The Global Carbon Cycle

Spec mapping (AQA 7037): Paper 1, §3.1.1 — The global carbon cycle: the major stores of carbon (lithosphere, hydrosphere, biosphere, atmosphere) and their sizes; factors driving change in their magnitude; the fast (biological) and slow (geological) carbon cycles. Synoptic links: the greenhouse effect connects to climate change and to §3.1.5 Hazards (volcanic outgassing); the carbon cycle couples to the water cycle through weathering, photosynthesis, and ocean circulation (Lesson 9). AOs: AO1 (stores, fluxes, processes), AO2 (relating store size to climatic importance), AO3 (interpreting a carbon-budget table or the Keeling Curve).

Carbon is the fundamental building block of life and, as carbon dioxide and methane, the master control on Earth's climate. The global carbon cycle describes the movement of carbon between four major stores — the lithosphere (rocks and fossil fuels), the hydrosphere (oceans), the biosphere (living organisms and soil), and the atmosphere — through biological, chemical, and physical processes. Like the water cycle, the global carbon cycle is effectively a closed system at the planetary scale: carbon atoms are conserved (neither created nor destroyed), but continuously redistributed between stores, with solar energy driving the transfers. The same systems vocabulary applies — stores, flows, feedback, dynamic equilibrium — and understanding the cycle is the foundation for explaining anthropogenic climate change, the defining environmental issue of the era.

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Carbon Stores

The four major stores vary enormously in size — by more than five orders of magnitude — which is itself geographically significant: the smallest store (the atmosphere) exerts the largest climatic leverage.

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 carbon in sedimentary rocks for millions of years. Yet the atmospheric store, though minuscule by comparison (~870 GtC, < 0.01% of the total), is the most climatically important: even small changes in atmospheric CO₂ drive large changes in global temperature via the greenhouse effect. A useful way to express this is that the cycle's leverage is inversely related to store size.

Residence Times and the Fast/Slow Distinction

As with water, residence time (store ÷ flux) determines how quickly each store responds. This sorts the cycle into two coupled sub-cycles:

Store	Approx. residence time	Sub-cycle
Atmosphere	~5 years (exchanges fast with biosphere/ocean)	Fast
Terrestrial vegetation	years to decades	Fast
Surface ocean	~10 years	Fast
Soil organic matter	decades to centuries	Fast (slowing)
Deep ocean	~350–1,000+ years	Intermediate
Fossil fuels	millions of years	Slow
Sedimentary carbonate rock	~150–200 million years	Slow

The fast (biological) carbon cycle moves carbon between the atmosphere, biosphere, and surface ocean over days to millennia; the slow (geological) carbon cycle moves it through rocks and fossil fuels over millions of years. The anthropogenic problem is fundamentally a coupling problem: humans are using combustion to transfer carbon from the slow store (fossil fuels) into the fast store (atmosphere) far quicker than the slow cycle can return it — overwhelming the dynamic equilibrium that held atmospheric CO₂ near 280 ppm for millennia.

The Closed System and Dynamic Equilibrium

Over the pre-industrial Holocene the carbon cycle sat in dynamic equilibrium: atmosphere–biosphere exchange (photosynthesis ~120 GtC yr⁻¹ in, respiration + decomposition ~119 GtC yr⁻¹ out) and atmosphere–ocean exchange (~90 GtC yr⁻¹ each way) were balanced, so the atmospheric store stayed constant despite enormous throughput. Treating the cycle as a closed system at the global scale is valid because carbon atoms do not leave the planet; what changes is the partitioning between stores — exactly the framework established in Lesson 1.

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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 1/40th to 1/100th of current anthropogenic emissions (~9.5 GtC/year).
• Over geological time, volcanic outgassing has been a critical source of atmospheric CO₂, maintaining temperatures above freezing and replenishing the carbon that weathering and burial steadily remove.

The contrast between volcanic and human fluxes is one of the most important quantitative points in the whole topic, because it is frequently misrepresented in public debate. Although large eruptions are spectacular, the steady background degassing of all the world's volcanoes and mid-ocean ridges together releases only a fraction of a gigatonne of carbon per year. Humans, by contrast, release roughly 40–100 times more CO₂ every year through fossil-fuel combustion. The geological record reinforces this: episodes of massive volcanic outgassing — the Siberian Traps at the end of the Permian (~252 million years ago) and the Deccan Traps around the end-Cretaceous — coincided with severe global warming and mass extinctions, but they unfolded over hundreds of thousands of years, far more slowly than the present human-driven rise. The lesson is that it is the rate as much as the amount of carbon transfer that matters: ecosystems and the slow carbon cycle can adjust to gradual change, but not to the geologically abrupt pulse humans are now imposing.

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. Note that this is a point of deep coupling with the water cycle: the weathering reaction needs liquid water both as a reactant (forming carbonic acid) and as the transport medium (rivers carrying bicarbonate to the sea), so the rate of this slow-cycle carbon transfer is set by the hydrological cycle.

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.

The thermostat may also help explain deep-time climate puzzles. The uplift-weathering hypothesis (Raymo & Ruddiman, 1992) proposes that the uplift of the Himalaya and Tibetan Plateau over the last ~40 million years exposed vast areas of fresh silicate rock to weathering, drawing down atmospheric CO₂ and contributing to the long-term Cenozoic cooling that led into the recent ice ages. Whether or not this specific mechanism dominated, it illustrates the principle the specification wants you to grasp: over geological time, the slow carbon cycle — mediated by tectonics, weathering, and the water cycle — has repeatedly reset Earth's climate, and the silicate thermostat is the negative feedback that has kept the planet broadly habitable for billions of years despite a steadily brightening Sun.

