The Paleozoic rise of land plants brought about increased burial of organic matter and a resulting increase in atmospheric oxygen concentrations. Levels as high as 30-35% O₂ were reached during the Permo-Carboniferous (Berner and Canfield, 1989; Berner, 2001). However, burning experiments (Watson, 1978) have challenged these results, the claim being that if the oxygen made up more than 25% of the atmosphere, the frequency and intensity of forest fires would increase sufficiently to prevent the continued existence of plant life. Since plants have persisted, it is possible that fires have served as a negative feedback against excessive oxygen levels.

An initial study of Paleozoic wildfire behavior via thermogravimetric analysis was conducted under ambient and enriched oxygen conditions to simulate present and ancient atmospheres. The tests focused on natural fuels, specifically tree leaves and wood, tree fern fibers, and sphagnum peat moss, simulating Permo-Carboniferous upland and swampland ecosystems, respectively. Paper, the only fuel that Watson burned, was included as a reference material.

Three conclusions are: (1) enriched oxygen increases the rate of mass loss during burning; (2) fuel chemistry (cellulose vs. lignin) influences burning patterns; and (3) in geometrically heterogeneous fuels, geometry affects burning rate significantly. Both geometrically and chemically, paper resists fire poorly; thus, we found that it loses its mass at lower temperatures than forest materials. In sum, we believe that rebuttal of the existence of O₂ levels above 25% needs to be verified by flame spread experiments focusing on natural fuels rather than paper.