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This lesson introduces the global carbon cycle as a fundamental Earth system, covering the major carbon stores, the fluxes between them, residence times, and the concept of carbon as a closed system at planetary scale. This material addresses the Edexcel A-Level Geography specification (9GE0), Paper 1, Topic 6: The Carbon Cycle and Energy Security. The overarching Enquiry Question is: "How does the carbon cycle operate to maintain planetary health?"
Carbon is the fourth most abundant element in the universe and the second most abundant element in the human body (after oxygen by mass). Its unique ability to form four covalent bonds means it can create long chains, branched structures and ring compounds — the basis of all organic chemistry.
Carbon exists in multiple forms:
The carbon cycle describes the continuous movement of carbon atoms between the atmosphere, hydrosphere, lithosphere and biosphere. Understanding this cycle is essential for explaining climate change, energy security and the sustainability of human development.
Exam Tip: The Edexcel specification requires you to understand carbon as a system. Always use systems terminology: stores (stocks/reservoirs), fluxes (flows/transfers), inputs, outputs, feedback loops, and dynamic equilibrium.
Geographers study the carbon cycle using a systems framework. At the planetary scale, the carbon cycle is a closed system — energy enters and leaves (solar radiation in, longwave radiation out), but matter (including carbon) does not enter or leave. The total amount of carbon on Earth has remained approximately constant for billions of years.
Within this closed system, carbon moves between stores via fluxes. The system can be in one of three states:
| System State | Description | Carbon Cycle Example |
|---|---|---|
| Dynamic equilibrium | Inputs and outputs are balanced over time; the system is stable | Pre-industrial carbon cycle with roughly constant atmospheric CO₂ (~280 ppm for millennia) |
| Positive feedback | A change is amplified, pushing the system further from equilibrium | Warming → permafrost thaw → CO₂/CH₄ release → more warming |
| Negative feedback | A change is counteracted, returning the system towards equilibrium | Increased CO₂ → enhanced plant growth (CO₂ fertilisation) → more carbon uptake → reduced CO₂ |
At sub-global scales (e.g. a single forest, an ocean basin), the carbon cycle is an open system — carbon enters and leaves through transfers with other parts of the Earth system.
Carbon is stored in four main Earth system components. The table below summarises the approximate size of each store:
| Carbon Store | Estimated Size (GtC) | Percentage of Total | Key Forms |
|---|---|---|---|
| Lithosphere (rocks and sediments) | ~100,000,000 | ~99.9% | Carbonate rocks (limestone, chalk, dolomite), fossil fuels (coal, oil, gas), kerogen |
| Oceans (hydrosphere) | ~38,000 | ~0.04% | Dissolved inorganic carbon (DIC), dissolved organic carbon (DOC), marine organisms, ocean sediments |
| Soils (pedosphere) | ~2,300 | ~0.002% | Soil organic matter, peat, humus, permafrost carbon |
| Atmosphere | ~880 (2024 value; ~750 pre-industrial) | ~0.001% | CO₂ (main), CH₄, CO, volatile organic compounds |
| Biosphere (living organisms) | ~560 | ~0.0006% | Plant biomass (~450 GtC), animal biomass, fungi, bacteria |
pie title Global Carbon Stores (GtC, excluding lithosphere)
"Oceans (38,000)" : 38000
"Soils (2,300)" : 2300
"Atmosphere (880)" : 880
"Biosphere (560)" : 560
The lithosphere is by far the largest carbon store. Carbon is locked in:
Carbon enters the lithosphere very slowly (through sedimentation and burial) and leaves very slowly (through weathering, volcanic outgassing, and metamorphism). The residence time of carbon in the lithosphere is typically 150 million years or more.
The oceans are the second-largest carbon store and the largest actively cycling store. Ocean carbon exists in three main forms:
The residence time of carbon in the surface ocean is about 6 years, but in the deep ocean it can be 1,000 years or more because deep water circulates very slowly (thermohaline circulation).
Soils store approximately 2,300 GtC — more than the atmosphere and biosphere combined. Key soil carbon stores include:
Exam Tip: Permafrost is a critical store that examiners love to ask about. Remember: permafrost contains approximately twice as much carbon as the current atmosphere. If it thaws due to global warming, it could release vast quantities of CO₂ and CH₄, creating a dangerous positive feedback loop.
The atmosphere is a relatively small but critically important carbon store. As of 2024, the atmosphere contains approximately 880 GtC, up from ~750 GtC in the pre-industrial era (~280 ppm CO₂). The current concentration exceeds 420 ppm (parts per million) — the highest level in at least 800,000 years based on ice core evidence.
Atmospheric carbon is mainly in the form of CO₂ (~78% of atmospheric carbon by mass), with smaller contributions from methane (CH₄), carbon monoxide (CO) and volatile organic compounds.
The residence time of a CO₂ molecule in the atmosphere is approximately 3–5 years before it is absorbed by oceans or vegetation, but the overall perturbation (excess CO₂) persists for centuries to millennia because the carbon is redistributed among stores rather than destroyed.
The biosphere stores approximately 560 GtC, overwhelmingly in terrestrial vegetation (~450 GtC), with forests accounting for the largest share:
| Biome | Approximate Carbon in Vegetation (GtC) | Notes |
|---|---|---|
| Tropical forests | ~200 | Highest biomass per hectare; Amazon alone stores ~150–200 GtC in vegetation and soils |
| Boreal forests (taiga) | ~90 | Low biomass per hectare but vast area; much carbon in soils |
| Temperate forests | ~60 | Moderate biomass; significant regrowth since 19th century deforestation |
| Other (grasslands, tundra, etc.) | ~100 | Grasslands store most carbon in roots and soil rather than aboveground biomass |
Marine biota hold only ~3 GtC, but their turnover rate is extremely high — phytoplankton have a residence time of just days to weeks, meaning they cycle carbon rapidly despite their small standing stock.
