The Pyrocene

How We Created an Age of Fire, and What Happens Next

Stephen J. Pyne

• Publisher: University of California Press
• Year: 2021
• ISBN:
  9780520383593
• Edition: 1
• Topic: Sciences biologiques
• Subtopic: Conservation et Protection de l'Envronnement

A provocative rethinking of how humans and fire have evolved together over time—and our responsibility to reorient this relationship before it's too late.? The Pyrocene tells the story of what happened when a fire-wielding species, humanity, met an especially fire-receptive time in Earth's history. Since terrestrial life first appeared, flames have flourished. Over the past two million years, however, one genus gained the ability to manipulate fire, swiftly remaking both itself and eventually the world. We developed small guts and big heads by cooking food; we climbed the food chain by cooking landscapes; and now we have become a geologic force by cooking the planet. Some fire uses have been direct: fire applied to convert living landscapes into hunting grounds, forage fields, farms, and pastures. Others have been indirect, through pyrotechnologies that expanded humanity's reach beyond flame's grasp. Still, preindustrial and Indigenous societies largely operated within broad ecological constraints that determined how, and when, living landscapes could be burned. These ancient relationships between humans and fire broke down when people began to burn fossil biomass—lithic landscapes—and humanity's firepower became unbounded. Fire-catalyzed climate change globalized the impacts into a new geologic epoch. The Pleistocene yielded to the Pyrocene. Around fires, across millennia, we have told stories that explained the world and negotiated our place within it. The Pyrocene continues that tradition, describing how we have remade the Earth and how we might recover our responsibilities as keepers of the planetary flame.

1

Fire Planet

Fire Slow, Fire Fast, Fire Deep

Earth alone holds fire. It’s worth pausing over this remarkable circumstance. Among planets fire is as rare as life, and for the same reason: fire on Earth is a creation of the living world. Life in the oceans gave Earth an oxygen atmosphere. Life on land gave Earth combustible hydrocarbons. As soon as plants rooted on land, lightning set them ablaze. They’ve been burning ever since.

Other planets have some oxygen—Mars, most notably. Others have combustibles; Saturn’s moon Titan has a methane atmosphere. The gaseous planets have lightning. But none have all the necessary elements, or not in ways that allow them to combine. We may find exoplanets that have life, that perhaps will have fire, that may even have intelligent species who can manipulate fire. But we know of none now, and if we find some, they will be so remote as to offer no comparative value. We live on the only viable fire planet. If we were to visit another at some distant future, it is most likely we would do so on plumes of flame.

How the Earth Got Its Fire

Fire on Earth has a history. It has its narrative. There was a time when fire didn’t exist, though it is difficult to imagine a time, short of the planet’s immolation by an erupting sun, when it will no longer flourish. The Earth would have to lose its lands, shed the oxygen in its atmosphere, end the electrical imbalances between land and air that kindle lightning, and find another way to convert energy into terrestrial matter, and hydrocarbon molecules back into energy. All that is possible in principle. It just isn’t plausible on this planet.

The history of fire, in brief, is the history of terrestrial life. Fire’s evolution, or elaboration, into new varieties and expressions, its arrangement into ever-changing biomes, its reorganization into new pyrogeographies, its inextricable entwining with the other elements—all this is not a process parallel to the evolution of life, but a coexistence and even coevolution with it so shared that it comes close to symbiosis. Fire is not alive, but because life called it into being and sustains it, it shares, much as a virus does, many of life’s properties. It feeds on living biomass, it spreads by a contagion of combustion. Fire of life is not a random metaphor.

Because it is a reaction, not a substance, fire is what its setting makes it. Fire’s history is the history of its parts and how they come together. Like a driverless car, it has no single pair of hands on the wheel: it synthesizes its surroundings. It takes its character from its context. It barrels down the road integrating everything around it. As air, water, earth, ignition, and life change over time, so does fire.

Begin with the oldest component, lightning. In truth, the Earth has plenty of sparks, and fires have started from rock falls, earth slides, volcanoes, spontaneous combustion, and the occasional meteor, but only lightning can account for the prevalence of combustion on a planetary scale. It has been continuous since the early Earth. Its relentless pervasiveness is a major reason why fire is elemental, ancient, and inescapable.

Lightning can seem capricious. It is not evenly spread over the Earth or across eons: it appears in clusters, crowding in time, bunching in space, tethered to places amenable to thunderstorms. Only a fraction of lightning is capable of kindling fires—only bolts that connect land to cloud, not cloud to cloud; bolts that strike something combustible, not rocky peaks or lakes; bolts with the proper electrical properties, throbbing and full of heat.

