The Carbon Cycle - Statistics & Facts

Carbon dioxide (CO₂) and methane (CH₄)

In recent years, carbon has been viewed in an increasingly negative context in the public psyche, as human-made climate change has largely been driven by the rise of carbon dioxide and methane in the atmosphere. Over 99 percent of our atmosphere is made up of nitrogen (N₂) and oxygen (O₂), while carbon dioxide and methane make up just 0.04 and 0.00017 percent respectively. However, individual CO₂ and CH₄ compounds in the air are much larger than nitrogen or oxygen molecules (i.e. 3-5 atoms of different elements vs two atoms of the same element), therefore they absorb and trap infrared waves of heat that are reflected off the earth's surface.

The carbon cycle was fairly balanced before the industrial revolution, and naturally occurring changes took place over millennia, but in recent centuries the burning of fossil fuels and conversion of land for agricultural or commercial purposes has seen vast quantities of carbon reintroduced into the carbon cycle. The share of these gases in our atmosphere may be less than 0.1 percent, but its sheer size of the atmosphere means that this is equal to almost 900 billion tons. Methane also traps and stores heat at a higher rate than CO₂ - greenhouse gases are measured by their global warming potential (GWP), and methane has a 20-year GWP of 86, meaning that one tone of methane will store the same amount of heat as 86 tons of CO₂ in this time. Methane's GWP falls gradually over time, and it is present in the atmosphere in much lower quantities than CO₂, but the urgency of the climate change threat means that it is often considered a more serious threat.

The carbon cycle in the biosphere

Plants, algae, and some bacteria use the process of photosynthesis to create energy - this is where they take in sunlight, carbon dioxide, and water from their surroundings, and convert this into energy in the form of glucose (carbohydrates), with oxygen expelled as a by-product. The chemical formula for photosynthesis is commonly written as : (sunlight) + 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂. If his formula is reversed (and sunlight removed), this gives the chemical formula for cellular respiration - the other major energy producing system in plants, whereby glucose and oxygen are used for energy, expelling CO₂ and water as by-products. In this way, plants are considered "autotrophic, as they have the ability to produce their own food internally. The difference between the O₂ and CO₂ output in plants is marginal - on average, oxygen output is slightly higher, however this difference was historically offset by surplus CO₂ emissions from other sources, such as oceanic and seismic activity, melting permafrost, or rock erosion, and the carbon cycle was generally balanced.

In contrast to plants, "heterotrophic" organisms (animals, fungi, and other bacteria) can not produce oxygen, yet they still respire and emit carbon dioxide. Perhaps surprisingly, the respiration of trillions of heterotrophic organisms on our planet is viewed negligibly in terms of the carbon cycle. This is because the carbon in CO₂ exhaled by these life forms is sourced from foods, in the form of (carbon-based) carbohydrates, fats, and proteins, which originally entered the food chain as autotrophic plants, algae, or bacteria - therefore the carbon exhaled by heterotrophs was originally taken from the atmosphere. In addition to energy, carbon is used for growth and maintenance in living organisms, and most have developed the ability to store carbon for later use, such as glycogen and fats in animals or starch in trees. When living organisms die, most of this carbon is then returned to the earth, where it is sequestered (absorbed) into the ground or used for sustenance by other living organisms.

The carbon cycle in the lithosphere

The lithosphere refers to the solid, outer layer of the earth's surface, comprised of rocks and minerals. The build up and compression of dead plant and animal matter over millions of years has led to the formation of fossil fuels within the lithosphere, primarily as solid coal, liquid oil, or natural gas. The atoms that once made up these life forms then bond to form hydrocarbons, such as methane in natural gas or paraffins in oil (coal is mostly solid carbon), which humans now burn for fuel. When fossil fuels are burned the heat causes oxygen atoms from the air to collide with carbon or hydrocarbon bonds within the fuel at an accelerated pace - this causes these chemical bonds to break down quickly, which then releases more heat, and the cycle continues until the fuel is depleted or conditions change. A simple chemical formula for the combustion of natural gas is: (heat) + CH₄ + 2O₂ → CO₂ + H₂O + (heat + light), however, fossil fuels also contain additional compounds that can impact the environment when they are released, such as sulfur, while carbon monoxide (CO) and various nitrogen oxides can be produced under certain conditions, which exacerbate many adverse effects of climate change.

