The Global Carbon Cycle

Introduction

Carbon moves constantly between air, oceans, living things, soils and rocks. Plants draw down CO₂ every summer and release it every winter, making the atmospheric record oscillate like a heartbeat. For thousands of years before industrialisation, these exchanges were nearly balanced. Burning fossil fuels transfers carbon from a slow geological store into the fast-cycling system in a matter of decades. Understanding the carbon cycle as a set of reservoirs and fluxes explains how much of that carbon remains in the air and where the rest goes.

Core explanation

Reservoirs. Approximate sizes, in gigatonnes of carbon (1 GtC = 10¹⁵ g):

Reservoir Carbon / GtC --- --- Atmosphere (CO₂ at 420 ppm) about 870–900 Terrestrial vegetation about 450–650 Soils and permafrost about 3000 including permafrost Surface ocean about 900 Intermediate and deep ocean about 37 000 Fossil fuel reserves several thousand Sedimentary rocks (carbonates, kerogen) tens of millions

A useful conversion is 1 ppm of atmospheric CO₂ ≈ 2.12 GtC.

Fast and slow cycles. The fast cycle operates over years to millennia. Photosynthesis on land takes up roughly 120 GtC per year, returned by plant respiration, decomposition and fire. Air–sea exchange moves about 80–90 GtC per year in each direction. The ocean's biological pump carries organic carbon from the sunlit surface to depth as sinking particles, while the solubility pump sends CO₂-rich cold water down in polar regions. The slow cycle involves rock weathering, carbonate burial and volcanic outgassing over hundreds of thousands to millions of years. Silicate weathering, summarised as CaSiO₃ + 2CO₂ + H₂O → Ca²⁺ + 2HCO₃⁻ + SiO₂, followed by carbonate precipitation in the ocean, is the planet's long-term thermostat.

The anthropogenic perturbation. Fossil fuel combustion and cement production now release about 10 GtC per year, and land-use change about 1 GtC per year. Of this, roughly 45% remains in the atmosphere (the airborne fraction ), about 25% dissolves in the ocean and about 30% is taken up by land vegetation, partly because higher CO₂ stimulates photosynthesis. The atmospheric increase of about 5 GtC per year corresponds to a rise of over 2 ppm per year.

Evidence the extra carbon is fossil. Fossil carbon contains no ¹⁴C and is depleted in ¹³C, because it came from ancient plants. The atmosphere's ¹³C/¹²C and ¹⁴C/¹²C ratios have both fallen (the Suess effect), and atmospheric O₂ is declining in step with combustion.

Ocean uptake chemistry. CO₂ dissolving in seawater reacts with carbonate ions: CO₂ + CO₃²⁻ + H₂O → 2HCO₃⁻. The carbonate ion acts as the ocean's buffer. As more CO₂ enters, carbonate is consumed and the buffer weakens, so each additional increment of CO₂ raises seawater CO₂ partial pressure more steeply. This is expressed by the Revelle factor , typically about 10: a 10% increase in seawater CO₂ produces only about a 1% increase in total dissolved inorganic carbon. Mixing into the deep ocean takes centuries, so uptake is limited by slow circulation.

Formulae

Residence time τ = reservoir size ÷ outflow flux. Revelle factor = (ΔpCO₂/pCO₂) ÷ (ΔDIC/DIC). 1 ppm CO₂ ≈ 2.12 GtC.

Step-by-step reasoning

To estimate the atmospheric response to an emission:

1. Convert the emission to GtC per year. 2. Apply the airborne fraction (about 0.45) to find how much remains in the air. 3. Divide by 2.12 GtC per ppm to convert to a concentration rise. 4. Allocate the remainder between ocean and land sinks. 5. Recognise that the sinks may weaken as the ocean's buffer declines and land responds to warming.

Visual explanation

Draw boxes for atmosphere, land plants, soils, surface ocean, deep ocean and fossil fuels, sized roughly by carbon content. Connect them with two-way arrows labelled with gross fluxes (120 GtC per year for photosynthesis, 80–90 GtC per year for air–sea exchange). Add a thick one-way red arrow from fossil fuels to atmosphere labelled 10 GtC per year and smaller red arrows showing the net uptake by ocean and land.

