The carbon cycle: reservoirs and fluxes: WACE Year 12 Earth and Environmental Science
“Explain the carbon cycle reservoirs and fluxes and how human activity disrupts the balance”
Carbon is stored in reservoirs (atmosphere, oceans, biosphere, soils and rocks) and moves between them through fluxes such as photosynthesis, respiration, ocean exchange, weathering and volcanism. A fast biological cycle exchanges carbon over years to decades, while a slow geological cycle moves it over millions of years. Humans disrupt the balance by burning fossil fuels and clearing land, transferring stored carbon into the atmosphere faster than natural sinks can absorb it.
What this dot point is asking
SCSA wants you to describe where carbon is stored, the processes that move it, and how human activity unbalances the system. This cycle directly links Unit 3's resource focus to Unit 4's climate content, because excess atmospheric carbon dioxide drives the enhanced greenhouse effect.
Carbon reservoirs
A reservoir is a store of carbon. The major ones differ enormously in size and turnover time.
- Atmosphere: carbon dioxide and methane; small but climatically crucial.
- Oceans: the largest fast-exchanging reservoir, holding dissolved carbon dioxide and bicarbonate.
- Biosphere: living plants and animals.
- Soils: decaying organic matter and stored carbon.
- Geosphere: by far the largest store, locked in carbonate rocks and fossil fuels, but exchanging extremely slowly.
The fast carbon cycle moves carbon through the atmosphere, biosphere, soils and surface ocean over years to decades. The slow carbon cycle moves carbon into rocks and fossil fuels and releases it through weathering and volcanism over millions of years. Burning fossil fuels takes slow-cycle carbon and injects it into the fast cycle almost instantly.
Carbon fluxes
A flux is a transfer of carbon between reservoirs.
- Photosynthesis removes carbon dioxide from the atmosphere and fixes it into plant tissue.
- Respiration and decomposition return carbon dioxide to the atmosphere.
- Ocean exchange moves carbon dioxide between air and sea, with cold water absorbing more.
- Weathering of rock slowly consumes carbon dioxide, while volcanism releases it.
- Burial of organic matter and carbonate locks carbon into the geosphere over geological time.
In a stable system these fluxes roughly balance, so reservoir sizes stay steady.
Reservoir: a store of carbon. Flux: a transfer of carbon between reservoirs. Sink: a reservoir taking up more carbon than it releases. Source: a reservoir releasing more than it takes up. Sequestration: long-term storage of carbon out of the atmosphere.
How humans disrupt the cycle
Human activity changes fluxes and shifts carbon between reservoirs.
- Burning fossil fuels transfers ancient geosphere carbon into the atmosphere within decades, far faster than it formed.
- Land clearing and deforestation release carbon stored in vegetation and soils, and remove a sink that would otherwise absorb carbon dioxide.
- The oceans and land vegetation absorb part of the extra carbon, acting as sinks, but cannot keep pace, so atmospheric carbon dioxide rises.
Ocean uptake of extra carbon dioxide also causes ocean acidification, a separate but linked impact on marine ecosystems such as coral reefs.
A diagram shows reservoirs with arrows for fluxes. To find whether the atmosphere is gaining carbon, add the incoming fluxes (respiration, fossil fuel burning, volcanism) and subtract the outgoing fluxes (photosynthesis, ocean uptake). If inputs exceed outputs, the atmospheric reservoir grows. Identifying fossil fuel burning as the added human flux is usually the key point.
Residence time and why it matters
Each reservoir has a characteristic residence time, the average time a carbon atom stays before moving on. Atmospheric carbon has a residence time of only a few years, surface-ocean carbon decades, deep-ocean carbon centuries to a millennium, and rock carbon hundreds of millions of years. Residence time explains why the system responds to a disturbance on very different timescales: a pulse of carbon dioxide added today is partly drawn down by the ocean and biosphere within decades, but a long tail persists for centuries because removal into the deep ocean and ultimately into rock is slow. This is why SCSA stresses that the climate consequences of present-day emissions are effectively committed for many human generations.
The blue carbon angle
Western Australian coastal ecosystems are significant carbon sinks. Seagrass meadows in Shark Bay, mangroves and tidal saltmarshes capture carbon dioxide through photosynthesis and bury it in waterlogged, oxygen-poor sediments where decomposition is slow, locking it away as blue carbon for centuries to millennia. Disturbing these systems, by dredging or coastal development, both stops the ongoing sink and can release previously buried carbon, converting a sink into a source. This links the carbon cycle directly to ecosystem services and to management decisions in Unit 3.
- Confusing reservoir and flux
- A reservoir is a store; a flux is a transfer between stores.
- Forgetting the slow cycle
- Rocks and fossil fuels are the biggest store, exchanging over millions of years.
- Saying oceans fully absorb emissions
- Oceans take up only part of the excess, and uptake causes acidification.
- Ignoring land clearing
- Deforestation both releases stored carbon and removes a sink.
Exam-style questions
Questions in the style of SCSA 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.
WACE 2021-style6 marksA carbon cycle diagram shows the following annual fluxes into and out of the atmosphere (gigatonnes of carbon per year): photosynthesis 120 out, plant and soil respiration 119 in, ocean uptake 92 out, ocean release 90 in, fossil fuel combustion 9 in, land clearing 2 in. Calculate the net annual change in the atmospheric carbon reservoir and explain which fluxes are responsible for the imbalance.Show worked answer →
A 6 mark data question rewards a correct calculation plus an explanation tied to human fluxes.
Calculation. Inputs to the atmosphere: GtC. Outputs: GtC. Net change GtC per year, so the atmosphere gains about 8 gigatonnes of carbon each year.
Explanation. The natural fluxes nearly balance: photosynthesis (120 out) is almost matched by respiration (119 in), and ocean uptake (92 out) nearly matches ocean release (90 in). The imbalance comes almost entirely from the human fluxes, fossil fuel combustion (9 in) and land clearing (2 in), which add carbon with no matching natural removal.
Markers reward the correct net figure with working shown, identification that natural fluxes roughly balance, and naming fossil fuel combustion and land clearing as the source of the surplus.
WACE 2023-style8 marksExplain how carbon moves through the fast and slow carbon cycles, and evaluate the claim that burning fossil fuels simply returns carbon that was already part of the natural cycle.Show worked answer →
An 8 mark extended response needs the two cycles described and a reasoned judgement on the claim.
- Fast cycle
- Carbon exchanges between the atmosphere, surface ocean, biosphere and soils over years to decades through photosynthesis, respiration, decomposition and air-sea exchange.
- Slow cycle
- Carbon is locked into rocks (carbonate sediments, limestone) and fossil fuels over millions of years through burial, and is released slowly by weathering and volcanism.
- Evaluating the claim
- It is technically true that fossil carbon was once atmospheric, fixed by ancient photosynthesis. However, the claim is misleading because of rate and reservoir. Fossil fuels are slow-cycle carbon that took millions of years to bury; burning them releases it within decades, far faster than slow-cycle removal (weathering, burial) can return it. The fast cycle and ocean cannot absorb the surplus quickly enough, so atmospheric carbon dioxide rises. The judgement: the claim ignores the mismatch between the slow rate of natural sequestration and the rapid rate of release.
Markers reward distinct descriptions of fast and slow cycles, a clear rate-and-reservoir argument, and an explicit evaluative judgement rather than mere description.