The Carbon Cycle Explained — Reservoirs, Fluxes and Climate

The Five Reservoirs

Carbon is stored in five main compartments. Their sizes differ by many orders of magnitude, and their turnover times range from days to millions of years:

Reservoir	Carbon stock (GtC)	Turnover time	Key form
Atmosphere	~870	Years–decades	CO₂ (~420 ppm in 2024), CH₄
Terrestrial biosphere	~2,600	Years–centuries	Organic C in plants (550) + soils (2,050)
Ocean (surface)	~900	Decades	Dissolved inorganic C (DIC), DOC
Ocean (deep)	~37,000	Centuries–millennia	Dissolved CO₂, HCO₃⁻, CO₃²⁻
Lithosphere	~66,000,000	Millions of years	Carbonate rock (limestone), coal, oil, gas

The key insight: the atmosphere holds only ~870 GtC — less than the annual plant biomass exchange, and a tiny fraction of the ocean and rock reservoirs. The atmosphere is thin and sensitive; small net transfers in or out change atmospheric CO₂ rapidly on geological timescales.

The Fast Carbon Cycle: Land and Biosphere

The fast cycle operates on timescales of years to centuries, driven by biology:

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Photosynthesis (GPP): Plants and phytoplankton absorb ~120 GtC/yr from the atmosphere, converting CO₂ + H₂O + sunlight into organic molecules (sugars, cellulose, lignin). This is the largest annual flux of carbon on Earth.

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Respiration: Plants themselves respire ~60 GtC/yr back to the atmosphere. Soil microbes and animals respire the rest (~60 GtC/yr). Over a full year on an undisturbed planet, the biosphere was approximately balanced.

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Fire: Wildfire returns ~2–3 GtC/yr to the atmosphere rapidly. A fraction of charred carbon becomes pyrogenic carbon (biochar) that persists in soils for thousands of years.

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Soil organic carbon: Dead plant matter accumulates in soil (humus, peat). Soil holds ~3× more carbon than living vegetation. Warming accelerates microbial respiration of soil carbon — a critical positive feedback.

Photosynthesis: 6 CO₂ + 6 H₂O + sunlight → C₆H₁₂O₆ + 6 O₂ Respiration: C₆H₁₂O₆ + 6 O₂ → 6 CO₂ + 6 H₂O + energy Net Primary Productivity (NPP) = GPP − plant respiration ≈ 60 GtC/yr (the carbon available to consumers, decomposers, and long-term storage)

The Ocean Carbon Cycle

The ocean is the planet's largest active carbon buffer. It absorbs and releases CO₂ depending on the partial pressure difference between the surface water and the atmosphere (solubility pump):

Cold surfaces dissolve more CO₂ (cold water → CO₂ sinks at high latitudes).
Warm surfaces release CO₂ (warm water → CO₂ outgasses at tropics).

The biological pump reinforces carbon storage: phytoplankton fix ~50 GtC/yr via photosynthesis in the sunlit zone. When they die, a fraction sinks as "marine snow" (organic particles and shells) into the deep ocean — sequestering carbon for centuries to millennia before it is remixed by thermohaline circulation.

The ocean's role in the current crisis: Since pre-industrial times the ocean has absorbed roughly 30% of all human CO₂ emissions — approximately 170 GtC of the ~650 GtC emitted. Without this absorption, atmospheric CO₂ would be ~85 ppm higher than it is today.

The Slow Carbon Cycle: Rock and Tectonics

On million-year timescales, carbon moves in and out of rock:

Weathering: Rainwater + CO₂ forms carbonic acid (H₂CO₃), which dissolves silicate rocks, releasing bicarbonate ions (HCO₃⁻) that rivers carry to the ocean. Marine organisms build shells from CaCO₃ — calcium carbonate — which, when dead, accumulate as limestone on the seafloor. Net effect: atmospheric CO₂ is consumed.
Volcanic outgassing: Plate subduction pulls carbonate rock into the mantle, where heat and pressure release CO₂ back to the atmosphere through volcanoes. Net effect: CO₂ is returned. Volcanoes emit ~0.1–0.3 GtC/yr — roughly 100× less than current human emissions.
Burial of organic matter: A tiny fraction of dead organisms escapes decomposition and is buried in sediment, forming — over tens of millions of years — coal, oil, and natural gas (fossil fuels).

The slow cycle acts as Earth's thermostat on geological timescales: if Earth warms, weathering accelerates (carbonic acid reacts faster), pulling CO₂ down and cooling the planet. If it cools, weathering slows, volcanic CO₂ builds up again. This thermostat operates over millions of years — far too slow to resolve a perturbation injected in decades.

Natural Fluxes: The Numbers

Natural annual fluxes (approximate, GtC/yr): Land photosynthesis (GPP): 120 ↓ atmosphere → biosphere Land respiration + decay: 118 ↑ biosphere → atmosphere Net land uptake: 2 (pre-industrial average ≈ balanced) Ocean gas exchange: 90 ↓ atmosphere → ocean surface Ocean outgassing: 88 ↑ ocean → atmosphere Net ocean uptake: 2 (pre-industrial average ≈ balanced) Volcanic outgassing: 0.1–0.3 ↑ rock → atmosphere Weathering: 0.2–0.4 ↓ atmosphere → ocean → rock Pre-industrial atmospheric CO₂: ~280 ppm (stable for millennia)

The pre-industrial carbon cycle was essentially in balance: the same amount of carbon leaving the atmosphere was returning each year. Atmospheric CO₂ stayed stable at ~280 ppm for the 11,700 years of the Holocene before industrialisation.

