The carbon cycle is a natural process that moves carbon through Earth's systems, including the biosphere, pedosphere, geosphere, hydrosphere, and atmosphere. Other important biogeochemical cycles are the nitrogen cycle and the water cycle. Carbon is a key part of living things and is also found in many rocks, such as limestone. The carbon cycle is essential for life on Earth because it describes how carbon moves through the biosphere, is reused, and is stored or released over long periods. In 2024, the average level of carbon dioxide in Earth's atmosphere reached 422.7 parts per million, a record high.

The carbon cycle can be divided into two types: the fast carbon cycle and the slow carbon cycle. The fast cycle, also called the biological carbon cycle, moves carbon between the atmosphere and biosphere quickly, often within years. The slow cycle, also known as the geological or deep carbon cycle, takes millions of years to move carbon through Earth's crust, soil, oceans, and atmosphere.

For many centuries, humans have disrupted the carbon cycle by changing land use and by burning fossil fuels like coal, petroleum, and natural gas. By 2020, carbon dioxide levels in the atmosphere had risen nearly 52% compared to pre-industrial times, causing global warming. This increase has also lowered the pH of the ocean, changing its chemistry. Carbon dioxide is necessary for plants to perform photosynthesis.

Main compartments

The carbon cycle was first described by Antoine Lavoisier and Joseph Priestley. It was made more well-known by Humphry Davy. The global carbon cycle is now usually divided into major storage areas (also called carbon pools) connected by pathways of exchange:

• Atmosphere
• Terrestrial biosphere
• Ocean, including dissolved inorganic carbon and living and non-living marine life
• Sediments, including fossil fuels, freshwater systems, and non-living organic material
• Earth's interior (mantle and crust). These carbon storage areas interact with other parts of the cycle through geological processes.

Carbon moves between storage areas because of chemical, physical, geological, and biological processes. The ocean holds the largest active pool of carbon near Earth’s surface. Natural movements of carbon between the atmosphere, ocean, land ecosystems, and sediments are fairly balanced, so carbon levels would stay roughly stable without human influence.

Carbon in Earth’s atmosphere exists mainly as carbon dioxide and methane. Both gases absorb and trap heat in the atmosphere and are partially responsible for the greenhouse effect. Methane has a stronger greenhouse effect per unit volume than carbon dioxide, but it exists in much lower amounts and lasts in the atmosphere for a shorter time. Therefore, carbon dioxide contributes more to the global greenhouse effect than methane.

Carbon dioxide is removed from the atmosphere mainly through photosynthesis and enters the terrestrial and oceanic biospheres. It also dissolves directly from the atmosphere into water bodies (such as oceans and lakes) and into raindrops as they fall. When dissolved in water, carbon dioxide reacts with water to form carbonic acid, which increases ocean acidity. This acid can be absorbed by rocks through weathering or can acidify other surfaces it touches or be washed into the ocean.

Human activities over the past two centuries have increased the amount of carbon in the atmosphere by nearly 50% by the year 2020, mainly in the form of carbon dioxide. This increase happened partly by changing ecosystems’ ability to remove carbon dioxide from the atmosphere and partly by directly releasing it, such as through burning fossil fuels and making concrete.

In the far future (2 to 3 billion years), the rate at which carbon dioxide is absorbed into soil through the carbonate–silicate cycle will likely increase due to changes in the Sun’s brightness as it ages. This increased brightness will likely speed up weathering on Earth’s surface. Eventually, most of the carbon dioxide in the atmosphere will be trapped in Earth’s crust as carbonate. Once the concentration of carbon dioxide in the atmosphere drops below about 50 parts per million (tolerances vary among species), C3 photosynthesis will no longer be possible. This is predicted to happen 600 million years from now, though models may differ.

Once Earth’s oceans evaporate in about 1.1 billion years, plate tectonics will likely stop because water is needed to lubricate the plates. Without volcanoes releasing carbon dioxide, the carbon cycle will end between 1 billion and 2 billion years from now.

