Carbon Cycle

Reservoirs and fluxes

Earth's carbon is divided among several reservoirs. According to the IPCC (2021), the preindustrial atmosphere held about 590 gigatonnes of carbon (GtC), land vegetation about 450–650 GtC, soils about 1,500–2,400 GtC and permafrost about 1,400 GtC. The ocean, at about 38,000 GtC, is the largest active reservoir, mostly dissolved inorganic carbon in the deep sea. Limestone and sediments in the lithosphere hold tens of millions of GtC, and fossil fuel reserves amount to thousands of GtC. Fluxes between reservoirs are large too. Land plants take up about 120 GtC a year through photosynthesis, and nearly as much returns to the air through respiration and decomposition. Ocean and atmosphere exchange about 80 GtC a year as gas. The sinking of organic matter produced by surface phytoplankton, carrying carbon to the deep ocean, is called the biological pump.

Fast and slow cycles

The fast carbon cycle, made up of photosynthesis, respiration and ocean–atmosphere exchange, operates over years to millennia. The slow carbon cycle operates over hundreds of thousands to millions of years. Atmospheric carbon dioxide dissolves in rain to form carbonic acid, which weathers silicate rocks; the products travel by river to the sea and are deposited as carbonate rock. When plates subduct, the carbon returns to the atmosphere through volcanism. Walker, Hays and Kasting (1981) described this process as a thermostat that stabilizes Earth's climate over long periods, since warmer temperatures speed weathering and remove more carbon dioxide. Berner (2003) showed how this long-term cycle and fossil-fuel formation shaped atmospheric composition over geological time.

Human disturbance and feedbacks

Revelle and Suess (1957) warned that burning fossil fuels amounted to a "large-scale geophysical experiment," and the measurements Charles Keeling (1960) began at Mauna Loa in Hawaii produced the Keeling curve showing atmospheric carbon dioxide rising year after year, from about 280 ppm before industrialization to about 420 ppm in 2023. According to the Global Carbon Budget (Friedlingstein et al., 2023), humans recently emitted about 11 GtC a year from fossil fuels and land-use change; roughly half stays in the atmosphere, about a quarter is absorbed by the ocean and about a quarter by land ecosystems (Sabine et al., 2004; Pan et al., 2011). Ocean uptake in turn causes ocean acidification, threatening corals and shellfish (Doney et al., 2009). There are also concerns about positive feedbacks in which warming thaws permafrost and increases soil respiration, releasing more carbon (Schuur et al., 2015). This understanding underpins research on the greenhouse effect and carbon neutrality policy.

FAQ

What is the carbon cycle in simple terms?

It is the planet-wide movement of carbon among air, oceans, plants and animals, soil and rock. Plants absorb carbon, while respiration and burning return it to the air.

How have humans changed the carbon cycle?

By burning fossil fuels and clearing forests, humans have rapidly released carbon long stored underground, raising atmospheric carbon dioxide about 50 percent above preindustrial levels.

Sources reviewed: 2026-10-26

References

1. IPCC (2021). Climate Change 2021: The Physical Science Basis, Chapter 5: Global Carbon and Other Biogeochemical Cycles and Feedbacks. Cambridge: Cambridge University Press.
2. Friedlingstein, P., et al. (2023). Global Carbon Budget 2023. Earth System Science Data, 15(12), 5301–5369.
3. Keeling, C. D. (1960). The concentration and isotopic abundances of carbon dioxide in the atmosphere. Tellus, 12(2), 200–203.
4. Revelle, R., & Suess, H. E. (1957). Carbon dioxide exchange between atmosphere and ocean. Tellus, 9(1), 18–27.
5. Berner, R. A. (2003). The long-term carbon cycle, fossil fuels and atmospheric composition. Nature, 426(6964), 323–326.
6. Walker, J. C. G., Hays, P. B., & Kasting, J. F. (1981). A negative feedback mechanism for the long-term stabilization of Earth's surface temperature. Journal of Geophysical Research, 86(C10), 9776–9782.
7. Sarmiento, J. L., & Gruber, N. (2006). Ocean Biogeochemical Dynamics. Princeton: Princeton University Press.
8. Falkowski, P., et al. (2000). The global carbon cycle: A test of our knowledge of earth as a system. Science, 290(5490), 291–296.
9. Sabine, C. L., et al. (2004). The oceanic sink for anthropogenic CO2. Science, 305(5682), 367–371.
10. Pan, Y., et al. (2011). A large and persistent carbon sink in the world's forests. Science, 333(6045), 988–993.
11. Archer, D. (2010). The Global Carbon Cycle. Princeton: Princeton University Press.
12. Doney, S. C., Fabry, V. J., Feely, R. A., & Kleypas, J. A. (2009). Ocean acidification: The other CO2 problem. Annual Review of Marine Science, 1, 169–192.
13. Schuur, E. A. G., et al. (2015). Climate change and the permafrost carbon feedback. Nature, 520(7546), 171–179.