French firefighters face 'pyrocumulonimbus' for first time

There was none like it since Canada became a nation, either: the exodus of May 3 was the largest, most rapid displacement of people due to fire in North American history. It took the form of an unbroken ribbon of vehicles crawling in ranks, like army ants, northward and southward out of the city while fire raged along the highway, in some cases right up to the breakdown lanes. Visible in every rearview mirror was a monstrous plume where their city should have been, as if the city itself had erupted. Many who saw this sight speculated that the entire city was lost. The fire plume, which was growing steadily larger, was actively changing the region’s meteorology. No longer simply a ground-level interface fire, it had become a force of Nature. As temperatures rose past 1,000°F, the air at the smoke column’s center rose ever more rapidly, driving upward, like smoke up a hot chimney. As this superheated air rose higher and faster, it created a vacuum into which cooler air was drawn from all sides at greater and greater velocity. Operating like a recirculating fountain, storm systems this large also generate powerful downdrafts along their outer edges, which, in the case of a wildfire, can cause it to burn even more intensely, like an atmospheric turbocharger.

Smoke columns behave like fountains in other ways, too: suffused within that swirling vortex, inconceivable in the face of so much fire, was a colossal amount of water—not just from moisture bound up in the forest, but also from melting ice, broken water lines, and fire hoses. In order for fuels to burn as explosively as they did in Fort McMurray, any residual moisture had to be removed by evaporation. All that water has to go somewhere, and it does: what looks from a distance like “smoke” is really a combination of soot, combustive gases, toxic chemicals, and steam. Hundreds of thousands of gallons of water vapor were being carried skyward though the smoke column, ten, twenty, thirty thousand feet above the fire, where it condensed and then froze. There, miles above the city, hurricane-force downdrafts hurled fusillades of black hail back to earth, just as they had done in ancient Egypt. Reduced to their most elementary ingredients, these carbon-infused ice pellets were all that remained of the trees and houses so recently devoured by the fire.

As the fire intensified, ash and glowing spruce needles grew into firebrands the size of work boots, and then branches, treetops, fence panels, and entire garden sheds—all flying through the air, on fire. Some of these were carried thousands of feet into the smoke column, just as they would in a tornado. Pilots flying over large wildfires have reported charred tree branches bouncing off their windshields at twenty thousand feet. A photo taken from an airplane window late on the night of May 3 shows a vast and luminous smoke cloud where the city had been while, high above, the northern lights blaze across the sky. In another age, this might have been an omen worthy of formal record, but that night, it was just one more illumination from the twenty-first century, captured in this smartphone-crowdsourced record of apocalyptic visions.

Other anomalies appeared as well, and, from this vantage, they sound more like details from the Old Testament or Greek mythology than events reported from one of the twenty-first century’s wealthiest industrial centers. Among them was a fire-borne thunderhead. Known to meteorologists as a pyrocumulonimbus cloud, or pyroCb, these massive formations can be two hundred miles wide and reach into the stratosphere. A fully developed pyroCb, like the one shrouding Fort McMurray on May 3, is so huge and energetic that its behavior is influenced by the coriolis effect—the rotation of the earth. In the Northern Hemisphere this will cause such a system to spin counterclockwise, just like a hurricane. Because of their size, particularly their height, pyroCbs are Nature’s most efficient delivery system for high-altitude pollutants, including carbon monoxide, hydrogen cyanide, ammonia, and vast amounts of carbon and other particulates. Once these smoke columns reach the lower stratosphere, between thirty thousand and forty thousand feet above the earth, the aerosols and particulates within them can be carried around the world on the jet stream, which circles the poles like a high-speed conveyor belt.

Breakthroughs in aerosol-sensing satellite technology have revolutionized scientists’ understanding of these phenomena. As recently as the 1990s, hemisphere-spanning aerosol clouds generated by enormous wildfires were mistakenly attributed to volcanic activity. In part because they were so rare, wildfire-generated pyroCbs have only been formally identified and studied as such since 1998, the dawn of this new era of twenty-first-century fire. One of the most exhaustively studied pyroCb events to date occurred during the Chisholm Fire, the same one American satellite data analysts initially suspected might be a nuclear bomb test. The plume it generated obscured an area of more than fifty thousand square miles, roughly the size of Greece. While they remain an atmospheric rarity, pyroCbs have become significantly more common over the past two decades, occurring around the world, in places they have never been observed before.

In addition to hail, pyroCbs can also generate their own lightning. “Pyrogenic lightning” has been described since ancient times, but almost exclusively in the context of large volcanic eruptions. While ember-generated fires are relatively easy to predict (they appear downwind, typically less than five miles from their source), fires caused by lightning can be ignited virtually anywhere within a fifty-mile radius of a pyroCb, where they are accompanied by all the hazards associated with electrical storms—tower strikes, power outages, and electrocution. By 4:00 p.m., as tens of thousands of citizens were making their slow escape, Fort McMurray was experiencing the same “darkness at noon” phenomenon associated with apocalyptic events recounted throughout the world’s histories and mythologies. With the forest already primed to burn, a pyroCb, combined with wind-driven embers and lightning, changed this fire from a localized conflagration into a perpetual motion machine of destruction operating on a regional scale. Given the long-term forecast, this fire could burn as long as the fuel held out, and, in these conditions, the boreal forest was nothing but fuel.

