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Extreme temperatures and a torrent of wildfires, many reaching hellish proportions, marked this summer in the northern hemisphere. An unprecedented number of these have burned through the threshold separating regular blazes from the far more serious class known as firestorms, or pyrocumulonimbus (pyroCbs).
Generating their own weather and puncturing the tropopause with monstrous plumes of smoke, pyroCbs are incredibly scary, and their sudden spike in prevalence sparks fears that they could become more frequent in the future.
“We’ve seen 75 to 80 pyroCbs this year in the northern hemisphere summer season, and this does seem to be a number which is exceeding previous seasons for which we have data,” Mike Fromm, a meteorologist with the US Naval Research Laboratory (NRL), explained to IFLScience. “I have to state pretty emphatically though that we can’t call this a trend. We don’t know if we have enough data to give trend information.”
This lack of data is largely due to the fact that, despite their staggering intensity, identifying pyroCbs is not easy. It’s only in the last two decades or so that researchers have been able to detect them from satellite measurements of smoke in the stratosphere.
“When I started at NRL in the mid-90s the term pyroCB didn’t even exist,” says Fromm. “In the work that we did, we stumbled upon smoke in the stratosphere. We now have to get good enough at identifying and quantifying pyroCbs to see if they are increasing in frequency.”
What Is A Pyrocumulonimbus?
As their other name "firestorm" suggests, a pyroCb is essentially a thunderstorm generated by a wildfire. When extremely large fires send hot air up into the atmosphere, it condenses into clouds. If the atmosphere is unstable, the smoke plume can continue to rise, generating powerful updrafts as cold air is sucked in to fill the void below. These winds fan the flames, creating a feedback loop resulting in more hot air being thrust skywards, causing the whole system to snowball.
If the smoke plume reaches the stratosphere, it can trigger a lightning storm. At this point, the fire can be said to be generating weather rather than the other way round. Once this occurs, a wildfire earns the title of pyroCb and becomes incredibly difficult to predict or control.
The highest wildfire smoke plume ever recorded belonged to the Australian New Year Super Outbreak (ANYSO) that raged between December 2019 and January 2020, sending smoke 34 kilometers (21.1 miles) into the atmosphere – well into the ozone layer. Winds generated by pyroCbs of this size can exceed 160 kilometers per hour (100 miles per hour), throwing firebrands far beyond the front of the blaze, where they can ignite new fires.
What Makes PyroCbs So Dangerous?
Once an unremarkable route passing through the sparsely populated Portuguese interior, Highway N 236-1 was dubbed the "Road of Death" following a pyroCb in June 2017 – the first ever recorded in Western Europe. As evening fell on the day of the fire, a downdraft caused a section of the road and the surrounding forest to become incinerated in the blink of an eye – along with many motorists attempting to flee.
Similar downward gusts have been observed in other firestorms, leading some scientists to theorize that changing atmospheric conditions may cause pyroCb smoke columns to “collapse”, causing sudden and catastrophic bursts of flames.
Paolo Fernandes, a professor of forestry and environmental science at the University of Trás-os-Montes and Alto Douro in Portugal, told IFLScience that prior to this fatal twist, “the smoke column [of the Portuguese pyroCb] had reached a maximum height of about 13 kilometers [8.08 miles] in the atmosphere.”
“There was a change in wind direction at about 6pm, and around two hours later the column collapsed, so this huge volume of smoke and air which was drawn up suddenly fell and hit the fire with extremely strong winds.”
Exactly what would cause such a colossal plume to suddenly collapse is debated. Fernandes speculates that this may have been precipitated by a sudden loss of fire intensity, whether due to a change in conditions or a reduction in fuel.
Fromm, meanwhile, is unconvinced by this hypothesis. “I’ll have to go on record as saying that I am a skeptic of the term ‘column collapse’,” he says. “As a meteorologist I don’t know what mechanism there would be for a collapse – a cloud doesn’t just fall down to the ground, the air doesn’t come crashing down like a can underfoot.”
