The largest fire in state history swept through the eastern slopes of the Cascade Range with explosive force last summer. The Carlton Complex Fire burned more than 250,000 acres, devouring everything in its path at the hypersonic pace of 3.8 acres per second.

Until then, the top slot in the state’s fire rankings belonged to the Tripod Fire, which burned up 175,000 beetle-infested acres in two months on the same slopes in 2006.

Carlton and Tripod are “megafires,” part of a wave of extreme fires that are transforming the great forests of the American West. By the end of the century, scientists say, megafires—conflagrations that chew up at least 100,000 acres of land—will become the norm. Which makes them of critical interest to researchers.

These infernos, once rare, are growing to sizes that U.S. Forest Service Chief Tom Tidwell describes as “unimaginable” two decades ago. Five alone have consumed more than five million acres in central Alaska since June. Washington, Oregon, California, Arizona, New Mexico and Colorado also experienced their worst wildfires in the past seven years.

So far in the Lower 48, none of the thousands of fires that have burned across the 11 Western states have grown into megafire size. But the most perilous weeks of fire season are still ahead. With extreme drought and sizzling temperatures searing the West, the only remaining component needed to turn low-threat fires into catastrophic ones is gusting wind. The Carlton Complex Fire, propelled by 30 mph gusts, took just one day to reach that status.

Bigger Burns

Megafires, those that burn more than 100,000 acres, have erupted more frequently in recent decades.

Megafires are remaking forests in ways that scientists are still struggling to understand. They incinerate habitat for songbirds like the yellow-rumped warbler, push already-vulnerable whitebark pine trees closer to extinction, and, when they are especially ferocious, burn down whole forests so thoroughly, they never grow back.

Superimpose an outline of the devastated acreage of the Carlton Complex and Tripod Fires on a map of the Okanogan National Forest and the extent of the damage comes into sharp focus. “You start to see these big burn patches,” says David Peterson, a University of Washington forest ecologist.

“We are starting to see them in the Pacific Northwest—Idaho, Oregon and other places. These patches are going to start running into each other. This is what will change the landscape.”

A change, he might have added, that will be probably be irrevocable.

“When fires are really large and severe and most of the trees burn up, it’s very difficult for a seed source to survive,” says Paul Hessburg, a Forest Service plant pathologist in Wenatchee, Washington. “Trees can take a century to regenerate. Meanwhile, fires will reoccur and keep those areas stuck in grass and shrub.”

What’s turning small fires into raging infernos is a stew of ingredients that includes government fire-fighting policies and the continued push by millions of people to set up housekeeping on the edge of national forests.

But the main driver is climate change. Rising temperatures exacerbate drought, spread beetle infestations and melt the snowpack earlier. Early snowmelt alone has lengthened the fire season by 70 days since 1970. (Read about the pine beetle epidemic.)

“These stresses are going to become more widespread,” says Craig Allen, a U.S. Geological Service forest ecologist in Los Alamos, New Mexico. “The drought itself is part of the natural variability here. What’s different is it’s a hotter drought than anything in the Northern Hemisphere in the last thousand years.”

A Natural History of Fire

Fire is part of the natural forest lifecycle. It thins trees, helps new seedlings take root, and removes decaying debris that has the potential to become tinder. So trees are well-adapted to flames. Ponderosa pines shed lower limbs and wear bark so thick they’re literally fire proof.  In the Southwest, scientists have discovered pine tree rings bearing scars of more than 30 individual fires they survived.

Over the decades, a subtle shift began to occur. Fires began to grow in size and trees took longer to regenerate.

The event that signaled the advent of megafires occurred in 1988, when firesin Yellowstone National Park caused by an unusually dry season burned 1.2 million acres—about 36 percent of the park.

Yellowstone recovered, but in the decades since, the number of big fires steadily multiplied. Today, wildfires, on average, burn twice as much land every year now as they did 40 years ago, according to an analysis of 42 years of U.S. Forest Service records, by Climate Central, a nonprofit research group. There have been an average ten megafires a year in the past ten years, according to statistics kept by the National Interagency Fire Center in Boise, Idaho.

The trend will only build as the planet warms. The National Research Council estimates that the amount of land burned in the West will quadruple for every degree Celsius (1.8 degrees Fahrenheit) that temperatures rise. According to government predictions, summer temperatures in the West will increase by 3.6-to-9 degrees Fahrenheit by mid-century.

