Systems concepts; the global water and carbon cycles, their stores, fluxes and feedbacks; the drainage basin and carbon budgets; and human impact on both cycles.

What this dot point is asking

AQA section 3.1.1 wants you to understand systems concepts (inputs, outputs, stores, flows, dynamic equilibrium and feedback), describe the global water and carbon cycles with their stores and fluxes, work quantitatively with the drainage-basin water budget and the carbon budget, and explain how human activity disturbs both cycles and drives climate change. The two cycles are taught together because they are physically coupled through vegetation, the ocean and a warming atmosphere.

Systems concepts

A system is a set of interrelated components linked by flows of energy and matter. Geographers describe systems using inputs, outputs, stores (components) and flows (transfers).

Feedback is the system's self-regulating mechanism. Negative feedback dampens a change and restores equilibrium (more evaporation increases cloud cover, which reduces incoming radiation and limits further warming). Positive feedback amplifies a change and pushes the system further from its original state. The classic example is ice-albedo feedback: warming melts reflective ice, exposing darker ocean or land that absorbs more radiation, causing more warming and more melting.

The global water cycle

The major stores, with their approximate share of all water, are the oceans (about 97 percent), ice caps and glaciers (the cryosphere, most of the remaining freshwater), groundwater, lakes and rivers, soil moisture and the atmosphere (a tiny but rapidly cycled store). The fluxes that move water are evaporation, transpiration (together evapotranspiration), condensation, precipitation, cryospheric exchange (melting, freezing, sublimation), surface runoff and groundwater flow.

The storm hydrograph links a rainfall event to river discharge: a short lag time and high peak follow when soils are saturated, the basin is small and steep, or impermeable surfaces (urbanisation) speed overland flow. Antecedent conditions, land use and basin shape all modify the hydrograph, which is why the water cycle topic connects directly to flood risk.

The carbon cycle

Carbon is stored in the atmosphere (mainly $CO_2$ and methane), the oceans (the largest active store, holding dissolved $CO_2$ and marine carbonate), soils, terrestrial biomass (the biosphere) and sedimentary rocks and fossil fuels (by far the largest store overall). Fluxes include photosynthesis (drawing $CO_2$ from the atmosphere into biomass), respiration, decomposition, combustion, ocean uptake and outgassing, weathering of rock by carbonic acid, and sequestration of carbon into sediments.

The cycle operates at two speeds. The fast carbon cycle moves carbon over years to centuries through living organisms, soils and the ocean surface, governed by photosynthesis and respiration. The slow carbon cycle moves carbon between rocks, the deep ocean and the atmosphere over millions of years through weathering, sedimentation, subduction and volcanic outgassing. Crucially, fossil-fuel burning transfers carbon from the slow store to the atmosphere far faster than the slow cycle can return it, which is the root of anthropogenic climate change.

Human impact and the link to climate

Burning fossil fuels moves carbon from the slow geological store into the atmosphere; deforestation reduces the biomass store and cuts photosynthetic uptake while releasing stored carbon through burning and decay; farming, drainage of peatlands and land-use change alter both cycles. Rising atmospheric $CO_2$ enhances the greenhouse effect, warming the climate. This couples the two cycles: warming changes evaporation rates, shifts precipitation patterns, melts ice (triggering ice-albedo feedback) and can release methane from thawing permafrost, a further positive feedback.

Try this

Q1. State the water-balance equation for a drainage basin and define each term. [3 marks]

• Cue. $P = E + Q + \Delta S$: precipitation, evapotranspiration, runoff and change in storage.

Q2. Explain one example of positive feedback in the carbon or water cycle. [3 marks]

• Cue. Ice melt lowers albedo, so more radiation is absorbed, causing more warming and more melt, amplifying the original change.

Q3. Distinguish between the fast and slow carbon cycles. [4 marks]

• Cue. Fast: years to centuries through life, soils and the ocean surface (photosynthesis, respiration). Slow: millions of years through rocks, the deep ocean and volcanism (weathering, sedimentation, outgassing).