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Carbon is the fundamental building block of all organic matter and plays a central role in regulating Earth's climate through the greenhouse effect. Like the hydrological cycle, the carbon cycle operates as a closed system at the global scale: carbon is neither created nor destroyed, but continuously transferred between stores. Understanding the carbon cycle quantitatively — with precise data on store sizes, flux rates, and the distinction between fast and slow cycles — is essential for AQA A-Level Geography.
Carbon is distributed across four major Earth system components. The following estimates are derived from IPCC AR5/AR6 and supporting literature.
The lithosphere is by far the largest carbon store:
| Component | Carbon (GtC) | Notes |
|---|---|---|
| Sedimentary rocks (carbonates) | ~60,000,000 | Limestone (CaCO₃), dolomite; formed over millions of years |
| Fossil fuels (organic sediments) | ~4,000 | Coal, oil, natural gas; accumulated over 300+ million years |
| Total lithosphere | ~60,004,000 | Overwhelmingly dominant store |
The ocean is the second-largest active carbon store:
| Component | Carbon (GtC) | Notes |
|---|---|---|
| Deep ocean | ~37,100 | Dissolved inorganic carbon (DIC); very slow turnover |
| Surface ocean | ~900 | Exchanges CO₂ with atmosphere; temperature-dependent solubility |
| Marine biota | ~3 | Phytoplankton, zooplankton, fish, etc.; tiny store but crucial flux |
| Dissolved organic carbon | ~700 | Dead organic matter in solution |
| Total ocean | ~38,700 | Second-largest store after lithosphere |
| Component | Carbon (GtC) | Notes |
|---|---|---|
| CO₂ | ~880 (2022) | Up from ~590 GtC pre-industrial (~280 ppm to ~420 ppm) |
| CH₄ (methane) | ~4 | Much smaller store but ~80× more potent GHG over 20 years |
| Total atmosphere | ~884 | Relatively small store but critical for climate regulation |
| Component | Carbon (GtC) | Notes |
|---|---|---|
| Soil organic carbon | ~1,500–2,400 | Includes humus, litter, peat; largest terrestrial store |
| Vegetation (biomass) | ~450–650 | Tropical forests hold ~55% of all vegetation carbon |
| Permafrost | ~1,400–1,700 | Frozen organic matter; vulnerable to thaw under warming |
| Freshwater systems | ~100–150 | Lakes, rivers, wetlands |
| Total terrestrial | ~3,450–4,900 | Significant uncertainty in estimates |
Key Point: The atmosphere contains a relatively small amount of carbon (~880 GtC), which is why even modest changes in fluxes to and from the atmosphere can cause significant changes in atmospheric CO₂ concentration and global climate.
The fast carbon cycle operates over timescales of days to thousands of years and involves exchanges between the atmosphere, biosphere, soils, and surface ocean.
| Flux | Rate (GtC/yr) | Direction |
|---|---|---|
| Photosynthesis (terrestrial) | ~120 | Atmosphere → biosphere |
| Respiration + decomposition (terrestrial) | ~120 | Biosphere → atmosphere |
| Ocean-atmosphere exchange (absorption) | ~80 | Atmosphere → ocean |
| Ocean-atmosphere exchange (release) | ~78 | Ocean → atmosphere |
| Net ocean uptake | ~2–3 | Atmosphere → ocean (absorbing excess CO₂) |
| Net land uptake | ~1–2 | Atmosphere → biosphere (fertilisation effect) |
Under pre-industrial conditions, the fast cycle was approximately balanced. Currently, the oceans and terrestrial biosphere act as net carbon sinks, absorbing roughly half of anthropogenic CO₂ emissions.
The fundamental equation of photosynthesis:
6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂ (using solar energy)
Respiration is essentially the reverse:
C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O (releasing energy)
The balance between these two processes determines whether an ecosystem is a net carbon sink or source.
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