OUR atmosphere could once have had a much higher proportion of oxygen – as much as 35 per cent – according to recent experiments. This contradicts the notion that wildfires would have obliterated plant life if oxygen levels had ever been far above today’s norm of 21 per cent, which was based on experiments igniting strips of dry paper. A group led by Richard Wildman, now at the California Institute of Technology, decided to mimic real forests more closely. They put pine needles in a chamber where the oxygen content and the amount of moisture could be controlled, and set them alight. The flames spread easily through dry needles and consumed them all, but when the moisture content was pumped up to match today’s normal forest conditions, the flames did not spread all the way across the chamber – even when the oxygen level was 35 per cent (Geology, vol 32, p 457).
This fits the fossil evidence of the late Palaeozoic, around 240 million years ago. The giant insects of the era are thought to have needed a lot of oxygen to breathe. Also the existence of plants with thick, fire-resistant bark implies that oxygen levels were high enough to make fires more common, though not to destroy all forests.