You are viewing a free preview of this lesson.
Subscribe to unlock all 10 lessons in this course and every other course on LearningBro.
Spec mapping: AQA 7037, Paper 1 (Physical), §3.1.1 — the carbon cycle as a closed system at the global scale, its major stores and the fluxes (sequestration, combustion, photosynthesis, respiration, decomposition, weathering) that link them, distinguishing the fast and slow cycles. This depth lesson assumes the basic store/flux vocabulary and pushes into quantified stores in GtC, the fast/slow flux budget, the biological, carbonate and solubility pumps, and lithospheric cycling. AOs exercised: AO1 (precise store sizes, the carbonate–silicate weathering chemistry, the ocean pumps), AO2 (explaining why the small atmospheric store is so climatically sensitive; how the pumps regulate atmospheric CO₂), AO3 (manipulating a GtC flux table; computing residence times and net sinks). Synoptic links run to Ecosystems/biomes (NPP), Hazards (volcanic outgassing) and Global systems (the international politics of emissions).
Carbon is the backbone of all organic matter and, as CO₂ and CH₄, the principal long-lived greenhouse-gas regulator of Earth's climate. At the global scale the carbon cycle is a closed system: carbon is conserved, merely transferred between stores over timescales from seconds (a leaf fixing CO₂) to hundreds of millions of years (limestone burial). The central insight of a depth treatment is that the cycle operates as two coupled sub-cycles of vastly different speed — a fast biological cycle and a slow geological cycle — and that human activity is, in effect, running the slow cycle in fast-forward by burning in decades what took the slow cycle hundreds of millions of years to bury.
Carbon is held in four great reservoirs. The estimates below follow IPCC AR5/AR6 and the Global Carbon Project; quote them with units (GtC = gigatonnes of carbon = 10⁹ tonnes).
| Component | Carbon (GtC) | Notes |
|---|---|---|
| Sedimentary carbonates | ~60,000,000 | Limestone (CaCO₃), dolomite; the dominant Earth store |
| Kerogen / organic sediments | ~15,000,000 | Dispersed organic carbon in shales etc. |
| Fossil fuels | ~4,000–5,000 | Coal, oil, gas; accumulated over >300 Myr |
| Component | Carbon (GtC) | Notes |
|---|---|---|
| Deep ocean (DIC) | ~37,000 | Dissolved inorganic carbon; slow turnover (centuries–millennia) |
| Surface ocean | ~900–1,000 | Exchanges CO₂ with the atmosphere; solubility temperature-dependent |
| Dissolved organic carbon | ~700 | Dissolved dead organic matter |
| Marine biota | ~3 | Tiny store but drives a huge flux (the biological pump) |
| Component | Carbon (GtC) | Notes |
|---|---|---|
| CO₂ | ~880 (≈420 ppm, 2020s) | Up from ~590 GtC (~280 ppm) pre-industrial |
| CH₄ | ~5 | Smaller store; GWP-20 ≈ 80× CO₂ |
| Component | Carbon (GtC) | Notes |
|---|---|---|
| Soil organic carbon (to ~1–2 m) | ~1,500–2,400 | Largest terrestrial store; humus, litter, peat |
| Vegetation (biomass) | ~450–650 | Tropical forests hold ~50–55% of vegetation C |
| Permafrost | ~1,400–1,700 | Frozen organic matter; thaw-vulnerable |
Key point: The atmosphere holds only ~880 GtC — roughly 0.0015% of the lithospheric store and far less than the soil store. Because it is so small, comparatively modest changes in the fluxes feeding or draining it cause large fractional changes in atmospheric CO₂ and hence in climate. The annual fossil-fuel flux (~10 GtC/yr) is over 1% of the entire atmospheric store every year — which is why concentration is rising so fast.
A unifying systems point: the ocean and the biosphere exchange enormous fluxes with the atmosphere each year (≈80 and ≈120 GtC respectively), yet the net exchange is small (a few GtC). The atmosphere's CO₂ is therefore the small residual of two huge, nearly balanced gross flows. Anthropogenic emissions of ~10 GtC/yr are tiny beside the gross fluxes but, being a one-way addition that the natural cycle cannot fully offset, they accumulate — about half remaining airborne. Recognising that climate change is a problem of a small imbalance in a large throughput is exactly the kind of AO2 framing examiners reward.
