The Global Carbon Cycle

Carbon is the fundamental building block of life on Earth. The global carbon cycle describes the movement of carbon between four major stores — the lithosphere, atmosphere, hydrosphere, and biosphere — through a series of biological, chemical, and physical processes. Like the water cycle, the global carbon cycle is effectively a closed system at the planetary scale: the total amount of carbon on Earth is fixed, but it is continuously redistributed between stores. Understanding the carbon cycle is essential for explaining climate change, one of the most important issues in contemporary geography.

---

Carbon Stores

The four major stores vary enormously in size:

Store	Sub-store	Estimated Carbon (GtC)	% of Total
Lithosphere	Sedimentary rocks (carbonates)	~65,000,000	~99.9%
	Fossil fuels (coal, oil, gas)	~4,130	<0.01%
Hydrosphere	Deep ocean dissolved inorganic carbon	~37,100	~0.06%
	Surface ocean	~900	<0.01%
	Marine biota	~3	negligible
Biosphere	Soil organic matter	~1,500	<0.01%
	Terrestrial vegetation	~560	<0.01%
	Peat	~500	<0.01%
Atmosphere	CO₂ (and CH₄, other gases)	~870 (2023)	<0.01%

Sources: Ciais et al. (2013), IPCC AR5 WG1; Friedlingstein et al. (2023)

Key Point: The lithosphere is by far the largest carbon store, locking up carbon in sedimentary rocks for millions of years. However, the atmospheric store, although tiny by comparison, is critically important because even small changes in atmospheric CO₂ drive major changes in global temperature through the greenhouse effect.

---

Geological Carbon Cycle (Slow Carbon Cycle)

The geological carbon cycle operates over timescales of millions of years. It involves transfers between the lithosphere, atmosphere, and hydrosphere through processes including:

Volcanic Outgassing

• Carbon locked in the mantle and in carbonate rocks is released to the atmosphere as CO₂ during volcanic eruptions and through mid-ocean ridge degassing.
• Annual volcanic CO₂ emissions are estimated at 0.15–0.26 GtC/year (Burton et al., 2013) — approximately 1/100th of current anthropogenic emissions.
• Over geological time, volcanic outgassing has been a critical source of atmospheric CO₂, maintaining temperatures above freezing.

Chemical Weathering

The Urey reaction (Harold Urey, 1952) describes the weathering of silicate rocks by carbonic acid:

CaSiO₃ + 2CO₂ + H₂O → Ca²⁺ + 2HCO₃⁻ + SiO₂

This process:

1. Removes CO₂ from the atmosphere (dissolved in rainwater as carbonic acid, H₂CO₃).
2. Produces dissolved bicarbonate ions (HCO₃⁻), which are transported by rivers to the ocean.
3. In the ocean, marine organisms (foraminifera, coccolithophores, corals) use calcium and bicarbonate to build calcium carbonate (CaCO₃) shells and skeletons.
4. When these organisms die, their shells accumulate on the ocean floor, forming carbonate sediments that eventually lithify into limestone.

This represents a transfer from the atmosphere → hydrosphere → lithosphere, locking carbon away for millions of years.

Key Definition: The silicate weathering thermostat (Walker et al., 1981) is a negative feedback mechanism: higher temperatures → more precipitation → more chemical weathering → more CO₂ removed from atmosphere → temperatures fall. This is believed to have regulated Earth's climate over geological timescales.

Tectonic Processes

• Subduction — oceanic plates carrying carbonate sediments are subducted at convergent plate boundaries. The carbon is eventually released back to the atmosphere through volcanic outgassing — completing the geological cycle.
• Metamorphism — heat and pressure in the Earth's crust can release CO₂ from carbonate rocks.

graph TD
    subgraph "Geological Carbon Cycle (millions of years)"
        V["Volcanic Outgassing
0.15-0.26 GtC/yr"] -->|"CO₂ released"| ATM["Atmosphere
~870 GtC"]
        ATM -->|"CO₂ dissolves in rain
(carbonic acid)"| W["Chemical Weathering
of silicate rocks"]
        W -->|"Dissolved HCO₃⁻
carried by rivers"| OC["Oceans
~38,000 GtC"]
        OC -->|"Marine organisms
build CaCO₃ shells"| SED["Seafloor Sediments"]
        SED -->|"Lithification
(millions of years)"| LITH["Limestone
~65,000,000 GtC"]
        LITH -->|"Subduction at
plate boundaries"| MANTLE["Mantle"]
        MANTLE -->|"Melting &
degassing"| V
    end

---

Biological Carbon Cycle (Fast Carbon Cycle)

The biological carbon cycle operates over timescales of days to thousands of years, involving living organisms, soils, the atmosphere, and the surface ocean.

Photosynthesis

Key Definition: Photosynthesis is the process by which green plants, algae, and cyanobacteria convert atmospheric CO₂ and water into glucose (C₆H₁₂O₆) and oxygen, using light energy.

6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