From 2016 onward, the fire seasons in both hemispheres have been relentless. In 2017, British Columbia, a huge coastal province bigger than Alberta, bigger than Chile, and more than twice the size of California, set a new global record. On July 7 alone, 142 separate wildfires ignited; by the end of the day, the province was in a state of emergency. A familiar combination of high heat, drought conditions, and wind caused many of those fires to grow rapidly into uncontrollable blazes. A month later, many of them were still burning. On August 12, four of the larger fires, along with one across the border in Washington State, erupted almost simultaneously into pyroCb thunderstorms, a phenomenon never observed before. David Peterson, a meteorologist at the U.S. Naval Research Laboratory in Monterey, California, speaking to the CBC, declared it “the most significant fire-driven thunderstorm event in history. Nothing else even comes close.” Once in the stratosphere, the mass of particulate was swept into the jet stream, where it circled the globe for four months.

The only mercy shown British Columbia that summer was that most of the fires ignited in sparsely inhabited areas. Even so, more than forty thousand people were displaced across the province, firefighting costs alone exceeded half a billion dollars, and almost five thousand square miles of forest burned. Though it was hundreds of miles from the biggest fires, the sky in Vancouver turned a burnt-orange color for weeks, and the air quality was rated some of the worst in the world. British Columbia’s historic aerosol injection, which has come to be known as the “Pacific Northwest Event,” was more than twice the size of any previously documented pyroCb.

But if twenty-first-century fire has taught us anything, it’s that there is no top end. It wasn’t long before the Pacific Northwest Event, “the mother of all pyroCbs,” had company. In 2020, the—once again—record-breaking fire seasons in Australia, California, and Oregon generated similarly volcanic pyroCbs. Australia’s, however, was—how many times can one say this?—unprecedented. Most readers will be familiar with the horrific fires that appeared to envelop that country in December and January of 2019–20, and with the shocking number of animals that perished. But high above Earth, something else was happening, too. An abstract describing it in the journal Communications Earth & Environment reads like a scientist’s description of a cataclysm from the Old Testament:

The Australian bushfires around the turn of the year 2020 generated an unprecedented perturbation of stratospheric composition…The resulting planetary-scale blocking of solar radiation by the smoke is [three times] larger than any previously documented wildfires and of the same order as the radiative forcing produced by moderate volcanic eruptions. A striking effect of the solar heating of an intense smoke patch was the generation of a self-maintained anticyclonic vortex measuring 1000 km. in diameter and featuring its own ozone hole. The highly stable vortex persisted in the stratosphere for over 13 weeks, travelled 66,000 km and lifted a confined bubble of smoke and moisture to 35 km altitude.

In other words, ferocious heat convection drove a climate-altering quantity of ash and particulate eight miles into the stratosphere, where it then formed an aerosol blob, six hundred miles wide and two miles thick. Because it contained so much water vapor and black carbon, it absorbed solar energy, which caused it to heat up and rise still further—en masse, like a black balloon the size of Texas—until it was more than twenty miles above the earth, twice as high as any previously known pyroCb injection. Once there, this half-million-cubic-mile pyrogenic carbon blimp drifted for more than three months around the Southern Hemisphere, covering forty thousand miles before finally dissipating.

In the 1990s, pyroCbs were a disturbing but exhilarating novelty wondered at, and discussed by, a small group of meteorologists. Now, they are not only a signature of major wildfires, they are actively growing in size and frequency—to the point that they are mimicking volcanoes, previously Earth’s most rapid and powerful climate-changing agents. PyroCbs are now being observed all over the world in places they have never been reported before. As these events multiply, they are altering, in significant and measurable ways, the chemical composition of what atmospheric scientists refer to as the “stratospheric overworld.”

This is the power of atmospheric CO2. It expresses itself through heat retention, and its “vocabulary” appears to be growing, most obviously through variations in weather, fire, and related phenomena, but in other, less visible ways as well, most notably in the oceans. The oceans absorb approximately 30 percent of all emitted carbon dioxide. Over the course of the Petrocene Age, this global system, home to more than half of the world’s species, has grown 30 percent more acidic, signaling the most rapid shift in ocean chemistry in the past 50 million years.

What the atmosphere and oceans are telling us is that carbon dioxide doesn’t get the respect it deserves. Others have been saying this, too—for a long time. Roger Revelle said as much to a congressional subcommittee in 1956. Eunice Foote said it to the American Association for the Advancement of Science in 1856. Any climate scientist or environmental studies teacher will tell you every chance they get. Until very recently, most of them have been tuned out, brushed off, or appeased in ways that bear a strong resemblance to the experience of people reporting incidents of sexism or racism: “Where? I can’t see it.”

The conclusion arrived at by the Communications article’s dozen authors is that a big enough pyroCb “eruption” could inject enough carbon into the stratosphere to alter the planet’s climate, just as large volcanic eruptions have done in the past. Pollution and air quality aside, the carbon dioxide generated by events of “planetary scale”—like twenty-first-century wildfires—exceeds the annual CO2 output of many states and countries. To put this in perspective, the CO2 emitted by the Australian bushfires of 2019–20 more than compensated for the global reduction caused by the coronavirus pandemic.

It hardly needs to be said that more CO2 leads to more heat retention, which leads to more fires, which leads to more pyroCbs…We are, right now, witnessing the early stages of a self-perpetuating and self-amplifying feedback loop, accompanied by myriad “cascade effects.” In human terms, this has been a long time coming, but in geologic terms it has taken place overnight—roughly seven human generations, or two life-spans. So limited are we by the brevity of our lives and, lately, by the kaleidoscopic swirl of technological advancement, further amplified by a twenty-four-hour news cycle, that it’s hard to appreciate how far we’ve come (and gone) in such an extraordinarily short time.