Water and Carbon Cycles
Carbon Cycle Transfers: Factors Driving Change in the Magnitude and Scale of Carbon Stores
Introduction
The carbon cycle is a dynamic system. Carbon is constantly transferred between the atmosphere, biosphere, oceans and lithosphere, but the amount (magnitude) stored in each part of the system changes over time and space.
Key flows and transfers, including photosynthesis, respiration, decomposition, combustion, weathering and sequestration, drive these changes.
At different scales, from a single plant, through vegetation succession (a sere), to entire continents, these processes alter how much carbon is stored in biomass, soils, oceans and rocks.
Understanding how these processes operate helps explain climate regulation, soil formation, ocean chemistry and long-term carbon balance.
1. Plant Scale
Individual plants are relatively small carbon stores, but they play an active role in the daily movement of carbon between the atmosphere and biosphere.
Photosynthesis
Plants absorb CO₂ and convert it into organic matter.
- Transfers carbon from atmosphere → biomass.
- Highest in warm, wet, sunny environments (e.g. tropical forests).
- Lowest in cold or dry regions.
- On a global scale, CO₂ levels fall slightly during the Northern Hemisphere summer when plant growth peaks.
Respiration
Plants and animals release CO₂ by breaking down carbohydrates for energy.
- Occurs day and night.
- Faster in warm, humid conditions.
- Ecosystems with high biodiversity and productivity (e.g. rainforests) have higher respiration rates than tundra or desert environments.
Combustion
- Wildfires or deliberate burning rapidly release stored carbon into the atmosphere.
- A tree that stored carbon for decades can lose it in minutes.
- Human combustion of fossil fuels releases geological carbon into the atmosphere, increasing atmospheric CO₂ quickly.
2. Sere Scale (Vegetation Succession)
A sere is the sequence of plant communities that develop during ecological succession (for example, a sand dune, a peat bog or a reclaimed quarry). As succession progresses, the capacity to store carbon increases.
Early stages
- Pioneer species such as mosses or marram grass store limited carbon.
- Soils are thin and nutrient-poor.
Developing stages
- Shrubs and trees increase biomass.
- Roots stabilise soil, and dead organic matter increases soil carbon.
Decomposition
- Decomposers break down dead material, returning CO₂ to the atmosphere.
- Some material becomes humus, increasing long-term soil carbon storage.
- Decomposition is fastest in warm, moist environments and slowest in cold, frozen or waterlogged conditions (e.g. peatlands).
Mature/climax communities
- Long-lived trees and deep soils in temperate and tropical forests store large amounts of carbon.
- Disturbance such as clearance, storm damage or fire can rapidly reduce carbon storage and restart succession.
- Example: In a dune succession at Studland Bay, Dorset, carbon storage increases markedly from sparse grasses near the shore to mature woodland further inland.
3. Continental and Global Scales
Weathering
Atmospheric CO₂ dissolves in rainwater, forming weak carbonic acid. This reacts with carbonate rocks such as limestone, transferring carbon into rivers and eventually the ocean. Weathering acts as a slow but continuous carbon sink linking the atmosphere → hydrosphere → lithosphere.
Ocean Sequestration
The oceans are the largest active carbon store on Earth.
- CO₂ dissolves into surface waters.
- Phytoplankton absorb carbon during photosynthesis; dead material sinks (biological pump).
- Cold, dense water sinks, storing carbon in deep oceans for centuries (physical pump).
- Some carbon becomes trapped in sediments or carbonate rocks, forming a long-term geological store.
Sedimentary Burial
Marine organisms build shells from carbonate. When they die, shells accumulate on the seabed and form sedimentary rocks such as limestone, locking carbon away for millions of years.
Volcanic Outgassing
Carbon stored in rocks returns to the atmosphere during volcanic eruptions and tectonic activity. Although small each year, this offsets long-term carbon removal through weathering and sedimentation.
Key Carbon Transfer Processes and Their Influence on Stores
| Process | What the process does | How it changes the magnitude of carbon stores | Conditions influencing the rate of change |
|---|---|---|---|
| Photosynthesis | CO₂ is converted into organic matter by plants. | Increases biomass and soil carbon; reduces atmospheric CO₂. | Highest in warm, sunny, moist environments with dense vegetation. |
| Respiration | Plants, animals and decomposers release CO₂. | Returns stored carbon to the atmosphere. | Faster in warm, humid climates with high biodiversity. |
| Decomposition | Dead organic matter is broken down by bacteria and fungi. | Releases CO₂, but also adds carbon to soils as humus. | Rapid in hot, moist conditions; slow in cold or waterlogged environments. |
| Combustion | Burning of vegetation or fossil fuels. | Rapidly reduces biomass stores and increases atmospheric CO₂. | Natural drought and heatwaves increase wildfire risk; human activity increases fossil fuel combustion. |
| Carbon sequestration in oceans and sediments | CO₂ dissolves in seawater; is stored in organisms, deep water or sediments. | Oceans act as a major sink; sediments lock carbon away for millions of years. | Influenced by temperature, circulation, marine productivity and nutrient levels. |
| Weathering | Carbonic acid dissolves carbonate rocks, moving carbon to oceans. | Transfers carbon from atmosphere to rivers, oceans and rock stores. | Faster in warm, wet climates with exposed carbonate rocks. |
Changing Carbon Stores Over Time
- Daily: Photosynthesis and respiration change atmospheric CO₂ levels.
- Seasonal: Forests store carbon in summer and release some in winter during leaf fall and decomposition.
- Decadal: Deforestation, afforestation, and land-use change alter terrestrial carbon storage.
- Geological timescales: Weathering, sedimentation and volcanic outgassing redistribute carbon between the atmosphere, oceans and rocks.
Summary
- At the plant scale, carbon enters and leaves biomass through photosynthesis, respiration and combustion.
- Across a sere, ecosystems accumulate more carbon as soils deepen and biomass increases.
- At the continental scale, weathering, ocean sequestration and sediment burial store carbon over extremely long timescales.
- These processes operate at contrasting spatial and temporal scales, maintaining the carbon cycle’s dynamic equilibrium.
- Human activity is now increasing atmospheric CO₂ faster than natural sinks can remove it, altering this balance.
Exam Tip
When answering A Level questions on systems:
1. Always refer to a scale
Examiners reward answers that explicitly reference plant, sere or continental scales:
- “At the plant scale…”
- “Across a sere…”
- “At the continental/global scale…”
2. Show cause → effect
Link each process to its impact on store size:
- “Photosynthesis increases biomass stores and reduces atmospheric CO₂.”
- “Combustion rapidly reduces biomass and transfers carbon to the atmosphere.”
3. Use correct terminology
Words like sequestration, weathering, humus, biomass, lithosphere, sediment burial, and respiration score higher than generic terms like “stored” or “trapped”.
4. Include climate or environmental controls
Examiners want to see why rates vary:
- “Decomposition is slow in cold or waterlogged soils, leading to long-term carbon storage, such as peat.”
5. Add a short example
Even a brief named example adds AO2 credit:
- “Peat bogs in northern Britain store large amounts of carbon due to slow decomposition.”
- “Tropical forests show high photosynthesis and respiration rates due to warm, wet conditions.”
6. Don’t describe processes in isolation
- Weaker answers list definitions.
- Stronger answers apply them to changes in store magnitude across time and space, exactly what the spec asks for.