I first heard a firefighter describe midafternoon as “the witching hour” in the Kootenai forest in western Montana. It was my first week of fighting fire. Our 20-person crew had hauled packs up a mountain, watching the blackened slope curve above us to a ridge. For all the heroic gloss on the job of fighting wildfires, most of it is hiking and digging. We hike. We tell jokes. We cut burning brush from not burning brush. And we dig to deprive advancing fire of fuel.
On a level rock outcropping, over the crackle of a torching fir, we got a briefing from the division leader. Smoke drifted across his boots, and behind him, a limp helicopter cargo net sagged around a stack of cardboard boxes. The boxes contained half a ton of explosives. Rather than hack through tough bear grass, we would string the sausage-like tubes of plastic explosive downhill of the fire.
We relayed the 50-pound boxes through wisps of smoke. I scrambled over rock outcroppings and jabbed my boots in sidehill. Groups of two traded off digging trenches down to mineral soil, stalling the flames before the fire reached the explosives.
My Pulaski practically bounced off the bear grass roots. Grass blades folded and burned. With each swing of our tools, soil scattered behind us.
I dug line at the edge of the flames until a boss told me to abandon it. We hiked to shelter, underneath a bluff at the shore of a mountain lake. Then we plugged our ears as someone more qualified than any of us blew the charges. The trees shook. The echo reverberated through the alpine bowl.
We camped on the mountaintop. We ate from a crate of paracargo that had accidentally crushed our crew boss’s tent, told ghost stories, and slept.
The next morning, cool air tamped the fire down. Open flames worked for long minutes to preheat brush, then ignite it. The fire smoldered up to the edge of the thin strip of mineral soil we had blown into the dirt. We could have stepped over shin-high flames into the cold ashes.
A helicopter slung two pump kits to the lake and we hauled the supplies up steep banks, trying to unroll canvas hoses before the fire spread. We skipped lunch. Since the fire was moving slowly, not all the rookies understood the hurry. Logan, my squad leader, explained: “We have to beat the witching hour.”
The witching hour happens later in the afternoon, when the sun has dried out the fuels and swung its heat across the valley floor, reversing the morning’s upslope winds. The temperature climaxes, the wind shifts.
When the witching hour hit on that fire, we had almost finished plumbing our section. Laying a system of hoses is known as “plumbing” a fire. Usually we use what’s called a “progressive hoselay,” in which 3-inch trunk hose carries the water, and firefighters can spray from a 1.5-inch “lat” or lateral hose. Typically, once crews cut brush and dig a trench around the outside of the fire to contain it, we lay hose around the fire’s edge to mop it up. We place a 100-foot lat every 200 feet. Then the whole edge of the fire is in range of a hose, and multiple people can all work the edge simultaneously with one water source and one pump. (A fire engine, obviously, has both. A lake and a Mark-3 pump work great.)
Logan and I finished plumbing, then started hiking back for more hardware — we had miscounted the number of reducers we would need to attach the 3-inch trunk hose down to the smaller lats. He wanted to connect them before lunch.
Then the wind died. Smoldering coals sprouted flames. New breezes eddied through the trees. Flames in the leaf litter quit idling and moved. A crackling sound accelerated from somewhere in the forest, rising with the roar of a Doppler effect on a highway. “Double time,” Logan said. He did not look back to check on me.
A column of fire had engulfed a tree, right above the gash blown in the dirt. Flames scampered across the canopy. Logan unrolled the lateral hose while I ran toward our stash of gear.
I heard a radio among the trees. “Throw me a reducer!” I yelled.
“One inch or inch and a half?” replied my crew boss.
I tried to remember. “For toy hose!”
He tossed a one-inch reducer between some tamaracks, I caught it and ran back to Logan. It was the wrong size. Failing to fit it to the hose was my first visceral reminder of a lesson good firefighters internalize: Get every detail right, as early as you can. Nothing else is in your control.
Two other rookies followed me as I ran back. The smoke was so thick we could only navigate by following the trunk hose at my boots. The correct reducer flew out of the smoke. Embers rained on our hard hats.
The fire had crossed the line. Adjacent to a torching tree, flames scampered in the branches of a green fir. Each flame stretched into a thin, tall strand, deposited flame on the branch above it, and rebounded to a fat, quivering teardrop. Someone on the radio charged the hose. Logan braced for water snaking through the lat. Under the rain of embers, holes materialized in the trunk hose, and they spouted like a fountain. The pressure sagged.
