The carbon cycle — AQA A-Level Geography
Test yourself on The carbon cycle with AQA A-Level practice questions.
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The carbon cycle explained
The global carbon cycle comprises five major stores of varying sizes and distributions.
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The lithosphere is the largest store, containing over 99.9% of global carbon (approximately 100,000,000 PgC), primarily in sedimentary rocks like limestone and fossil fuels. The hydrosphere is the second largest (approximately 38,000 PgC), with most carbon dissolved in the deep ocean. The biosphere (approximately 3,190 PgC) stores carbon in living vegetation, soils, and peat, distributed heavily in tropical rainforests and boreal forests. The cryosphere stores carbon in permafrost (approximately 1,400 PgC), mainly in tundra regions. The atmosphere is the smallest major store (approximately 750 PgC) but is highly active, distributing carbon globally.
Factors driving change in the magnitude of these stores over time and space, including flows and transfers at plant, sere and continental scales. Photosynthesis, respiration, decomposition, combustion, carbon sequestration in oceans and sediments, weathering.
Carbon store magnitudes change over time and space through dynamic flows operating across multiple scales. At the plant scale, photosynthesis removes atmospheric carbon, while respiration returns it. At the sere scale, ecosystems transition over time, altering carbon storage through decomposition and biomass accumulation. At the continental scale, weathering of silicate rocks and combustion of vast forests or fossil fuels drive massive carbon transfers. Carbon sequestration in oceans involves the physical and biological pumps transferring carbon to deep waters, while sedimentary sequestration locks carbon in the lithosphere for millions of years. These interconnected processes continuously alter the balance of global carbon reservoirs.
Changes in the carbon cycle over time, to include natural variation (including wild fires, volcanic activity) and human impact (including hydrocarbon fuel extraction and burning, farming practices, deforestation, land use changes).
The carbon cycle fluctuates over time through natural and anthropogenic drivers. Natural variations include volcanic activity, releasing lithospheric carbon dioxide, and wildfires, which rapidly transfer biospheric carbon to the atmosphere while reducing photosynthetic capacity. Human impacts significantly accelerate these changes. Hydrocarbon fuel extraction and burning shift vast quantities of fossilised carbon into the active cycle. Farming practices, such as cattle rearing and ploughing, release methane and oxidise soil carbon. Deforestation removes vital carbon sinks, and broader land use changes, like urbanisation, seal soils and alter natural fluxes, disrupting the long-term equilibrium of the global carbon cycle.
The carbon budget and the impact of the carbon cycle upon land, ocean and atmosphere, including global climate.
The carbon budget is the balance between carbon emissions and removals across all global reservoirs. Understanding this budget is crucial for assessing the carbon cycle's impact on Earth's systems. On land, increased atmospheric carbon stimulates plant growth through carbon fertilisation, but alters soil dynamics and thaws permafrost. In the oceans, absorbing excess carbon dioxide causes ocean acidification, lowering pH and threatening marine calcifying organisms like corals. In the atmosphere, rising carbon dioxide and methane concentrations enhance the greenhouse effect. This directly drives changes in the global climate, resulting in rising temperatures, altered precipitation patterns, and more frequent extreme weather events.
Your focus
- Rank the major global carbon stores by their relative sizes.
- Describe the geographical distribution of carbon within the biosphere and cryosphere.
- Explain the composition of the lithosphere and hydrosphere as carbon reservoirs.
Show all 12 objectives
- Identify and define the key flows of the carbon cycle including photosynthesis, respiration, and combustion.
- Explain how carbon transfers operate at plant, sere, and continental scales.
- Analyse the mechanisms of carbon sequestration in oceans and sediments and their impact on store magnitudes.
- Categorise the natural and human drivers of change in the carbon cycle.
- Explain the mechanisms by which farming practices and land use changes alter carbon stores.
- Compare the relative impacts of volcanic activity and hydrocarbon combustion on atmospheric carbon levels.
- Define the global carbon budget and calculate net fluxes between major stores.
- Explain the mechanisms driving ocean acidification and its ecological consequences.
- Evaluate the interconnected impacts of an altered carbon budget on land, oceans, and the global climate.
The carbon cycle exam tips
Marking Points
- Identify the lithosphere as the largest carbon store, containing sedimentary rocks and fossil fuels.
- Describe the hydrosphere's role, noting that the deep ocean stores significantly more carbon than the surface ocean.
- Explain the distribution of the biosphere, highlighting the concentration of terrestrial carbon in tropical and boreal biomes.
- Recognise the cryosphere as a significant store of frozen organic carbon in permafrost regions.
- Compare the relative sizes of the stores, noting the atmosphere is comparatively small but crucial for regulating global temperatures.
- Explain how photosynthesis and respiration act as opposing flows at the plant scale, regulating local atmospheric carbon.
- Describe how decomposition breaks down organic matter, transferring carbon from the biosphere to the pedosphere and atmosphere at the sere scale.
