Comprehensive Study Guide on Carbon and the Global Carbon Cycle

General Overview and Fundamental Significance of Carbon

Carbon is an essential element for life on Earth. All living organisms require carbon for either their physical structure or their energy requirements, though humans specifically utilize it for both purposes. In the context of the wider universe, carbon is the fourth most abundant element and is found in many diverse forms on Earth. These forms range from gaseous carbon dioxide ($CO_2$), which is categorized as a greenhouse gas, to solid forms such as diamond and graphite.

The element carbon is identified by the atomic symbol $6$ and an atomic mass of $12.0107$. It is fundamental for the existence of life on Earth because of its unique ability to combine with other important elements, such as oxygen, nitrogen, and phosphorus, as well as with hydrogen. These combinations form complex molecules that are essential for cellular metabolism and reproduction. Carbon is ubiquitous and can be found in rocks and soils, in the oceans, in all known life forms, and in the atmosphere. Without carbon, life would not exist as we know it.

Distribution and Storage of Carbon Reservoirs

Carbon can be stored in a variety of places across the planet. In the atmosphere, carbon is stored primarily in the form of gases, specifically carbon dioxide ($CO_2$). This means that carbon found in the atmosphere is most commonly present as atmospheric carbon dioxide.

The oceans also serve as a significant storage site for carbon. Atmospheric carbon dioxide has the ability to dissolve in water, where it reacts with calcium present in the water to form calcium carbonate ($CaCO_3$). Calcium carbonate is a critical component for the structures of various marine organisms, such as the shells of clams. Despite the visibility of atmospheric and oceanic carbon, most of the carbon on the planet is actually contained within rocks, minerals, and other sediments buried deep beneath the surface of the planet.

In biological terms, the human body is comprised of approximately $20\%$ carbon. The provided statistical breakdown of the human body's elemental composition indicates that Oxygen accounts for $65\%$, Carbon accounts for $18\%$ (with the text characterizing it as about $20\%$), Hydrogen accounts for $10\%$, Nitrogen accounts for $3\%$, and other elements make up the remaining $4\%$. This carbon is not present as a pure element but is joined with atoms of other elements, such as hydrogen and oxygen. Consequently, all living things consist mostly of carbon-containing compounds.

The Mechanics and Principles of the Carbon Cycle

A fundamental principle of the carbon cycle is that carbon cannot be created nor destroyed. Because carbon is not an endless resource, it must be recycled continuously to allow for the creation and growth of new organisms. As a result, carbon exists in a constant state of movement from one place to another. The carbon cycle is the term used to describe this continuous movement of carbon through the environment.

The cycle involves several critical processes: photosynthesis, fossilisation, decomposition, feeding, combustion, and respiration. These processes facilitate the movement of carbon between different states, such as carbon dioxide in the air, carbon compounds in plants and animals, carbon compounds in microorganisms (decomposers in soil), and carbon compounds stored in fossil fuels.

Carbon Uptake and Transfer through Biological Systems

Once carbon dioxide is present in the atmosphere, it must return to living organisms to continue the cycle. This is primarily achieved through photosynthesis. Plants take carbon dioxide out of the atmosphere and convert the carbon into glucose and other carbon compounds using sunlight and water, releasing oxygen as a byproduct. Additionally, carbon dioxide is removed from the atmosphere when it dissolves into the sea.

After carbon is incorporated into plant matter, it is passed on to animals and microorganisms through feeding. Animals feed on plants, thereby passing the carbon along. This carbon can then move through the entire food chain as animals feed on each other. It is important to note the vital role of plants in this cycle: without plants, carbon cannot be incorporated into the bodies of animals. If there were no plants to perform this conversion, there would be no animals.

Release and Return Pathways to the Atmosphere

Carbon is returned to the atmosphere in the form of carbon dioxide through the process of respiration, which is carried out by plants, animals, and microorganisms. Furthermore, when organisms produce waste such as urine and faeces, or when they die, the carbon compounds are processed by decomposers in the soil, such as microorganisms.

Combustion, which is the process of burning, also releases carbon. Burning any carbon-containing compound releases carbon dioxide into the atmosphere. Specifically, when fossil fuels are burned, the carbon within them combines with oxygen, and carbon dioxide is released into the atmosphere. The increased global use of fossil fuels is directly contributing to an increase in the carbon dioxide content of the Earth's atmosphere.

Long-term Sequestration and Fossilisation

Fossilisation is a long-term storage process within the carbon cycle. If animals and plants die in specific conditions where decomposing microorganisms are not present, the carbon contained within their bodies is not immediately released. Instead, over millions of years and under significant pressure, this carbon can be converted into fossil fuels. Examples of these high-pressure carbon stores include oil, natural gas, and coal.

Human Impact on the Carbon Cycle

Nature generally maintains a balance in carbon levels, but scientists believe that human activity has upset this balance. It is important to clarify that humans have not created more carbon on Earth; rather, human activity moves carbon from one reservoir to another much more quickly than would occur naturally. This acceleration of movement has significant consequences for the planet's climate.

There are three primary ways humans impact the cycle. First, burning fossil fuels increases the amount of carbon dioxide in the atmosphere by releasing carbon-containing gases that were previously sequestered underground. Second, cutting and burning trees (deforestation) moves carbon from land plants to the atmosphere; when trees are burned, the carbon stored in their physical structures is released as carbon-containing gases. Third, deforestation reduces the total amount of photosynthesis occurring globally, which decreases the Earth's natural ability to remove carbon dioxide from the atmosphere.