The terrestrial biosphere includes organic carbon in all land-living organisms, both alive and dead, as well as carbon stored in soils. About 500 gigatons of carbon are stored above ground in plants and other living organisms, while soil holds approximately 1,500 gigatons of carbon. Most carbon in the terrestrial biosphere is organic, while about a third of soil carbon is stored in inorganic forms, such as calcium carbonate. Organic carbon is a major part of all living organisms on Earth. Autotrophs take carbon dioxide from the air and convert it into organic carbon, while heterotrophs receive carbon by eating other organisms.

Because carbon uptake in the terrestrial biosphere depends on living factors, it follows daily and seasonal cycles. This pattern is visible in CO2 measurements, such as the Keeling curve. It is strongest in the northern hemisphere because this hemisphere has more land than the southern hemisphere, allowing more ecosystems to absorb and release carbon.

Carbon leaves the terrestrial biosphere in several ways and over different time scales. Burning or respiration of organic carbon releases it quickly into the atmosphere. It can also be carried into the ocean through rivers or remain stored in soil as inert carbon. Carbon stored in soil can stay there for thousands of years before being washed into rivers by erosion or released into the atmosphere through soil respiration. Between 1989 and 2008, soil respiration increased by about 0.1% per year. In 2008, the global total of CO2 released by soil respiration was roughly 98 billion tonnes, about three times more carbon than humans now release annually by burning fossil fuels (this does not mean a net transfer of carbon from soil to atmosphere, as respiration is largely balanced by inputs to soil carbon). The most likely reason for this trend is that rising temperatures have increased the breakdown of soil organic matter, increasing CO2 flow. The length of time carbon stays in soil depends on local climate conditions and changes as climate changes.

The ocean can be divided into a surface layer, where water frequently interacts with the atmosphere (daily to annually), and a deep layer below the typical mixed layer depth (a few hundred meters or less), where water interacts with the atmosphere much less often (centuries between interactions). Dissolved inorganic carbon (DIC) in the surface layer exchanges rapidly with the atmosphere, maintaining balance. The deep ocean holds much more carbon than the surface layer, mainly because of its larger volume, even though its DIC concentration is about 15% higher. The deep ocean contains 50 times more carbon than the atmosphere, but it takes hundreds of years to reach equilibrium with the atmosphere. The slow movement of carbon between the two layers, driven by thermohaline circulation, causes this delay.

Carbon enters the ocean mainly through the dissolution of atmospheric carbon dioxide, a small portion of which becomes carbonate. It can also enter the ocean through rivers as dissolved organic carbon. Organisms convert it into organic carbon through photosynthesis, which can be passed through the food chain or settle into the ocean’s deeper, carbon-rich layers as dead tissue or in shells as calcium carbonate. This carbon circulates in the deep layer for long periods before either being deposited as sediment or returned to the surface through thermohaline circulation.

Oceans are slightly basic (with a current pH of 8.1 to 8.2). The increase in atmospheric CO2 lowers the ocean’s pH in a process called ocean acidification. The ocean’s absorption of CO2 is one

Types of dynamic

There are two types of carbon cycles: a fast cycle and a slow cycle. The fast cycle happens in the biosphere, which includes living things, while the slow cycle involves rocks and the Earth's crust. The fast cycle can complete in years, moving carbon between the atmosphere and the biosphere, then back to the atmosphere. The slow cycle can take millions of years to complete, moving carbon through the Earth's crust between rocks, soil, oceans, and the atmosphere.

The fast carbon cycle includes short-term natural processes between the environment and living organisms in the biosphere. It involves the movement of carbon between the atmosphere and land and ocean ecosystems, as well as soils and ocean floor sediments. The fast cycle includes yearly processes like photosynthesis and processes that happen over decades, such as plant growth and decay. How the fast carbon cycle responds to human activities will influence many immediate effects of climate change.