Global warming intensifies wildfires. We’re seeing more intense and longer droughts, stronger winds and hotter temperatures. According to 2016 research by the environmental engineer Anthony LeRoy Westerling, the fire season — the time between when the first large fire ignites and the last one is contained — in western North America lasts 84 days longer than it did in the 1970s.
The average area burned in the West grew from 2 million acres each year in the 1980s to 8 million acres a year this decade. Researchers project that by mid-century, the number of “very large fire” weeks, where conditions are favorable toward wildfires that are 12,000 acres or larger, would increase 600% in some areas. That’s practically optimistic. Since the 1970s, Westerling has noted, the burned acreage in the Pacific Northwest has increased by 5,000%. Fourteen of the hottest 15 summers have occurred since 2000. In only nine days this summer in California, fire burned more than three times what it does on average in an entire season. Fires consume larger swaths of forests worldwide, even in equatorial Africa, which does not have North America’s history of systematic fire suppression (and subsequent accumulation of underbrush).
Worse, in the Mountain West, global warming is expected to cause wetter springs and drier summers, feeding a yearly burst of vegetation and then drying it to tinder. Whether precipitation falls at a temperature above or below 32 degrees Fahrenheit has outsize consequences. If it’s warmer, it falls as rain, not snow. Without a snowpack to release a steady trickle of water through the summer, living trees may retreat into a drought stance, storing no water in the sapwood and becoming dry fuel.
“When I hear the climatologists talk, they’re as worried about snowpack as anything else,” John Bailey, a forestry professor at Oregon State told me. Ecosystems, he explained, need “the slow recharge of melting snowpacks into groundwater supplies.”
The United Nation’s Intergovernmental Panel on Climate Change recently concluded that we have less time than it takes for a child to reach middle school before global warming triggers catastrophic consequences. Just as cheap fossil fuels, interstate highways and air conditioning enabled modern America to grow and expand, climate change and wildfires will drive the next mass movements of people in the country.
That’s the big picture. For me, global warming starts with an ember.
When the witching hour hit that afternoon in the Kootenai, wind and the warming air dramatically increased fire activity. That’s because temperature influences what firefighters call “probability of ignition” (POI).
The forest service and affiliated universities, like the University of Montana, have spent the last century researching the way fires behave. Fires are exceptionally complex physics problems — researchers quip that rocket science has fewer variables. Scientists test everything from the way the steepness of a slope affects the rate of spread to the speed at which drying sticks match their moisture content to the surrounding air.
A function of fuels, relative humidity and temperature, POI describes the odds that an ignition source, like a sizzling match, will catch and spread. An hour’s rise in temperature and dryness can make the POI jump. We consult it to gauge the danger of drifting embers, the speed that flames could turn on us or if a backburn is likely to succeed.
Ten degrees Fahrenheit can mean the difference between an ember dying in cool grass or flames charging uphill. I’ve seen a controlled burn blacken the sawtooth outline of trees into a meadow because fire ripped through grass in sunlight but smoldered impotently when it hit shade. A spruce’s shadow stopped it.
All combustion requires oxygen, fuel and heat. Thicker fuels, like tree trunks, resist ignition more than thinner ones, like twigs. At a given ignition point, the volume of a tree trunk reduces the ratio of oxygen to fuels. A big log’s surface-to-volume ratio also slows the log’s rate of moisture evaporation. So ideally, a fire barely scorches a tree trunk, consumes leaf litter and torches underbrush.
Raise the probability of ignition, and you increase fire behavior. If the sun has dried and warmed fuels, the fire expends less energy to preheat and combust them. The fire spreads faster.
All things being equal, a faster fire is a hotter fire. When fuels burn simultaneously instead of one twig at a time, they combine their heat. A hotter fire has a chance to combust thicker fuels, which store more energy. A high probability of ignition, raised by warmer temperature or drier air, doesn’t just mean a lit match will start a fire. It can trigger a cascading chain of energy release in which the fire will burn more destructively.
In other words, fire only catches in the right conditions. Conditions are changing. I grew up in Alaska, where global warming is obvious to anyone who spends time outdoors. Treelines are climbing hillsides. The past few years, Anchorage has barely escaped with a white Christmas. When I worked in Prudhoe Bay in college, permafrost melted under the foundation of a warehouse. I remember a long day with my sleeves rolled up, loading and stacking tons of hydraulic oil tools outside so contractors could pump expanding foam under the foundation to stabilize the building.
As temperatures increase, water evaporates faster. Even if forests experience the same amount of precipitation, the trees experience dryer conditions. In the Western United States, fuels are not only dry, they are growing denser. Credit a destructive force in forest ecology: me.