The fast cycle operates over days to a few thousand years, exchanging carbon between atmosphere, biosphere, soils and surface ocean.
| Flux | Rate (GtC/yr) | Direction |
|---|---|---|
| Terrestrial photosynthesis (GPP) | ~120 | Atmosphere → biosphere |
| Terrestrial respiration + decomposition | ~118–119 | Biosphere → atmosphere |
| Ocean–atmosphere absorption | ~80 | Atmosphere → ocean |
| Ocean–atmosphere release | ~78 | Ocean → atmosphere |
| Net ocean uptake | ~2.5–3 | Atmosphere → ocean (excess CO₂) |
| Net land uptake | ~1.5–3 | Atmosphere → biosphere (CO₂ fertilisation, regrowth) |
Pre-industrially the fast cycle was close to balance. Today the ocean and land act as net sinks, together removing roughly half of anthropogenic emissions — the single most important fact in the carbon budget, because it both slows warming now and represents a service that may weaken in future.
6CO2+6H2OlightC6H12O6+6O2
Respiration is essentially the reverse, releasing the stored energy:
C6H12O6+6O2→6CO2+6H2O.
The balance between fixation and release determines whether an ecosystem is a net sink or source, developed quantitatively (GPP, NPP, NEP) in the next lesson.
flowchart LR
ATM[Atmosphere ~880 GtC] -->|photosynthesis ~120| BIO[Vegetation ~500 GtC]
BIO -->|litterfall| SOIL[Soils ~1500-2400 GtC]
BIO -->|autotrophic respiration| ATM
SOIL -->|decomposition / heterotrophic respiration| ATM
ATM -->|absorption ~80| SOC[Surface ocean ~900 GtC]
SOC -->|release ~78| ATM
SOC -->|biological + solubility pumps| DEEP[Deep ocean ~37000 GtC]
The slow cycle operates over millions to hundreds of millions of years, exchanging carbon between atmosphere, ocean and lithosphere. Its fluxes are tiny (~0.1–0.3 GtC/yr) but, integrated over geological time, they have set Earth's long-term climate.
Silicate (chemical) weathering — carbonic acid, formed when atmospheric CO₂ dissolves in rainwater, weathers silicate minerals, consuming CO₂:
CaSiO3+2CO2+3H2O→Ca2++2HCO3−+H4SiO4.
The dissolved calcium and bicarbonate are carried to the sea.
Carbonate formation and burial — marine organisms (foraminifera, coccolithophores, corals) precipitate calcium carbonate:
Ca2++2HCO3−→CaCO3+CO2+H2O.
Their shells sink and accumulate as limestone, locking carbon away for millions of years. (Note that calcification releases one CO₂ per CaCO₃ precipitated, but the dominant long-term effect of the weathering→burial pathway is net CO₂ drawdown.)
Volcanic outgassing — subduction of carbonate-bearing oceanic crust returns CO₂ to the atmosphere via volcanism, at ~0.1–0.3 GtC/yr — roughly 100× less than current anthropogenic emissions (~10 GtC/yr). (This single comparison decisively rebuts the "volcanoes emit more CO₂ than humans" misconception.)
Fossil-fuel formation — incomplete decay of buried organic matter under heat and pressure makes coal, oil and gas over hundreds of millions of years, removing carbon from the active cycle — the store humans are now rapidly reversing.
The weathering thermostat: Because silicate weathering speeds up in a warmer, wetter climate, it acts as a slow negative feedback stabilising Earth's CO₂ over geological time. Crucially, it operates over 10⁴–10⁶ years and so is far too slow to counteract anthropogenic emissions — a key evaluative point in feedback and management questions.
The ocean holds ~50× more carbon than the atmosphere and absorbs ~25–30% of anthropogenic emissions. Three "pumps" move carbon from surface to depth.
The biological pump transfers on the order of ~10 GtC/yr of organic carbon out of the surface layer. Without it, model estimates suggest atmospheric CO₂ would be roughly 200 ppm higher — it is a major reason the pre-industrial atmosphere was habitable at ~280 ppm rather than far higher.
