Carbon pathways refer to the continuous movement of carbon, a fundamental element, through Earth’s various systems. This element is essential for all life, forming the building blocks of living organisms and playing a central role in maintaining the planet’s climate. Carbon’s journey is a dynamic and intricate cycle, constantly shifting between different global reservoirs. Understanding these pathways is important for comprehending the natural processes that sustain our environment.
Natural Carbon Cycling Processes
Carbon moves through the environment via interconnected natural processes. Plants, algae, and some bacteria absorb carbon dioxide from the atmosphere or dissolved in water through photosynthesis. This process converts carbon dioxide into organic compounds, forming the basis of food chains. Approximately 110 gigatons of carbon are absorbed by plant life annually.
Conversely, living organisms release carbon dioxide back into the atmosphere through respiration, breaking down organic compounds for energy. When plants and animals die, decomposers like bacteria and fungi break down their organic matter. This decomposition releases carbon back into the atmosphere as carbon dioxide and methane, with about 60 gigatons released annually from decaying biomass. These biological exchanges occur over days, months, or years.
The ocean exchanges carbon with the atmosphere as carbon dioxide dissolves into surface waters and is released back. On average, oceans absorb around 2.9 gigatons of carbon from atmospheric carbon dioxide each year. This exchange is influenced by water temperature and carbon dioxide concentration in both the atmosphere and ocean.
Over longer geological timescales, carbon moves through the Earth’s crust. Fossil fuels, like coal, oil, and natural gas, form over millions of years as dead organic matter is buried and transformed under pressure and heat. This process sequesters carbon from the active cycle for extended periods. Volcanic activity also releases carbon dioxide from the Earth’s interior into the atmosphere. Global volcanic carbon dioxide emissions, estimated between 0.13 and 0.44 gigatons per year, are smaller than human-caused emissions.
Earth’s Carbon Storage Locations
Carbon is stored in various reservoirs throughout the Earth system, each holding different amounts and retaining carbon for varying durations. The largest reservoir is the lithosphere, encompassing sedimentary rocks like limestone and fossil fuel deposits. Sedimentary rocks alone can store up to 100 million gigatons of carbon, while fossil fuels hold about 4,000 gigatons. This carbon is typically sequestered for millions of years.
The oceans represent the second-largest carbon sink, containing an estimated 38,000 to 40,000 gigatons of carbon, primarily as dissolved inorganic carbon. This vast oceanic reservoir holds about 50 times more carbon than the atmosphere. While surface waters exchange carbon rapidly, carbon can be stored for centuries in the deep ocean.
On land, carbon is stored in living biomass and in soils. The terrestrial biosphere contains an estimated 540 to 610 gigatons of carbon, with plants accounting for a significant portion. Soil is another substantial terrestrial reservoir, holding between 1,500 and 1,600 gigatons of carbon in organic matter. Soil inorganic carbon, largely calcium carbonate, contributes another 2,305 gigatons globally. Much of this carbon is stored in nonliving organic matter within soils and deadwood, remaining sequestered for hundreds to thousands of years.
The atmosphere contains approximately 750 gigatons of carbon, mainly as carbon dioxide. Its concentration has increased significantly, reaching 427 parts per million in 2024, representing 3341 gigatons of carbon dioxide. This atmospheric carbon plays a direct role in regulating Earth’s temperature and is a dynamic component of the carbon cycle.
Human Activities and Carbon Pathways
Human activities have altered the natural balance of carbon pathways by releasing large amounts of stored carbon into the atmosphere. The combustion of fossil fuels—coal, oil, and natural gas—is the leading cause of increased atmospheric carbon dioxide. Global energy-related carbon dioxide emissions reached a record 37.4 billion tonnes in 2023. Since the Industrial Revolution, over 1.5 trillion tonnes of carbon dioxide have been emitted.
Deforestation and other land-use changes also contribute to atmospheric carbon. When forests are cleared or burned, carbon stored in trees and soil is released. Tropical deforestation alone led to an average annual emission of 5.3 gigatons of carbon dioxide equivalent between 2001 and 2019. Current deforestation rates release approximately 1.9 gigatons of carbon per year.
Agricultural practices, including changes in land use and farming methods, also influence carbon pathways. They can affect the amount of carbon released from soil organic matter. These human-induced releases of carbon lead to an increased concentration of carbon dioxide in the atmosphere. The cumulative effect of these activities impacts the overall carbon cycle, influencing Earth’s climate and ecosystems.