The carbon cycle: The harmonious interaction of oceans, forests and subsoils

Carbon fluxes between the atmosphere, the ocean, the terrestrial biosphere and geological reservoirs — Illustration showing the exchanges of carbon (CO2) between the **atmosphere**, **forests** (via photosynthesis and respiration), the **oceans** (via dissolution and plankton), and **deep reservoirs** such as sediments and fossil fuels.
Image source: astronoo.com (new window), AI-generated image, public domain.

Scientific abstract

The carbon cycle is the set of physical, chemical and biological processes that ensure the circulation of carbon between the four main terrestrial reservoirs: the atmosphere (about 880 GtC), the biosphere (vegetation, soils, animals), the hydrosphere (oceans, about 38,000 GtC) and the lithosphere (rocks, sediments, fossil fuels, more than 60,000,000 GtC). It operates over a wide range of time scales (from rapid annual exchanges such as photosynthesis to slow processes spanning millions of years such as volcanic subduction). This superposition of fast and slow dynamics, including natural feedbacks (such as silicate weathering), naturally regulates Earth's climate and makes it a planetary thermostat.

What is the carbon cycle and how does it regulate Earth's climate?

The carbon cycle refers to the ceaseless circulation of carbon between the atmosphere, the oceans, the biosphere and the rocks. But how does this phenomenon regulate Earth's climate? The answer lies in a subtle balance between fast fluxes and slow geological processes. Annual exchanges, such as photosynthesis and oceanic dissolution, constantly redistribute tens of billions of tons of carbon. Meanwhile, millennial mechanisms such as silicate weathering and volcanism slowly adjust the atmospheric CO₂ content. This dual dynamic gives the carbon cycle the role of a planetary thermostat, capable of damping climatic disturbances over long periods. Understanding this system is essential to assess the impact of human emissions on the climate.

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Carbon: A structuring element of the Earth system

Carbon, the foundation of organic life

Carbon is much more than a simple chemical element (C, atomic number 6). It is the foundation of organic life and one of the main regulators of our planet's climate.

A major regulator of the planetary climate

Its perpetual journey between the great terrestrial reservoirs: the atmosphere, the hydrosphere (oceans), the biosphere (forests, soils) and the geosphere (subsoils), forms a system of remarkable complexity and harmony that directly influences the climate.

Evolution of the scientific understanding of the carbon cycle

The first experiments on CO₂

The carbon cycle was progressively understood as a complex network of fluxes and stocks, linking biogeochemistry and climatology. The identification of CO₂ (carbon dioxide) as a gas participating in exchanges between atmosphere, oceans and biosphere began with the experiments of Joseph Priestley (1733–1804) and Antoine Lavoisier (1743–1794), who demonstrated the role of plants in producing oxygen and absorbing CO₂.

The discovery of the thermal effect of CO₂

In the mid-19th century, Eunice Newton Foote (1819–1888) showed that CO₂ absorbs solar heat, highlighting its role in the warming and thermal regulation of the atmosphere, a key element for understanding the carbon cycle.

The formalization of the cycle in the 20th century

Also in the 19th century, studies on animal respiration and organic decomposition showed that carbon could circulate between living organisms and the atmosphere. In the 20th century, the quantification of oceanic and terrestrial fluxes made it possible to formalize the carbon cycle, distinguishing fast reservoirs (biosphere, atmosphere, surface oceans) from slow ones (sediments, fossil fuels, lithosphere).

Modern climate models

Today, the use of climate models and isotopic measurements makes it possible to precisely track the transfer of carbon between reservoirs and to understand the impact of human activities on atmospheric CO₂ (carbon dioxide) and global warming.

The carbon cycle: The different time scales

A circulation between four reservoirs

The carbon cycle is the set of physical, chemical and biological processes that ensure the circulation of carbon between the main reservoirs of the Earth: atmosphere, biosphere, hydrosphere and lithosphere.

Highly varied time scales

The carbon cycle operates on highly varied time scales: from rapid exchanges between the atmosphere, biosphere and surface oceans (years to decades) to slow transfers toward sediments and the lithosphere (thousands to millions of years). This superposition of fast and slow dynamics regulates Earth's climate and determines the response to natural or anthropogenic disturbances.

The table below presents the main carbon reservoirs, their stocks, the annual fluxes exchanged, the associated chemical transformations and the characteristic time scales.

