soils hold most of the carbon in the boreal region, and for now they seem very vulnerable.

Meanwhile, in the boreal forests of Siberia, intensifying fires have mutated recently into multimillion-acre monsters that threaten to release huge reserves of ancient carbon from the permafrost. Those burns are turning some forests into shrublands or grasslands, which store less carbon, says Heather Alexander of Auburn University in Alabama. Yet the switch to a lighter-colored landscape also has a cooling effect, because it reflects more sunlight than darker forest—especially when blanketed by winter snow. The bottom line for climate, Alexander says: “Unknown.”

Each region is its own case, but the threat to forests is general and global. ‘There’s just red flag after red flag,’ ecologist Jennifer Baltzer says. 

The Amazon rainforest presents a clearer and more urgent picture. It produces much of its own rain, recycling water vapor over and over. The clearing of forest for cattle ranches and soy farms has accelerated again under President Jair Bolsonaro, and climate change may be hastening the approach of a dangerous tipping point. Grueling droughts in 2005, 2010, and 2015-16 killed billions of trees outright and helped spread fires that killed more. As forest is logged, burned, or dried out, that reduces rainfall in a self-reinforcing spiral. Some scientists fear that spiral threatens to send the world’s biggest rainforest hurtling toward a transition to a savanna.

Each region of the world faces its own particular challenges, but the threat to forests is general and global. “There’s just red flag after red flag where these forested ecosystems are being pushed right to their limit,” Baltzer says.

Yet increasingly, governments from Japan to the United Kingdom are setting up complex trading schemes that allow businesses to offset fossil fuel emissions by protecting forests rather than to cut emissions at the smokestack. Often those schemes don’t account adequately for the possibility that forests may not be protectable. As I was visiting sequoias last year, a wildfire in Oregon was releasing carbon that tech giant Microsoft had purchased to offset its own emissions.

No one knows what awaits this summer, or next. But it’s time we embraced our new reality. We can no longer forestall rapid changes to some forests. The planet won’t stop warming until we completely halt fossil fuel emissions, and that will take decades. As Craig Allen witnessed in New Mexico and Nate Stephenson has seen with giant sequoias, some changes may be drastic.

But we can keep things from getting even worse. To start, we must halt the destruction of native forests, especially tropical, boreal, and temperate old-growth forests. The benefits they provide aren’t replaceable. The good news: Many are still healthy, for now.

For example, humans have cleared far less of the Congo rainforest, the world’s second largest, than of tropical forests in Asia or South America. The forest is getting less precipitation, but it’s showing resilience. While some trees in Gabon produce less fruit, providing less food for forest elephants, the Congo has avoided widespread tree mortality. Even in Brazil and Southeast Asia, millions of square miles of lush forest remain intact.

“We need to protect the forests we have,” says Robin Chazdon, a restoration expert with the University of Connecticut. “That’s number one.”

We also need to manage forests better, especially for fire. In cooler, dry months in northern Australia’s Arnhem Land, Indigenous rangers carry drip torches or drop fire starters from helicopters to ignite ground-crawling blazes in the tall grass. So far, that has dramatically curbed explosive late-summer forest fires. In the U.S., the White House announced plans in January to help government and private landowners start more prescribed burns and thin more forests, where appropriate, with logging. The aim is to reduce fire risks on four times more land, up to 50 million acres, over 10 years—if Congress provides the money.

But that’s not enough. We also need to restore damaged forests, primarily in equatorial regions, where native trees can come back quickly, but elsewhere too. The infrastructure bill signed by President Joe Biden last fall authorizes billions of dollars to increase nursery and seed-growing capacity and kick-start the largest U.S. reforestation campaign in history by replanting four million acres in a decade.

And of course we need to break our fossil fuel addiction, quickly.

On my last day in Yellowstone with Turner, we visited old burns from another 2016 fire. This one had ripped across a plateau above the Madison River, which also had burned in 1988. The recent blaze had so scorched the landscape that it even incinerated downed trunks, leaving nothing but lines of white ash that stretched like shadows across blackened soil. Turner called them “ghost logs.” In 30 years of traipsing through fire scars, she’d never seen ground so pummeled by fire.

Do we want even more of this?

This spring marks 150 years since President Ulysses S. Grant signed the act creating Yellowstone, America’s first national park. It required “preservation, from injury or spoliation” and “retention in their natural condition” of the park’s wonders. The effort that entails has expanded since Grant’s day, when threats were direct and local. Turner projects that if global temperatures were to rise four degrees Celsius (7.2 degrees F) from preindustrial values, the region’s high-elevation spruces and subalpine firs, such as those near the Snake River’s headwaters, could be wiped out. Forest cover could drop by half by 2100. The density of what remains would drop even more.

That’s far from inevitable. If the world’s nations keep their current promises, the planet will warm less than three degrees Celsius (5.4 degrees F). Stabilizing emissions closer to two degrees or less could limit forest losses in Yellowstone to 15 percent. High-elevation trees would still struggle, and there’d be more Douglas firs and aspens. But some old growth would persist. Yellowstone’s forests, like many in the world, will never be the same. But they might be close.

Senior writer Craig Welch has been reporting on climate change for more than 20 years. In the past year he has written cover stories on electric cars and the culture of whales.

This story appears in the May 2022 issue of National Geographic magazine.