Aim for FiveEnvironmental Science

Unit 1 · Topic 1.4

1.4 The Carbon Cycle

The carbon cycle tracks how carbon moves among the air, ocean, living things, soil, rocks and fossil fuels. Photosynthesis and respiration move carbon quickly, while burning fossil fuels releases carbon that took millions of years to store, raising atmospheric CO₂.

Key terms

  • reservoir
  • carbon sink
  • photosynthesis
  • cellular respiration
  • fossil fuels

Reservoirs, sources and sinks

Carbon is stored in reservoirs (also called pools): the atmosphere, the ocean, living things (biomass), soil, sedimentary rocks and fossil fuels. Carbon moves between them through processes called fluxes.

A carbon sink is a reservoir that takes in more carbon than it releases, like a growing forest or the ocean today. A carbon source releases more than it takes in, like a burning forest or a power plant burning coal. The same reservoir can switch roles: a forest is a sink while it grows but becomes a source if it's cut and burned.

By far the largest reservoir is sedimentary rock, such as limestone. The ocean is the largest reservoir that exchanges carbon quickly with the air.

The fast cycle: photosynthesis and respiration

Photosynthesis pulls CO₂ out of the air: 6CO₂ + 6H₂O + light energy → C₆H₁₂O₆ + 6O₂. Producers build the carbon into sugar and then into their bodies.

Cellular respiration does the reverse: C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + energy. Plants, animals and decomposers all respire, returning carbon to the air. Decomposition of dead matter by bacteria and fungi is a big part of this.

Because photosynthesis and respiration happen constantly, carbon moves between air and living things in days to decades. Measured CO₂ in the air even rises and falls each year: it dips during the Northern Hemisphere summer when plants grow and rises in winter.

The ocean and the slow cycle

The ocean absorbs CO₂ directly from the air where they meet. Some dissolved carbon is used by algae; some is used by organisms like corals and plankton to build calcium carbonate (CaCO₃) shells. When those organisms die, their shells and remains sink and pile up as sediment. Over millions of years, burial and pressure turn sediments into sedimentary rock like limestone.

Carbon leaves rocks slowly through weathering and volcanic eruptions. Plant and algae remains buried quickly without oxygen, over millions of years, became the fossil fuels coal, oil and natural gas. This slow part of the cycle runs on timescales of thousands to millions of years.

How people change the cycle

Extracting and burning fossil fuels moves carbon from a long-term underground reservoir into the atmosphere much faster than natural processes can take it back out. Clearing and burning forests adds even more CO₂ and removes a sink.

The result is a steady rise in atmospheric CO₂, from about 280 ppm (parts per million) before the Industrial Revolution to about 317 ppm in 1960 and about 425 ppm by 2024. Roughly half of the CO₂ people emit stays in the air; the ocean and land absorb the rest. The extra CO₂ dissolving in the ocean is making seawater more acidic.

Worked examples

Try each one yourself first, then open the solution.

  1. Example 1

    Into or out of the air?

    For each process, state whether it moves carbon into the atmosphere or out of it: (a) photosynthesis by grass, (b) a cow's respiration, (c) burning gasoline, (d) shells sinking to the seafloor and forming sediment, (e) a volcanic eruption.

    Show the solution
    1. Step 1: Ask whether CO₂ is being released as a gas or captured into something else.
    2. Step 2: (a) Photosynthesis captures CO₂ into sugar: out of the atmosphere.
    3. Step 3: (b) Respiration breaks down sugar and releases CO₂: into the atmosphere.
    4. Step 4: (c) Combustion of a fossil fuel releases CO₂: into the atmosphere.
    5. Step 5: (d) Sedimentation locks carbon away in the seafloor: out of the atmosphere (by way of the ocean).
    6. Step 6: (e) Volcanoes release CO₂ from deep rock: into the atmosphere.

    Answer: Out: (a) and (d). Into: (b), (c) and (e).

  2. Example 2Calculator allowed

    Percent change in CO₂

    Atmospheric CO₂ at Mauna Loa, Hawaii averaged about 317 ppm in 1960 and about 425 ppm in 2024. Calculate the percent increase.

    Show the solution
    1. Step 1: Use percent change = (new − old) ÷ old × 100.
    2. Step 2: New − old = 425 − 317 = 108 ppm.
    3. Step 3: 108 ÷ 317 = 0.341.
    4. Step 4: 0.341 × 100 = 34.1%.

    Answer: About a 34% increase.

  3. Example 3Calculator allowed

    Carbon versus carbon dioxide (trap)

    A small power plant burns coal containing 2.5 metric tons of carbon. How many metric tons of CO₂ does this produce? (Atomic masses: C = 12, O = 16.)

    Show the solution
    1. Step 1: The trap is answering 2.5 tons. Each carbon atom picks up two oxygen atoms when it burns, so the CO₂ weighs more than the carbon did.
    2. Step 2: Molar mass of CO₂ = 12 + 2(16) = 44. Carbon makes up 12 of those 44 units.
    3. Step 3: Mass of CO₂ = 2.5 t C × (44 ÷ 12) = 2.5 × 3.67.
    4. Step 4: 2.5 × 3.67 ≈ 9.2.

    Answer: About 9.2 metric tons of CO₂.

Common mistakes

  • Saying the atmosphere is the largest carbon reservoir. Sedimentary rocks hold the most; the ocean is the largest fast-exchanging reservoir.
  • Saying only animals respire. Plants respire all the time too; they just photosynthesize more than they respire while growing.
  • Calling a reservoir a sink without checking the balance. A sink must take in more carbon than it releases.
  • Forgetting that burning fossil fuels is a problem because of speed: carbon stored over millions of years is released in decades.

On the exam

  • Be ready to name processes that move carbon between two specific reservoirs, like ‘photosynthesis moves carbon from the atmosphere to biomass’.
  • Calculation questions may ask for percent change in CO₂ or converting between mass of carbon and mass of CO₂. Show the setup to earn the point.

Connected topics

Videos

Check yourself

5 questions on 1.4 The Carbon Cycle. Pick an answer to see if you got it, and why.

Time of dayCO₂ concentration above the canopy (ppm)
00:00434
04:00440
08:00428
12:00418
16:00420
20:00430

Hypothetical data from a summer day over a deciduous forest

Question 1 of 5

Which statement best explains why the CO₂ concentration is lowest near midday?

Question 2 of 5Calculator allowed

What is the percent decrease in CO₂ concentration from 04:00 to 12:00?

Question 3 of 5

Which process moves carbon from a reservoir that holds it for millions of years into the atmosphere most quickly?

Carbon flowCarbon per year (billion metric tons)
Released by burning fossil fuels and making cement9.5
Released by deforestation and other land-use change1.5
Taken up by the ocean2.5
Taken up by land plants and soils3.0

Hypothetical global estimates, rounded

Question 4 of 5Calculator allowed

Based on the table, how much carbon is added to the atmosphere each year?

Question 5 of 5Calculator allowed

About what percentage of the carbon released by human activities stays in the atmosphere each year?

0 of 5 answered