The carbon cycle (carbon stores, sinks and sources, fluxes, human disturbance): VCE Environmental Science Unit 4
“The carbon cycle including the main carbon stores (reservoirs), the fluxes between them, the role of fast and slow cycling, and how human activities have altered the cycle”
The carbon cycle is the continuous movement of carbon between stores (atmosphere, oceans, biosphere, soils and rocks) through fluxes such as photosynthesis, respiration, decomposition and ocean exchange. Humans have disturbed it by burning fossil fuels and clearing land, moving carbon from long-term stores into the atmosphere faster than natural sinks can absorb it.
What this dot point is asking
VCAA wants you to describe the main carbon stores (reservoirs), the fluxes that move carbon between them, the difference between fast and slow cycling, and how human activity has disturbed the cycle to enhance the greenhouse effect. Understanding sources and sinks underpins both climate science and mitigation strategies.
Carbon stores (reservoirs)
Carbon is held in several stores or reservoirs, which differ enormously in size and how long they hold carbon:
- Atmosphere. Carbon as carbon dioxide and methane. A relatively small but climate-critical store.
- Oceans (hydrosphere). The largest active carbon store, holding dissolved carbon dioxide, carbonate ions and marine life. Oceans absorb a large share of human emissions.
- Biosphere. Carbon in living plants and animals, especially forests, and in marine organisms.
- Soils. Carbon stored in dead organic matter and humus, a large terrestrial store.
- Lithosphere (rocks and fossil fuels). By far the largest store, locked in limestone, sediments and fossil fuels over geological time.
A carbon source releases more carbon than it absorbs (burning fossil fuels, deforestation, respiration). A carbon sink absorbs more than it releases (growing forests, oceans, soils). The balance of sources and sinks sets the amount of carbon dioxide in the atmosphere.
Fluxes: how carbon moves
A flux is the transfer of carbon from one store to another. The main fluxes are:
- Photosynthesis. Plants and phytoplankton remove carbon dioxide from the atmosphere and fix it into organic matter. This makes the biosphere a sink.
- Respiration. Living organisms release carbon dioxide back to the atmosphere as they use energy.
- Decomposition. Decomposers break down dead matter, returning carbon to the atmosphere and soil.
- Ocean exchange. Carbon dioxide dissolves into and outgasses from the ocean surface; marine organisms incorporate carbon into shells.
- Combustion. Burning of biomass or fossil fuels rapidly releases stored carbon to the atmosphere.
- Sedimentation and weathering. Very slow fluxes that move carbon into and out of rocks over millions of years.
Fast and slow cycling
The carbon cycle operates on two very different timescales:
- The fast (biological) cycle moves carbon between the atmosphere, biosphere, soils and surface ocean over days to centuries, through photosynthesis, respiration, decomposition and ocean exchange. This cycle is roughly in balance naturally.
- The slow (geological) cycle moves carbon into and out of rocks and fossil fuels over millions of years, through sedimentation, burial and volcanic release. It locks carbon away for vast periods.
This distinction is the key to the human problem: fossil fuels are part of the slow cycle, carbon that was removed from the atmosphere over millions of years and stored deep underground.
How humans have disturbed the cycle
Human activities have shifted carbon from long-term stores into the atmosphere faster than natural sinks can remove it:
- Burning fossil fuels. Combustion takes carbon out of the slow geological store and releases it as carbon dioxide in seconds, returning to the atmosphere carbon that took millions of years to bury. Australia's reliance on coal-fired power, such as the Latrobe Valley brown-coal stations, is a major source.
- Deforestation and land clearing. Removing forests releases stored carbon and reduces the biosphere's capacity to absorb carbon dioxide. Land clearing in Queensland and the box-ironbark woodlands of Victoria has reduced this sink.
