Our Earth’s Carbon Cycle

The Carbon and Water Cycles are the support systems for the Earth and are, unsurprisingly, at the heart of everything. As they are the basis for everything, it’s important that they are running at the highest level of efficiency, which they are. Actually, let’s rephrase. They were running at the highest level of efficiency, before humans had their input. Now they’re struggling. 

If we are to understand how we are impacting the Earth’s cycles and the way it ‘runs’, then we need to understand those very cycles and processes. That way, as consumers we can see how our daily choices are having an effect. 

Focusing on the carbon cycle, there are multiple effects that humans have had. We all know that the Earth is a complex creation, that it has its own inner workings. Let’s compare the Earth to humans, so we can start to understand how – if we are to get sick, then our bodies produce antibodies, raise our core temperature, and we rest for a few days. They do this without medication, but if our immune system is struggling, we help it. 

Our Earth has support systems that keep it running which react to changes in its ‘health’. One of these is the weathering thermostat. CO2 traps the heat within the atmosphere – higher levels mean the Earth fries whereas lower levels can lead to an Ice Age. As carbon dioxide is crucial to the Earth’s Climate and has the power to alter it, the Earth can regulate it through weathering. Put simply, more CO2 in the atmosphere (hotter climate), means more chemical weathering occurs transferring carbon to long-term stores. Less CO2 in the atmosphere (cooler climate), means less chemical weathering and less transfers of carbon to long-term stores, so more carbon remains in the atmosphere. So when temperatures are hotter, the chemical weathering speeds up, carbon levels fall in the atmosphere, ensuring the cooling of the climate. Colder temperatures slow down the chemical weathering process, which prevents the climate from becoming too cold. Thus proving that our Earth can fight off sickness. 

Sound familiar? But now it can’t keep up with the changes in temperature, because they’re not natural, they’re not gradual, they’re not small. Our Earth’s immune system needs help to get back to full strength. It needs medication to aid its fight; that medication can only come from us. 

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The carbon cycle is the process which enables carbon to travel between different stores and sinks. They are connected by the flows of carbon, in which the levels vary. There are a number of stores, but they all hold different amounts of carbon in different states. For example, the atmosphere as CO2, oceans as dissolved CO2 and carbonic acid and sedimentary rocks as carbon. The flows of carbon vary, as some processes take longer than others and of course the stores can’t all hold the same level of carbon. What is important to remember, is that the Earth’s processes are relative to the natural state of the Earth, as it is designed to function in a state of equilibrium. 

Before the industrial revolution in 1765, it is thought that the global carbon cycle was in a state of equilibrium. Since then however, there has been a continual increase in human activity pumping unnatural levels of CO2 into the atmosphere. Therefore, our Earth’s global carbon cycle finds itself in a state of disequilibrium. The Earth is trying to respond and self-regulate to restore equilibrium, but the human-made inputs are too high and have been introduced too quickly for the Earth to respond with any success. 

So the processes which are explained, are enough for the natural flows of carbon, but not the addition of human activity/combustion. Of course human-made carbon inputs are absorbed, but the levels absorbed and stored do not even nearly match the levels emitted.

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So carbon, what actually is it? It’s a chemical element, which has two main solid forms – diamond and graphite. It is also found in impure forms in the more commonly known charcoal, soot, and coal. As a result of chemical reactions and natural processes, carbon can change its solid state to gas and liquid compounds. This is where the problems arise, as the chemical state of the carbon is crucial in its storage and transfer, thus influencing the resulting Climate Change. 


The global carbon cycle itself breaks down into two cycles. These are used to describe the length of time it takes for the carbon to circulate through the stores and transfers, along with the time carbon remains in the stores and the levels that can be stored. They are therefore, unsurprisingly, called the Slow Carbon Cycle and the Fast Carbon Cycle. The difference between the two is as simple as it sounds – the fast carbon cycle transfers carbon 1000 times faster than the slow carbon cycle, essentially amounting to the day to day carbon fluxes; whilst the slow carbon cycle stores the carbon not needed for life on Earth for 150 million years, and only lets it out during tectonic movements, like volcanic eruptions. This is where the human activity around carbon gets messy, as we are interfering with the fast carbon cycle in relation to deforestation, but the slow carbon cycle with our use of fossil fuels and discarding of plastics into the oceans. 