Anthropogenic Flux: Why 10 GtC/yr Matters

Human activities currently add approximately 10–11 GtC/yr (37–40 GtCO₂/yr) to the atmosphere — primarily from fossil fuel combustion (~9 GtC/yr) and land-use change (deforestation) (~1–1.5 GtC/yr).

Compare to natural fluxes of ~200 GtC/yr: human emissions are about 5% of gross natural exchange. That sounds small. The critical difference is balance: natural gross fluxes nearly cancel out. Humanity injects a net addition with no corresponding removal, causing CO₂ to accumulate in the atmosphere.

Human emissions: +10.5 GtC/yr Absorbed by ocean: −2.5 GtC/yr (~25% of emissions) Absorbed by land: −3.0 GtC/yr (~30% of emissions) Remaining in atm: +5.0 GtC/yr (~45% of emissions) 2.3 ppm CO₂ rise per year = 5 GtC/yr ÷ 2.13 GtC/ppm 2024 atmospheric CO₂: 424 ppm (pre-industrial: 280 ppm) Net increase since 1850: +144 ppm (+51%)

The airborne fraction: Only ~45% of human CO₂ stays in the atmosphere; the rest is absorbed by oceans and land within years. However, as oceans warm and absorb CO₂, their uptake efficiency decreases. If the land and ocean carbon sinks weaken — as observations suggest is beginning — more of each year's emissions will stay in the atmosphere.

Ocean Acidification

When CO₂ dissolves in seawater it does not simply sit there inertly. It forms carbonic acid, which dissociates:

CO₂ (aq) + H₂O ⇌ H₂CO₃ (carbonic acid) H₂CO₃ ⇌ H⁺ + HCO₃⁻ (bicarbonate) HCO₃⁻ ⇌ H⁺ + CO₃²⁻ (carbonate) More CO₂ dissolved → more H⁺ ions → lower pH Ocean surface pH, pre-industrial: 8.18 Ocean surface pH, 2024: 8.05 Ocean surface pH, 2100 (RCP8.5): ~7.8 (0.4 unit drop) Each 0.1 pH unit = 26% more acidic (pH is logarithmic)

The drop from 8.18 to 8.05 since industrialisation represents a 26% increase in hydrogen-ion concentration. This is deeply significant because carbonate ion (CO₃²⁻) is the building block that corals, oysters, sea urchins, and pteropods use to build calcium carbonate (CaCO₃) shells. As CO₃²⁻ concentration falls, organisms must spend more energy building shells and, in severe cases, existing shells dissolve.

Great Barrier Reef bleaching: Bleaching is caused by warming (not acidification directly), but the combination of warmer, more acidic, lower-oxygen water under a high-CO₂ scenario represents a multi-stressor threat. The reef has experienced five mass bleaching events since 2016 — a pace unprecedented in the instrumental record.

Climate Feedbacks

The carbon cycle interacts with climate through positive feedbacks that can amplify warming beyond the direct effect of CO₂:

Permafrost thaw: Northern permafrost soils hold ~1,500 GtC — almost double the current atmosphere. As permafrost thaws, microbes decompose ancient organic matter, releasing CO₂ and CH₄. This is a positive feedback as warming drives more release drives more warming.
Amazon dieback: Reduced rainfall (partly driven by deforestation) stresses the Amazon, converting forest to savanna. Less forest = less evapotranspiration = less local rainfall = accelerating dieback. The Amazon may be approaching a tipping point at 20–25% deforestation (currently ~17–20%).
Methane hydrates: Vast amounts of methane are frozen in seafloor sediments as gas hydrates — stable only at low temperature and high pressure. Deep-ocean warming could destabilise these over centuries.
Ice-albedo feedback: Arctic sea ice reflects 80–90% of incoming sunlight. Open ocean absorbs 93–95%. Melting ice exposes dark ocean, absorbing more heat, melting more ice. Not a direct carbon feedback, but amplifies warming that drives carbon feedbacks.

The Remaining Carbon Budget

The carbon budget is the total cumulative CO₂ capable of being emitted while keeping warming below a target temperature above pre-industrial (with a given likelihood):

Remaining carbon budget from January 2024 (IPCC AR6 + subsequent updates): 1.5 °C target (67% probability): ~200 GtCO₂ ≈ 5 years at current rates 1.5 °C target (50% probability): ~360 GtCO₂ ≈ 9 years 2.0 °C target (67% probability): ~1200 GtCO₂ ≈ 30 years Current global emissions: ~37–40 GtCO₂/yr

At current emission rates, the 1.5 °C budget is likely already exhausted or will be within this decade. Staying below 2 °C requires deep and immediate decarbonisation — roughly halving global emissions by 2030 and reaching net-zero by ~2070.

"Net zero" means human CO₂ emissions are balanced by deliberate removal (reforestation, direct air capture, enhanced weathering) — not zero emissions. The natural carbon cycle will then slowly draw down the excess atmospheric CO₂ over centuries to millennia; a return to 280 ppm will not occur on any human-relevant timescale without active intervention.