Before that heartbreaking night at the end of July, she was a ghost bear tramping the backwoods shade, a scared specter at her wit's end. She and her three cubs, all woefully thin and eking out a diet from grass and shoots, were so unwell that they wore their winter coats through the full, high heat of summer. In a lean year for grizzlies, they stood last in line, going without a solid meal of deer or elk or the staple of Yellowstone's bears, whitebark pine seeds. Those seeds, rich and fleshy, had grown for centuries on the crowns of the staunchest trees in North America: gnarled, obdurate pines that survived 50-below winters and laughed off killing winds on western peaks. Nothing could slay those trees, neither fire nor ice, until the region started warming around 1980. Now 80 percent of the Rockies' whitebark pine groves stand dead or dying in ghost-gray swaths, and the bears who ate their fruit and kept out of harm's way have bumbled down the hills in search of food. Among their number was the sow with three cubs and teats running dry of milk. With winter two months off, she had to somehow bulk up fast or watch her yearlings starve.

To her credit, she hadn't become a "problem bear," the park officials' term for hundreds of hungry grizzlies who venture into town prowling for food. Though Yellowstone's 600 bears aren't confined to the park itself – they're given free run of the greater ecosystem, an area that stretches from central Wyoming to the forests of northern Montana – there simply wasn't enough alternative food to see all of them through the summer. And while full-grown males have the brawn and bravado to venture off the range in search of meat, a mother grizzly rarely leaves the safety of her turf, lest a wolf pack or another bear kill her cubs. Timidity had its virtues: She wasn't one of the 80 or so bears shot last year while picking apples off a tree or nosing through trash in someone's backyard, or given a lethal injection by U.S. Fish and Wildlife vets for grazing on the bluegrass near a school.

Six weeks before, the first shoe dropped. On June 17, an adult male bear (or boar, as they're called by biologists) killed a veteran hiker who had the wretched luck to cross his path. Erwin Evert, a botanist and and retired science teacher, had spent most of his career studying Yellowstone's flora and had just brought out his life's masterwork, the first comprehensive catalog of plants in the area in more than a hundred years. On his daily hike near Wyoming's Kitty Creek, the easternmost of the park's gateways, he wandered into a copse where a team of federal researchers had trapped and sedated a bear. Alas, they hadn't posted warning signs or waited until the boar was sufficiently roused to pad back into the brush. Dazed and in pain (he'd been darted three times with a chemical cognate of PCP, then had blood, teeth, and hair pulled for study reasons), the bear bit Evert through the skull and skittered off; he was shot two days later by marksmen in a chopper who tracked his radio signal. There hadn't been a bear-caused fatality in the park in 24 years, though given the grim developments of the prior decade – a 10-year run of extreme drought and heat, and a glut of famished grizzlies – the screw was bound to turn. On July 28, it turned again, and this time it wasn't about human error or the caprices of nature's law. This time, it was a taste of things to come.

Sometime after midnight on a streamside slope near the northeast end of the park, the sow and her three cubs entered Soda Butte Campground, drawn by the lingering smell of broiled fish. After trying in vain to pry the tamper-proof lids off food bins and garbage cans, the sow poked her nose under the fly of a tent. She bit the leg of its occupant, Ronald Singer, who managed to drive her off with panicked blows. A short while later, around 2:15 am, Deborah Freele awoke in her tent at No. 11 to find the sow gnawing on her arm. She shrieked and fought back, but the bear bit down harder, snapping bones. By now, there was tumult in adjacent sites, people dashing around and honking car horns in warning, and the sow let go of Freele and ran away. A couple of hours later, rangers and deputies scoured the pitch-dark camp. Near the western end, 600 yards from Freele's tent, they came upon the gnawed remains of a man named Kevin Kammer. Kammer, a medic from Grand Rapids, Michigan, whose lifelong dream was to fish Yellowstone's streams, had been dragged from his tent, killed by several bites, then consumed from chest to groin. There were several sets of prints on his flattened tent – the sow's and at least one of her cubs'.

