Earth’s Life Support Systems
The Distribution and Size of the Major Stores in the Carbon Cycle
Carbon is a fundamental element for all life on Earth. It is stored and cycled between the atmosphere, oceans, lithosphere, biosphere, and cryosphere, forming the global carbon cycle. The total amount of carbon on Earth remains constant, but it is continuously exchanged between stores through natural and human processes.
1. Overview
The global carbon cycle operates as a closed system; carbon is neither created nor destroyed, only transferred between stores (sinks). However, at local and regional scales (such as forests or soil systems), it functions as an open system, in which carbon enters or leaves through processes such as photosynthesis, respiration, and combustion.
Understanding the size, distribution, and interaction of carbon stores is crucial for explaining global climate regulation and the enhanced greenhouse effect.
The table below provides a summary of the major stores in the carbon cycle.
| Store | Approximate size (gigatonnes of carbon, GtC) | Key Features |
|---|---|---|
| Lithosphere (sedimentary rocks and fossil fuels) | ~100,000,000 GtC | The largest carbon store, held in carbonate rocks such as limestone and fossil fuels (coal, oil, gas). Exchange with the atmosphere occurs over geological timescales. |
| Oceans | ~38,000 GtC | The second largest store. Carbon is stored as dissolved CO₂, bicarbonates, and marine biomass. The ocean acts as both a source and sink of carbon, exchanging with the atmosphere through diffusion. |
| Biosphere | ~2,000 GtC | Includes all living organisms and dead organic matter. Carbon is cycled rapidly through photosynthesis, respiration, and decomposition. |
| Soil and peat | ~1,500 GtC | Organic carbon stored in decomposed plant material. Soils are an important medium-term carbon store, influenced by land use and climate. |
| Atmosphere | ~830 GtC | Present mainly as carbon dioxide (CO₂) and methane (CH₄). This store regulates Earth’s temperature through the greenhouse effect. |
| Cryosphere | ~400 GtC (approx.) | Frozen organic matter trapped in permafrost. Warming temperatures risk releasing this carbon as methane and CO₂. |
2. The Lithosphere (Rocks, Sediments, and Fossil Fuels)
The lithosphere is by far the largest carbon store, containing both organic and inorganic carbon.
| Type of carbon | Approximate Store Size (Gigatonnes of Carbon – GtC) | Key Features |
|---|---|---|
| Sedimentary rocks (e.g. limestone, chalk) | ~100,000,000 GtC | The main long-term store of carbon. Formed by the burial and compaction of marine organisms and shells rich in calcium carbonate (CaCO₃). |
| Fossil fuels (coal, oil, natural gas) | ~4,000 GtC | Carbon accumulated over millions of years from partially decomposed organic matter. Released through combustion and extraction. |
| Soil organic carbon | ~1,500 GtC | Formed by decomposed plant and animal material. Plays a major role in nutrient cycling and plant growth. |
Importance of the lithosphere:
- Acts as a long-term carbon sink, storing carbon for millions of years.
- Processes such as weathering, volcanic activity, and human extraction (e.g. burning fossil fuels) move carbon from this store to the atmosphere and oceans.
3. The Oceans (Hydrosphere)
The oceans are the second largest store of carbon, holding about 38,000 GtC. Carbon is stored in dissolved forms, in marine organisms, and in ocean sediments.
| Ocean Carbon Store | Approximate Store Size (Gigatonnes of Carbon – GtC) | Key Features |
|---|---|---|
| Surface ocean | ~900 GtC | Exchanges CO₂ rapidly with the atmosphere through diffusion. Acts as both a source and sink of carbon. |
| Deep ocean | ~37,100 GtC | Holds most oceanic carbon as dissolved inorganic carbon (carbonates and bicarbonates). Exchange occurs over hundreds to thousands of years. |
| Marine organisms | ~3 GtC | Store carbon in soft tissue and shells (CaCO₃). When organisms die, carbon sinks to the ocean floor and forms sediment. |
Key processes:
- Biological pump: Carbon is absorbed by phytoplankton through photosynthesis and transferred to deeper layers as organisms die.
- Physical pump: Cold water absorbs CO₂, which is transported to depth by ocean currents.
- Solubility pump: CO₂ dissolves more readily in cold water; warming reduces this uptake.
4. The Atmosphere
The atmosphere is a relatively small but highly active carbon store, containing approximately 830 GtC.
| Main Carbon Gases | Approximate Proportion | Role |
|---|---|---|
| Carbon dioxide (CO₂) | ~400 ppm (0.04%) | Main greenhouse gas responsible for trapping heat and regulating Earth’s temperature. |
| Methane (CH₄) | ~1.9 ppm | More potent than CO₂ but present in much smaller amounts. Produced by wetlands, agriculture, and fossil fuel use. |
Key points:
- Atmospheric carbon is vital for maintaining a habitable temperature through the greenhouse effect.
- Carbon is exchanged rapidly with the biosphere and oceans through photosynthesis, respiration, and diffusion.
- Human activities such as burning fossil fuels and deforestation are increasing atmospheric CO₂ concentrations, enhancing global warming.
5. The Biosphere
The biosphere includes all living and dead organic matter — plants, animals, and soils. It stores approximately 2,000 GtC.
| Carbon Component | Approximate Proportion | Key Features |
|---|---|---|
| Vegetation | ~560 GtC | Main greenhouse gas responsible for trapping heat and regulating Earth’s temperature. |
| Soils and peat | ~1,500 GtC | More potent than CO₂ but present in much smaller amounts. Produced by wetlands, agriculture, and fossil fuel use. |
| Humus and litter | Minor | Organic material on the forest floor or within upper soil layers, forming part of the active carbon pool. |
Importance of the biosphere:
- Links the atmosphere and lithosphere through photosynthesis and decomposition.
- Acts as a short-term store — carbon cycles through plants and soils over decades to centuries.
- Deforestation and land-use change reduce the biosphere’s capacity to store carbon.
6. The Cryosphere
The cryosphere stores relatively small but significant amounts of carbon — around 400 GtC — mainly as frozen organic matter trapped in permafrost.
Key facts:
- Found in Arctic tundra regions such as Siberia, Alaska, and northern Canada.
- When permafrost thaws, CO₂ and CH₄ are released, acting as a positive feedback that accelerates global warming.
- Cryospheric carbon is part of the long-term terrestrial carbon store, but rapid thawing due to rising temperatures is making it increasingly active.
7. The Distribution of Carbon Stores
- The lithosphere and oceans dominate in terms of size and long-term storage.
- The biosphere, atmosphere, and cryosphere play key roles in short-term carbon cycling.
- Distribution varies geographically:
- Tropical regions have large biospheric stores (vegetation and soils)
- Polar regions hold carbon in frozen soils.
- Oceans near high latitudes act as major carbon sinks due to colder water temperatures.
Continuous exchanges between the lithosphere, oceans, biosphere, atmosphere, and cryosphere drive the carbon cycle.
While the lithosphere and oceans contain the vast majority of Earth’s carbon, smaller stores, particularly in the atmosphere and biosphere, are more dynamic and critical in regulating the global climate. Human activity is now accelerating carbon fluxes, particularly through fossil fuel combustion and deforestation, leading to an imbalance that underpins modern climate change.