To Fight or Not To Fight

Ironically, the century-old government policy of fighting fires instead of letting them burn has led to a build-up of the very tinder that lesser fires once destroyed as part of the natural process.

“It’s a bit of magical thinking that if you keep putting fires out, fires will go away,” Hessburg says. “What happens is they get more severe, because you just keep loading a powder keg.”

In New Mexico and Arizona, for example, where small fires were the norm, forest debris built up over decades finally touched off the Southwest’s largest fires in the early 2000s.

So what to do about it? Clearing forests of excess vegetation through “managed burns,” might improve the odds of keeping fires small. But there is little money to do so.

Tidwell told Congress 58 million acres of national forests are at risk for severe fire, but there is only enough funding to clean up 11.3 million acres; priority goes to areas where housing development pushes against wilderness. (Between 1940 and 2000, the number of houses within a half-mile of a national forest rose from 484,000 to 1.8 million, according to Forest Service figures.)

Owls Lose, Woodpeckers Win

Meanwhile, scientists are still sorting out what Western forests might look like in the next century. Uncertainties remain. How much change in climate can forests tolerate before reaching a tipping point?

“We think forests can hang on for a long time when the climate has slipped out from under them,” says Steve Running, a University of Montana fire ecologist. “What wipes the slate clean is fire.”

Forests of the future, growing in hotter, drier environs, will be sparser, with fewer trees per acre, and fewer small trees in the understory. They will also be younger, as fire “resets” forest age. In cases where forests cannot recover, the land may give way to open range, with grass and shrub.

“We will see a reduction in the total area of forest and changes in distributions of tree species,” says Monica Turner, a University of Wisconsin ecologist who has tracked Yellowstone’s changes since the fire. “So it is critical that we keep an eye on places like Yellowstone which will be key to our ability to see how species adapt.”

There will be clear winners and losers in what Peterson calls “a re-arrangement” of species. Spotted owls, which nest in old-growth forests, are high on the list of losers, if events like the 500,068-acre Biscuit Fire that destroyed their habitat in the Siskiyou National Forest in southern Oregon and northern California in 2002, are good predictors. That’s just one of many large fires that have already forced owls to relocate.

Woodpeckers are among the winners, along with small hawks and other birds that like to call bug-filled, hollowed out dead trees home. Deer and elk prefer open spaces created when forests burn down, but denuded terrain also attracts invasive plants, which crowd out native species. One of the most notorious of these is cheat grass.

Southern Idaho, where much of the forest has burned in the last quarter-century, has seen a dramatic transformation from forest to open range.

“We’re seeing the migration of the Great Plains ecosystems northward into Idaho now,” says Dick Bahr, deputy director of the Interior Department’s wildlands fire office. “People are going, ‘whoa, what happened?”

WATCH: National Geographic photographer Mark Thiessen describes capturing unique images of a controlled but fast-moving forest fire in Canada’s Northwest Territories.

Rocky Barker, an Idaho journalist and author of Scorched Earth: How the Fires of Yellowstone Changed America, says a scientist friend once advised to him to move north.

“He told me if I liked my life in Boise, I might think about moving to Coeur d’Alene because Idaho forests are going to follow in the same direction.”

Aftermath of a Fire

It is too early by decades to assess the impact of the Carlton Complex Fire. But based on recent patterns, Bahr and others think its stately stands of Ponderosa may be gone for good.

The Carlton Complex began as four lightning-sparked fires last July 14 that coalesced into one. The blaze roared out of the mountains and across the shrub steppe to the Columbia River, burning through 123,000 acres – half the territory it would claim – in nine hours. The fire didn’t kill anyone, but destroyed 300 homes.

Carlene Anders, who has fought hundreds of fires in her 30 year-long career, has never seen anything like it.

“It was like a tidal wave,” she says. “There were fires within fires.”

In spring, the shrubs and bitter bush greened up. Kayakers and cyclists came back to celebrate the Methow Valley’s rebirth. But high on hillsides, broad stands of charred Ponderosa pines are not far out of sight.

“That was the part of the fire that made me heartsick,” says Susan Prichard, a University of Washington fire ecologist who is tracking the fire’s aftermath. “Ponderosa pine is built to withstand fire. What we saw in Carlton was 100 percent mortality, even on open ground. The heat just made the trees succumb.”