Over the next days and weeks, all manner of havoc ensued. The media descended from as far away as Finland, asking pointed questions about "killer bears" and the safety of the park's guests. Park Service wardens, who trapped the sow and dispatched her via lethal injection, denounced her as a rogue whose "predatory" act was indefensible but rare. (Her cubs were transported across the state for permanent residence in a zoo.) Test after test was conducted, post-mortem, to establish her motivation. Was she rabid? No. Exotic diseases? None. Maddened by injury or wounds? The federal Interagency Grizzly Bear Study Team, which took charge of the investigation, needed a month to conclude that there was "no clear explanation for the behavior of this bear," though a lucid possibility fairly leaped off her chart. Her weight at the time of death was 216 pounds, or about 80 pounds less than average for a full-grown sow. Like her cubs, called malnourished by the zoo's curator, and countless other bears forced downhill by hunger, she was a forerunner of the turmoil that awaits us all: species pushed to breaking by climate change.

The numbers come at you in a solid wall, wave after wave of peer-reviewed findings, each set worse than the last. The planet, heating steadily since the Industrial Revolution and growing warmer by leaps from 1980 onward, has suffered through 12 of the hottest years in history over the past 20. The decade just completed was the hottest on record, in which worldwide temperatures shot up a half degree. (For comparison, 1 degree was the total gain amassed during the entire 20th century.) The cause is well settled and the consensus broad; only a handful of crackpots and political opportunists deny that heat-trapping gases from fossil fuel burn-off are to blame for the bulk of the warming or insist that the spike is a statistical deviation brought on by cosmic rays and rogue currents.

The sharpest rebuke to the junk-science shills is the disaster now unfolding in the American West. States from the Dakotas on down to Nevada weathered devastating jumps of almost 2 degrees, or roughly double the rate of the planet's rise. The northern Rocky winters got radically milder, the summers started sooner and were drier and longer, and wildfires burned through vast tracts of timber weeks after the usual fire year ended. The damage to natural resources has outstripped the worst predictions of climate scientists everywhere: Lake Mead and Lake Powell, the largest reservoirs in the States and the lifeblood of Las Vegas and Los Angeles, respectively, stand half- empty and notching record lows, thanks to shrinking snowmelt. The country's greatest trout streams have been closed to anglers during parts of eight of the past 10 years, and the keystone trees of the interior West – the aspen, whitebark, and lodgepole pines – are dropping dead in holocaust numbers, felled directly by the surge of heat or by insect infestations spawned by it. And this is a mere prelude to the hellfire that's coming: a regional warming of as much as seven to 10 degrees by the end of this century, bringing permanent drought plains, coastal tsunamis, and widespread human dispersal. "Without a countershift the equal of the Industrial Revolution, we'll see mass migrations in our grandkids' lifetimes," says Steve Running, a renowned ecologist at the University of Montana and a lead author of the United Nations report on global warming in 2007. "Major cities in the West, like Phoenix and Las Vegas, may have to be abandoned as badlands."

Running is scarcely an outlier voice. "What we've seen out here is like nothing on record, and our tree-ring studies go back a thousand years," says Jonathan Overpeck, the co-director of the Institute of the Environment at the University of Arizona and one of the go-to climate scientists in the country. "The winters are shorter, the snowpacks melt early, and the drying seasons are longer and hotter, which leads to terrible wildfires and tree death. One of the many things that worry me, as the heat increases, is that this region becomes a second Outback. Already, our water supply is severely strained, and with decade-long droughts like the one we're in now, it's hard to see how we'll avoid it."

If the West is ground zero for the unholy experiment being conducted on weather shifts, then Yellowstone is first up on the blasting range. The oldest and most magical of our national parks, its 2 million acres stretch to three states, boast a spectacular chain of rivers, lakes, and creeks, and sit, a vast chunk of them, on a supervolcano that spawns half the world's geysers and hot springs. Among the last menageries of charismatic wildlife in the northern temperate zone, its grasslands feed herds of wapiti, moose, and bison, which in turn feed grizzlies, wolves, and mountain lions – a matchless array of sovereign predators. Four million people visit the park each year to fish its waters, run its trails, and climb its summits; to get anywhere near the top of the Gallatin Mountains and look down on those silver-backed buttes and falls is to know, in your bones, what beauty is. There is grandeur on all sides of you, but graveyards, too: mile after mile of zombified forests, dead from the roots but still standing.

