24 December 2016
A wondrous transformation
Posted by Callan Bentley
It’s bonfire season here in the Fort Valley.
I live in a forest, and that forest is full of dead and downed wood. Motivated by a desire to (a) reduce forest fire risk and (b) clear out some of the area under the trees for unobstructed recreation, I gather it up and periodically burn it off in batches. We time these blazes to the weather – before or after after a heavy rain or snow, with the bonfire structure pre-built and kept dry by wrapping it in a tarp. My family and I gather round and safely enjoy the convivial conflagration. It’s nice, both primordially comforting and exciting.
We turn something like this…
…into something like this:
I hope you notice the change in volume on the bonfire between those pictures. Believe me, there’s a change in mass too – it’s a proper workout hauling hundreds of pounds of wood in from the surrounding land to burn on this spot, but after we burn the fire, there’s only a wee pile of ashes left. Where does it all go?
Matter is neither created nor destroyed in a backyard bonfire. But a chemical reaction does take place – a highly exothermic one! The wood itself is a distillation of the air. Plants perform a profound magic trick: Their chlorophyll molecules are capable of the most incredible transformation. Water sucked up through the plants’ roots is sutured in a crucible of sunshine to an invisible gas in the air. This trace gas, carbon dioxide, is carefully extracted and forced to bond to the water, yielding sugar. Plants use this sugar to make their bulk, a compound called cellulose.
Chemically, this is what’s going on:
CO2 + H2O → C6H12O6 + O2
(photosynthesis; needs a lot of energy input)
C6H12O6 + C6H12O6 + C6H12O6 + C6H12O6, etc. → (C6H10O5)n + H2O
(formation of cellulose; growth of the tree)
C6H12O6 + O2 → CO2 + H2O
(bonfire; results in a lot of energy output)
Trees are made of atoms, and about half of those atoms came into the tree via water sucked up by its roots. And the other half were filtered out of the air, entering the tree through its leaves.
Not to put too fine a point on it, but this is utterly amazing. We can build houses out of stuff that used to be air and water.
I can work for hours cutting and hauling wood, working up a sweat and tiring my muscles. But all that heavy wood turns into mere gas once I set it on fire. The atoms still have the same mass, but their relationship to my weary back is completely transformed.
Those trees in the background of these photos? Those trees are made of carbon that used to be present in the atmosphere, floating around being selectively transparent, absorbing and redirecting infrared radiation. As the trees survive and bulk up, they hoard carbon in a place where it can’t participate in the greenhouse effect. When a forest fire takes hold, it burns the living and the dead, and sends it all back into the sky. (When the tree dies and fungi consume it, the same thing happens, but at a much, much slower rate.) This is the short-term carbon cycle.
If the trees weren’t growing on a mountainside in the Appalachians but instead in a bayou on the Mississippi Delta, then the wood would be in a better position to be buried by rising seas and smothered under layers of mud and sand and carbonate. If that were to happen, the carbon would enter the long-term carbon cycle. It could be sequestered in the Earth, transformed to coal, potentially for hundreds of millions of years. When we extract coal from the crust and use it to generate electricity, we’re basically doing the same thing as I do in my yard with the bonfires. The difference is that the ‘trees’ we burn did their photosynthetic magic in the Paleozoic. Their cellulose formed 300 million years ago. But it’s the same essential transformation: Hutton’s Principle of Uniformity, right? Then and now, plants pull carbon from the air. Young and old, we set them on fire and return the carbon to the air.
And it’s even more profound than that. The magically disappearing pile of wood is a potent reminder of the atomic nature of reality. Carbon is made of atoms, and they engage in all sorts of chemical relationships. Promiscuous carbon doesn’t limit its dalliances with the atmosphere and trees. Those same atoms are just as happy dissolved in the ocean, supplying skeletal material to a coral, locked up in a limestone, subducted into the mantle, erupted from a volcano, farted from a cow, scratched from a pencil lead onto the field notebook of a geologist, imprisoned in methane hydrates in the deep cold sea, shimmering in a diamond ring, or making up neurons in your brain. These aren’t different carbon atoms – a single carbon atom could successively be part of all these entities, all these processes.
This is one of the greatest aspects of Tyler Volk’s excellent book CO2 Rising. In it, he tracks the fate of a single hypothetical carbon atom, “Dave,” following its biography through the eons. I think one can’t have a decent grasp on modern anthropogenic climate change unless they internalize this perspective. As far as Earth goes, carbon is pretty much eternal.
If you want to get deeper into the wonder of a bonfire, consider the source of the carbon atoms themselves. Carbon is an element that is formed in the heart of stars. Specifically, the atoms are born through the thermonuclear fusion of helium atoms in ancient extinct, dismembered suns. Here’s a video I made on the basics of how carbon got to Earth in the first place (6 minutes):
Forged in dead star guts, by golly! Carbons, like all atoms heavier than hydrogen, are palpable atomic artifacts from a stellar predecessor. These durable atoms persist throughout the lifetimes of multiple generations of solar system formation and collapse. We inherit our carbon atoms from former worlds; worlds blasted utterly into oblivion, dissociated into their smallest components.
Carbon survives, an active participant in terrestrial goings-on. Its atoms are almost everywhere we care to look on this dynamic planet. Carbon is critical to keeping our planet warmer than outer space. Shifting from one terrestrial reservoir to another, it pushes on the boundaries of the climate system. Winnowed from a sea of nitrogens and oxygens, it gets woven into the fabric of organisms, crossing from the lifeless into the realm of the animated, and passed up the food chain from one being to another through consumption and digestion.
The blaze of the bonfire is a glowing signal to our intellect – something extraordinary is going on here!