Here’s something I wrote last week to explain the role of the carbon cycle in our lives in more detail. I promise to tell a story next time!
The carbon in the atmosphere, the oceans, the trees, the soils, us and everything else is constantly in motion, flowing in a giant circle from air to land and back to air again in an unending, closed loop. The Law of the Conservation of Matter says that in a closed system matter can neither be created or destroyed. It can only cycle and recycle. The Earth has been a closed system almost from its origin, with only solar energy, an occasional electromagnetic pulse from the sun, and stray bits of asteroids entering the atmosphere from space (to burn up). What’s here today has always been here, including carbon, whose total amount is essentially the same as it was when Earth formed 4.5 billion years ago.
The ancient Greek philosophers understood all this intuitively, proclaiming that ‘nothing comes from nothing.’ Epicurus wrote “the totality of things was always as it is now, and always will be.” Nothing can be created or destroyed. This observation was explained scientifically by none other than Monsieur Lavoisier, who discovered that although matter may change its form or shape – a diamond into gas – its mass always remains the same.
So it is with carbon. And what carbon does is cycle – a process essential to life on Earth. It’s a carefully regulated process too, so that the planet can maintain critical balances. Call it the Goldilocks Principle: not too much carbon, not too little, but just the right amount. For instance, without CO2 and other greenhouse gases, Earth would be a frozen ball of rock. With too many greenhouse gases, however, Earth would be like Venus. Just right means balancing between the two extremes, which helps to keep the planet’s temperature relatively stable.
It’s like the thermostat in your house. If it gets too warm, the cycle works to cool things off, and vice versa. Of course, the planet’s thermostat gets overwhelmed at times, resulting periods of rapid warming or cooling (think Ice Ages). No matter what happens, the miraculous carbon cycles keeps working, scrubbing excess CO2 out of the atmosphere, or adding more if necessary. The carbon cycle never sleeps.
Who does all this regulatory work? Two quick answers: green growing plants and evolution. Photosynthesis is the process by which carbon is transferred from sky to soil. It’s what makes the Goldilocks principle tick. Evolution is the process by which life changes over succeeding generations – what lives, what dies, which population expands, which one contracts. It keeps the Goldilocks principle ticking over time – long periods of time. The two work in concert. The quantity of carbon in the environment influences the course of evolution and vice versa.
The effects of an excessive build up of CO2 in the atmosphere, for example, will impact the fate of generations of living things. Carbon and evolution interact and adjust to each other, regulating and responding in a sophisticated dance. Carbon chooses the music, if you will, while evolution dictates the steps in a planet-wide choreography. It is a dance with a profound effect on audience members.
During the Carboniferous Period of Earth’s history, for instance, which lasted from 350 to 300 million years ago, the music was turned up very loud. A potent combination of swampy terrain, warm temperatures, high humidity, and unprecedented levels of oxygen caused an explosion of life across the planet. Insects grew to huge sizes. Modern-looking fish evolved. Birds, reptiles and mammals began to lay eggs on solid ground for the first time – in a fateful evolutionary leap. It was the vegetation, however, that really went wild. As the Period’s name implies, massive amounts of carbon-bearing trees grew during this time, many of which toppled into swamps when they died becoming entombed in muck. Layer after layer of trees and muck piled up, creating, 300 million years later, the rich coal seams that we exploit today for our energy (for better or worse).
Carbon is not the only dance on the planet, of course. Our world is full of cycles – water, energy, nutrients, nitrogen, phosphorus, and many more – each interacting with each other in complicated ways. Some cycles are short, like a song, while some are long, like a symphony, or a mass. Carbon has both. Its short, or fast, cycle revolves around green plants and photosynthesis – the process by which carbon is separated from oxygen, stored in roots and soils, or released back into the atmosphere via death and decomposition. Its long, or slow, cycle is geologic – what happens when carbon is released after being trapped or frozen in layers of rock for millions of years. In the case of the slow cycle, the symphony is really long – carbon can take between one to two hundred million years to rotate fully through rocks, soil, ocean, and atmosphere.
