- The carbon cycle is a fundamental biogeochemical process that describes the continuous movement of carbon atoms through the Earth’s atmosphere, biosphere, hydrosphere, and geosphere.
- As a key element of life and climate regulation, carbon exists in various forms—including carbon dioxide (CO₂), methane (CH₄), and organic molecules—and cycles through living organisms, air, water, and rocks. This cycle ensures that carbon is recycled and reused, playing an essential role in sustaining life and maintaining Earth’s environmental stability.
- The carbon cycle operates through both natural processes and human influences. One major component is the atmospheric carbon pool, primarily in the form of CO₂ and CH₄. These gases are absorbed by plants, algae, and cyanobacteria during photosynthesis, converting atmospheric carbon into organic matter. In terrestrial ecosystems, plants store carbon in their biomass and transfer it to herbivores and decomposers through the food web. In aquatic environments, phytoplankton perform a similar role, supporting marine life and contributing to long-term carbon storage through the biological pump.
- Carbon returns to the atmosphere through respiration by plants, animals, and microbes, as well as through decomposition of dead organisms and organic waste. This process releases CO₂ and CH₄ back into the air or water, completing the short-term carbon cycle. Combustion of organic materials—such as wood or fossil fuels—also releases carbon, particularly when humans burn coal, oil, and natural gas for energy. This has significantly increased the concentration of atmospheric CO₂, intensifying the greenhouse effect and driving global climate change.
- Another key part of the carbon cycle is the ocean-atmosphere exchange. Oceans absorb vast amounts of atmospheric CO₂, some of which reacts with seawater to form carbonic acid, bicarbonate, and carbonate ions. Marine organisms, such as corals and shellfish, use these ions to build calcium carbonate shells, which eventually sink and form limestone deposits on the ocean floor. This long-term storage of carbon in sediments—along with the burial of organic material—comprises the geological carbon cycle, which operates on timescales of thousands to millions of years.
- Volcanic activity and tectonic processes also play a role in the geological cycle. Over geologic time, subducted carbonates are returned to the atmosphere via volcanic eruptions, which release CO₂. In contrast, weathering of rocks (especially silicate weathering) removes CO₂ from the atmosphere and contributes to carbon storage in soils and sediments.
- Human activity has profoundly altered the natural balance of the carbon cycle. Since the Industrial Revolution, anthropogenic emissions from fossil fuel combustion, deforestation, and land-use changes have accelerated the input of carbon into the atmosphere. This imbalance has led to a net increase in atmospheric CO₂, contributing to global warming, ocean acidification, and ecosystem disruption.
- In response, scientists and policymakers are exploring ways to mitigate carbon emissions and enhance carbon sequestration. Approaches include reforestation, soil carbon enrichment, carbon capture and storage (CCS) technologies, and restoration of blue carbon ecosystems (such as mangroves, seagrasses, and salt marshes).
