Water and Carbon Cycles
Global Distribution and Size of Major Stores of Carbon
Introduction
Carbon is fundamental to life on Earth and a key regulator of the planet’s climate system. It circulates continuously between five major stores: the lithosphere, hydrosphere, cryosphere, biosphere, and atmosphere.
Although the total global carbon stock is relatively constant, its distribution between stores varies over time due to both natural processes and human activity.
Together, these stores and the flows between them form the global carbon cycle, which operates over fast (biological) and slow (geological) timescales.
1. Overview of the Global Carbon Budget
Carbon exists in a variety of forms, including carbon dioxide (CO₂), methane (CH₄), calcium carbonate (CaCO₃), and organic molecules such as carbohydrates and hydrocarbons.
- The total global carbon stock is around 100 million gigatonnes (GtC).
- Most is stored in the lithosphere and deep oceans, while the atmosphere and biosphere contain much smaller but more active stores.
- Carbon residence time, the average time a molecule stays in a store, varies from hours in the biosphere to millions of years in rocks.
| Carbon Store | Approx. Carbon Stock (GtC) | Residence Time | Percentage of Total Global Carbon | Key Processes |
|---|---|---|---|---|
| Lithosphere | ~100,000,000 | 10–100 million years | ~99.9% | Weathering, volcanic outgassing, burial, lithification |
| Hydrosphere | ~38,000 | 100–1,000 years | ~0.04% | Solubility and biological pumps, ocean–atmosphere exchange |
| Cryosphere | ~1,400 | Centuries–millennia | ~0.001% | Carbon stored in permafrost and trapped air bubbles |
| Biosphere | ~2,000 | Hours–decades | ~0.002% | Photosynthesis, respiration, decomposition |
| Atmosphere | ~800 | ~6 years | ~0.001% | Exchange with ocean and vegetation, combustion, respiration |
2. The Lithosphere
Description
The lithosphere is Earth’s largest carbon store, containing over 99% of all carbon, mainly in sedimentary rocks and fossil fuels. Carbon here is part of the slow carbon cycle, locked away for millions of years.
| Lithospheric Sub-store | Description | Approximate Carbon Content (GtC) |
|---|---|---|
| Sedimentary rocks and fossil fuels | Carbon stored as limestone, chalk, coal, oil, and natural gas. | ~100 million |
| Soil organic carbon | Carbon from decomposing plant and animal material. | ~1,500–2,400 |
| Peat | Partially decomposed organic matter under anaerobic conditions. | ~500 |
Key Processes
- Chemical weathering of rocks absorbs atmospheric CO₂, forming bicarbonate ions.
- Volcanic outgassing releases CO₂ from subducted carbonates.
- Burial and lithification trap carbon in sediments over geological time.
3. The Hydrosphere
Description
The oceans are the second largest active carbon store, holding around 38,000 GtC.
Most oceanic carbon exists as dissolved inorganic carbon (DIC), with smaller amounts as organic matter.
Oceanic Carbon Distribution
| Zone | Description | Residence Time |
|---|---|---|
| Surface ocean (mixed layer) | Rapid exchange with the atmosphere. | Days–years |
| Deep ocean | Carbon transported via currents; slow circulation. | 100–1,000 years |
| Marine sediments | Long-term geological storage. | 10,000+ years |
Processes
- Biological pump: Phytoplankton absorb CO₂; carbon sinks as organic matter.
- Solubility pump: Cold, high-latitude waters absorb CO₂, which sinks with currents.
- Outgassing: Warm surface waters release CO₂ back to the atmosphere.
4. The Cryosphere
Description
The cryosphere includes frozen ground, ice sheets, and glaciers.
Although it stores a small proportion of global carbon, it plays a key role in climate feedback mechanisms.
| Cryospheric Store | Location/Example | Approximate Carbon Stock (GtC) | Notes |
|---|---|---|---|
| Permafrost | Arctic tundra (Siberia, Alaska, Canada) | ~1,400 | Organic carbon is trapped in frozen soils; released as CO₂ and CH₄ when thawed. |
| Glacial ice | Greenland, Antarctica | Less than 1 | Indirect storage via trapped air bubbles and dust particles. |
Importance
- Permafrost thawing releases greenhouse gases → positive feedback.
- Acts as a temporary carbon sink during colder climatic periods.
5. The Biosphere
Description
The biosphere comprises all living organisms and decaying organic matter on land and in oceans.