For millions of years, fossil fuels sank deeper into the earth's lithosphere, becoming removed from the global carbon cycle. Since the 1800s, industrialization drove coal consumption on a larger scale, releasing excess CO₂ and CH₄ into the carbon cycle in bulk. Oil rose to prominence during the Second World War and overtook coal as the most common fuel source in the early 1960s, and natural gas consumption rose gradually throughout the late 20^th century. While natural gas is considered the cleanest and most efficient fossil fuel, its collection and distribution leads often results in leakage of methane into the atmosphere. Apart from fossil fuels, the other major way carbon is unnaturally redistributed from the lithosphere to the atmosphere is through mining and cement production.

Carbon sinks in the hydrosphere and pedosphere

The volume of carbon in the air is dwarfed by that stored in the earth's waters (the hydrosphere). The ocean alone has almost 50 times more carbon than the atmosphere, most of which is inorganic and dissolved in the form of carbon dioxide, carbonic acid (H₂CO₃), and carbonates. Oceans absorb most carbon dioxide by waves simply coming into contact with the air, as well as through rain, the erosion of rocks (including along rivers and coastlines), and the decay of organic matter. It is believed that the ocean may have historically been a net emitter of CO₂, but this changed in the past two centuries as it adapted to sequester more carbon from the air, especially in the northern hemisphere where there is more human activity. Carbon in the ocean does not store heat in the same way as atmospheric CO₂, but the temperature of the water itself has risen as a result of atmospheric heat retention. Additionally, higher levels of carbon in the hydrosphere is causing ocean acidification, which threatens the ability of coral, animals, and other organisms to use calcium carbonate to build shells or skeletons. Rising temperatures and acidification are just two of many ways that human activity is threatening marine life faster than it can adapt or evolve.

While the ocean contains the largest volume of carbon in the carbon cycle, terrestrial sources have been the largest carbon sinks in recent centuries. Vegetation takes in CO₂ from the atmosphere, but soils (the pedosphere) are also major carbon sinks on land. Similarly to oceans, soils takes in CO₂ through contact with the air and water, as well as absorbing dead plant and animal matter. In terms of human-made climate change, land-use conversion is the second-largest contributor of CO₂ emissions, especially for agricultural activities such as soil tilling or the expansion of grazing land which leads to higher livestock emissions. Deforestation is especially detrimental to the environment as it affects both the biosphere and pedosphere sub-cycles by removing autotrophic life while releasing carbon stored in soil and plants (when burned). In many cases, especially near rainforests such as those in southeast Asia, areas that once acted as the world's most effective carbon sinks have now been terraformed into some the world's most polluting regions through commercial agricultural and logging.

Another source of excess carbon emissions from the ground is permafrost, which is ground that was frozen below 0°C for at least two years, (although much has been frozen for thousands of years), most commonly found in the northern hemisphere. Ground thawing was a natural, cyclical process in the past, yet permafrost is now thawing at a faster rate than it can refreeze. This has more severe consequences for climate change than regular soil emissions, as permafrost contains large numbers of methanogens (methane-producing microbes) and rotting vegetation, which have the potential to release vast quantities of methane into the atmosphere at rates that cannot be contained or controlled by humans.

Re-balancing the carbon cycle

Although carbon sinks in the earth and ocean adapted to sequester higher quantities of carbon than in previous centuries, emissions are being released at a much higher rate than these sinks can absorb, and land-use conversion has long-exceeded sustainable levels. Over the 2010s, the difference in combined carbon emissions from fossil fuel combustion and land-use change to the combined sink in the ocean and land was more than 5.1 gigatons in each year. Because of this, governments, international organizations, and scientific institutions are working together to limit the effects of climate change, but the focus until now has been to reduce emissions and to slow the pace of global warming. As CO₂ remains in the atmosphere for centuries, returning global warming to pre-industrial levels may be impossible within our lifetimes -however, initiatives to halt or even reverse global warming, such as carbon capture and storage, could provide a solution to this problem, although their large-scale implementation likely remains many years in the future.