The carbon cycle as a system; stores in the atmosphere, oceans, biosphere, soils and lithosphere; fluxes including photosynthesis, respiration, decomposition, combustion and sequestration; the fast and slow carbon cycles; and the carbon budget.

What this dot point is asking

AQA section 3.1.1 wants you to treat carbon as a system, identify its major stores and the size differences between them, explain the fluxes that move carbon, distinguish the fast and slow carbon cycles, and work with the carbon budget. This is the carbon-specific half of the compulsory topic; it pairs with the water cycle and feeds the climate discussion.

Carbon stores

Carbon sits in five major stores, which differ enormously in size and in how fast they exchange carbon:

• Atmosphere. A relatively small store of about 800 GtC, held mainly as carbon dioxide ($CO_2$) with some methane ($CH_4$), but the one we change directly.
• Oceans. The largest active store, holding dissolved $CO_2$, bicarbonate and marine carbonate; the ocean surface exchanges carbon with the atmosphere within years, the deep ocean over centuries to millennia.
• Soils. A large terrestrial store of organic carbon, sensitive to land use and warming.
• Terrestrial biomass (biosphere). Carbon locked in plants and animals, drawn down by photosynthesis.
• Lithosphere. Sedimentary rocks and fossil fuels, by far the largest store of all, but exchanged only over geological time.

Carbon fluxes

The fluxes that move carbon between stores are:

• Photosynthesis: plants draw $CO_2$ from the atmosphere into biomass.
• Respiration: living things return $CO_2$ to the atmosphere.
• Decomposition: micro-organisms break down dead matter, releasing $CO_2$.
• Combustion: burning biomass or fossil fuels releases stored carbon.
• Ocean uptake and outgassing: $CO_2$ dissolves into and escapes from the sea (cold water absorbs more; warm water releases more).
• Weathering: carbonic acid (rain plus $CO_2$) slowly dissolves rock, drawing carbon from the atmosphere.
• Sequestration: long-term burial of carbon in sediments, soils and the deep ocean.

The fast and slow carbon cycles

The cycle operates at two very different speeds.

The fast carbon cycle moves carbon over years to centuries through living organisms, soils and the ocean surface, governed by photosynthesis and respiration, decomposition and air-sea exchange. Because biological processes are quick, this is where seasonal and decadal change shows up: atmospheric $CO_2$ even rises and falls each year as Northern Hemisphere vegetation grows and dies back.

The slow carbon cycle moves carbon over millions of years between rocks, the deep ocean and the atmosphere through weathering, sedimentation (carbonate and organic matter settling and lithifying), subduction and volcanic outgassing. Over geological time the slow cycle keeps atmospheric $CO_2$ and climate broadly balanced.

The crucial exam point: fossil-fuel combustion transfers carbon from the slow geological store into the atmosphere far faster than the slow cycle can return it. The carbon released in a century took the slow cycle millions of years to bury, which is the root cause of rising atmospheric $CO_2$ and anthropogenic warming.

Try this

Q1. Name the five major carbon stores. [3 marks]

• Cue. Atmosphere, oceans, soils, terrestrial biomass (biosphere) and the lithosphere (rocks and fossil fuels).

Q2. Explain how photosynthesis and respiration move carbon in the fast cycle. [4 marks]

• Cue. Photosynthesis draws $CO_2$ from the atmosphere into biomass; respiration and decomposition return it. The balance determines whether vegetation is a sink or a source.

Q3. Explain why burning fossil fuels raises atmospheric $CO_2$. [3 marks]

• Cue. Combustion transfers carbon from the slow geological store to the atmosphere faster than weathering and sedimentation can return it, so the atmospheric store grows.