The slow (or deep) carbon cycle includes long-term processes related to the rock cycle. The exchange of carbon between the ocean and atmosphere can take centuries, and the breakdown of rocks can take millions of years. Carbon in the ocean can settle to the ocean floor, forming sedimentary rock and being pushed deep into the Earth's mantle. Mountain-building events bring this carbon back to the Earth's surface. There, rocks break down, and carbon is released into the atmosphere through volcanic activity or into the ocean through rivers. Some carbon returns to the ocean through hot water releasing calcium ions. Each year, between 10 and 100 million tonnes of carbon move through the slow cycle. This includes carbon released into the atmosphere by volcanoes as carbon dioxide. However, this amount is less than 1% of the carbon dioxide released into the atmosphere from burning fossil fuels.

Processes within fast carbon cycle

The movement of carbon from land to water in the water cycle is shown in the diagram and explained below:

• Tiny particles in the air help clouds form by acting as surfaces for water droplets to gather.
• Raindrops absorb carbon from the air as they fall to Earth, including both organic and inorganic types.
• Burning and volcanic eruptions release black carbon and greenhouse gases like carbon dioxide (CO₂) back into the atmosphere.
• Plants on land take in CO₂ from the air through photosynthesis. Some of this carbon is released back into the air through respiration. Lignin and cellulose, which are parts of plant cell walls, make up most of the organic carbon in forests and pastures.
• Dead plant material and root carbon mix with soil to form organic soils. Microbes and fungi break down this carbon, storing or changing it over time.
• Water carries dissolved organic and inorganic carbon from plants and the air as it moves through forests (throughfall) and along tree trunks (stemflow). When water soaks into soil or flows over saturated ground, chemical changes occur.
• Carbon from land and produced by plants is broken down by microbes in rivers and streams. This process releases CO₂ into the air, with amounts similar to the carbon stored by plants each year. Large molecules like lignin and black carbon are broken into smaller pieces, eventually becoming CO₂, energy sources, or new life.
• Lakes, reservoirs, and floodplains store large amounts of carbon and sediments. However, they also release CO₂ into the air, though less than rivers. Methane is often produced in oxygen-free sediments in these areas.
• Rivers carry nutrients to the ocean, which can boost plant growth near river mouths. However, estuaries still release CO₂ into the air.
• Coastal marshes store and release "blue carbon," a type of carbon from wetlands. Globally, marshes and wetlands release about the same amount of CO₂ into the air as rivers.
• The ocean and continental shelves usually take in CO₂ from the air.
• The ocean’s biological pump stores some of the absorbed CO₂ as organic carbon in ocean sediments.

Land and ocean ecosystems are connected mainly through rivers, which carry materials from land to the sea. Processes like plant growth and decay help balance carbon and oxygen levels in ecosystems.

Rivers transport carbon from land to the ocean, mainly as dissolved organic carbon (DOC) and particulate organic carbon (POC). Some DOC returns to the air during chemical reactions, while the rest moves to the ocean. In 2015, global rivers were estimated to release 0.50 to 0.70 petagrams of inorganic carbon and 0.15 to 0.35 petagrams of organic carbon each year. Particulate organic carbon can stay buried in ocean sediments for long periods, with an estimated annual global transfer of 0.20 petagrams of carbon from land to the ocean.

The ocean’s biological pump moves carbon from the air and land to the deep ocean and seafloor sediments. This process involves many steps, such as phytoplankton using CO₂ to grow, forming organic matter, and sinking to the ocean floor. Phytoplankton release dissolved organic matter (DOM) and are eaten by zooplankton. Zooplankton, like copepods, produce fecal pellets that sink faster, carrying carbon deeper. Some DOM is used by bacteria, while the rest moves into the deep ocean.

