Richard Feynman was an inspiring science communicator. In one of his television broadcasts [embedded below], he talks about a log burning on an open fire and provides one of the most eloquent explanations of the physical chemistry of wood combustion [full video and transcript below].
Feynman explains that throughout its life, a tree accumulates a great amount of potential energy in the form of carbon stripped by photosynthesis from its preferred molecular partner oxygen thanks to energy from the sun – that at a tree grows from the air, and not from the ground. It is only once it is provided with a source of ignition that carbon atoms in the tree and oxygen atoms in the air gain enough energy to reunite, a process that releases a great amount of energy, sustaining high temperatures and producing a chain reaction that is a fire. As Feynman put it, “the light and heat that’s coming out, that’s the light and heat of the sun that went in. So it’s stored sun that’s coming out when you burn a log!”
An interpretation of Feynman’s discussion is that combustion re-balances the imbalance caused by photosynthesis. The carbon that was taken from the atmosphere, and the energy that was taken from the sun by the tree, is quickly released back to the atmosphere during a fire – a closed cycle, where the regrowth of a new tree begins the process once again. If we extend this interpretation to the burning of ancient and fossilised carbon stores, such as peat, coal, oil, and gas; we can appreciate that the energy released comes from sunlight that once shone down on our planet many thousands or millions of years ago.
In a separate post on ‘Why wildfires fuel climate change’, I discuss how this natural component of the carbon cycle has been tipped far from equilibrium by human activity.
The rainforest is being cleared and burned in Borneo. A majestic Meranti tree is close to the edge of the forest. As flames from the nearby fires rage closer to this ancient tree, water quickly evaporates from its leaves, transforming the foliage into dry fuel for the fire. When turbulent, convective winds within the canopy bring hot air to the immediate surroundings of our tree, long-chain cellulose molecules in the desiccated foliage begin to crack, producing hydrocarbon gases and solid char deposits; with a flash, the gases ignite, forming the flames of the fire.
This brief, catastrophic episode at the end of this tree’s life quickly rebalances a century of organic chemistry. Throughout its life, the tree had accumulated a great amount of potential energy in the form of carbon stripped from its preferred partner oxygen thanks to energy from the sun through photosynthesis. Once provided with a source of ignition, carbon atoms in the tree and oxygen atoms in the air gain enough energy to reunite, releasing a great amount of energy, sustaining high temperatures and producing a chain reaction that is a fire. As the great physicist Richard Feynman once put it, “the light and heat that’s coming out [of a log fire], that’s the light and heat of the sun that went in. So it’s stored sun that’s coming out when you burn a log!”
According to Feynman, one could explain that a forest fire is ‘recycled sunlight’, a natural component of the carbon exchange between the biosphere and atmosphere. After all, fire is endemic to many of our planet’s natural environments, essential for regeneration, fertilisation and germination of many plant species. But does all of this burning have any influence on our climate? Continue reading “Why wildfires fuel climate change”