The carbon cycle describes the continuous movement of carbon atoms between Earth’s major spheres, a process that has historically maintained a long-term planetary balance. This movement involves geological processes that take millions of years and biological exchanges that happen over days or years. Combustion, the process of burning carbon-containing materials, represents a rapid and relatively new pathway, largely driven by human activity, that significantly alters this natural cycle. The speed and scale of combustion introduce a powerful flow of carbon that changes the atmospheric composition.
The Global Carbon Reservoirs
Carbon is stored in four primary reservoirs, each holding the element in different forms and for varying lengths of time. The atmosphere contains carbon primarily as carbon dioxide gas, representing the smallest of the major reservoirs but having the most immediate influence on global temperature. The hydrosphere, mainly the oceans, serves as the largest active reservoir, holding dissolved inorganic compounds and exchanging carbon quickly with the atmosphere.
The terrestrial biosphere stores carbon in living organisms, like plants and animals, and in dead organic matter within soils. The largest overall reservoir is the lithosphere, which includes rocks like limestone and sequestered fossil fuels. Carbon moves between these reservoirs through processes like photosynthesis, which transfers atmospheric carbon to the biosphere, and respiration, which returns it to the air. Slow, natural processes, such as rock weathering and the burial of organic matter, keep the cycling of carbon in equilibrium over vast geological timescales.
Defining Combustion as a Rapid Flux
Combustion is a high-speed chemical reaction where a carbon-based fuel reacts with oxygen, releasing energy as heat and light. This rapid process is also known as oxidation, resulting in carbon atoms quickly combining with oxygen to form carbon dioxide ($\text{CO}_2$). This conversion is often accompanied by the formation of water vapor.
Unlike slow processes such as decomposition or deep ocean circulation, combustion is nearly instantaneous. It acts as a powerful flux, moving carbon from its stored state directly into the active atmospheric pool. This rapid transfer is the defining characteristic that gives combustion its role in the modern carbon cycle.
Distinguishing Between Carbon Sources
The environmental consequence of combustion depends heavily on the source of the carbon being burned. Fossil fuels, such as coal, oil, and natural gas, are considered ancient carbon because they were formed from organic matter buried and transformed over millions of years. This carbon has been locked away in the lithosphere, outside of the active atmosphere-biosphere cycle for eons. Burning fossil fuels represents a net addition of carbon to the atmosphere, introducing stored carbon back into the system.
In contrast, biomass, which includes wood and agricultural waste, is considered modern or biogenic carbon. This carbon was recently absorbed from the atmosphere during the plant’s growth through photosynthesis. The combustion of biomass, such as during a wildfire or controlled burning, returns this carbon to the atmosphere. If biomass is harvested as part of a sustainable system where new plants are immediately regrown, the process is theoretically carbon neutral over time, as new growth draws the equivalent amount of carbon back out.
Overwhelming the Natural Balance
Human activities, primarily the burning of fossil fuels, have significantly accelerated the rate at which carbon is transferred into the atmosphere, surpassing the capacity of natural systems to reabsorb it. This introduction of ancient carbon disrupts the long-established equilibrium between the reservoirs. Natural carbon sinks, including the land biosphere and the oceans, remove a substantial fraction of the carbon dioxide that humans emit, but they cannot keep pace with the current emissions rate.
The remaining carbon dioxide accumulates in the atmosphere, increasing its overall concentration. This accumulation has caused atmospheric $\text{CO}_2$ levels to reach record highs in just a few centuries, compared to the millions of years it took for the carbon to be sequestered. The oceans absorb excess atmospheric carbon, but this absorption has a secondary effect. When $\text{CO}_2$ dissolves in seawater, it forms carbonic acid, which lowers the ocean’s pH in a process known as ocean acidification. This change threatens marine organisms, particularly those that build shells and skeletons, by making the necessary minerals more difficult to acquire.