Fire looks to be one big contributor to global warming — not directly through its heat, although in some barely discernable way that certainly adds to warmth, but by what it does to the planet's landscape.
A new study that appeared in the April 24 edition of the magazine Science suggests that as much a fifth of mankind's carbon dioxide emissions in the industrial era can be traced to intentionally burning down forests. That doesn't include burning firewood to heat your hut, and it doesn't count wildfires (even those from arson or controlled burns gone awry).
And those intentional fires help create the feedback loop that fosters unintentional fires like the wildfires that char Australia or the Western U.S. each summer. Beyond the release of carbon dioxide, "landscape fires" of all stripes incinerate carbon-storing plants and pump sunshine-absorbing soot into the global atmosphere.
And so the vicious circle is described thusly: "These deforestation-related fires contribute substantially to the global burden of greenhouse gases, and the associated global warming that they will cause is projected to increase extreme fire weather, leading to further spikes of carbon emissions."
Despite this, fire really hasn't factored into comprehensive examinations of climate change, argue the authors of the Science paper — led by David Bowman of the University of Tasmania and Jennifer Balch of the National Center for Ecological Analysis and Synthesis at the University of California, Santa Barbara.
"Progress in understanding fire on Earth has been hampered by cultural aversions to accepting fire as a fundamental global feature and disciplinary parochialism," they write in calling for a more sophisticated understanding, both technologically and psychologically, of fire's role in climate change. "... Such an integrated perspective is necessary and timely, given that a diversity of fragmented research programs have identified the pervasive influences of fire on the Earth system."
“It's not new information," Balch said of the paper itself, "but it's been fragmented. What we've done is pull all these threads together to state the obvious."
Shadow Life Form
A conversation with Bowman suggests the ambitions of the paper — "Fire in the Earth System" — barely fit its four pages of narrative. The work of 22 authors, ranging from physicists, epidemiologists and ecologists to cultural historians, the paper aims to push beyond the "fire community" to present landscape fire into a central position in the earth sciences — "it demands that we think about its history."
The phenomenal output of the greenhouse gas carbon dioxide that the paper traces to intentional burning is the hook, Bowman stated bluntly, but the overriding message is that fire has been a "shadow life form" on Earth that predates civilization by 420 million years — when the first plants started popping up on land.
Fire, he said, has been a potent force for evolution ever since. "I like to say 'fire is the one Darwin dropped,'" Bowman said, reflecting on the naturalist's visit to fire-configured island of Tasmania on the same trip that landed him on the Galapagos. "He saw everything he needed to see, but he didn't get it."
While the ecological landscape in Hobart in Tasmania actually requires routine visits by fire to thrive, the paper suggests the Earth itself evolved in part by growing up alongside fire. "Earth is an intrinsically flammable planet owing to its cover of carbon-rich vegetation, seasonally dry climates, atmospheric oxygen, and widespread lightning and volcano ignitions," it reads.
As Balch said, "Earth is a fire planet. Fire is what happens when you have carbon burn, an ignition source, and a hot and dry climate. And we are a fire species. But part of what is tricky for people is that we use fire very imperfectly — we don't control it."
And so that's why fire has such a major influence now on accelerating climate change: Humans have unleashed what they thought was something tamed only to discover it remained feral.
Fire has been part of the human toolkit for between 50,000 and 100,000 years, with reliable evidence of at least occasional human use going back hundreds of thousands of years earlier. Once hunter-gathers with fire became agriculturalists with fire, the record shows they started using fire to shape their surroundings, through slash-and-burn efforts, to manage wildlife or to permanently take land from the forests for cities and fields.
"Assuming that deforestation and fire are synonymous — and we do — fire is the cheapest and easiest way to replace forest with pasture or crop land," said Balch. Those fires produce greenhouse gases and put soot in the atmosphere, but in some cases they also change the land's reflectiveness — its albedo — from dark forest to brighter snowfields or pastures. In addition, inefficient burning often left lots of carbon still rooted, often literally, in the ground. Some estimates even see the cooling from albedo canceling out the heating from soot in the air.
That canceling out is taken as a given in naturally occurring wildfires, Balch said of the paper's estimates. "We assume that all other types of fires are in equilibrium. That's why we focus on deforestation fires, because they are outside the norm."
The Hard Part
There was also a practical reason to focus on deforestation fires, Bowman said. It was something that could be figured out, even if the effort might prove tortuous. Describing the meeting on fire where the Science paper was born, the idea arose to calculate the contribution fire might be making to the warming of the atmosphere.
Such a calculation, with a billion or so cook fires every night, wildfires, deforestation fires and others in the mix, "would be devilishly complicated for a climate model," Bowman recalled the thinking. "That's going to be really, really hard. Maybe we just use deforestation fire." He summed up the consensus. "Intentional deforestation fire - that is something we do know, a clearly described domain, with an historical certainty and an historical precedent."
The starting point for the data gathering was the Industrial Revolution, circa 1750, which is also the starting point for many other examinations of greenhouse gases and climate change.
As it stands, the authors estimate that all sources of fire, from hut warming to ginormous peat fires in Indonesia, emit an amount of carbon dioxide equal to half that from the burning fossil fuels, the traditional villain in climate change scenarios.
But lots of unknowns and perhaps unknowables remain, and the authors acknowledge that this paper is but a first stab at the issue, "a first estimate, a crucial step in quantifying and recognizing the issue," Balch said. "What we are calling for is a complete treatment of fire in the next generation of climate models and an acknowledgement that fire is, and has been, an important actor for a very long time."
“We all know that climate affects fire," said Bowman, "but we're only now realizing that fire affects climate. Fires are blindingly obvious, but their effects on the Earth's systems (are) deeply subtle, and humans have the interesting possibility that they're part of the feedback.
"That's the scary bit, that if we serio
usly disturb that equilibrium ... then we have the capacity for feedback. And that plays into the climate projections that we're using, that we're debating right now, may, sadly, be an underestimate."
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