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.

Tectonics is what closes the loop of the slow carbon cycle, and it is a genuine synoptic link to the plate-tectonic theory underpinning §3.1.5 Hazards. Without subduction returning buried carbonate carbon to the mantle, and without volcanism degassing it back to the atmosphere, the weathering and burial fluxes would steadily strip CO₂ from the atmosphere until photosynthesis — and hence life — failed. The geological carbon cycle is therefore a tectonically-driven conveyor: weathering and biological burial remove atmospheric CO₂ and lock it into rock; plate tectonics carries that rock down into the Earth; and volcanism returns the carbon to the atmosphere, ready to begin again. That this conveyor has kept atmospheric CO₂ within habitable limits for billions of years, despite a Sun that has brightened by ~30% over that time, is one of the most remarkable features of the Earth system — and a reminder that the carbon cycle operates on timescales stretching from days (a single breath) to the age of the planet itself.

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

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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

• Terrestrial photosynthesis fixes approximately 120 GtC/year from the atmosphere.
• Marine photosynthesis (by phytoplankton) fixes approximately 50 GtC/year.
• These represent the primary biological pathway by which carbon moves from the atmosphere to the biosphere.

Respiration

Key Definition: Respiration is the metabolic process by which organisms release energy from organic compounds, returning CO₂ to the atmosphere. It is, in effect, the chemical reverse of photosynthesis.

C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + energy

• All living organisms respire — plants, animals, fungi, and microorganisms.
• Soil respiration (by bacteria, fungi, and root systems) is a major flux: approximately 60 GtC/year from soils globally (Bond-Lamberty & Thomson, 2010).
• Under natural (pre-industrial) conditions, photosynthesis and respiration were broadly in balance.

Decomposition

When organisms die, their organic matter is broken down by decomposers (bacteria and fungi). The rate of decomposition depends on:

Factor	Effect on Decomposition Rate
Temperature	Higher → faster (up to ~40°C, beyond which enzymes denature)
Moisture	Moderate moisture → fastest; waterlogged conditions → anaerobic decomposition → slower, may produce CH₄
Oxygen availability	Aerobic decomposition is ~10× faster than anaerobic
Soil pH	Acidic conditions slow decomposition (fewer decomposer organisms)
Carbon-to-nitrogen ratio	Low C:N ratio (e.g., grass) → fast decomposition; High C:N ratio (e.g., wood) → slow

The Carbon Pathway Through an Ecosystem

It is worth tracing carbon's journey through a terrestrial ecosystem as a sequence of coupled fluxes, because this is what makes a biome a store with a particular turnover:

1. Fixation: photosynthesis converts atmospheric CO₂ into glucose and then into plant tissue (Gross Primary Productivity, GPP).
2. Autotrophic respiration: the plant respires part of this back to the atmosphere to power its own metabolism; what remains as new biomass is Net Primary Productivity (NPP).
3. Litterfall and root exudates: dead leaves, branches, and roots transfer carbon to the soil/litter store.
4. Decomposition and heterotrophic respiration: soil bacteria, fungi, and detritivores break down dead organic matter, respiring much of it back to the atmosphere; the rest accumulates as stable soil organic carbon (humus) or, in waterlogged conditions, as peat.
5. Disturbance: fire, harvest, or land-use change can short-circuit this slow cycle and return carbon rapidly to the atmosphere.

The balance of fixation against the various respiration and disturbance losses determines whether the ecosystem is a net sink (accumulating carbon) or net source (losing it) — a distinction developed fully in Lesson 7.

Net Primary Productivity (NPP)

Key Definition: Net Primary Productivity (NPP) is the rate at which plants produce biomass (organic matter) after accounting for their own respiratory losses. NPP = Gross Primary Productivity (GPP) − Plant Respiration.

Biome	Approximate NPP (gC/m²/year)
Tropical rainforest	900–2,200
Temperate forest	600–1,200
Boreal forest (taiga)	200–600
Tropical savanna	200–2,000
Temperate grassland	200–1,500
Tundra	10–400
Desert	0–200
Open ocean	2–400
Continental shelf	200–600

Source: Whittaker (1975), updated estimates

The geographical pattern in NPP is itself a story of the coupling between the water and carbon cycles (Lesson 9): productivity is highest where both warmth and water are abundant (the tropical rainforest, ~900–2,200 gC m⁻² yr⁻¹) and lowest where either is limiting (deserts lack water; tundra lacks warmth; the open ocean lacks nutrients). This is why NPP maps so closely onto climate, and why changing the water cycle (drought, altered rainfall) directly changes the carbon cycle through productivity. It also means the largest terrestrial carbon-fixing flux on the planet — tropical-forest photosynthesis — depends on rainfall that the forest itself partly generates, a feedback that makes the tropics both the most productive and the most vulnerable biome.

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The Oceanic Carbon Pump

The ocean is the second-largest active carbon store (~38,000 GtC) and absorbs approximately 2.5 GtC/year of anthropogenic CO₂ (Friedlingstein et al., 2023).

The Solubility Pump