It contains roughly 2,000 GtC, actively exchanged with the atmosphere through photosynthesis and respiration.
| Ecosystem Type | Approx. Carbon Stock (GtC) | Key Notes |
|---|---|---|
| Forests | ~550 | Major terrestrial carbon sink; Amazon and Congo key examples. |
| Soils | ~1,500–2,400 | Huge store of organic matter; vulnerable to disturbance. |
| Grasslands and savannas | ~200 | Carbon balance affected by grazing and fire. |
| Marine biomass | ~3 | Rapid turnover through phytoplankton productivity. |
Processes
- Photosynthesis: Converts atmospheric CO₂ into biomass.
- Respiration and decomposition: Release CO₂ back to the atmosphere and soil.
- Sequestration: Long-term storage through forest growth or soil accumulation.
Example:
The Amazon Rainforest sequesters roughly 0.5 billion tonnes of carbon annually, though drought and deforestation have weakened its role as a carbon sink.
6. The Atmosphere
Description
The atmosphere stores about 800 GtC, mainly as carbon dioxide (CO₂) and methane (CH₄), the key greenhouse gases driving climate regulation.
| Gas | Approximate Concentration (2024) | Warming Potential (100-year timescale) |
|---|---|---|
| CO₂ | ~419 ppm | Baseline (×1) |
| CH₄ | ~1.9 ppm | ×28 |
| N₂O | ~0.33 ppm | ×265 |
Processes
- Photosynthesis and ocean uptake remove CO₂.
- Respiration, combustion, and volcanic activity add CO₂.
- Ocean-atmosphere exchange balances short-term fluctuations.
Trend:
CO₂ levels have risen from 280 ppm (pre-industrial) to over 419 ppm (2024), the highest in 800,000 years.
7. Seasonal Variation in Carbon Storage
Carbon exchange between the atmosphere and biosphere fluctuates seasonally, especially in the Northern Hemisphere:
- During spring and summer, plant growth increases photosynthesis, drawing down atmospheric CO₂.
- In autumn and winter, decomposition and reduced photosynthetic activity release CO₂ back to the atmosphere.
- This produces a seasonal CO₂ oscillation of about 6-8 ppm each year, recorded in data such as the Keeling Curve (Mauna Loa Observatory).
- These fluctuations demonstrate how short-term biological processes influence atmospheric carbon on a global scale.
8. Interconnectedness of Carbon Stores
All five stores are linked through flows (fluxes) of carbon.
- The fast carbon cycle involves exchanges between the atmosphere, biosphere, and hydrosphere (timescale: days–decades).
- The slow carbon cycle links the lithosphere and deep ocean (timescale: thousands to millions of years).
This system maintains a dynamic equilibrium, though human activity is now increasing the transfer rate between stores — particularly from the lithosphere (fossil fuels) to the atmosphere.
9. Summary Table
| Carbon Store | Relative Size (GtC) | Residence Time | Key Role |
|---|---|---|---|
| Lithosphere | ~100,000,000 | 10–100 million years | Long-term geological carbon storage. |
| Hydrosphere | ~38,000 | 100–1,000 years | Oceanic regulation of atmospheric CO₂ via dissolution and biological processes. |
| Cryosphere | ~1,400 | Centuries–millennia | Stores frozen organic carbon; potential feedback source. |
| Biosphere | ~2,000 | Hours–decades | Active exchange of carbon through photosynthesis and respiration. |
| Atmosphere | ~800 | ~6 years | Greenhouse gas regulation of Earth’s climate. |
Summary
- The lithosphere is the largest carbon store, containing over 99% of Earth’s total carbon.
- The hydrosphere acts as a key regulator, exchanging carbon rapidly with the atmosphere.
- The biosphere and atmosphere are smaller but more dynamic, influencing short-term climate change.
- The cryosphere stores frozen carbon and plays an increasingly important role in feedback loops as global temperatures rise.
- Understanding the distribution and interactions of these stores is essential for explaining changes in Earth’s carbon balance and climate system.
Exam Tip
When answering questions on global carbon stores:
- Always quantify (use GtC).
- Mention residence time to show temporal understanding.
- Link stores using flows (e.g. photosynthesis, weathering, ocean exchange).
- Use examples like the Amazon Basin, deep ocean, and Arctic permafrost.
- Refer to both spatial (where carbon is stored) and temporal (how long it remains there) dimensions.