Geography (9635) | The carbon cycle

Welcome to the Carbon Cycle!

Hello! This chapter explores one of the most critical systems on Earth: the Carbon Cycle. Carbon is more than just a chemical element; it’s the building block of all life (including you!) and the main driver of global climate regulation.

Understanding how carbon moves around the planet—where it is stored and how it flows—is fundamental to understanding modern environmental challenges, especially climate change. Don't worry if this seems tricky at first; we will break it down using the simple concept of a natural system.
Let’s get started!

Quick Review: Systems in Geography

Remember that a system is a set of linked components. The carbon cycle is an open system on Earth's surface (though often treated as a closed system globally) consisting of stores (where carbon waits) and flows (how carbon moves).

1. Global Stores of Carbon

The carbon cycle involves five main "spheres" or stores where carbon resides. The size of these stores is measured in Gigatonnes of Carbon (GtC). One Gigatonne is 1 billion metric tonnes—a huge amount!

It’s important to know the relative size of these stores (the magnitude) and where the carbon is held.

The Five Major Carbon Stores (LHBCA Mnemonic)

We can rank them from largest to smallest. Think of them as giant savings accounts, some active (quick flow) and some inactive (slow flow).

1. The Lithosphere (Earth's Crust)
   • Size: By far the largest store (estimated >100,000,000 GtC).
   • Form: Stored in rock (like limestone, which is mostly calcium carbonate), sediments, and fossil fuels (coal, oil, gas).
   • Activity: This is the slowest, most inactive store. Carbon here can be locked away for millions of years.
2. The Hydrosphere (Oceans)
   • Size: Second largest (estimated ~38,000 GtC).
   • Form: Stored as dissolved inorganic carbon (mostly CO₂ dissolved in seawater).
   • Activity: The ocean acts as a massive carbon sink, absorbing CO₂ from the atmosphere. The carbon is exchanged between surface waters (active, fast exchange) and deep waters (inactive, slow circulation).
3. The Pedosphere/Biosphere (Land and Life)
   • Size: Significant (estimated ~3,000 GtC).
   • Form: Held in living vegetation (plants), dead organic matter, and soil (soil organic carbon is a massive component of this store).
   • Activity: Relatively fast. Carbon moves quickly between the atmosphere and the biosphere via plant processes.
4. The Atmosphere (Air)
   • Size: Smallest major store (estimated ~850 GtC today).
   • Form: Primarily stored as Carbon Dioxide (CO₂), Methane (CH₄), and other greenhouse gases.
   • Activity: Extremely active and crucial. Even small changes in this store have huge effects on global climate (the Greenhouse Effect).
5. The Cryosphere (Frozen Areas)
   • Size: Smallest store mentioned in the syllabus, but linked to the lithosphere/biosphere (e.g., permafrost).
   • Form: Frozen organic material and methane trapped in permafrost soils.
   • Activity: Historically stable, but now a concern. As permafrost melts due to warming, this stored carbon and methane is released rapidly into the atmosphere.

2. Factors Driving Flows and Transfers

Carbon is constantly flowing between these stores. These flows are driven by various physical and biological processes that operate at different speeds (time and space scales).

A. Biological Flows (The Fast Cycle)

These processes involve living things and move carbon quickly, often in days or years, operating at plant, vegetation community, and continental scales.

1. Photosynthesis (Atmosphere to Biosphere)
   • What it is: Plants use sunlight, water, and atmospheric CO₂ to create their own food (glucose) and grow.
   • Transfer: This process takes carbon out of the atmosphere and locks it into plant structures (biomass).
   • Analogy: Plants are the ‘lungs’ of the planet, inhaling CO₂.
2. Respiration (Biosphere/Pedosphere to Atmosphere)
   • What it is: The opposite of photosynthesis. Both plants and animals release CO₂ back into the atmosphere when they break down food/energy.
   • Transfer: Carbon is released back as a gas.
3. Decomposition (Biosphere/Pedosphere to Atmosphere/Soil)
   • What it is: Bacteria and fungi break down dead organic matter.
   • Transfer: If oxygen is present, CO₂ is released (respiration by decomposers). If oxygen is absent (like in peat bogs or swamp sediments), methane (CH₄) is released.
4. Combustion (Burning) (Biosphere/Lithosphere to Atmosphere)
   • What it is: Rapid oxidation (burning) of organic material.
   • Transfer: Releases large amounts of carbon back into the atmosphere as CO₂, whether from a natural wildfire or human burning of wood or fossil fuels.

B. Geological and Oceanic Flows (The Slow Cycle)

These processes move carbon much slower, sometimes over thousands or millions of years, often involving oceans and sediments (carbon sequestration).

1. Weathering (Atmosphere/Hydrosphere to Lithosphere)
   • What it is: The slow breakdown of rocks by physical, chemical, and biological means.
   • Focus: Chemical weathering is key for the carbon cycle. Rainwater absorbs CO₂, creating weak carbonic acid, which dissolves rocks (especially limestone).
   • Transfer: This carbon is washed into rivers and eventually reaches the ocean, where it is used by organisms or sinks to the seabed to form sediments. This is a crucial long-term atmospheric regulation mechanism.
2. Carbon Sequestration in Oceans and Sediments
   • What it is: The process of capturing and storing atmospheric CO₂.
   • Oceanic Sequestration:
     • Biological Pump: Marine organisms (like plankton) use dissolved carbon to build shells. When they die, they sink, locking carbon in deep water or sediments.
     • Physical Pump: CO₂ dissolves more easily in cold water. In polar regions, cold, dense water sinks, carrying dissolved CO₂ down to the deep ocean circulation.
   • Transfer: This process moves carbon from the fast atmospheric store to the slow oceanic and lithospheric stores (sediments).

