The carbon cycle describes the continuous movement of carbon atoms between the atmosphere, oceans, land, and living organisms on Earth. Carbon, a fundamental element for all life, travels through various reservoirs, including the atmosphere as carbon dioxide, in oceans as dissolved forms, within terrestrial ecosystems, and stored in rocks and sediments. This biogeochemical cycle ensures that carbon is recycled and reused, underpinning Earth’s ability to sustain diverse life forms.
The natural carbon cycle maintains a delicate balance, where the absorption and release of carbon dioxide are roughly equal over long periods. Plants absorb carbon dioxide from the atmosphere through photosynthesis, while respiration from plants, animals, and microbes returns carbon dioxide to the atmosphere. Oceans also continuously exchange carbon dioxide with the atmosphere, absorbing more than they release under natural conditions. This natural equilibrium helps regulate Earth’s global temperature and climate, preventing extreme fluctuations that would be inhospitable to life.
Releasing Ancient Carbon Reserves
Human activities have significantly altered the natural carbon cycle, primarily by releasing carbon that has been stored underground for millions of years. The burning of fossil fuels, such as coal, oil, and natural gas, represents the largest contribution to this disruption. These fuels formed from ancient organic matter that was buried and subjected to immense heat and pressure over geological timescales, effectively locking away vast quantities of carbon. When combusted for energy, this stored carbon is rapidly converted into carbon dioxide and released into the atmosphere.
Fossil fuels are extensively used across various sectors, including electricity generation, transportation, and industrial processes. Most electricity is still generated by burning coal, oil, or gas, which produces carbon dioxide and nitrous oxide. Manufacturing and industry also produce emissions, mostly from burning fossil fuels to produce energy for making things like cement, iron, steel, electronics, plastics, and clothes. Global fossil fuel carbon emissions in 2022 primarily came from coal (40%), oil (32%), natural gas (21%), and cement (5%). The rapid injection of this “ancient” carbon into the atmosphere overwhelms the natural processes that typically absorb and store carbon, leading to an accumulation of atmospheric carbon dioxide.
Beyond fossil fuel combustion, industrial processes like cement production also contribute to the release of stored carbon. Cement manufacturing is highly energy- and emissions-intensive because of the extreme heat required to produce it. To produce cement, limestone (calcium carbonate) and other clay-like materials are heated in a kiln at about 1400°C to form clinker, releasing carbon dioxide as a byproduct. This chemical process, called calcination, accounts for approximately 50% to 63% of all emissions from cement production. The remaining emissions come from burning fossil fuels to heat the kilns, which typically rely on coal, natural gas, or oil. This dual source of emissions makes the cement industry a significant contributor to global carbon dioxide emissions, accounting for approximately 7% to 8% of the worldwide total.
Transforming Terrestrial Carbon Storage
Human actions also significantly transform the Earth’s land-based carbon storage, known as terrestrial carbon sinks. Deforestation is a major contributor to this disruption, as forests are vast reservoirs of carbon stored in trees, other vegetation, and soils. When forests are cleared, often for agricultural expansion, urbanization, or logging, the carbon they hold is released back into the atmosphere. This release occurs through the burning of biomass, which immediately liberates carbon dioxide, or through the decomposition of cleared vegetation and exposed soil organic matter. For instance, tropical deforestation annually contributes significant carbon emissions.
The conversion of natural ecosystems, such as forests, into agricultural land further diminishes the planet’s capacity to store carbon. These natural landscapes typically contain substantial amounts of carbon in their soils, accumulated over centuries. When these lands are tilled or developed, soil carbon is exposed to oxygen and rapidly oxidizes, transforming into carbon dioxide that escapes into the atmosphere. Studies indicate that converting forest land to cultivated land can result in a loss of soil organic carbon stocks by around 41.2%, with changes being more pronounced in surface soils. This alteration of land use reduces the long-term carbon sequestration potential of these areas.
Agricultural practices themselves contribute to greenhouse gas emissions, impacting the broader carbon cycle. Methane, a potent greenhouse gas, primarily comes from livestock digestion, known as enteric fermentation, and the way livestock manure is managed. Nitrous oxide, another powerful greenhouse gas, results mostly from agricultural fertilizer application to soils and from manure management. These emissions, alongside those from deforestation and land-use change, collectively diminish terrestrial carbon storage and add to atmospheric carbon concentrations, contributing to climate change.
Modifying Ocean Carbon Absorption
Human activities indirectly and directly impact the ocean’s capacity to absorb carbon dioxide from the atmosphere. The ocean absorbs about 30% of the carbon dioxide that is released into the atmosphere from human activities. This absorption leads to ocean acidification, an ongoing decrease in the pH of the Earth’s ocean. When carbon dioxide dissolves in seawater, it forms carbonic acid, which increases the concentration of hydrogen ions and lowers the ocean’s pH. Between 1950 and 2020, the average pH of the ocean surface fell from approximately 8.15 to 8.05, representing a 30% increase in acidity.
Ocean acidification also poses a substantial threat to marine life. Many marine organisms, such as mollusks, corals, and shellfish, rely on carbonate ions to build their shells and skeletons. The increased acidity reduces the availability of these carbonate ions, making it more difficult for calcifying organisms to form and maintain their structures. This can weaken marine food webs and ecosystems, potentially disrupting the biological processes that naturally help regulate oceanic carbon storage and impacting species that rely on these calcifying organisms for food and habitat.
Furthermore, rising ocean temperatures, largely driven by human-induced climate change, diminish the ocean’s capacity to absorb atmospheric carbon dioxide. Warmer water cannot hold as much dissolved carbon dioxide as colder water, similar to how a warm soda loses its fizz faster. As ocean temperatures climb, the solubility of carbon dioxide in seawater decreases, meaning the ocean can absorb less of the gas. This reduced solubility creates a feedback loop where warmer oceans absorb less carbon dioxide, leaving more in the atmosphere, which in turn contributes to further warming and further limits the ocean’s absorption capacity. While the ocean has a large overall capacity, its ability to take up carbon dioxide weakens as more is added.