A carbon source is defined as any entity, activity, or process that releases a net amount of carbon into the atmosphere, meaning it releases more carbon than it absorbs. The carbon is typically released in the form of carbon dioxide ($\text{CO}_2$) or methane ($\text{CH}_4$), both of which are greenhouse gases. Understanding these sources is fundamental to analyzing the global carbon balance and addressing the increasing concentration of atmospheric carbon.
Carbon’s Journey Through the Environment
Carbon is constantly exchanged among four major global reservoirs: the atmosphere, the oceans, the land biomass (biosphere), and the lithosphere (rocks and sediments). This continuous movement is known as the carbon cycle, encompassing both rapid biological and slow geological processes. For instance, plants absorb atmospheric $\text{CO}_2$ through photosynthesis and release it back through respiration or decomposition.
Historically, the flow of carbon between these reservoirs maintained a natural equilibrium. The amount of carbon released by natural sources was approximately equal to the amount absorbed by natural sinks. Disrupting this balance by adding large quantities of carbon previously stored underground fundamentally alters the Earth’s climate system.
Natural and Human-Driven Sources
Carbon sources are categorized as either natural or resulting from human industrial activity. Natural sources include the decomposition of organic matter in soils and wetlands, which release $\text{CO}_2$ and methane as microbes break down material. Volcanic eruptions also release carbon trapped in the Earth’s crust, though their contribution is less than one percent of annual human emissions.
The current atmospheric imbalance stems primarily from human-driven, or anthropogenic, sources. The largest of these is the combustion of fossil fuels, such as coal, oil, and natural gas, for electricity, transportation, and industry. Burning these fuels introduces carbon that was stored in the lithosphere for millions of years back into the active cycle, overwhelming natural systems.
Land use changes represent another anthropogenic source of emissions. Deforestation releases stored carbon when trees are cut down and burned or allowed to decay. Additionally, industrial processes, like cement production, chemically release $\text{CO}_2$ as a byproduct.
The Role of Carbon Sinks
The opposite of a carbon source is a carbon sink, which is any process or reservoir that absorbs and stores a net amount of carbon from the atmosphere. The largest natural sinks are the oceans and terrestrial ecosystems, primarily forests and soil. Oceans absorb atmospheric $\text{CO}_2$ through physical and biological processes, having taken up about a quarter of all human-generated emissions since the Industrial Revolution.
Forests act as terrestrial carbon sinks by utilizing $\text{CO}_2$ to build biomass, storing carbon in their wood, leaves, and roots. Soil also stores carbon in the form of organic matter from decayed plant material. Achieving climate stability requires reducing carbon sources while simultaneously strengthening the capacity of these natural sinks to absorb excess atmospheric carbon.