Wildfires and oxygen

Ray Bradbury wrote a seminal political fiction in the 1950s, called Fahrenheit 451.  It is about a repressive regime that tries to snuff out dissent by burning books, the title referring to the temperature (233ºC) at which paper spontaneously bursts into flame in the modern atmosphere.  With no reference to book burning by some future oligarchy, geoscientists have speculated on the possibility of higher atmospheric oxygen contents being able to induce massive conflagration of green vegetation after lightning strikes or meteorite impacts.  One often cited case is at the K/T boundary, where the thin layer that signifies the mass extinction event contains a high proportion of sooty particles.  Late Cretaceous air probably had significantly higher oxygen content than now, generated by pole-to-pole luxuriant vegetation, and the idea of a global wildfire gained much support when first mooted.  During the Carboniferous, there is very good evidence that oxygen levels were as high as 35% compared with 21% today.  It was a time of giant flying insects, whose size is limited by the availability of oxygen. Carboniferous and Permian strata contain much charcoal, which suggests that indeed fires then were a great deal fiercer and more capable of spreading.  They might have destroyed vegetation, despite evidence that the tree-sized plants of the period had developed fire-resistant structures.  Experiments to simulate the effects up to now have used strips of paper in different oxygen levels, and showed a strong correlation between the minimum energy for ignition and oxygen concentration.  US geologists, foresters and engineers have repeated the experiments using a range of natural plant materials as well as paper (Wildman, R.A. et al. 2004.  Burning of forest materials under late Paleozoic high atmospheric oxygen levels.  Geology, v. 32, p. 457-460).  Their results approximately confirm Bradbury’s fictional paper-combustion temperature, but monkey-puzzle (Araucaria) leaves are more easily set alight.  However, the temperature for ignition does not change as oxygen levels increase, although burning is faster.  How natural materials burn depends on their relative proportions of cellulose and lignin, the higher the latter, the greater the temperature for complete combustion.  They behave very differently from paper.  Another finding was that the rate at which burning spreads did not rise as dramatically as expected for Carboniferous conditions.  The limiting factor is moisture content, although that for no-burn does increase with oxygen levels.  This is particularly important for the firing of dead vegetation lying on the surface, which is essential for catastrophic wildfires.  Natural fires are started by lightning, and that occurs during heavy rainfall, when surface debris is thoroughly saturated.  Fires in the canopy would have occurred at higher frequencies and with greater intensities, but the authors consider they would not have seriously threatened plant life.

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