Calcifying organisms export CaCO₃ to the deep sea and sediments. This pump interacts with ocean chemistry and is directly threatened by ocean acidification (developed in the next lesson), which lowers the carbonate-ion saturation organisms need to build shells.
flowchart TB
SURF[Surface ocean: CO2 absorbed] -->|biological pump:
sinking marine snow| MID[Mid/deep water:
remineralisation -> DIC]
SURF -->|carbonate pump:
sinking CaCO3| SED[Sea-floor sediments]
SURF -->|solubility pump:
cold dense water sinks| DEEPW[Deep water formation]
MID --> STORE[Deep-ocean store ~37000 GtC]
DEEPW --> STORE
SED --> SLOW[Slow cycle:
limestone burial]
The thermohaline circulation (the "global conveyor") is driven by density contrasts from temperature (thermo) and salinity (haline).
Climate-change link: Freshwater from melting Greenland ice lowers surface density in the NADW formation regions, threatening to weaken the overturning. A weaker circulation slows the solubility pump, reducing the ocean's CO₂ uptake — a positive feedback explored in the feedback lesson.
Using the figures above:
Atmospheric residence time of carbon. With an atmospheric store of ~880 GtC and a gross outflow (photosynthesis + ocean absorption) of ~120 + 80 = 200 GtC/yr:
Tr=FS=200880≈4.4 years.
A given CO₂ molecule is therefore exchanged every few years — but this is not the same as how long an emissions pulse perturbs the climate (which is centuries to millennia, because the perturbation must be removed by the net sink, not the gross exchange). Distinguishing molecular turnover from perturbation lifetime is a sophisticated AO3 point.
Net sink as a fraction of emissions. If fossil-fuel + land-use emissions total ~11 GtC/yr and the ocean and land take up ~3 and ~3 GtC/yr respectively, the airborne fraction is:
1111−(3+3)×100=115×100≈45%.
So roughly 45% of emissions remain airborne and ~55% are absorbed — consistent with the observed ~50% airborne fraction.
Explain and evaluate. The sinks are doing immense work, but they are responses to elevated CO₂ (Henry's-Law uptake, CO₂ fertilisation) and may weaken: a warmer ocean dissolves less CO₂, and stressed/disturbed forests can flip to sources. The flux figures also carry real uncertainty (the land sink in particular is the residual of large, imprecise terms), so quoting them as approximate and noting the risk of sink saturation is what secures top marks.
The single most important dataset in carbon science is the Keeling Curve from Mauna Loa Observatory, Hawaii, where Charles David Keeling began continuous CO₂ measurement in 1958. Mauna Loa was chosen because, at ~3,400 m on a mid-Pacific volcano far from local pollution and vegetation, it samples well-mixed background air representative of the Northern Hemisphere.
Two signals are superimposed in the record. The long-term trend rises inexorably from ~315 ppm (1958) to ~424 ppm (mid-2020s) — the fingerprint of fossil-fuel emissions accumulating in the small atmospheric store. The annual sawtooth of ~5–7 ppm amplitude is the fast carbon cycle made visible: Northern-Hemisphere vegetation draws CO₂ down through spring and summer (photosynthesis exceeds respiration as forests leaf out) and releases it up through autumn and winter (respiration and decomposition dominate while plants are dormant). The Northern Hemisphere dominates this cycle because it holds far more land and vegetation than the Southern. The amplitude is the planet's terrestrial biosphere "breathing" once a year — a direct, observable demonstration of the GPP/respiration balance and of why the net atmospheric concentration is the small residual of two huge opposing fluxes. Deploying the Keeling Curve to evidence both the human trend and the fast-cycle seasonality is a hallmark of a strong carbon answer.
The Amazon and the tropical peat swamps of Southeast Asia illustrate the active carbon store and its vulnerability.
The Amazon rainforest holds on the order of 150–200 GtC in biomass and soils and accounts for a large share of tropical GPP. In an undisturbed state it has been a modest net sink, but it sits close to the source–sink boundary: in drought years (2005, 2010, 2015–16) it has temporarily flipped to a net source as tree mortality and fire overwhelmed uptake. This makes it the clearest example of an active store whose sign (sink or source) depends on conditions — the conceptual heart of the next lesson.
Subscribe to continue reading
Get full access to this lesson and all 10 lessons in this course.