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Spec mapping (AQA 7037): Paper 1, §3.1.1 — Stores and fluxes of the carbon cycle: detailed examination of each store; the processes of sequestration, photosynthesis, respiration, decomposition; the oceanic carbon pumps; weathering. Synoptic links: ocean acidification connects to marine ecosystems and coasts; the soil/peat store couples to the water cycle (Lesson 9); fossil-fuel formation links to geological time. AOs: AO1 (store sizes, flux mechanisms), AO2 (relating sequestration to biome and ocean processes), AO3 (interpreting a carbon-budget table, calculating an airborne fraction).
Building on the overview in Lesson 6, this lesson dissects each of the four major carbon stores — the lithosphere, atmosphere, hydrosphere, and biosphere — and the flows (fluxes) that connect them, together with the factors that set the rate and direction of each transfer. Two ideas frame everything: (1) a store is an amount (GtC) whereas a flux is a rate (GtC yr⁻¹), and (2) whether a store acts as a sink (net carbon in) or a source (net carbon out) depends on the balance of opposing fluxes — a balance that human activity is now tipping. Keeping these two ideas in mind prevents the most common errors in this part of the topic: confusing the size of a store with the rate at which carbon moves through it, and assuming a store is "fixed" when in fact it is dynamically maintained by two opposing fluxes that can fall out of balance. Quantifying stores and fluxes is the prerequisite for evaluating human impacts (Lesson 8) and the cycles' coupling (Lesson 9).
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.
This is worth pausing on, because it shows the carbon cycle operating as a rock-forming process driven by life. Marine organisms — coccolithophores, foraminifera, corals, molluscs — extract dissolved carbon and calcium from seawater to build calcium carbonate (CaCO₃) shells and skeletons. When they die, these settle on the seafloor and, over millions of years, are compacted and cemented into limestone and chalk. Vast thicknesses of the Earth's sedimentary record are, in effect, fossilised carbon that was once dissolved in the ocean and ultimately drawn from the atmosphere. This biological burial is the principal way the slow carbon cycle removes carbon from the active system over geological time, and it is why the lithosphere holds ~10⁷ GtC. The reverse flux — the return of this carbon to the atmosphere — occurs slowly through the chemical weathering of exposed carbonate and the metamorphic and volcanic release of CO₂ where carbonate rocks are heated or subducted. Over hundreds of millions of years these opposing fluxes have stayed roughly in balance, which is why atmospheric CO₂ has remained within habitable bounds despite the colossal size of the rock store.
| 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 |
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.
It helps to see fossil fuels as ancient, concentrated solar energy and carbon. Each lump of coal or barrel of oil is the product of photosynthesis that fixed atmospheric CO₂ into living tissue tens to hundreds of millions of years ago — Carboniferous swamp forests for coal, plankton-rich ancient seas for oil and gas. Crucially, this organic carbon escaped the normal fast-cycle fate of being respired or decomposed back to CO₂; instead it was buried before it could decompose, usually in oxygen-poor (anoxic) conditions, and then compressed and heated over geological time into concentrated hydrocarbons. In effect, the slow burial of this carbon over hundreds of millions of years gradually drew down atmospheric CO₂ and built up the lithospheric store. Combustion reverses that entire history in an instant of geological time: it re-oxidises the buried carbon back to CO₂, returning to the fast atmospheric store the carbon that the slow cycle painstakingly removed. This framing makes the temporal mismatch vivid — we are unburying, and re-releasing in centuries, the carbon that the biosphere took aeons to sequester.
The atmosphere contains approximately 870 GtC (as of 2023), predominantly as carbon dioxide (CO₂) but also as methane (CH₄) and other trace gases.
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."
