The main carbon stores are the atmosphere, oceans, soils, living organisms, and fossil fuels. Carbon moves between these stores through flows including photosynthesis, feeding, respiration, death, decomposition, combustion, and ocean-atmosphere exchange.
This is IB ESS Subtopic 2.3: Biogeochemical cycles and is core content for both SL and HL.
Carbon Stores and Their Main Flows
A store is a location where carbon is held. A flow is a process that moves carbon between stores; it may be a transfer that changes location or a transformation that changes chemical form.
| Carbon store | Form of carbon | Important flows |
|---|---|---|
| Atmosphere | Mainly carbon dioxide and methane | Carbon dioxide enters plants through photosynthesis and returns through respiration, decomposition, and combustion. |
| Living organisms | Organic carbon compounds in biomass | Feeding transfers carbon through food chains; death and defecation transfer it to soils and decomposers. |
| Soils | Dead organic matter and soil carbon | Decomposition releases carbon dioxide; some carbon remains stored for long periods. |
| Oceans | Dissolved carbon dioxide and other dissolved carbon compounds | Carbon dioxide dissolves from the atmosphere and may return through outgassing. Marine organisms also take up carbon. |
| Fossil fuels | Coal, oil, and natural gas | Combustion rapidly transfers long-stored carbon into the atmosphere as carbon dioxide. |
Photosynthesis transforms atmospheric carbon dioxide into organic compounds. Feeding then transfers this carbon between trophic levels, while cellular respiration and decomposition transform organic carbon back into carbon dioxide.
A common misconception is that carbon disappears when organisms respire or fuels burn. Carbon is conserved and redistributed between stores; unlike matter, energy flows through ecosystems and is eventually lost as heat.
The Amazon rainforest illustrates how human activity alters the cycle: deforestation reduces photosynthetic uptake and releases stored carbon, while also affecting Indigenous and local communities dependent on forest resources.
Exam Technique
For a systems diagram, draw stores as boxes and flows as labelled arrows. If asked to explain, identify the starting and ending stores and state the process linking them; do not merely list flow names.