The Great Peshtigo Fire of 1871 was always going to happen. Settlers in the United States Upper Midwest had practically built their own funeral pyre: after clearing land for crops, they left hardwood slash where it lay. Thinnings from the great virgin pine forests, they piled into incendiary windrows, and much of the industry around the small Wisconsin town—itself built entirely of wood— was housed in sawmills and timber yards. Workers building a railroad from Milwaukee to Michigan’s Upper Peninsula simply left the cleared brushwood on either side.
Already, on that hot October day, small fires were burning all over Wisconsin—slash and burn was standard practice. Then a cold front rolled in. Blustering westerlies whipped the blazes into a concerted firestorm that raged over nearly 1.5 million hectares. It roared from town to town along the railway. A dozen settlements vaporised in a rampant inferno. Survivors recounted seeing railcars and houses lifted high by the fierce vortex. People hurled themselves into rivers and wells. At least 1500 perished in America’s worst conflagration, but so did the county’s records, so it’s thought the toll could have been as many as 2500. A mass grave in modern-day Peshtigo holds 350 victims, unidentified because nobody was left alive to recognise them.
Wildfires broke out that day all over the Midwest (the Great Chicago Fire destroyed most of that city’s business district the same night).
Firestorms are nothing new, but they’re becoming more frequent, and more intense. A study published recently in Nature compared fires between 1979 and 2013 and found that fire weather seasons across a quarter of the planet are now nearly 20 per cent longer.
According to the US Forest Service-led team, the total global burnable area affected by those longer seasons has doubled. During the second half of the study period, the number of wildfires jumped by 53 per cent, an increase the researchers blamed on climate change.
As temperatures rise and droughts worsen, the planet becomes ever more flammable. And more wildfires mean more wildfires: in 1997, blazes across Indonesia released carbon equivalent to as much as 40 per cent of the world’s average annual fossil-fuel emissions.
Climate change is fuelling more fires, which in turn spur climate change.
Much of what we understand about wildfires comes from US and Canadian studies, driven by pressing need. California, currently withering in its worst drought for a millennium, has already entered a whirling spiral. There were 1000 more fires than usual in 2014, and a recent National Park Service study showed that yearly carbon losses from the state’s burning woodlands account for as much as seven per cent of its annual emissions. That’s enough to offset much of the mitigation that would have otherwise seen California meet its 2020 emissions-reduction targets (and, incidentally, reverse much of the progress made on air quality).
Modelling by Harvard University has found that California’s infamous Rim Fire of 2013—which burned for two months in the Sierra Nevada mountain range, scorching more than 100,000 hectares—may well herald a new norm.
In early August, more than 3800 firefighters were still struggling to contain a monster: the 170 kilometre-wide Rocky Fire of northern California, which simply vaulted everything—highways, firebreaks, rivers—that should have contained it. Experts have kept re-writing wildfire models to keep up. In five hours, the fire mushroomed by 8900 hectares; a blowout that according to conventional wisdom should have taken a week.
“In the last 10 years, I’ve seen fire behaviour that I had never seen in my entire career,” Capt. Ron Oatman, a firefighter of 30 years, told The New York Times.
By 2050, researchers estimated, wildfires may burn an extra 65 per cent of the Pacific Northwest. Fire footprints in the Eastern Rocky Mountains and Great Plains will nearly double.
Here in New Zealand, NIWA has researched climate-driven fire risk, and expects our own fire season to stretch out at both ends, too. At present, about 3000 fires start in rural New Zealand each year (notably in Northland, where more than 200 rage every season, just under half deliberately lit). They burn some 7500 hectares of vegetation—barely a backyard bonfire by Californian standards, but blazes often hit fragmented and vulnerable remnant populations of birds, lizards and invertebrates that evolved in far-less-flammable times.
Warmer temperatures, lower humidity and more droughts, says NIWA, will dry scrub and brushwood into prime kindling. Meteorologists expect that climate change will create more energetic weather systems, which raises the likelihood of lightning strikes (a 1994 US study reported that, under a doubling of atmospheric CO2, lightning-struck fires could increase by nearly 45 per cent).
Higher wind speeds will fan flames and drive fires further, faster. But a wildfire, once it’s big enough, needs no further urging. It becomes its own, self-sustaining meteo-physical phenomenon—a firestorm. As a fire consumes more biomass, it needs vast amounts of oxygen, such that it creates its own fanning winds. These ‘in-drafts’ rush from all points of the compass at once into the base of a fire at close to 100 kilometres per hour. Fires in steep terrain are especially difficult to contain, because in-drafts flowing uphill are slower than those coming from above. That creates a lop-sided pressure gradient that can send flames racing uphill at terrifying speed. In 1949, at Mann Gulch in Montana, 13 firefighters were killed when a forest blaze did just that.
Occasionally, wind speeds are so great they form a tornado inside the blaze. A spectacular fire whirl—a ‘firenado’— erupts from the seat. Firefighters dread these, because they cast burning embers far and wide, starting—or ‘spotting’ in firefighting lexicon—new fires. Like ordinary tornadoes, firenadoes can travel some distance, flinging burning trees, vehicles and even houses into the air. Witnesses to a firestorm in west Canberra in 2003 reported firenadoes that covered 600 metres in less than a minute. “The whole sky was full of flame,” said one. “It sounded like a jumbo jet was flying down our street.”
Big blazes can create their own thunderstorms, too, in much the same way as they appear over erupting volcanoes. Superheated air rises from the fire until it stabilises at some cooler, moister altitude. Here, it forms towering, turbulent ‘pyrocumulus’ cloud. No-one’s quite sure how or why these clouds produce lightning, but it may happen when charged particles are separated by violence within the cloud, and the presence of so much ash. Ironically, some fires have been quenched by rain from clouds they created.
When a firestorm gorges itself on so much oxygen, combustion—and therefore temperatures—inside it go through the roof, up to 1200ºC. Heat that intense can radiate well ahead of the fire front:
Witnesses recounted trees around Peshtigo simply exploding, before flames had even reached them. People’s hair spontaneously burst into flames. Sand along the banks of the Peshtigo River was fused into glass.
Even if human firefighters could withstand a blaze that hot, aiming water at it is plainly futile—it would evaporate long before it reached it.
Large blazes are today fought from above by tanker aircraft carrying special chemical retardants—usually some mix of huntite, hydromagnesite, aluminium hydroxide or magnesium hydroxide. When aluminium hydroxide contacts a heat source it dehydrates, releasing stored water vapour and absorbing heat. In this way, a retardant robs a fire of one of its three staple needs.
Nowadays, we model the behaviour of large fires in powerful computers, their physics in pyrotechnic wind tunnels. We know infinitely more than the poor folk of Peshtigo, but in one sense we’re still making the same mistakes. The inner rings of centuries-old tree trunks reveal that fires were, even a few hundred years ago, routine in the great forests of the world. But the outer rings, laid down since the 1900s, show no similar record because we stopped forests from burning: we cut firebreaks, lit managed burns and stamped out blazes before they could take hold.
In doing so, we altered the very composition of the forests, so that nowadays the extra brushwood inflames infernos, such as the disastrous Black Saturday firestorm around Melbourne in 2009. When fires take hold, they’re far more destructive, and now, with climate change ensuring that more sparks will catch, there will be more of them.