One very warm week in early October, I took a four-day tour of Yellowstone's peaks and ravines with people who love the park dearly. My principal guide, an attorney named Matt Skoglund, was a rising star at a law firm in Chicago until he shucked it, in '08, to become a wildlife advocate, taking a job in Montana with the Natural Resources Defense Council at a fraction of his former wages. The NRDC, along with partner nonprofits like Earthjustice and the Rocky Mountain Climate Organization, has led the fight against climate change and its collateral damage. It has sued the federal government on behalf of bears, funded a landmark study on the collapse of alpine woods, and released a report on the warm-up in the West that reads like slasher sci-fi but isn't.

Skoglund, a supremely fit Ironman entrant who fell hard for the area on college fishing trips, has learned this landscape the honest way: by power-hiking its hills and rowing its rapids. Giving me a day to reset my lungs to the air at elevation (Yellowstone's low point is a mile above sea level; its mountains can reach 11,000 feet), he arranged for a bird's-eye view of the park, flying in a pilot from Colorado. The wind was flapping hard when we boarded a Cessna at an airfield near the town of West Yellowstone. Bruce Gordon, who's flown and photographed the West to document the damage done over the course of the past two decades, punched through thermal gusts as we banked and wobbled over the park's western rim. Below us were tracts of aspens framing Hebgen Lake, their gold crowns bending in the breeze. I started to praise their beauty, but Gordon stopped me: "Those are dead trees that rusted out. The drought and heat stressed them; then the bugs and spores killed them. From here to Colorado, there's miles of aspens gone, and for all we know, they'll never grow back again."

Indeed, in five years, more than half a million acres of young and old aspens in the Rockies have been nuked. The phenomenon is so new that it's inspired an acronym (SAD, or Sudden Aspen Death) but no clear means of reversal. Beetles and borer larvae, rarely seen at these elevations, have moved up the Rockies as the winters have warmed and feasted on the pulp of these regal trees. "Strip the bark," said Skoglund, "and you'll see tracks and channels where they've set up shop by the thousands. Then fungi enter through the holes the beetles carve and finish off the trees through infections."

Flying north, over the foothills of the Madison Range, we passed lodgepole pines whose evergreen crowns were the color of undercooked lamb. Shaped like closed umbrellas at a hundred feet tall, these, too, were desecrated by beetles, who carry in their mouths a vicious bacteria that drains the trees of sap and nutrients. Beyond the sick lodgepoles were legions of dead ones, their gray trunks tinder for trouble. Though Yellowstone's fire season used to end in September, three big blazes raged that morning, having burned through the weekend unchecked. "With winters ending early and summers stretching longer, you're getting bone-dry conditions that last for months," says Gordon, who, as founder of the nonprofit EcoFlight, flew arborists over Yellowstone dozens of times to compile the data on whitebark-pine death for the NRDC. "With all this deadwood, a couple of lightning strikes and there go thousands of acres."

In years of normal climate, or what used to pass for normal, summer fires served as reset buttons, purging old trees to make way for young ones and clearing new groves for herds to graze. But the decade of high heat here has set the stage for cataclysm: superfires that leap past all containment. Montanans speak grimly of the summer of 2000, when the Bitterroot National Forest lost a fifth of its acres to 100 new fires a day, and of 1988, when a third of Yellowstone's trees were devoured in a months-long inferno. Dire though those were, things could have been worse. "The Big Blowup of 1910 basically leveled Montana, burning everything in sight, including towns," said Skoglund. "Everything's in place for another fire-of-the-century event. All we've lacked – so far – is gale winds."