Both fire and lightning highlight a complex choreography between wet and dry. Some moisture is needed to make storms, and so lightning; too much moisture prevents ignition. Some moisture is needed to grow fuels; too much renders fuel unburnable, and too little means fire can’t spread. Areas dense with thunderstorms rarely equate with areas rife with lightning-kindled fires (central Florida is a notable exception). Dry lightning, in which rain evaporates or is separated from the bolts, starts more fires than wet lightning, whose spark must survive a downpour.

Compared with the number of flashes, fires are relatively rare, but when conditions are right, they can kindle fires in swarms. The epicenter of lightning fires in the United States is the Southwest, where drought and monsoon, mountain and desert, make ideal circumstances for dry lightning. Yet outbreaks can come even to places that experience them rarely. In Northern California, the 1987 fire bust ignited 4,161 fires, of which 92 grew larger than 300 acres. In the siege of 2008, lightning started almost 3,600 fires, of which 88 swelled over 1,000 acres. In 2020, in excess of 10,000 strikes were documented amidst a record heat wave, and they kindled between 400 and 500 fires, most around the Coast Range.¹

The interaction between life and lightning is unequal. Lightning is a phenomenon of geophysics, not of life. Plants adapt to lightning, lightning does not adapt to plants. Lightning can occur on Jupiter or Uranus as readily as Earth; a bolt can select a limestone cliff as handily as a black spruce. There seem few ways for the living world to influence the electrical discharge except very indirectly, as seen when taller trees are struck more than short ones. Lightning can exist without life; fire cannot, and so it shares life’s evolution. If life were removed altogether, lightning would continue. Fire would expire.

Fire needs more than ignition, and life contributed the two other ingredients—oxygen and fuel—that stir together to make combustion and, in the right context, fire. Life in the oceans first stuffed the atmosphere with oxygen, and life on land lathered the surface with fuels. When those products of the living world met lightning’s spark, fire resulted. It is not just that planets like Mars lack life because they lack the proper conditions, but that they lack those conditions because they lack life.

The varieties of organisms that captured and tamed oxygen came late in the process. The earliest forms of life (in oceans) appeared in the absence of oxygen; the first photosynthesizers were anaerobic; many genera today still thrive in anaerobic settings in swamps, lakes, and seas below an oxygenated zone, and there are whole ecosystems in the deep oceans around thermal vents that operate on a fundamentally different chemistry. For all of these biomes, free oxygen could prove toxic.

The dominant life-forms of the time, however, managed to turn oxygen from a threat into an opportunity. They captured it, contained it, trained it to their purposes—a template for what hundreds of millions of years later hominins would do with fire. They didn’t merely accept oxygen, they produced it. A competition commenced between organic sources and geologic sinks. While life evolved new ways to release oxygen, rocks found new ways to absorb it. Then between 2.35 billion years ago and 700 million years ago, during what has become known as the Great Oxidation Event, sources overwhelmed sinks, as atmospheric oxygen increased and stabilized, and aerobic photosynthesis and respiration became the norm for terrestrial life. What had been a chemical poison evolved into a biochemical necessity. Ever after, across the swells and troughs of deep time, the Earth’s atmosphere has contained oxygen in quantities that range between 14 and 16 percent, below which it is difficult to get biomass to burn, and 30 and 35 percent, at which it is hard to halt burning.²

A new process, combustion, became possible, then pervasive. Note that its chemistry is a biochemistry: it occurs within living organisms and by means created by life. Within cells everything about the reaction is tightly constrained to prevent free-roaming oxygen from wreaking havoc. Across landscapes, however, little is confined, and fire responds to winds and humidity, to seasons and rains and droughts, to terrain that ranges from gorges to mountains, and to infinitely complex arrangements of biomass over which humans have little power.

Life on land had to find ways to control fire, or at least influence its character, or everything that grew might be scavenged by burning. What happened with oxygen thus happened with fire: what began as a potential toxin became a norm, and then a necessity, and life even moved to enhance its presence. Terrestrial life and fire coevolved within a shared biotic matrix; they became interdependent and, in curious ways, codependent. In brief, fire was not something imposed on life, like wind or flood. It emerged from life’s very character.