- Analyse the role of combustion, both natural and human-induced, in rapidly transferring carbon from the biosphere and lithosphere to the atmosphere.
- Detail the process of carbon sequestration in oceans, including the biological pump and physical downwelling.
- Explain how chemical weathering of rocks transfers carbon from the atmosphere to the hydrosphere and eventually to ocean sediments.
- Explain how volcanic activity transfers carbon from the lithosphere to the atmosphere as carbon dioxide.
- Describe the dual impact of wildfires: immediate release of carbon dioxide and long-term reduction in the biosphere's capacity to sequester carbon.
- Analyse how hydrocarbon extraction and combustion move slow-cycle carbon into the fast cycle, creating an atmospheric surplus.
- Evaluate the impact of farming practices, such as methane emissions from livestock and carbon loss from soil ploughing.
- Assess how deforestation and land use changes (e.g., urbanisation) reduce biomass carbon storage and alter soil carbon fluxes.
- Define the carbon budget as the difference between the inputs and outputs of carbon within a specific reservoir or the global system.
- Explain the impact of the carbon cycle on land, including carbon fertilisation effects on vegetation and the release of carbon from thawing permafrost.
- Describe the process of ocean acidification, where increased dissolved carbon dioxide lowers ocean pH, impacting marine ecosystems.
- Analyse the atmospheric impact of the carbon cycle, specifically the role of carbon dioxide and methane in the enhanced greenhouse effect.
- Evaluate how changes in the atmospheric carbon store drive global climate change, including temperature increases and shifting weather patterns.
Examiner Tips
- 💡Learn approximate values in petagrams of carbon (PgC) or gigatonnes of carbon (GtC) to support comparisons of store sizes.
- 💡When discussing distribution, link specific stores to geographical biomes, such as the biosphere to tropical rainforests.
- 💡Use the term 'store' or 'reservoir' rather than 'sink' when referring to the absolute amount of carbon held in these spheres.
- 💡Explicitly link specific flows to their spatial scales, such as photosynthesis to the plant scale and weathering to the continental scale.
- 💡Use systems terminology, clearly distinguishing between stores (e.g., oceans) and the transfers or flows (e.g., sequestration) that connect them.
- 💡When evaluating changes in magnitude, consider both the rate (fast or slow) and the volume of the carbon transfer.
- 💡Use specific data, such as gigatonnes of carbon (GtC), to compare the magnitude of natural versus human-induced carbon fluxes.
- 💡When discussing wildfires, acknowledge that they can be both a natural variation and exacerbated by human-induced climate change.
- 💡Structure essay answers by contrasting the timescales of natural variations (often geological) with the rapid pace of recent human impacts.
- 💡Link the concept of the carbon budget to tipping points, such as the irreversible thawing of permafrost.
- 💡Use the chemical process of carbon dioxide dissolving in seawater to form carbonic acid to demonstrate a deep understanding of ocean impacts.
- 💡When evaluating impacts on global climate, discuss both temperature changes and secondary effects like sea-level rise and altered atmospheric circulation.
Common Mistakes
- Confusing the sizes of the biosphere and atmosphere; the biosphere is significantly larger than the atmosphere.
- Ignoring the cryosphere as a distinct carbon store; ensure permafrost is recognised as a major, vulnerable carbon reservoir.
- Assuming the surface ocean holds the most marine carbon; correct this by stating the deep ocean holds the vast majority of dissolved inorganic carbon.
- Confusing plant respiration with photosynthesis; remember that respiration releases carbon dioxide while photosynthesis absorbs it.
- Treating weathering as a rapid process; correct this by describing weathering and sedimentary sequestration as slow carbon cycle transfers operating over geological timescales.
- Overlooking the sere scale; ensure you can explain how carbon dynamics change as a plant community undergoes ecological succession.
- Error: Stating that volcanic eruptions are the primary cause of recent atmospheric carbon increases. Correction: Clarify that human activities, particularly hydrocarbon burning, release significantly more carbon annually than volcanic activity.
- Error: Confusing the greenhouse effect with ozone depletion when discussing hydrocarbon burning. Correction: Explicitly link hydrocarbon combustion to the enhanced greenhouse effect and global warming, not the ozone hole.
- Error: Treating deforestation and land use change as identical processes. Correction: Distinguish deforestation (tree removal) from broader land use changes, which include urbanisation, afforestation, and agricultural conversion.
- Error: Defining the carbon budget solely as human emissions. Correction: Define the carbon budget as the net balance of all carbon fluxes (both natural and human) between the atmosphere, land, and oceans.
- Error: Stating that ocean acidification involves the ocean becoming a strong acid. Correction: Explain that the ocean remains slightly alkaline, but its pH is decreasing (becoming less alkaline) due to carbonic acid formation.
- Error: Confusing the natural greenhouse effect with the enhanced greenhouse effect. Correction: Specify that the enhanced greenhouse effect, driven by a surplus in the atmospheric carbon budget, is responsible for recent global climate change.