Real-world analogy

The atmosphere resembles a bath with the taps and plughole both wide open, so the level barely changes. Burning fossil fuels is like adding a small extra hose. The flow is small compared with the gross exchanges, but because it is not matched by extra drainage, the water level rises steadily.

Real-world example

The Mauna Loa record, begun by Charles Keeling in 1958, shows CO₂ rising from about 315 ppm to over 420 ppm, with a seasonal cycle of several ppm driven by Northern Hemisphere vegetation. The upward trend and the declining ¹³C signal together identify fossil fuels as the cause.

Why?

Why does the ocean take up less than its enormous size suggests? Only the surface layer is in rapid contact with the air, and deep-water exchange takes centuries to a millennium. The surface layer's capacity is further limited by carbonate chemistry, because consuming carbonate raises the Revelle factor.

Common misconception

"CO₂ has an atmospheric lifetime of only a few years, so emissions would vanish quickly." A single molecule is exchanged with ocean and plants within about 4–5 years, but it is replaced by another. The excess concentration decays over centuries, and about 20% persists for many thousands of years until weathering removes it.

Worked example

Question: The atmosphere holds 880 GtC and exchanges about 210 GtC per year with land and ocean in total. Estimate the residence time of a CO₂ molecule. Then estimate the annual concentration rise from 11 GtC per year of emissions with an airborne fraction of 0.45.

Reasoning: τ = 880 ÷ 210 ≈ 4.2 years. Carbon remaining = 0.45 × 11 = 4.95 GtC per year. Concentration rise = 4.95 ÷ 2.12 ≈ 2.3 ppm per year.

Answer: About 4 years for a molecule; about 2.3 ppm per year increase.

Quick check

1. Why do both ¹³C/¹²C and ¹⁴C/¹²C ratios in atmospheric CO₂ decrease as fossil fuels are burned? Answer: Fossil carbon is plant-derived, so depleted in ¹³C, and so old that all its ¹⁴C has decayed.

Exam focus

Know the approximate reservoir sizes, the difference between gross and net fluxes, the airborne fraction and the partitioning of emissions. Be able to calculate residence time and the ppm–GtC conversion, and explain the carbonate buffer reaction limiting ocean uptake.

Advanced insight

Carbon-cycle feedbacks may amplify warming: thawing permafrost releases CO₂ and CH₄, warmer oceans dissolve less CO₂, and drought or fire can turn forests from sinks into sources. Earth system models disagree most about the land response, which is why estimates of the remaining carbon budget for a given temperature target carry substantial uncertainty.

Summary

Carbon is stored in the atmosphere, vegetation, soils, oceans and rocks, with large gross fluxes that were balanced before industrialisation. Fossil fuel and land-use emissions of about 11 GtC per year are partitioned into about 45% atmosphere, 25% ocean and 30% land. Isotope evidence confirms a fossil source. Ocean uptake is limited by slow mixing and by the carbonate buffer, quantified by the Revelle factor.

Practice questions

1. State the approximate partitioning of anthropogenic CO₂ among atmosphere, ocean and land. Answer: About 45% remains in the atmosphere, about 25% enters the ocean and about 30% is taken up by the land biosphere. 2. Write the equation for CO₂ uptake by the carbonate buffer in seawater. Answer: CO₂ + CO₃²⁻ + H₂O → 2HCO₃⁻. 3. Explain what the Revelle factor measures and why it increases with continued CO₂ uptake. Answer: It measures how much seawater pCO₂ rises per fractional change in dissolved inorganic carbon; as carbonate is used up, buffering weakens and the factor increases. 4. Describe how silicate weathering removes CO₂ on geological timescales. Answer: CO₂ dissolved in rain weathers silicate rocks to give Ca²⁺ and HCO₃⁻, which rivers carry to the ocean where they precipitate as CaCO₃ and are buried in sediments.