The carbon cycle including human influences — AQA A-Level Environmental Science

The carbon cycle is a fundamental biogeochemical cycle that regulates planetary climate and biological productivity by redistributing carbon among four primary spheres: the lithosphere, hydrosphere, atmosphere, and biosphere. Natural equilibrium relies on balanced fluxes, including photosynthesis, aerobic respiration, oceanic gas exchange, and geological sedimentation, which keep atmospheric concentrations relatively stable over millennial timescales.

Human industrial activities have significantly accelerated transfer rates from long-term geological reservoirs into active atmospheric and surface ocean stores. Understanding these perturbations is vital within AQA Environmental Science for assessing climate change impacts, evaluating ocean acidification, and designing evidence-based mitigation strategies such as carbon capture and storage (CCS) and ecological restoration.

• →Reservoirs and Residence Times: The lithosphere is the largest carbon reservoir (over 99% of global carbon, mainly in carbonate rocks), followed by deep oceans, fossil reserves, soils, the atmosphere, and terrestrial biomass.
• →Dynamic Equilibrium: Under undisturbed conditions, continuous biological and geochemical fluxes maintain balanced atmospheric CO2 concentrations through natural negative feedback mechanisms.
• →Oceanic Carbon Pumps: Oceanic uptake operates via the physical solubility pump (downwelling of cold, CO2-rich polar water) and the biological pump (organic sedimentation and carbonate precipitation).
• →Anthropogenic Flux Shifts: Fossil fuel extraction and combustion, agricultural soil degradation, and deforestation bypass slow geological pathways, injecting fossil carbon directly into the fast carbon cycle.

• Defines carbon sequestration as the capture and long-term storage of carbon so it is removed from the atmosphere.
• Identifies a biological example such as forests, peatlands or soils storing carbon in biomass and organic matter.
• Identifies a geological or technological example such as injecting captured carbon dioxide into deep rock formations.
• Distinguishes sequestration from emissions reduction by noting that sequestration removes carbon already present rather than avoiding new emissions.

• 💡Look for the words capture and long-term storage when identifying a correct sequestration option.
• 💡Reject options that describe only avoiding emissions rather than removing and storing carbon.
• 💡Check whether an example stores carbon in a stable, long-lived store before selecting it.
• 💡Always specify named carbon compounds (e.g., carbon dioxide, methane, calcium carbonate) rather than referring generally to 'carbon' when discussing chemical transformations.
• 💡Use standard units when quoting carbon reserves and fluxes: petagrams (Pg) or gigatonnes of carbon (GtC).
• 💡When explaining feedback loops, explicitly identify whether the loop is positive (self-amplifying) or negative (self-regulating) and provide a step-by-step mechanism.

• Confuses sequestration with reducing emissions; clarify that sequestration captures and stores carbon already in the atmosphere or in flue gases.
• Assumes any tree planting guarantees permanent storage; recognise that forests can be cut down or burn, releasing stored carbon.
• Treats short-term storage as sequestration; require that carbon is held securely for a long period.
• Equating respiration with simple breathing rather than a cellular catabolic process that releases CO2 from all living cells, including plants at night.
• Assuming afforestation creates a permanently secure carbon sink, overlooking that biomass carbon is vulnerable to rapid release via wildfire, disease, or harvesting.
• Believing that global warming and ozone depletion are the same mechanism within the carbon cycle, rather than distinct atmospheric processes.

1. 1Stage 1: Memorise the major carbon reservoirs by relative size (lithosphere > ocean > pedosphere > atmosphere > biosphere) and define residence time.
2. 2Stage 2: Diagram the natural fast and slow cycles, labeling key biological and geological fluxes including dissolution, sedimentation, and subduction.
3. 3Stage 3: Detail human activities (combustion, deforestation, peat drainage, cement manufacture) and quantify their impacts on natural flux rates.
4. 4Stage 4: Analyse marine dynamics, specifically the solubility pump, biological pump, and carbonate buffering reactions.
5. 5Stage 5: Practice 6-mark and 9-mark questions focusing on dynamic equilibrium disruption, carbon feedback mechanisms, and carbon capture strategies.

• 📋Quantitative flux calculations: Calculating residence times, percentage changes in reservoir mass, or net flux imbalances using provided diagrammatic data.
• 📋Mechanistic explanations: Explaining step-by-step how specific human practices (e.g., ploughing, permafrost thaw) alter biological or geochemical carbon pathways.
• 📋Evaluative extended response: Assessing the efficacy and ecological viability of carbon sequestration approaches such as biochar, afforestation, or industrial CCS.