The atmospheric store, then, is best understood as a small, fast-turnover reservoir through which carbon flows rapidly — it exchanges roughly a quarter of its entire content with the biosphere and ocean each year. This rapid exchange is why the seasonal sawtooth is visible: the biosphere's annual cycle of photosynthesis and respiration is large enough, relative to the small atmospheric store, to move its concentration measurably within months. It is also why the atmosphere responds so quickly to changes in the balance of its inputs and outputs. Under pre-industrial conditions, inputs (respiration, decomposition, ocean and volcanic outgassing) and outputs (photosynthesis, ocean uptake, weathering) were balanced, holding the store near 280 ppm. Human emissions added a persistent extra input with no matching output, and because the store is small, even ~5 GtC of annual surplus accumulates into a rapid, measurable rise — the upward trend of the Keeling Curve. The atmospheric store thus behaves like a small bath with the taps and drain nearly balanced: open one extra tap (fossil-fuel CO₂) and the water level climbs steadily, even though the flow is modest compared with the great circulating fluxes.
The contrast between CO₂ and CH₄ has important management implications. Because methane is so potent but so short-lived, cutting methane emissions delivers rapid climate benefit — the gas is removed from the atmosphere within a couple of decades, so reductions show up quickly in slower warming. CO₂, by contrast, persists for centuries to millennia, so a fraction of every tonne emitted today will still be warming the planet in the year 3000. This is why some argue that methane cuts (from leak repair, agriculture, and landfill) are the most effective near-term lever, while CO₂ cuts are indispensable for the long-term climate. The choice of time horizon (20-year vs 100-year Global Warming Potential) therefore materially changes how we weigh the two gases — a subtle but examinable point about how the carbon cycle interfaces with policy.
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.
The physics is worth stating precisely, because it underlies the entire climate problem. Incoming solar radiation is mostly shortwave (peaking in the visible), to which the atmosphere is largely transparent — so sunlight reaches and warms the surface. The warmed surface re-radiates energy as longwave (infrared) radiation. Greenhouse-gas molecules — CO₂, CH₄, H₂O, N₂O — have molecular vibrations that absorb specifically at infrared wavelengths; they absorb this outgoing longwave radiation and re-emit it in all directions, including back downward. This traps energy in the lower atmosphere, raising surface temperature above what it would otherwise be. Adding more greenhouse gas absorbs a larger fraction of the outgoing longwave radiation and raises the effective emitting altitude, forcing the surface to warm to restore the planet's radiation balance. Because CO₂ is well-mixed and long-lived, a small change in its concentration (parts per million) has a global, persistent effect — which is exactly why the tiny atmospheric carbon store wields such disproportionate climatic power.
The hydrosphere (primarily the oceans) holds approximately 38,000 GtC of dissolved inorganic carbon, making it the largest active (non-lithospheric) carbon store.
| 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 |
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.
Key controls:
The ocean currently absorbs approximately 2.5 GtC/year of anthropogenic CO₂ — about 25% of total human emissions (Friedlingstein et al., 2023). This oceanic uptake is a crucial carbon sink, but it comes at a cost: ocean acidification.
When CO₂ dissolves in seawater, it forms carbonic acid:
CO₂ + H₂O → H₂CO₃ → H⁺ + HCO₃⁻
The release of hydrogen ions (H⁺) lowers the ocean's pH. Since the pre-industrial era, average ocean pH has declined from ~8.2 to ~8.1 — a 26% increase in acidity (because pH is a logarithmic scale).
Consequences:
The ocean sink illustrates a recurring theme of the carbon cycle: the same process can be both beneficial and harmful. By absorbing ~25% of human emissions, the ocean has substantially slowed the rise of atmospheric CO₂ and the pace of climate change — without it, warming would already be far worse. But this service comes at the cost of acidification, which threatens the calcifying organisms (corals, shellfish, coccolithophores) at the base of marine food webs and, by some estimates, the security of fisheries on which hundreds of millions of people depend. Moreover, the sink is self-limiting: as surface water absorbs CO₂, its chemistry changes (the Revelle factor rises) so that each additional tonne is absorbed less readily, and warming further reduces solubility and strengthens the stratification that slows the export of carbon to depth. The ocean has therefore done humanity an enormous, under-appreciated favour — but it is a favour with both an ecological price and a finite limit, which is precisely why projecting the future strength of the ocean sink is one of the central uncertainties in climate science.
The biosphere stores approximately 2,060 GtC — split between soil organic matter (~1,500 GtC) and living vegetation (~560 GtC).
Carbon is stored in plant biomass: trunks, branches, leaves, and roots. The amount varies enormously by biome:
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