A carbon source is any component or process that releases carbon into the atmosphere or another part of an environmental system. Identifying these sources is fundamental to understanding the global carbon cycle and how carbon moves through Earth’s various reservoirs.
Forms of Carbon in the Environment
Carbon exists in diverse forms across Earth’s major reservoirs. In the atmosphere, carbon is primarily found as carbon dioxide (CO2) gas. Oceans store vast amounts of carbon, predominantly as dissolved inorganic carbon, including bicarbonate (HCO3-) and carbonate (CO3^2-) ions, which are formed when atmospheric CO2 dissolves in water.
Living organisms, both plants and animals, contain carbon in organic compounds like carbohydrates, proteins, and fats. When these organisms die, their organic matter is incorporated into soils and sediments. Over millions of years, deeply buried organic matter can transform into fossil fuels like coal, oil, and natural gas, which are concentrated carbon stores.
Natural Processes Releasing Carbon
Several natural processes continually release carbon into the atmosphere and other parts of the environment. Cellular respiration is a universal biological process carried out by most living organisms. During respiration, organisms break down organic compounds for energy, releasing carbon dioxide as a byproduct into the atmosphere.
Decomposition of organic matter also acts as a natural carbon source. When plants and animals die, decomposers like bacteria and fungi break down organic molecules. This breakdown releases carbon dioxide and, in anaerobic conditions, methane (CH4) into the atmosphere. Volcanic eruptions represent a geological carbon source, expelling large quantities of carbon dioxide and other gases directly into the atmosphere from Earth’s interior. Additionally, oceans naturally release dissolved carbon dioxide back into the atmosphere when surface waters warm, reducing the gas’s solubility.
Human Activities as Carbon Sources
Human activities have become substantial contributors to atmospheric carbon, altering natural carbon balances. The combustion of fossil fuels is the largest anthropogenic source, involving the burning of coal, oil, and natural gas for electricity generation, transportation, and industrial processes. This process rapidly converts stored geological carbon into atmospheric carbon dioxide. Industrial processes, such as cement production, also release significant amounts of carbon dioxide as a direct byproduct of chemical reactions.
Deforestation is another major human-induced carbon source. Forests store large quantities of carbon in their biomass and soils. When forests are cleared, especially through burning, the stored carbon is rapidly released into the atmosphere as carbon dioxide. Even when not burned, the decomposition of cleared vegetation and disturbed soil organic matter contributes to carbon release over time. Agricultural practices, including livestock farming, can also lead to the emission of methane.
Identifying Carbon Sources in a System
Identifying a carbon source within a diagram or conceptual model of the carbon cycle involves looking for components or processes that show carbon moving out of them into another reservoir. These are typically represented by arrows pointing away from a “box” or labeled area. A box depicting a factory with a smokestack, for instance, would represent a carbon source because it shows carbon dioxide being emitted into the atmosphere. Similarly, an arrow originating from a forest and pointing towards the atmosphere could indicate carbon release from deforestation or forest fires.
Another common depiction of a carbon source is a box representing an animal or microbe, with an arrow indicating respiration releasing carbon dioxide. A decomposing log or a landfill might also be shown as a source, with arrows pointing towards the atmosphere, signifying the release of carbon dioxide and methane from decay. The key is to recognize any element within the system that acts as an exporter of carbon to another part of the cycle, rather than an importer or storage unit.