Past the town of Gardiner, we crossed the Gallatin Mountains, their brown pates glinting like copper pots. Twenty years ago, these peaks were crowned with snow no later than mid-October and, packing drifts up to 10 feet, were closed to hikers. Now people climb them in shorts and Tevas clear through Columbus Day. Gordon swooped in closer, to a couple hundred yards of the tree line; there, as ragged as week-old stubble, stood vistas of whitebark pine. Even in health, they are queer-shaped things – tall, gnarled stalks screwed into the hill, with crowns like Druids' hoods. These were far from well, though, either draped in red (dying) or the end-stage gray of rigor mortis. "Seven years ago, this was solid green. Now it's all deadfall and ashes," muttered Gordon. "It's like the roof's blown off and the animals have fled. But where do they go – where do any of us go – when it's all this gray down there?"

The roof's blown off. There's a hole in the ceiling. In the course of my reporting, I heard versions of those tropes from half a dozen eco-literate people. It's a resonant image but precisely wrong: The roof isn't porous. It's padded.

Suspended over the Earth, like the bolt of cheap foam that underlies a living-room rug, are trillions of gas molecules that shake when light hits them, creating heat and sending it earthward. Those molecules, produced by natural activities like plant respiration and volcanoes, have hung there since the planet became livable, post-Ice Age; absent carbon dioxide, methane, and other dense vapors, Palm Springs would still be permafrost. And as we've learned by drilling holes into Arctic ice sheets to read their chemical profiles, the ratio of those particles had held remarkably firm for 10,000 years or so, balancing the energy retained from the sun with the amount sent back into space. But since the construction, in 1750, of the first coal-fired factories (and the invention, a century later, of the internal-combustion engine), the density of greenhouse gases has increased by a third, holding in much of the solar radiation that bounces off sidewalks and snowcaps. This set up a vicious feedback loop, in which the extra heat was reflected down into the soil or got stored in those giant holding vats, our oceans. "If we could somehow scale back carbon, we'd still stay hot for centuries, manifesting the energy trapped in seas," says Running, the University of Montana ecologist. "But if we don't scale it back, we'll soon cross a threshold where all of the sea ice melts – and then there's no telling how high the oceans will get or any known way to make it stop."

For eons, nature balanced its own emissions by capturing some of the gases in carbon sinks like marshes, soil, and forests. Trees are particularly deft sponges of carbon: Their leaves or needles convert it to sugars that feed them from crown to root, and they go on sopping up noxious particles until they rot, burn down, or get logged. But when clear-cutting commenced on a massive scale in the middle of the 19th century, the planet doubled down on its carbon load, making much more and trapping less. Roughly half the world's woodlands have vanished since then. There's a net loss the size of Greece each year – and no effort under way to start replanting.

I was thinking about that when I stopped on a knoll halfway up the spine of the Gallatin Mountains. It was a masterpiece day, the sky spit-shined of haze, the air so clear it crackled. We'd hiked all morning up trails of steep scree and toeholds of bone-dry sage, and now, looking up from 8,000 feet, Skoglund pointed to the carpet of forest rising from our ledge. "Listen," he said. "Do you hear what I hear?"

I craned my ear but heard nothing.

"Exactly," he said. "It's peak time for wildlife, and nothing's stirring up here. These woods should be packed with birds and squirrels filling their larders for winter, and loads of bear scat as the grizzlies come up and scour for seeds before denning. Instead, they're still down there looking for food, and here it's just pin-drop quiet."

We'd been told, hours earlier, about low-ranging bears when we drove to the B Bar Ranch in Tom Miner Basin. "Seen a bunch of 'em in the meadows," warned Mark Waite, the manager, before we began our ascent through the hills behind his spread. "Have your bear spray out, ready to rock and roll. It's crazy around here these days."

On the hard trek up, we'd encountered no bears, though a mound of recent droppings at 7,000 feet suggested we'd missed one within the day. Now, as we knocked off lunch on the knoll and pushed toward 9,000 feet, the woods closed in again, walling off light and cloaking us in noonday gloom. We'd entered whitebark country, a cathedral of stern shapes that starts at elevations of 8,500 feet. In front of us, stripped of their bark and cones, were ash-gray corpses of centuries-old pines, their trunks contorted like petrified demons fleeing from the wind. Further on were clumps of newly killed trees, their dull bark dotted by purulent holes where beetles bored in to attack. The white stains signified attempts by the trees to save themselves from bugs; when beset by invaders, they exude a pasty resin to try to flush them out. But they were overmatched by the scope of the onslaught: thousands of bore holes on every trunk, where adult and larval insects arrived in swarms to eat their way down to the heartwood.