Like emitting carbon, fighting fire is a form of procrastination. Fuels can burn in small fires now or big fires later. When we stop every fire we can, fuels accumulate until they carry fires we can’t stop.
Left alone, a lightning fire in the northern Rockies might crawl through underbrush for weeks, pumping up by daylight and settling at night. Eventually it would die in September rains, or blow up on a hot day and rage through a pocket of fuels. Our society has decided we can’t risk that blow-up happening near homes. So firefighters catch fires early. Underbrush grows dense. A lot of fuels accumulate over a century of fire suppression. Of the three requirements for combustion — fuel, oxygen and heat — two are growing. In locations where climate change strengthens winds, that’s all three.
In the late 1700s and early 1800s, a scientist and explorer named Alexander von Humboldt climbed mountains from Europe to South America. He noticed that on every continent, similar species grew at similar latitudes and elevations. This is called elevation banding, or zonation.
Say you drove from the Great Plains up to a summit in the Rockies. You would pass through bands of forest stacked as predictably as colors in the rainbow. In the beginning, piñon pine and juniper would colonize the grasses. Ponderosa pine would transition in next. Thick stands of Douglas firs grow in cooler air above the pondies, towering and shutting out light. In the cold wind of high elevations, the forest shrinks to subalpine fir and aspen. Above that, rocks.
The exact elevation of each layer varies according to latitude. The ponderosa band starts higher in New Mexico than it does in Montana. Even in the same canyon, upper treelines stripe the north-facing slopes lower than on south-facing slopes.
Elevation bands are migrating.
Grasslands are overtaking dying trees at lower elevations. Upper treelines are running out of room on the tops of mountains. Researchers in France observed that even in temperate forests, last century more than 100 species climbed 60 feet higher every 10 years. Microclimates are warming so fast that trees are struggling to colonize fast enough to catch up. The favorable climate for a tree could outrun the tree’s ability to spread seeds to it. In Yellowstone, bristlecone pines have stranded themselves at the tops of mountains. The next suitable habitats are peaks in British Columbia — distant islands in terms of germination.
Seeds can chase migrating climate conditions, but adult trees are stuck. In Montana’s Bitterroot Valley, researchers are finding that where grasslands yield to Ponderosa pines, drier conditions are killing juvenile trees. Adult pines survive by drawing water from deep roots.
Changed climate conditions can weaken adult trees and expose them to insects, disease and competitive new species. Between 2010 and 2018, 147 million trees died in California. Because conditions will no longer support their survival, more than half the tree species in central California are expected to die off. Dead trees rot, or they burn.
It’s a question of overlapping odds. Every year, each band of habitat experiences a range of days with temperatures hot enough to facilitate combustion of its lightest fuels. For a big fire to start, a human spark or lightning strike must coincide with those hot days. To spread, that ignition must also coincide with a place in which fuels have accumulated since the last fire, or a year in which drought has dried out fuels that are usually too wet to combust. Maybe an ecosystem evolved to burn every 10 to 30 years, like an Arizona ponderosa pine forest, maybe a thousand or more, like the rainforest on islands off the coast of Washington. Scientists call this fire return interval.
In 2016, along with thousands of firefighters from across the country, I fought California’s Soberanes Fire in Big Sur. Grass fires ripped past oaks that were 200, 300 years old. While we monitored a backburn at night, stars sharpened behind the dissipating smoke. Flames receded into a hillside of glowing embers. In the dark, without a reference for scale, the embers looked like the lit windows of a vast mountainside city.
Coals pulsed, sketching contours of the ravine. Every few hours, a crack would shatter the silence, followed by a thud. These were limbs dropping off ancient trees. We edged out into clearings.
In wetter, cooler summers, the oaks would have repelled the flames. The fact that they had survived so long, but this fire had killed them, meant it was a once-in-200-year fire.
The rare odds of a campfire coinciding with both high temperatures and drought-dried fuels had protected the oaks for centuries. Those odds have changed. Warmer temperatures and droughts will increase the number of dry, hot days with high POI, which increase the odds of catastrophic fire. Saplings will regrow where the big oaks burned, but fire will return for them in much less than 200 years. A different zone of plants will probably replace Northern California’s oak savanna, maybe from as far south as Mexico.
Climate conditions migrate past forests, stranding trees in fire circumstances they didn’t evolve to thrive in. They are left weakened, dry and dense. Evolutionarily speaking, the role of fire is to clear the unfit. And the hotter it gets, the fewer trees are fit.