In the slow cycle, carbon in the atmosphere combines with water vapor to form carbonic acid (in a weak solution) that falls to the ground with rain events and begins to dissolve rocks – a process called chemical weathering. This process releases minerals, including potassium, sodium, calcium, and magnesium, all of which are carried by streams and rivers to the ocean. There, it provides the calcium carbonate necessary for shell-making creatures, such as corals and plankton to grow – a key to life underwater. When these organisms die, they fall to the sea floor where they become, over time, carbonate rocks, such as limestone. Then, after more time (a lot more), carbon is returned to the atmosphere via volcanic activity. Ejecta flies upward into the air in the form of ash, lava or other material. Volcanism also releases trapped carbon dioxide – and the cycle starts all over. Round and round, very slowly. If too many volcanoes go off at once, the process of chemical weathering will rebalance things again – but only after hundreds of thousands of years.
The fast carbon cycle involves sunlight, green plants, water, nutrient minerals, and soil microbes and has four basic dance steps:
Photosynthesis: This is the process by which energy in sunlight is transformed into biochemical energy, in the form of a simple sugar called glucose, via green plants – which use CO2 from the air and water from the soil, releasing oxygen as a by-product.
Resynthesis: Through a complex sequence of chemical reactions, glucose is resynthesized into a wide variety of carbon compounds, including carbohydrates (such as cellulose and starch), proteins, organic acids, waxes, and oils (including hydrocarbons) – all of which serve as “fuel” for life on Earth.
Exudation: Carbon created by photosynthesis can be exuded directly into soil to nurture the microbes that grow plants and build healthy soil. This process is essential to the creation of topsoil from the lifeless mineral soil produced by the weathering of rocks over time. The amount of increase in organic carbon is governed by the volume of plant roots per unit of soil and their rate of growth. More active green leaves mean more roots, which mean more carbon exuded.
Humification: or the creation of humus – a chemically stable type of organic matter composed of large, complex molecules made up of carbon, nitrogen, minerals, and soil particles. Visually, humus is the dark, rich layer of topsoil that people generally associate with stable wetlands, healthy rangelands, and productive farmland. Once carbon is sequestered as humus it has a high resistance to decomposition, and therefore can remain intact and stable for hundreds or thousands of years.
A lack of humus can mean that the carbon exuded from plant roots simply oxidizes and recycles back to the atmosphere as CO2. Additionally, humus-rich soils can be disturbed by human activity, such as plowing, which exposes the stored carbon to air, facilitating its release. In each case, oxygen combines with sugar to release water, carbon dioxide, and energy.
The key to creating humus are a class of microbes called mycorrhizal fungi, which get their energy in liquid form, as soluble carbon, directly from actively growing plant roots. In turn, these fungi facilitate the transport of essential nutrients, such as phosphorus, zinc and nitrogen, into plant roots in exchange for carbon. In this way, these mycorrhizal fungi help turn atmospheric carbon into humus, often quite deep in the soil profile. When mycorrhizal fungi are functioning properly, say scientists, 40-50% of the carbon fixed in the leaves of plants can be channeled directly into soil as soluble carbon – which is why people get excited about the prospect of storing excess CO2 in the soil. Not only is it possible, on a practical level, all it requires are the processes that create life, including cycles – and life is something that Earth does very, very well.
By the way, this complex interplay of carbon, microbes, nutrients, and water in the soil is nearly identical to what happens in the digestive gut of humans, livestock, and other animals. It is not a coincidence either. The ‘purpose’ of what goes on in the soil is the same as what goes on in our gut: to create the optimal conditions for life. The chemical, physical, and biological components of the human ecosystem also require regulation and balancing, often through slow-and-fast cycles of our own. We are star dust, after all, just like every other living organism on the planet. And just like a watershed or a population of animals or the microbial universe in the soil, the way this balance is expressed is by health.