Hatchet in hand, Skoglund chipped at the bark. It broke off easily, in two-foot strips, and there, in the pinkish pulp of the tree, was a series of trenches, called galleries, that the beetles had furrowed as their base of operations. Devouring the sap-rich tissue of the trunk, they had bled these pines of the carbon-based sugars that fed them from the needles down and infected the trees with a microbe called blue-stain fungus that choked off circulation. It took 60 to 80 years for these slow-growing giants to sprout their first cones and reproduce – and all of five days for the winged marauders to suck the life from them. With the tip of my pinkie, I dug one out of its quarter-inch-wide trench. Though dead at least a year, it stuck to my nail, a speck of black malevolence in a dusty shell.

"Mountain beetles have been here forever, but they could never stick it out at this elevation," said Skoglund. "Every winter would have cold snaps of 50 below, which wiped out any bugs inside the trunks. But now it only drops to 20 below, max, and beetles can easily live through that – their larvae produce a kind of antifreeze that protects them to 30 below."

We hiked another hour toward Packsaddle Peak, the split-rock summit high above tree line on the humpbacked mountain. The farther we went, though, the bleaker things got: stand after stand of rust-colored pine, the red hue both a last sign of life and a coating of bug-shit and sawdust. Equally distressing was the absence of birds – most important, Clark's nutcrackers, who built these woods. Unlike other pines, whitebarks can't spread their seeds, which are locked inside tightly woven cones. For that they need nutcrackers to pierce the cones, then bury the seeds under the rocky soil to eat at a later time. It's an arrangement that's served everyone well for eons: The birds get a store of food for the winter, new pines are born from the seeds they forget, and at the end of every summer, so many new cones have sprouted that bears swarm up here to raid the stashes that red squirrels hide in the dirt. But the few healthy trees now were far too young to produce and drop their seeds, and the only cones we saw were decomposed husks that lay on the ground like mulch. "We've lost lots of pine before in abnormally hot spells like the 1930s and '70s," says Jesse Logan, a retired Forest Service scientist who's one of the world's foremost whitebark experts, "but it always came back when the weather cooled because the nutcrackers stuck around. This time it's different, because we won't be cooling down, and with no seeds to eat, they'll find other places to live. That'll be the final knell for whitebark."

Logan, in tandem with other top-shelf arborists, is trying very hard to stave that off. He's lent his legwork and expertise to an NRDC petition to get whitebark protected under the Endangered Species Act. If the plea is granted, a federal recovery plan will be drafted and funded to try to salvage these forests. Not that anyone knows how to do that or whether it's still feasible. "Whitebark evolved to fight cold, not beetles, and there's nothing we can spray or rub on pines to drive the beetles out," says Logan. "We could maybe breed these trees on north-facing slopes, which stay colder because there's no direct sun, or see if there's potential in a small group of whitebark whose resin is truly toxic to beetles. But either of those projects would be very expensive and take political vision, and I'm not sure that we've got either."

Indeed, if the fight to save bears is any model, it's going to be a hard sell for whitebark. In 2007, Fish and Wildlife decided, by fiat, to take Yellowstone's grizzlies off the endangered species list, saying it had met its recovery goals by more than doubling their population in 30 years. Only a swift lawsuit by Earthjustice and NRDC prevented trophy hunters in Wyoming and Montana from immediately shooting bears in the park. The case slogged through federal court, where, in 2009, a district judge dealt the feds an emphatic drubbing, saying their scientists had misread their own studies and that the bears were endangered – by whitebark loss. That relisting incensed Fish and Wildlife and its big-game backers, who appealed. A decision on grizzlies is expected this summer, but what's clear already is the derision in which government officials hold the ESA. "It costs them billions to protect and count the bears and prevents them from building subdivisions or drilling for gas where bears roam for food," says naturalist Doug Peacock, who spent most of two decades living with Yellowstone's bears and wrote the now classic Grizzly Years: In Search of the American Wilderness. "Whether it's Bush or Obama or the state game commission, they hate having their hands tied from doing business as usual, and no animal or stupid pine tree is going to stop them."