A flux is a transfer of carbon between stores, measured in GtC per year. The table below summarises the main natural and anthropogenic fluxes:
| Flux | Direction | Rate (GtC/year) | Process |
|---|---|---|---|
| Photosynthesis | Atmosphere → Biosphere | ~120 | Plants absorb CO₂ and convert it to organic carbon |
| Respiration (autotrophic) | Biosphere → Atmosphere | ~60 | Plants release CO₂ through their own metabolism |
| Respiration (heterotrophic) and decomposition | Biosphere/Soils → Atmosphere | ~60 | Animals, fungi and bacteria break down organic matter |
| Ocean–atmosphere exchange (absorption) | Atmosphere → Oceans | ~90 | CO₂ dissolves in surface waters |
| Ocean–atmosphere exchange (outgassing) | Oceans → Atmosphere | ~88 | CO₂ is released from warming surface waters |
| Net ocean uptake | Atmosphere → Oceans | ~2.5 | Oceans are currently a net carbon sink |
| Fossil fuel combustion | Lithosphere → Atmosphere | ~9.5 | Burning coal, oil and gas |
| Land use change (deforestation) | Biosphere → Atmosphere | ~1.5 | Clearing forests releases stored carbon |
| Volcanism | Lithosphere → Atmosphere | ~0.1 | Volcanic eruptions release CO₂ |
| Weathering | Atmosphere → Lithosphere | ~0.3 | Chemical weathering of silicate rocks consumes CO₂ |
| Sedimentation | Oceans → Lithosphere | ~0.2 | Burial of organic and inorganic carbon in ocean sediments |
flowchart LR
A[Atmosphere<br>880 GtC] -->|"Photosynthesis<br>120 GtC/yr"| B[Biosphere<br>560 GtC]
B -->|"Respiration &<br>Decomposition<br>~120 GtC/yr"| A
A -->|"Ocean absorption<br>90 GtC/yr"| C[Oceans<br>38,000 GtC]
C -->|"Ocean outgassing<br>88 GtC/yr"| A
D[Lithosphere<br>100M GtC] -->|"Volcanism<br>0.1 GtC/yr"| A
A -->|"Weathering<br>0.3 GtC/yr"| D
D -->|"Fossil fuel<br>burning<br>9.5 GtC/yr"| A
B -->|"Sedimentation<br>& burial"| D
B -->|"Soil carbon<br>transfer"| E[Soils<br>2,300 GtC]
E -->|"Decomposition"| A
The residence time of carbon in a store is the average length of time a carbon atom remains in that store before being transferred elsewhere. Residence time can be calculated as:
Residence Time = Store Size ÷ Flux Rate
For example:
| Store | Typical Residence Time |
|---|---|
| Atmosphere | 3–5 years (molecule); centuries (perturbation) |
| Surface ocean | ~10 years |
| Deep ocean | ~1,000 years |
| Soils | 25 years (labile); 1,000+ years (peat/permafrost) |
| Terrestrial vegetation | 1–100 years (leaves vs trunks) |
| Lithosphere | 100+ million years |
Exam Tip: Examiners often ask you to compare stores and fluxes. Remember that the largest stores (lithosphere, deep ocean) have the longest residence times and the slowest fluxes. The smallest stores (atmosphere, biosphere) have the shortest residence times and the fastest fluxes. This is why human additions to the atmosphere have such a rapid and significant effect — the atmospheric store is small and sensitive to perturbation.
At the planetary scale, the Earth's carbon cycle is a closed system for matter. No significant amount of carbon enters or leaves the Earth system. The total carbon budget — approximately 100 million GtC — has remained essentially constant since the formation of the Earth 4.5 billion years ago.
However, the distribution of carbon between stores has changed dramatically over geological time:
The current anthropogenic disruption is fundamentally a transfer of carbon from the lithosphere (where it has resided for millions of years) to the atmosphere (where its residence time is short but its climatic impact is immediate).
This lesson addresses the Edexcel Enquiry Question: "How does the carbon cycle operate to maintain planetary health?" Key points to remember:
Exam Tip: In a 12-mark question, always provide specific data (e.g. "the lithosphere stores approximately 100,000,000 GtC" or "photosynthesis transfers approximately 120 GtC per year from atmosphere to biosphere"). Quantitative precision distinguishes A/A* answers from generic descriptions.
| Term | Definition |
|---|---|
| Carbon store | A reservoir where carbon accumulates and is held for a period of time (also called a stock, pool or reservoir) |
| Carbon flux | A transfer of carbon from one store to another, measured in GtC per year |
| Residence time | The average length of time a carbon atom remains in a particular store |
| Closed system | A system that exchanges energy but not matter with its surroundings |
| Open system | A system that exchanges both energy and matter with its surroundings |
| Dynamic equilibrium | A state in which the inputs and outputs of a system are balanced, so the system remains stable |
| Positive feedback | A feedback mechanism that amplifies change, pushing the system further from equilibrium |
| Negative feedback | A feedback mechanism that counteracts change, returning the system towards equilibrium |
| GtC | Gigatonnes of carbon — 1 GtC = 1 billion (10⁹) tonnes of carbon |
| ppm | Parts per million — the standard unit for measuring atmospheric CO₂ concentration |