The most important carbon stores across the cycles are the atmosphere, oceans, sedimentary rocks, seafloor sediments, fossil fuels, plants (terrestrial biomass) and soils. Each of these hold a different amount of carbon in different states, for varying lengths of time. Each of the stores are connected by transfers, and have transfers and processes within them. The atmosphere and oceans are part of both cycles, with the ocean being the primary carbon sink leading to a store. This is crucial, as it is the key to all the long-term carbon storage. 

There is no start to the carbon cycle, however we all seem to think of the atmosphere as the start, as that’s where the majority of human-made carbon dioxide goes once produced. As we all know, human made carbon dioxide is not a natural process – yes we take the fossil fuels from the sedimentary rock store, which means we can argue that it is part of the carbon cycle. However, as this transfer would not occur naturally through human combustion, it is not part of the Earth’s carbon cycle – it is part of the human carbon cycle. However, the current inputs are relevant to the global stores and transfers. 


So, let’s start with the atmosphere. Despite the amount of carbon dioxide that currently resides in the atmosphere, it is by no means the highest global carbon store. In fact, it is the smallest aside from the plants store (terrestrial biomass). The atmosphere has a store of approximately 600 Gt (1 Gt = 1 billion tonnes). Despite this, the atmosphere is a busy place within the carbon cycle. Even though it is a small store in relation to the rest of the cycle, there are a large number of transfers that occur to ensure that this store remains small. Therefore, the residence time is the shortest within the cycle, only 6 years. As mentioned earlier, the Earth uses carbon to regulate its temperature. So the level in the atmosphere is crucial. This is why, unlike the other stores, there are transfers linking up to four other stores (five if we include the human combustion transfer) – opposed to only one or two around the other stores. So what are the linked stores? The terrestrial biomass (plants), soil, sedimentary rocks and the oceans. What’s more, the transfers that link these stores are not just reducing the atmospheric store, they are also adding to it; this ensures the cycle is unending and non-reliant (on just volcanic eruptions for example), as human emissions are not a natural process. So this means, that despite the levels that we input into cycle, they are not needed. 

In relation to the atmosphere, there are ten main transfers across the stores. The first is with a store that we’re all familiar with; terrestrial biomass. This store is actually the smallest of all stores, holding 560 Gt of carbon with a residence time of 18 years (the second shortest in the cycle). This encompasses land plants, for example grasses, trees, flowers and shrubs. Three transfers are associated with this store – respiration and decomposition which add to the atmospheric store and photosynthesis, which deducts from it. Respiration and photosynthesis are the reverse of each other, which is why one absorbs and one releases CO2. In order for photosynthesis to take place, carbon is needed. This process involves light energy being transferred to chemical energy, with the additional need of water and CO2 from the atmosphere. Photosynthesis is a continual process (continually absorbing CO2 from the atmosphere), which provides the plants with energy to ensure plant growth, reproduction and respiration for example. This is their form of ‘eating’ just like we eat to generate energy. Much in the same way as us, plants also respire. Whilst this involves mainly oxygen being released into the atmosphere which we of course breath in, small amounts of CO2 are also released. This therefore, is then adding to the atmospheric store on a relatively small scale. At the end of a plant’s life, they decompose into the soil, transferring the carbon to the soil store. However, some is released back to the atmosphere as CO2.  This though, takes us to the next store in the cycle – the soil. 

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There are three transfers regarding the soil store, two of which are in relation to the terrestrial biomass store and the other is back to the atmosphere. This store holds up to 2300 Gt of carbon, with the largest transfer being back to the atmosphere. Decomposition is the main transfer; it not only ensures that the carbon is transferred from the terrestrial biomass, but is also returned back to the atmosphere. When decomposition occurs at the end of a plants life, whatever isn’t released back into the atmosphere through the release of energy from the decomposer organisms, is protected in the soil. So, there is a transfer from the terrestrial biomass to the soil store though decomposition. There is a reverse to this – small amounts of carbon are absorbed back to the terrestrial biomass by the plants, however this is minor in relation to the other transfers. However, decomposition also occurs within oxidation (decomposition and natural combustion). Decomposer organisms (bacteria and fungi for example) in the soil consume the plants and exert energy in the form of CO2, minerals and nutrients. This natural process means that the CO2 absorbed throughout the plant’s life, is then released once again. The other half of oxidation is natural combustion, which put simply is fires. Bushfires are a natural occurrence for example, and we all know that a byproduct of fires is CO2 (as well as other forms of carbon). This therefore is another way CO2 is transferred back to the busy atmosphere. When it reaches the atmosphere, the fast carbon cycle on land has been completed. 