Earth’s is an atmosphere of combustion, primed to burn, but also an outcome of combustion. As James Lovelock memorably put it, “It is not too far-fetched to look on the air as like the gas mixture that enters the intake of an internal combustion engine: combustible gases, hydrocarbons, and oxygen mixed. The atmospheres of Mars and Venus are like the exhaust gas, all energy spent.” The nature of this interdependence is tricky, and an inquiry might usefully begin with the question—a template for all the others that fire seems to raise—of whether fire has influenced the atmosphere within which it burns. How does the fast combustion of fire compare to slow combustion of respiration in shaping the planet? Is free-burning fire a vital process in the global oxygen cycle, or simply a geochemical afterthought? By regulating carbon, has fire also regulated oxygen? Geologic eras with more fossil charcoal suggest higher oxygen; those with scanty charcoal hint at lower. As oxygen rises and falls, so does the planetary condition for fast combustion.³

What was the character of burning across deep time? Surely, the Earth burned in the past, as it does today, with a medley of fires, some of which flare at certain times and dim at others, but all of which depend on the character of the biomass available to burn. Oxygen content would have to rise significantly for wet logs to burn, and it would have to drop hugely to prevent ignition in small, dry grasses. By whatever feedback fire shapes the atmosphere, it seems to do so through fuels. After all, the photosynthesizing plants that pump oxygen into the air are the same ones that stoke free-burning fire.⁴

It doesn’t take long before the question begins to turn on itself like a Möbius strip. And it’s not just oxygen. There is a perhaps unavoidable circularity to fire that may not be inappropriate for a phenomenon whose very character is to interact and integrate.

The Fire of Life

Fire’s biological makeup is fundamental. It takes apart what photosynthesis puts together. When that occurs in cells, we term it respiration; oxygenation occurs among tightly scripted and bounded molecules. Call the process slow combustion. When it occurs in the wide world, we label it fire; oxygenation takes place amid an essentially unbounded milieu of roughened terrain, turbulent air masses, and an endlessly evolving biota. Call it fast combustion. These processes have gone on since the Devonian, over 420 million years. Call that chronicle deep combustion. Fire slow, fire fast, fire deep—they are as basic to Earth as the waters that flow through its valleys and the plants that clothe its slopes.

This vision can seem bizarre to people who live in cities in the industrial world and whose fire power resides in machines. They experience fire mostly virtually, through monitors. They no longer use flame to do routine work—to light scenes, to cook food, to heat rooms, to jolt fields and pastures to life, to protect themselves against wildfire. There are practical reasons, of course, to restrict flame in built landscapes, but there are cultural biases as well. European thinkers have long regarded the use of fire as a stigma of primitivism, while European agronomists identified agricultural burning with prerational superstition. Even the term developed country refers, in shorthand, to a nation that has replaced open flame with the closed combustion encased in fire machinery. Out of sight, out of mind.

If such residents can imagine fire, they see it as a solely physical process. They define fire as the oxygenation of hydrocarbons, a chemical event shaped by its physical surroundings. They imagine it as a phenomenon that physics and chemistry can deconstruct and then implant into devices like stoves, candles, and furnaces. They can have light without heat, heat without smoke, smoke without fire, fire without flame. They can live in a setting that may burn but only as accident or arson and almost always as disaster. They have broken down fire into its components and refashioned them to support a world of combustion without fire. If they think about fire on the land, they likely imagine it as another spasm of physical force like a tornado, tidal wave, or flood. To them it is something imposed on landscapes from outside; an ecosystem may adapt to it, as a river’s channel to flooding, but life has no say in that jolt of energy any more than it does with an erupting volcano or an earthquake.

Yet fire is intrinsically different from these other disturbances. It emerges out of the character of living landscapes. In the living landscape all the factors that literally feed fire converge. While ecological science formally regards fire as a disturbance, this is a fiction of modeling, for fire is a “disturbance” in the same way rain is. Even fire’s chemistry is a biochemistry. Hurricanes and floods can occur apart from anything living. Fire cannot. It derives its power by feeding on its biotic matrix. It more resembles a fast herbivory than a wind or ice storm. Like a virus, fire is not itself alive, but it relies on the living world to propagate. We often speak of an epidemic spreading like wildfire, but it makes equal sense to speak of a fire spreading like a plague, a contagion of combustion.

· • ·

Ignition in the form of lightning is the oldest contributor to the reaction we call fire, but it is the patchiest in any particular place. Oxygen arrived next, but it is universal—the same around the globe for any particular era. The final ingredient is fuel. Life had to leave the oceans in order to burn, and when it did, combustion found a new abode in which to happen. Combustion could spread between organisms; fast combustion, fire, could complement and compete with the slow combustion of respiration. The terrestrial biosphere has three ways to decompose biomass: microorganisms, herbivores, and fire. All three rely on forms of combustion. In the living landscape all the factors that literally feed fire converge.⁵

The complexity of combustion increased exponentially. Oxygenation took place not only as slow combustion among tightly scripted an...