3. Changes in the Carbon Cycle Over Time

The magnitude of carbon stores and the intensity of flows change constantly due to natural events and, increasingly, human activity.

A. Natural Variations

Even without human influence, the cycle changes over geological time and shorter periods:

• Wild Fires: These are natural (e.g., lightning strikes) or caused by humans. They cause rapid, intense combustion, transferring huge amounts of carbon from the biosphere and soil directly to the atmosphere as CO₂ and soot.
• Volcanic Activity: Volcanoes release carbon that has been stored deep in the lithosphere. Gas emissions, including CO₂, are released during eruptions, transferring carbon to the atmosphere.

  Note: While large single eruptions (like Pinatubo) can temporarily cool the planet due to dust blocking the sun, volcanic activity over geological time is a significant, long-term source of atmospheric carbon.

B. Human Impact (The Enhanced Greenhouse Effect)

Since the Industrial Revolution, human actions have rapidly accelerated transfers, especially those moving carbon from the slow lithospheric store (fossil fuels) and the biosphere (forests) into the fast atmospheric store. This is the primary cause of the enhanced greenhouse effect and climate change.

1. Hydrocarbon Fuel Extraction and Burning
   • Impact: This is the biggest single impact. We extract oil, gas, and coal (carbon stored for millions of years in the lithosphere) and burn them for energy.
   • Transfer: The carbon is instantly transferred to the atmosphere as CO₂.
2. Deforestation and Land Use Changes
   • Deforestation: Forests are massive carbon stores (part of the biosphere). When forests are cut down (especially using slash-and-burn techniques), the immediate combustion releases stored carbon to the atmosphere. The long-term loss of trees also reduces the capacity for photosynthesis (a key flow that removes CO₂).
   • Land Use Changes: Converting natural land to urban areas, or intensive farming, disrupts soil structure, releasing soil organic carbon into the atmosphere.
3. Farming Practices
   • Livestock: Ruminant animals (cows, sheep) produce large amounts of methane (CH₄), a potent greenhouse gas, through digestion.
   • Plowing: Deep plowing exposes soil carbon to oxygen, accelerating decomposition and respiration, which leads to greater CO₂ release.
   • Fertilisers: Use of nitrogen fertilisers releases nitrous oxide (N₂O), another powerful greenhouse gas.

Key Takeaway: Human activity essentially acts like an accelerator pedal, shifting carbon from the stable, long-term stores (Lithosphere/Biosphere) into the unstable, short-term atmospheric store.

4. The Carbon Budget and Climate Impact

The Carbon Budget is the balance between the inputs of carbon into a store (like the atmosphere) and the outputs of carbon removed from that store over a set period.

Maintaining a Balance

In a healthy, stable cycle (dynamic equilibrium), the total carbon inputs to the atmosphere should roughly equal the total outputs (e.g., photosynthesis + oceanic sequestration = respiration + volcanic activity).

The Modern Imbalance

Since the Industrial Revolution, the budget has become unbalanced due to human flows (burning fossil fuels, deforestation). The inputs to the atmosphere now significantly outweigh the outputs.

The result? The atmospheric store is growing rapidly.

Impact of the Carbon Cycle Imbalance

The increase in atmospheric CO₂ has critical impacts on the three receiving stores:

1. Impact on the Atmosphere (Global Climate)
   • Increased concentrations of CO₂ and methane enhance the Greenhouse Effect.
   • This traps more long-wave radiation, leading to Global Climate Change (warming temperatures, changing weather patterns).
2. Impact on the Ocean (Hydrosphere)
   • The ocean tries to compensate for high atmospheric CO₂ by absorbing more of it (physical pump).
   • When CO₂ dissolves in water, it forms carbonic acid, leading to Ocean Acidification.
   • The problem: Acidification hinders marine life, especially organisms that build carbonate shells (like corals and plankton), disrupting the biological pump and food chains.
3. Impact on Land (Biosphere and Soils)
   • Warming temperatures stress ecosystems (e.g., changes in biome distribution).
   • Climate shifts increase the frequency of natural hazards like wild fires, further releasing stored carbon.
   • Melting permafrost in the cryosphere/lithosphere releases old, long-stored carbon and methane, creating a positive feedback loop (warming causes release, release causes more warming).

5. Human Intervention and Mitigation

Human interventions are strategies designed to influence carbon transfers and mitigate the impacts of climate change by trying to balance the carbon budget once again.

Strategies to Influence Carbon Transfers

These strategies fall into two main types: reducing inputs (flows into the atmosphere) and increasing outputs (flows out of the atmosphere, often via sequestration).

1. Reducing Inputs (Tackling the Source)
   • Energy Shifts: Switching from fossil fuels (lithosphere burning) to renewable energy sources (solar, wind).
   • Carbon Taxes/Legislation: Policies to make burning fossil fuels more expensive, discouraging combustion.
   • Improving Transport: Promoting electric vehicles and public transit to reduce hydrocarbon consumption.
2. Increasing Outputs (Enhancing Sinks)
   • Afforestation/Reforestation: Planting new forests or restoring old ones. This increases the biosphere store and enhances the flow of photosynthesis, drawing CO₂ out of the atmosphere.
   • Land Management: Sustainable farming practices (like no-till farming) that keep soil carbon locked away instead of releasing it.
   • Carbon Capture and Storage (CCS): Technological solutions where CO₂ is captured directly from industrial sources (e.g., power plants) and pumped deep underground into geological formations (a form of artificial sequestration into the lithosphere).

Final Key Takeaway: The carbon cycle is deeply interconnected with the water cycle and climate (3.3.1.4). Understanding this system is vital because our reliance on fossil fuels has triggered a massive imbalance, making climate mitigation one of the most pressing geographical challenges today.