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The other half of the fast carbon cycle takes place in the oceans, which itself breaks down further into two main processes. The CO2 in the atmosphere is diffused into the surface layer of the oceans, and is carried through the currents, allowing the processes to begin (also creating the foundation for the slow carbon cycle). The ocean surface waters globally hold 700 Gt for 25 years, compared to the deep waters which hold 38,000 Gt for 1,250 years. Once the carbon is diffused from the atmosphere (known as dissolved CO2) the currents within the oceans carry the carbon to areas of downwelling. Here, it sinks to the deep ocean over a period of time until it is carried to areas of upwelling, where it once again rises to the ocean surface. It is here that (whatever has not been consumed during the process by marine organisms) is diffused back into the atmosphere. This therefore absorbed the carbon from the atmosphere, but also releases it a number of years later. The second transfer is very similar to that on land. Phytoplankton live in the ocean surface waters, were they are exposed to sunlight. They combine the dissolved carbon with the sunlight and water through photosynthesis, just like land plants. At the end of their life, they either remain as the fast cycle or are added to the slow cycle – if they decompose they remain as the fast cycle. Again just like land plants, when they decompose they release CO2 back into the oceans; this is then used in the slow carbon cycle, or released back to the atmosphere.  

When looking at the rest of the stores, it is easier to look at them as the slow carbon cycle as a whole. There are only a small number of transfers, however they are crucial, as this is the only way that the huge level of CO2 can be returned to carbon. This is the only way in which it can be stored long term. There is a slight cross over between the two stores here, as the atmosphere to oceans diffusion transfer applies to both. In the slow carbon cycle, the dissolved CO2 is used by marine life. Marine organisms, like clams and corals, fix the dissolved carbon with calcium in the water to form their shells and skeletons (CaCo3, also known as limestone). When these organisms die, they sink to the seafloor and accumulate. The carbon is locked within the shells and skeletons, and once on the seafloor they become seafloor sediments, thus being the next carbon store in the cycle. This store holds 6000 Gt of carbon, which is transferred to the ‘end’ store. Over millions of years the sediments are transferred to sedimentary rock as a result of immense heat and pressure. It is here that carbon is stored in its elemental form, for 150 million years. This is why this is the ‘end’ store. Once there (in the natural cycle) it’s where it remains. The sedimentary rock itself stores between 60,000,000 – 100,000,000 Gt, whilst fossil fuels trapped within only store 4130 Gt. 

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Even though the natural cycle ‘ends’ here, there is another transfer, otherwise it wouldn’t be unending and non-reliant. This is again back to the atmosphere. At tectonic plate boundaries carbon rich sedimentary rock can subduct into the mantle, where it is then vented out through volcanic eruptions, transferring that carbon back into the atmosphere. Carbon is also released when the rock is exposed, or near the surface as a result of erosion or tectonic movements, by chemical weathering. One of these chemical processes is carbonation, which includes rain charged with CO2 in the atmosphere, creating a weak acid. This acid attacks limestone and chalk, producing a bicarbonate. This then passes through stores within the water cycle and ends up in the oceans. It is here that it combines with the calcium in the oceans to form limestone once again, sinking to the seafloor as being stored as seafloor sediments. In the same way as the marine organisms, this also forms sedimentary rocks. 

The carbon cycle is being largely affected by the human carbon cycle. It is clear that the cycle doesn’t run on human combustion – that’s not natural. The fossil fuels are meant to remain locked in the sedimentary rocks. As a result we are altering the stores and transfers. Deforestation and change in land use is also leading to a change in the terrestrial biomass store, which is critical in keeping the atmospheric store levels under control. 



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