There are broadly two types of chemical processes in nature: one-way, and two-way. One-way chemical reactions are irreversible, meaning that they cannot be undone. Most importantly, once they begin, one-way reactions are very difficult to stop. Fire, which is the oxidation of hydrocarbons within plants, is one of the most spectacular examples of a one-way chemical reaction. It will only stop when either one, or both, of the raw materials of the reaction (in this case trees and atmospheric oxygen) run out. They are consumed and converted into the products of combustion: carbon, nitrogen, water vapour and CO2. This reaction is classified as one-way, because putting together nitrogen, water vapour, carbon and CO2 cannot give you back complex hydrocarbons under normal natural conditions — and it certainly won’t give you back the trees.
Is it Magic or a Chemistry Trick?
One-way reactions would have been a death sentence for life on Earth, had they not been interlinked in such an ingenious configuration whereby the products of one reaction, become the substrates (raw materials) for the next. Hundreds of reactions can be linked head-to-tail in a chain in this way, until one final, critical reaction regenerates the original substrate that was used up by the very first reaction in the chain. This is much like a circular Chinese Whispers game where the last person in the chain somehow always guesses the correct word, against all odds, re-starting the circle. The “magic” about this regeneration reaction loop is that, unlike the short-lived one-way reactions it is composed of, it can go on literally forever, as long as conditions do not change. In the case of fire, the “antidote” reaction to combustion is complex hydrocarbon polymer synthesis: the laying down of cellulose and lignin fibres by plant cell walls to make wood, as trees and other plants grow. This is the last step in a long reaction loop that begins with CO2, that noxious by-product of fire, being re-absorbed by the leaves. It is reacted with water through photosynthesis with the help of sunlight, initially to create simple hydrocarbons like sucrose, an energy source for the plant. These simple hydrocarbons, made from sunlight, water and CO2, eventually lead, through a cascade of more chain reactions, to the eventual creation of the precious lignin molecule: the building block for timber.
The relationship between fire, lignin synthesis and photosynthesis is just one of the hundreds of examples of how Earth’s biotic systems have evolved to interlink multiple chemical reactions that are normally quite vigorous, destructive and unidirectional, in a way that they essentially compete with each other through to eternity. No one ever wins, not the fire, or the trees. The circle continues, and the one-way reaction has magically become reversible.
Circle of Immortality
It is this circularity that lies behind every aspect of the reincarnation of resources that we observe on a healthy, pre-human Earth. Every life-giving chemical that is used up, is somehow eventually regenerated, through this chemistry magic. The Earth’s “carbon cycle” is just one of these circular loops, comprised of millions of one-way reactions that have been wired up to each other, nurturing and decomposing every single living organism on the planet. The human body is 18.5% carbon by weight. All 8 million species on the planet are interlinked through carbon not just figuratively but literally, through a mega chain reaction that includes even the carbon spewed out of volcanos or deep sea vents.
In contrast to one-way reactions, other processes are much more easily reversible. The melting of ice into water may not be a chemical reaction, but it is a reversible phase change that regulates almost every chemical reaction that takes place in the planet’s living and non-living systems. Until recently, the amount of ice vs water on Earth was in balance. The two-way conversion of ice into water and back, was in what is called a “dynamic equilibrium”. This means that, although the relative proportions of ice and water stay relatively the same over time, vast amounts of each are constantly being converted back and forth.
The Illusion of Stability
Dynamic equilibriums are very deceptive, because they give the false impression of stability, like two people constantly lending each other exactly the same amount of money. Their total capital may appear stable as a net value, but in reality they are constantly exchanging vast amounts of money between each other. If either of them makes a small mistake, one of them will suddenly end up with either a huge surplus or a massive debt. Dynamic equilibriums are extremely sensitive to perturbation.
Whether in fact we look at one-way or two-way reactions and processes on Earth, all of them until recently (pre-Anthropocene) were in a dynamic equilibrium — even trees and CO2. Trees were constantly converted into CO2 by natural forest fires and senescence, and an equal amount of CO2 was constantly converted back into trees and other organisms through photosynthesis and other processes. But there was never any “stability”. Everything on Earth constantly moves and shifts. Our planet is in fact one chaotic, dynamic, rapidly changing place. We just don’t notice it because of the two-way dynamic equilibriums that prevail: the carbon cycle, the water cycle, the phase changes of water. Earth’s systems, and the life forms that inhabit them, are in fact incredibly unstable. We live on a constant, sharp knife edge.
Terminal Velocity for Civilisation
Human civilisation is the only one-way chemical process on Earth that has not been integrated into a circle. Civilisation converts trees and fossil fuel into excess CO2, resulting in an ever increasing surplus of the latter. It converts hydrocarbons into plastic, a material that cannot function as a substrate for any natural chemical reaction and does not belong to any circle. Moreover, the climate change created by the excess CO2 is shifting the dynamic equilibrium of water and ice, towards more water and less ice. While humans are part of all of Earth’s cycles, they are quickly converting all of those cycles from reversible, perpetual ones to one-way irreversible processes that are approaching their end state.
An important fact to keep in mind about one-way processes is that they begin at maximum velocity, but slow down very quickly once substrates begin to run out and reaction products increase. Some of the substrates that our civilisation has already almost exhausted are arable land, fresh water and most importantly biodiversity, the latter being the planet’s “library” of millions of biochemical reactions that maintain the planet’s cycles. We are not simply throwing these cycles into imbalance, and destroying the dynamic equilibriums that exist. By making species extinct we are destroying the biological enzymes that catalyze many of the chemical reactions that sustain the planet’s prized cycles. We are taking the cycles apart, one chemical reaction at a time, every time a species is driven to extinction. The rise in temperature further exacerbates this, as it affects reaction conditions. For example a higher temperature means lower pH, more acidic oceans, and sea creatures unable to lay down coral, shells and other structures. Guess which chemical element is crucial for these structures: carbon.
As data emerges that the summation of man-made material (products) on Earth now exceeds the amount of biomass (substrates), we are more than half-way through the one-way, irreversible reaction called Human Civilisation.
(from the book Disposable Earth)
George is an author, researcher, podcaster, chemist, molecular biologist and food scientist. You can follow him on Twitter @99blackbaloons , listen to his Spotify podcast George reads George, join his mailing list, or enjoy his books