Phytoplankton cells sink very slowly, taking years to reach the ocean floor. However, when they form larger groups or are trapped in fecal pellets, they sink much faster, reaching the deep ocean in days. Only about 1% of these particles reach the ocean floor, where they are consumed, released as CO₂, or buried in sediments. This process moves carbon from the surface ocean to deeper layers, maintaining a balance of carbon in the ocean. Over time, deep ocean carbon is returned to the air through slow ocean currents, and some carbon is stored in Earth’s mantle for millions of years.

Viruses influence the carbon cycle by affecting the food web and microbial life. They contribute about 8.6% to the Earth’s carbon cycle, with smaller roles in oceans (1.4%) compared to land (6.7%) and freshwater (17.8%). Human activities and climate change over the past 200 years have changed how viruses affect carbon cycling.

Processes within slow carbon cycle

Slow or deep carbon cycling is an important process, but it is not as well understood as the faster movement of carbon through the atmosphere, land, oceans, and Earth's solid layers. The deep carbon cycle is closely connected to how carbon moves across Earth's surface and into the atmosphere. Without this process, carbon would stay in the atmosphere and build up to very high levels over long periods. By helping carbon return to Earth, the deep carbon cycle helps keep conditions on Earth suitable for life.

The process is also important because it moves huge amounts of carbon through the planet. Studies of basaltic magma and carbon dioxide released by volcanoes show that the amount of carbon in Earth's mantle is about 1,000 times greater than the amount on Earth's surface. However, studying deep Earth carbon processes is very difficult because the lower mantle and core extend from 660 to 2,891 km and 2,891 to 6,371 km deep, respectively. Scientists know little about carbon's role in the deep Earth, but some evidence from lab experiments suggests how carbon moves into the lower mantle and the forms it takes under extreme temperatures and pressures. Techniques like seismology have also helped scientists better understand the possible presence of carbon in Earth's core.

Carbon mainly enters the mantle through carbonate-rich sediments on ocean crust tectonic plates. These sediments are pulled into the mantle during subduction. Scientists know little about how carbon moves within the mantle, especially in the deep Earth, but many studies have tried to improve understanding. For example, a 2011 study found that carbon cycling reaches the lower mantle. The study examined rare, super-deep diamonds from Juina, Brazil, and found that the diamonds' inclusions matched the expected results of basalt melting and crystallization under lower mantle conditions. This suggests that basaltic oceanic lithosphere is the main way carbon is transported into Earth's deep interior. These subducted carbonates can interact with lower mantle silicates, eventually forming super-deep diamonds.

However, carbonates descending into the lower mantle can also form other materials. In 2011, carbonates were tested under conditions similar to those 1,800 km deep in the Earth. This experiment produced magnesite, siderite, and various types of graphite. Other studies and observations support this, showing that magnesite is the most stable carbonate form in much of the mantle due to its high melting temperature. Scientists have concluded that carbonates are reduced as they move into the mantle and then stabilized by low oxygen environments. Metals like magnesium and iron help balance this process. The presence of carbon in reduced forms, such as graphite, shows that carbon compounds are reduced as they move deeper into the mantle.

The different forms of carbon affect its stability at various depths. Laboratory experiments and calculations suggest that tetrahedrally coordinated carbonates are most stable near the core-mantle boundary. A 2015 study found that the high pressure in the lower mantle causes carbon bonds to change from sp² to sp³ hybridized orbitals, making carbon bond tetrahedrally to oxygen. CO₃ trigonal groups cannot form large networks, while tetrahedral CO₄ groups can, leading to changes in carbon's coordination number and altering carbonate properties in the lower mantle. For example, theoretical studies suggest that high pressure increases the viscosity of carbonate melts, reducing their movement and causing large carbon deposits to form deep in the mantle.

Carbon can stay in the lower mantle for long periods, but it often returns to the lithosphere through a process called carbon outgassing. This happens when carbonated mantle undergoes decompression melting or when mantle plumes carry carbon compounds upward toward the crust. As carbon rises toward volcanic hotspots, it is oxidized and released as CO₂ to match the oxidation state of erupting basalts.