“But what does happen, because you get very strong updrafts, you also get very strong vertical circulation. This means you get air coming down to compensate for the air going up, and that might be what people observe at the ground level.”
By Fromm’s own admission, lack of consensus regarding the column collapse theory highlights the fact that “we still know very little” when it comes to the inner workings of pyroCbs.
Are PyroCbs Becoming More Frequent?
In 2010, less than ten pyroCbs had been reported in the scientific literature – although satellite data had revealed a surprising number of unexplained layers of smoke in the stratosphere. For decades these were attributed to volcanic eruptions, yet an eye-opening study co-authored by Fromm revealed that many were actually the product of firestorms.
“In the literature one can find [many] cases wherein stratospheric aerosol layers are attributed to volcanic eruptions when no clear evidence of such an event exists,” wrote the study authors.
For example, aerosol layers detected in the atmosphere in August 1989 were originally recognized as the result of a volcano that erupted in Guatemala the month prior, yet Fromm’s analysis revealed that this eruption was not large enough to send particles into the stratosphere. Wildfires that raged in Canada at that time, however, were intense enough to have done so.
The authors, therefore, claim that “pyroCbs offer a plausible alternate explanation for phenomena that were previously assumed to involve volcanic aerosols,” and go on to identify 17 previously undetected firestorms that had occurred in North America in summer 2002.
Based on this analysis, they conclude that “pyroCb events occur surprisingly frequently, and they are likely a relevant aspect of several historic wildfires.” In other words, pyroCbs are not new, but our ability to recognize them is. Only by identifying more past firestorms using satellite data will we gain a clear picture of just how unusual this year’s count is, and whether or not these terrifying infernos are becoming more frequent.
Can We Prevent PyroCbs From Occurring?
As with all fires, key factors contributing to pyroCb formation are heat, dryness, and wind. As average global temperatures continue to rise and droughts become more common, it seems logical to assume that extreme fires will only increase in frequency. An enormous firestorm that engulfed the Maule region of central Chile in January 2017, for example, is widely accepted to have been facilitated by a multi-year drought. According to one analysis, five consecutive dry years preceding the blaze extended the local fire season by 67 days a year, helping to make the resulting pyroCb something of an inevitability.
Further research, however, suggests that land use changes over the past few decades have led to the creation of highly flammable landscapes, and by altering certain rural practices we may be able to reduce the likelihood of firestorms.
In central Chile, for instance, an initiative to plant pine and eucalyptus trees for use in the paper pulp industry was initiated in the 1970s. By the time of the 2017 fire, these highly flammable foreign species had replaced 80 percent of native trees in Maule. Subsequent studies revealed that these forest plantations had made the landscape considerably drier than it would otherwise have been, and the prevalence of both species significantly increased the countryside's combustible nature.
A similar story unfolded in Portugal, where according to Fernandes, pine and eucalyptus now account for 60 percent of all trees in the country – including most surrounding the Road of Death. A struggling rural economy has further exacerbated the problem by driving people away from the countryside, enabling incendiary forests to take over abandoned farms.
“In the past, farming created buffers of low-flammability vegetation, so the continuity for fire spread was interrupted,” explains Fernandes. “These forests were not there 70 years ago so we had no biomass, but all this changed when the landscape become devoid of people, and that partly explains our fire problem.”
Likewise, parts of Maule lost 25 percent of their population between 1974 and 2015, allowing huge swathes of pine and eucalyptus forests to dominate the countryside. To prevent a repeat of the recent hellish firestorms, Fernandes says we need to buck this trend and start creating more diverse landscapes – yet doing so will naturally require people to return to rural areas.
“Economy plays an important role here, because if you want people to again take control of the landscape, they need a way to make a living off the land,” says Fernandes. Offering financial incentives to return to the countryside and creating more diverse ecosystems may therefore represent Portugal’s best shot at reducing pyroCb outbreaks.
A cautiously hopeful Fernandes says that plans for such an initiative have been muted, yet while politicians continue to stall on the issue, fuel for the next major firestorm continues to accumulate in the country’s heavily forested interior.
