The carbon cycle is a fundamental Earth process, continuously moving carbon atoms among various reservoirs. This ongoing exchange plays a significant role in regulating the planet’s climate and supporting all forms of life.
The Natural Carbon Cycle
Carbon exists naturally in several major reservoirs: the atmosphere, oceans, land (in soils and vegetation), and geological deposits such as fossil fuels. In the natural carbon cycle, carbon moves between these reservoirs through various processes. Plants absorb carbon dioxide (CO2) from the atmosphere for photosynthesis, converting it into organic compounds. Organisms, including animals and microbes, release CO2 back into the atmosphere through respiration as they break down organic matter for energy.
When plants and animals die, decomposers break down their organic material, releasing carbon back into the soil and atmosphere. Oceans also play a significant role by absorbing CO2 from the atmosphere, where it dissolves in the water. Over vast geological timescales, carbon can be stored in sediments and form fossil fuels like coal, oil, and natural gas.
Human-Caused Carbon Releases
Human activities have significantly altered the natural carbon cycle, primarily by releasing large amounts of stored carbon into the atmosphere at an accelerated rate. A major contributor is the burning of fossil fuels—coal, oil, and natural gas—for energy generation. These fuels, formed from ancient organic matter over millions of years, release CO2 when combusted for electricity, transportation, and industrial processes, returning sequestered carbon to the atmosphere much faster than natural processes can remove it.
Industrial activities also contribute to atmospheric carbon. Cement production, for instance, releases CO2 during a chemical process where limestone (calcium carbonate) is heated to form clinker, a component of cement. This chemical conversion accounts for a portion of the cement industry’s emissions, with additional CO2 released from the energy used in production.
Deforestation and changes in land use further exacerbate carbon releases. When forests are cleared for agriculture, development, or logging, the carbon stored in trees and soil is released into the atmosphere. Burning cleared vegetation immediately releases CO2, while decomposition of remaining plant material and disturbed soils also contributes to emissions. These changes are among the largest sources of human-induced CO2 emissions after fossil fuel combustion.
Human Influence on Carbon Absorption
Beyond releasing carbon, human activities also reduce Earth’s natural capacity to absorb atmospheric CO2, disrupting the carbon cycle’s removal side. Continued deforestation is a significant factor, as forests act as vital carbon sinks, absorbing CO2 through photosynthesis. The destruction of forests not only releases stored carbon but also diminishes the planet’s ability to draw down future emissions.
Increased atmospheric CO2 leads to ocean acidification, where oceans absorb a significant portion of this excess carbon. When CO2 dissolves in seawater, it forms carbonic acid, lowering the ocean’s pH. This acidification reduces the availability of carbonate ions, making it harder for marine organisms like corals, mollusks, and some plankton to build and maintain their calcium carbonate shells and skeletons. This impacts marine food webs.
Land degradation, including soil erosion and desertification, also reduces the soil’s capacity to store carbon. Soils naturally hold more carbon than the atmosphere and biomass combined. Practices such as intensive tillage in agriculture disturb soils, releasing stored carbon into the atmosphere and diminishing the soil’s ability to sequester carbon. This loss of soil organic carbon weakens the land’s function as a carbon sink.
The Environmental Outcome of Carbon Imbalance
The human alteration of the carbon cycle has broad environmental consequences, primarily by increasing the concentration of greenhouse gases in the atmosphere. Elevated levels of atmospheric carbon dioxide trap more heat, leading to a rise in global temperatures, known as global warming. This warming results from human activities, with CO2 being the primary cause.
Rising global temperatures contribute to changes in weather patterns. This can manifest as more frequent and intense heatwaves, altered precipitation, and an increase in the severity of extreme weather events. These shifts disrupt natural systems and affect ecosystems and human societies.
Ocean acidification, caused by the absorption of excess atmospheric CO2, continues to impact marine ecosystems. The decrease in ocean pH makes it difficult for many marine species, particularly those that form shells or skeletons from calcium carbonate, to survive and reproduce. This disruption can alter marine food chains and biodiversity.