In the end, of course, there won't be a national day of mourning if or when the whitebark dies out. It's a commercially useless tree, too far up to interest loggers or summon camera-snapping tourists from Dallas. Furthermore, those forests won't remain bare long; at some point, firs and spruces will move up to replace them, chased higher by the ever-building heat. Much like the grizzly bears that its seeds support, whitebark registers as a low-end worry in an age of uppercase dread. Why would anyone underwater on his mortgage in Tucson expend a moment's thought on the matter?

Well, for starters, that house in Tucson (and Denver and San Diego) will be worthless when the spigots run dry. Among its unsung virtues, whitebark shelters the shrinking snowpack that feeds the water table in the West. Without the deep shade of its wide-armed canopy, the high snow would melt in torrents each spring, causing floods and mud slides in April and May and hellacious drought each summer. "In hydrology, when is as crucial as how much; you need slow, regular runoff that lasts you through August, when reservoirs are drained and farms need water," says Running. "An inch of snow is worth five of rain, because snow soaks deeper and evaporates slower. But with temperatures rising, we'll get less and less snow – to the point where it'll only stick in the high mountains."

Second, whitebark matters to the millions of people who fish the gin-clear waters of the Rockies. Early, heavy snowmelt makes for raging spring rapids – and streams too thin to fly-fish by mid-August. On a daylong boat float down the Yellowstone River with Skoglund and Laura Ziemer, an attorney who directs the Montana Water Project – an initiative of the nonprofit Trout Unlimited, which preserves and restores fisheries – I learned firsthand about the impact of warming on the region's signature species, the cutthroat trout. "We've seen massive fish-kills since the serious heat-up started a decade ago," said Ziemer. "As cold-water fish who've spent centuries here in 50- or 60-degree streams, the cuts can't adapt to late-summer waters that hit 80 degrees and higher." Rivers have been closed to anglers for weeks, trout populations have taken a staggering hit, and the thousands of outfitters who depend on fish "got kicked in the teeth," said Ziemer. Her organization has teamed with ranchers and Forest Service officials to offset some of the damage, resuscitating streams where cutthroats lay their eggs and modernizing water use on farms. But the changes in the environment are too systemic to be rolled back by grassroots groups alone. Says Craig Matthews, owner of the world-famous Blue Ribbon Flies, the go-to hook-and-bait shop in West Yellowstone: "We used to have 30 frost-free nights a year. Now it's 70, so the water gets hot and doesn't drop down after dark. Everywhere you look, these fish are pushed to the edge – and you can go right on up the line with other species."

One of those species is the grizzly. The park's bears used to fish for trout at the end of summer before heading up the hills to gorge on pine seeds. Now they're reduced to squirrels and berries and the occasional road-kill deer. A hundred years ago, there were roughly 50,000 bears living west of the Mississippi River. These days, there are maybe 1,500, and it's hard to imagine how Yellowstone's bruins will make it to the end of this century. So desperate have they become that they run toward gunfire, having learned that hunters leave gut piles after a kill. Their main chance may lie in one day quitting the park and heading north toward the Yukon Territory. There's a consensus building among wildlife groups to try to carve out a corridor to western Canada, in which bears, wolves, and lynx could come and go freely, roaming where there's food and cover. It's a plan fraught with peril – they'd have to cross three states, four superhighways, and two Canadian provinces – and opposed by a formidable cast of lobbies, primarily big ranchers and mining firms. But the winters there are arctic, the forests are robust, and the glacier-fed lakes teem with fighting trout and pike the size of beagles. We owe our bears nothing less than safe passage there, after trashing their habitat here. They'd better start now, though, to beat the rush. We'll all be heading north soon enough.