Carbon cycle: reservoirs, stocks and annual fluxes
Source reservoir	Stored quantity (GtC)	Target reservoir	Process	Annual flux (GtC/yr)	Dominant transformation	Time scale
Atmosphere (CO₂)	≈ 880	Plant biosphere	Photosynthesis	≈ 120	\( CO_2 + H_2O \xrightarrow{\text{solar energy}} CH_2O + O_2 \) (Reduction of mineral carbon into organic carbon)	Years
Plant biosphere	≈ 450 to 650	Animal biosphere	Transfer of organic carbon	≈ 60	\( CH_2O_{\text{plant}} \rightarrow CH_2O_{\text{animal}} + CO_2 \)	Years to decades
Biosphere (plant and animal)	≈ 2,000 (including soils)	Atmosphere (CO₂)	Respiration, decomposition	≈ 120	\( CH_2O + O_2 \rightarrow CO_2 + H_2O + \text{energy} \) (Biological oxidation of organic carbon)	Years
Atmosphere (CO₂)	≈ 880	Surface oceans	Oceanic dissolution	≈ 90	\(CO_2 + H_2O \rightleftharpoons H_2CO_3 \rightleftharpoons H^+ + HCO_3^- \rightleftharpoons 2H^+ + CO_3^{2-}\) (CO₂ / HCO₃^- / CO₃^2- equilibria)	Months to centuries
Oceans (dissolved carbon)	≈ 38,000	Marine sediments	Biomineralization	≈ 0.5 to 1	\(Ca^{2+} + HCO_3^- \rightleftharpoons CaCO_3(s) + H^+\) (Formation of CaCO₃ carbonates)	Centuries to millennia
Biosphere and sediments	≈ 15,000	Fossil fuels	Burial, diagenesis	≈ 0.1	\(CH_2O_{\text{organic matter}} \rightarrow C_{\text{fossil}} + H_2O + CO_2\) (Concentration and fossilization of carbon)	Millions of years
Fossil fuels	≈ 4,000	Atmosphere (CO₂)	Combustion	≈ 10	\(C_6H_{12}O_6 (\text{fossil}) + 6\,O_2 \rightarrow 6\,CO_2 + 6\,H_2O\) (Rapid oxidation of fossil carbon)	Instantaneous to decades
Lithosphere (carbonate rocks)	>60,000,000	Atmosphere (CO₂)	Subduction, volcanism	≈ 0.1	\(CaCO_3(\text{sediment}) + \text{magma} \rightarrow CaO + CO_2(\text{volcanic})\) (Mantle degassing of carbon)	Millions of years

The biosphere: carbon sink and source

Vegetation absorbs atmospheric CO₂ through photosynthesis, but it also releases it through respiration and the decomposition of organic matter. Thus, the biosphere acts both as a carbon sink and source, with a net balance that depends on seasonal fluxes and ecological disturbances.

The geological thermostat: volcanic degassing and silicate weathering

Climatic equilibrium on a geological scale results from the feedback between volcanic degassing (which releases CO₂ from lithospheric reservoirs) and silicate weathering (which captures atmospheric CO₂). Warming accelerates weathering, reducing CO₂ and cooling the climate. This mechanism links carbon stocks and fluxes and acts as a slow but effective planetary thermostat.

References

FAQ: Everything you need to know about the carbon cycle

What are the four main carbon reservoirs on Earth?

The four main reservoirs are: 1) the atmosphere (about 880 GtC, mainly in the form of CO₂), 2) the biosphere (vegetation, soils, animals, about 450 to 2,000 GtC of organic carbon), 3) the hydrosphere (oceans, about 38,000 GtC of dissolved carbon), and 4) the lithosphere (carbonate rocks and fossil fuels, more than 60 million GtC). These reservoirs exchange carbon through processes such as photosynthesis, respiration, oceanic dissolution or volcanism.

What is the difference between the fast and the slow carbon cycle?

The fast cycle (years to decades) involves intense exchanges between the atmosphere, the biosphere (photosynthesis and respiration, ~120 GtC/yr) and surface oceans (dissolution, ~90 GtC/yr). The slow cycle (thousands to millions of years) concerns the transfer of carbon to marine sediments (biomineralization), burial and the formation of fossil fuels (diagenesis), as well as the return of lithospheric carbon to the atmosphere via volcanism and subduction.

How does the carbon cycle act as a planetary "thermostat"?

The carbon cycle has natural feedbacks that regulate the climate. A key mechanism is silicate weathering: climatic warming accelerates rock weathering, which captures atmospheric CO₂. The decrease in CO₂ then cools the climate. Conversely, volcanism releases lithospheric CO₂, which can warm the planet. This negative feedback loop, acting over millions of years, maintains long-term climatic equilibrium.

What is the role of the oceans in the carbon cycle?

The oceans constitute the largest surface carbon reservoir on the planet. They absorb atmospheric CO₂ through dissolution and through the biological activity of plankton. The dissolved carbon is then transported to the depths by thermohaline circulation and the biological pump, where it can be stored for centuries to millennia. This role as an oceanic sink slows the accumulation of CO₂ in the atmosphere, but the growing acidification of the oceans threatens this equilibrium.

How do human activities disrupt the carbon cycle?

Human activities, particularly the combustion of fossil fuels and deforestation, inject into the atmosphere carbon that was once stored in the lithosphere and the biosphere. This rapid transfer, of about 10 GtC per year, far exceeds the capacity of natural sinks (oceans, forests) to reabsorb this surplus. The result is a continuous increase in the atmospheric concentration of CO₂, which intensifies the greenhouse effect and disrupts the climatic equilibrium established over millions of years.

What is silicate weathering and what is its role in climate regulation?

Silicate weathering is a slow chemical process by which silicate rocks react with atmospheric CO₂ and water to form carbonates. This mechanism consumes CO₂ and traps it in mineral form. During periods of warming, weathering accelerates, which decreases atmospheric CO₂ and gradually cools the climate. This phenomenon constitutes a major negative feedback that acts as a natural thermostat on geological time scales.