The natural sinks (oceans and forests) absorb roughly half of human emissions, but the rest accumulates in the atmosphere, raising carbon dioxide from about 280 parts per million before industrialisation to over 420 parts per million today, as recorded at the Cape Grim baseline station in Tasmania. This extra atmospheric carbon strengthens the enhanced greenhouse effect. Ocean uptake also causes ocean acidification, which harms shell-forming organisms such as corals on the Great Barrier Reef.
Mitigation strategies work by reversing these flows: protecting and growing forests, restoring soils and wetlands (blue carbon), and leaving fossil carbon in the ground all keep carbon out of the atmosphere.
- Confusing stores and fluxes
- A store is where carbon sits (ocean, atmosphere); a flux is the process moving it (photosynthesis, combustion). Markers want both used correctly.
- Forgetting the fast versus slow distinction
- The human problem is moving carbon from the slow geological store (fossil fuels) into the fast cycle far too quickly for sinks to cope.
- Saying oceans simply solve the problem
- Oceans absorb much carbon dioxide but this causes acidification, and they cannot keep pace with emissions.
- Treating the natural cycle as the problem
- The natural cycle is roughly balanced. Human activity disturbs it by adding carbon faster than it can be removed.
Exam-style questions
Questions in the style of VCAA exam questions on this dot point, each with a worked answer. They are written by ExamExplained unless tagged "Past paper"; the year shows the paper a question is modelled on.
2025 VCAA-style3 marksCape Byron Power runs biofuel plants that burn wood residues and sugarcane milling waste, and grows energy crops specifically for biomass fuel. Compare the environmental impact of using biomass energy to that of using coal in terms of the carbon cycle.Show worked answer →
A 3 mark answer contrasts where the carbon comes from and whether it is reabsorbed.
1 mark: burning coal releases carbon that was locked in the slow (geological) carbon store for millions of years, adding 'new' carbon dioxide to the atmosphere that natural sinks cannot quickly reabsorb, so it increases atmospheric carbon.
1 mark: biomass releases carbon dioxide that the plants recently took out of the atmosphere by photosynthesis (part of the fast biological cycle).
1 mark: conclude that because the energy crops can be regrown and reabsorb a similar amount of carbon dioxide, biomass is close to carbon neutral over its cycle, whereas coal is a net source that permanently raises atmospheric carbon. (Biomass is not perfectly neutral once harvesting, transport and processing emissions are counted.)
2023 VCAA-style3 marksA state government aims to reduce net carbon dioxide emissions to zero. Option one: afforestation (planting trees where they have not historically grown). Option two: maintaining an existing forest thousands of years old and protecting it from land clearing. Considering the carbon cycle, state whether afforestation or protection of existing forests would be more effective in meeting the aim. Justify your decision.Show worked answer →
A 3 mark answer makes a clear choice and justifies it through carbon fluxes and stores.
1 mark: state the choice. Both have merit, but a strong answer argues afforestation (option one) actively removes additional carbon dioxide from the atmosphere as the new trees grow, increasing the size of the biosphere carbon sink.
1 mark: explain that protecting the existing old forest (option two) is also vital because clearing it would release a very large store of carbon built up over thousands of years; protection prevents that flux to the atmosphere but adds little new uptake, since a mature forest is roughly carbon neutral.
1 mark: justify by linking to the net-zero aim. Afforestation creates a new sink that draws down carbon dioxide, while protection prevents a large new source. The best programs do both: protect existing stores and add new sinks.
2022 VCAA-style2 marksA regional area has begun extracting coal seam gas for use in gas-fired turbines to generate electricity. Describe the consequence of the combustion of coal seam gas on the carbon cycle.Show worked answer →
A 2 mark answer traces the carbon from store to atmosphere.
1 mark: coal seam gas is mostly methane, a fossil fuel held in the slow geological carbon store. Burning it (combustion) releases this long-stored carbon as carbon dioxide (plus water).
1 mark: this transfers carbon from the lithosphere store into the atmosphere far faster than natural sinks can remove it, increasing atmospheric carbon dioxide and contributing to the enhanced greenhouse effect.