Carbon Stores and Flows

Building on the overview of the global carbon cycle, this lesson examines each of the four major carbon stores — the lithosphere, atmosphere, hydrosphere, and biosphere — in greater detail, analysing the flows (fluxes) between them and the factors that control the rate and direction of carbon transfers. Understanding the magnitude of stores and the speed of flows is essential for evaluating human impacts on the carbon cycle and predicting future climate change trajectories.

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The Lithospheric Carbon Store

The lithosphere is overwhelmingly the largest carbon store on Earth, containing approximately 65,000,000 GtC in sedimentary rocks alone. This carbon is effectively locked away from the active carbon cycle for millions of years.

Carbonate Rocks

• Limestone (CaCO₃) and dolomite (CaMg(CO₃)₂) are the most abundant carbon-bearing rocks.
• They form from the accumulation of marine organisms' shells and skeletons (foraminifera, coccolithophores, corals) on the ocean floor, which are then compacted and lithified.
• The White Cliffs of Dover are composed of Cretaceous chalk — essentially the compressed remains of billions of coccolithophores that lived ~66–100 million years ago.
• Carbon in limestone has a residence time of 150–200 million years before being recycled through subduction and volcanic outgassing.

Fossil Fuels

Fuel	Formation	Estimated Carbon Store (GtC)
Coal	From tropical swamp forests buried 300–360 million years ago (Carboniferous Period); compressed and heated to form peat → lignite → bituminous coal → anthracite	~3,510
Oil (petroleum)	From marine microorganisms (phytoplankton, zooplankton) buried in anoxic ocean sediments; heated under pressure over millions of years (source rock → migration → trap)	~230
Natural gas	Similar origin to oil; lighter hydrocarbons (mainly methane, CH₄) formed at higher temperatures or through methanogenesis	~390

• Fossil fuels represent ancient photosynthetically-fixed carbon that was buried before it could be decomposed and returned to the atmosphere.
• Human extraction and combustion of fossil fuels transfers this carbon from the lithosphere to the atmosphere at a rate far exceeding natural geological flows.

Exam Tip: When discussing fossil fuels in the context of the carbon cycle, emphasise that they represent a geological store of carbon that was sequestered over hundreds of millions of years, but which humans are releasing to the atmosphere in just a few centuries. This temporal mismatch is the fundamental cause of anthropogenic climate change.

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The Atmospheric Carbon Store

The atmosphere contains approximately 870 GtC (as of 2023), predominantly as carbon dioxide (CO₂) but also as methane (CH₄) and other trace gases.

Carbon Dioxide (CO₂)

• Pre-industrial atmospheric CO₂ concentration: ~280 ppm (parts per million).
• Current (2024) concentration: ~424 ppm — a 51% increase.
• Annual rate of increase: approximately 2.5 ppm/year (accelerating from ~1 ppm/year in the 1960s).
• CO₂ has a long atmospheric residence time (~100–1,000 years for full removal).

The Keeling Curve, established by Charles David Keeling at the Mauna Loa Observatory in Hawaii (1958–present), provides the iconic record of rising atmospheric CO₂. It also reveals a distinctive seasonal oscillation: CO₂ drops ~6 ppm each Northern Hemisphere summer (photosynthesis draws down CO₂) and rises each winter (respiration and decomposition dominate). This oscillation is sometimes called the "breathing of the biosphere."

Methane (CH₄)

• Current atmospheric concentration: ~1,900 ppb (parts per billion) — more than doubled since pre-industrial times.
• Methane is approximately 80 times more potent as a greenhouse gas than CO₂ over a 20-year period (GWP₂₀ = 80), but has a much shorter atmospheric residence time (~12 years).
• Sources include: wetlands, rice paddies, livestock (enteric fermentation), landfill, natural gas leaks, permafrost thaw, and termites.

The Greenhouse Effect

graph TD
    SUN["Incoming Solar Radiation
(shortwave, ~342 W/m²)"] --> SURF["Earth’s Surface"]
    SUN -->|"~30% reflected
(albedo)"| SPACE["Space"]
    SURF -->|"Longwave (infrared)
radiation emitted"| GHG["Greenhouse Gases
(CO₂, CH₄, H₂O, N₂O)"]
    GHG -->|"Absorb and
re-emit in all
directions"| SURF
    GHG -->|"Some escapes
to space"| SPACE
    SURF -.->|"Enhanced greenhouse
effect: more GHGs
trap more heat"| WARM["Surface Warming"]

The natural greenhouse effect raises Earth's average surface temperature from approximately −18°C (without an atmosphere) to +15°C — a 33°C warming that makes life possible. The enhanced greenhouse effect refers to additional warming caused by anthropogenic increases in greenhouse gas concentrations.

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The Hydrospheric Carbon Store

The hydrosphere (primarily the oceans) holds approximately 38,000 GtC of dissolved inorganic carbon, making it the largest active (non-lithospheric) carbon store.

Forms of Carbon in the Ocean

Form	Description	Approximate Amount
Dissolved inorganic carbon (DIC)	CO₂, bicarbonate (HCO₃⁻), carbonate (CO₃²⁻) ions	~37,100 GtC
Dissolved organic carbon (DOC)	Organic molecules produced by marine organisms	~700 GtC
Particulate organic carbon (POC)	Dead organisms, faecal pellets sinking as marine snow	~30 GtC
Marine biota	Carbon in living marine organisms	~3 GtC

Ocean-Atmosphere CO₂ Exchange

The exchange of CO₂ between the ocean and atmosphere is governed by Henry's Law: the solubility of a gas in a liquid is proportional to its partial pressure in the atmosphere above the liquid.