Although carbon in Earth's core is well understood, recent studies suggest large amounts of carbon might be stored there. Seismic waves moving through the inner core travel about 50% slower than expected for iron-rich alloys. Since the core is believed to be an alloy of crystalline iron and a small amount of nickel, this suggests the presence of light elements like carbon. Studies using diamond anvil cells to simulate core conditions show that iron carbide (Fe₇C₃) matches the inner core's wave speed and density. This supports the idea that the core may hold up to 67% of Earth's carbon. Another study found that carbon dissolved in iron under core conditions forms a stable phase with the same Fe₇C₃ composition but a different structure. In summary, while the exact amount of carbon in the core is unknown, recent research suggests that iron carbides could explain some seismic observations.

Human influence on fast carbon cycle

Since the Industrial Revolution and especially after World War II, human activities have significantly changed the global carbon cycle by moving large amounts of carbon from Earth's rocks and soil. People have also altered natural processes in land ecosystems by changing plant life and land use. Man-made carbon compounds, created and produced in large quantities, remain in the air, water, and soil for many years as pollutants. Climate change is causing more indirect human changes to the carbon cycle through various positive and negative effects.

Current climate trends are increasing ocean temperatures and acidity, which affects marine ecosystems. Acid rain and polluted runoff from farming and industry also change the ocean's chemical balance. These changes can harm sensitive ecosystems like coral reefs, reducing the ocean's ability to absorb carbon from the atmosphere in certain areas and decreasing ocean biodiversity worldwide.

The movement of carbon between the atmosphere and other parts of Earth's systems, known as the carbon cycle, currently helps reduce the impact of human-caused carbon emissions on climate change. Land and ocean carbon sinks absorb about one-quarter of human-caused carbon emissions each year.

These natural reductions are expected to become weaker in the future, increasing the impact of human-caused carbon emissions on climate change. However, the extent of this weakening is uncertain, as Earth system models predict a wide range of land and ocean carbon absorption even under the same conditions. Human-caused global warming also indirectly increases Arctic methane emissions, which further affects the carbon cycle and contributes to more warming.

The largest and fastest-growing human impact on the carbon cycle and biosphere is the extraction and burning of fossil fuels, which directly move carbon from Earth's rocks and soil into the atmosphere. Carbon dioxide is also released during the process of heating limestone to make cement, which is used in construction.

By 2020, about 450 gigatons of fossil carbon had been extracted, an amount close to the total carbon in all Earth's living plant life. Recent rates of carbon emissions into the atmosphere have exceeded the ability of plants and oceans to absorb it. These natural systems are expected to remove about half of the added carbon within a century. However, the ocean's ability to absorb carbon changes over time, and 20–35% of added carbon is projected to stay in the atmosphere for centuries or longer.

Halocarbons are chemicals used in various industries, such as solvents and refrigerants. Although their concentrations in the atmosphere are small (parts per trillion), these gases contribute about 10% of the total direct warming effect from long-lived greenhouse gases in 2019. Chlorofluorocarbons also damage the ozone layer in the upper atmosphere. International agreements like the Montreal Protocol and Kyoto Protocol aim to control the production and use of these harmful gases. Safer alternatives, such as hydrofluoroolefins, are being developed and used increasingly.

Since the start of agriculture, humans have gradually changed the carbon cycle over long periods by altering plant life on land. Over the past few centuries, human-caused changes to land use and land cover have reduced biodiversity, weakening ecosystems' ability to withstand environmental stress and absorb carbon from the atmosphere. Directly, these changes often release carbon from land ecosystems into the atmosphere.

Deforestation for farming removes forests, which store large amounts of carbon, and replaces them with farmland or cities. These new land types store much less carbon, leaving more carbon in the atmosphere. However, these effects can be reversed through reforestation efforts.