amazon.co.ukWater and Carbon Cycles
A* revision for the water and carbon cycles: stores, fluxes and feedbacks, the Amazon case study, and climate change links for AQA 7037, Edexcel and OCR.
🎯 Learning Objectives
- Explain the systems concept (inputs, outputs, stores, fluxes) applied to the water and carbon cycles, distinguishing open, closed and dynamic equilibrium systems (AQA 3.1.1.1; Edexcel 2.1; OCR 1.2.a)
- Describe and explain the global water budget, the water balance equation and the relationship between drainage basin stores, flows and hydrographs (AQA 3.1.1.2-3.1.1.3; Edexcel 2.2; OCR 1.1)
- Compare the magnitude and location of global carbon stores and evaluate the role of the carbon budget in planetary temperature regulation (AQA 3.1.1.4; Edexcel 2.4; OCR 1.2.b)
- Analyse positive and negative feedback loops linking the water and carbon cycles, including at least one Amazon-specific feedback (AQA 3.1.1.5; Edexcel 2.1, 2.7; OCR 1.3)
- Evaluate the impact of deforestation on the Amazon's water and carbon cycles using specific data (AQA 3.1.1.6; Edexcel 2.5-2.6; OCR 1.3)
- Assess the role of the two cycles in regulating planetary climate and the consequences of anthropogenic change for stores and fluxes (AQA 3.1.1.7-3.1.1.8; Edexcel 2.7; OCR 1.3)
- Evaluate local and global management strategies for maintaining water and carbon security (AQA 3.1.1.9; Edexcel 2.8; OCR 1.4)
📋 Key Definitions and Core Concepts
Systems concept: Modelling the water and carbon cycles as stores linked by flows (fluxes), with inputs and outputs, so that dynamic equilibria and feedbacks can be analysed.
Dynamic equilibrium: A state in which inputs and outputs balance so stores remain broadly constant over time; if a change disturbs the balance, the system responds through feedback until a new equilibrium is reached.
Water balance (water budget): The accounting relationship for a drainage basin: P = E + Q ± ΔS; differences between precipitation and evapotranspiration determine soil moisture recharge, utilisation, surplus and deficit.
Carbon sequestration: The long-term transfer and storage of carbon from the atmosphere into a store such as biomass, soils or geological reservoirs, e.g. by photosynthesis or carbon capture and storage (CCS).
Positive feedback: A loop in which a change in one component is amplified further around the cycle, destabilising the system, e.g. warming increases evaporation; water vapour (a greenhouse gas) traps more heat, causing further warming.
Negative feedback: A loop in which a change is dampened, pushing the system back toward equilibrium, e.g. extra atmospheric CO2 boosts plant photosynthesis, increasing carbon uptake from the atmosphere.
Fast / slow carbon cycle: The fast cycle moves carbon between the atmosphere, biosphere, soils and ocean surface over seconds to millennia (photosynthesis, respiration, decomposition); the slow cycle transfers carbon through rock weathering, ocean burial, subduction and volcanism over 100,000s to millions of years.
Overland flow: Water flowing across the land surface to the channel, occurring when rainfall intensity exceeds infiltration capacity (Hortonian) or when the soil is saturated (saturation-excess); rises sharply with deforestation and urbanisation.
Permafrost carbon feedback: Warming thaws permafrost (~1,400-1,700 Gt of frozen organic carbon), allowing microbial decomposition to release CO2 and CH4, which drives further warming - a positive feedback and potential tipping point.
Ocean acidification: The falling pH of seawater (around 0.1 units since the pre-industrial era) as the ocean absorbs excess atmospheric CO2, reducing carbonate ion availability for shell- and skeleton-building organisms.
📌 Key Points
Hydrosphere: 96.5% ocean; freshwater dominated by ice sheets (~68.7%) and groundwater (~30.1%), not rivers
Geological carbon store dwarfs all others; ocean is the largest active (near-surface) store
Fast carbon cycle (photosynthesis ~120 GtC/yr) vs slow carbon cycle (weathering, burial, volcanism ~0.1-0.3 GtC/yr)
Fossil fuel flux ~10 GtC/yr; sinks absorb only ~50%, so CO2 rose from 280 to >420 ppm
Amazon: ~2,000 mm/yr rainfall, 50-75% recycled by transpiration, ~120-150 GtC in biomass
Positive feedbacks: water vapour, ice-albedo, permafrost methane, forest dieback; negative: CO2 fertilisation, water-use efficiency
The systems concept applied to the water and carbon cycles (AQA 3.1.1.1; Edexcel 2.1; OCR 1.2.a)
Both cycles are best revised as systems: stores (reservoirs), flows/fluxes (transfers between stores), inputs and outputs. The water cycle as a whole is a closed system for matter (water is neither created nor destroyed on Earth; energy still arrives from the sun), while a drainage basin is an open system - water enters as precipitation and leaves as evapotranspiration and runoff. The carbon cycle is open on human timescales (fossil carbon is transferred from a geological store into the atmosphere) but closed over geological time because subduction and volcanism return carbon to the mantle-crust reservoir.
Systems language to deploy in essays: inputs (precipitation; volcanic degassing of CO2), outputs (river discharge; sedimentation/carbon burial), stores (ocean, ice, soil moisture, groundwater, atmosphere; biomass, ocean, fossil fuels, limestone), fluxes (evaporation ~505,000 km3/yr globally; photosynthesis fixes roughly 120 GtC/yr). A system in dynamic equilibrium has balancing inputs and outputs; change one flux and the system self-regulates via feedback or shifts to a new equilibrium. Examiners reward applying these concepts to specific stores and fluxes with magnitudes, not reciting the diagram.
| Water cycle | Carbon cycle |
|---|---|
| Closed at global scale (matter conserved); open for drainage basins | Open on human timescales (fossil fuel combustion); closed over geological time |
| Key store: ocean (96.5% of hydrosphere) | Key store: geological (sedimentary rock ~99.9% of carbon) |
| Fastest flux: evaporation-precipitation over oceans | Fastest flux: photosynthesis-respiration (fast cycle) |
| Solar energy drives phase changes | Photosynthesis converts solar to chemical energy |
The water cycle: global budget, stores, flows and the water balance (AQA 3.1.1.2-3.1.1.3; Edexcel 2.2; OCR 1.1)
Global water budget: ~1,386 million km3 of water on Earth. Oceans hold 96.5%; of the ~2.5% freshwater, most is locked in ice caps/glaciers (~68.7%) and groundwater (~30.1%), leaving lakes, rivers, soil moisture, atmosphere and biota as tiny fractions. The global hydrological cycle is a closed system: annual evaporation approximates global precipitation, but there is a net flux of ~37,000-40,000 km3/yr of vapour from ocean to land, returning as runoff. Only ~0.001% of Earth's water is in the atmosphere at any instant, yet it drives weather.
Drainage basin stores and flows: stores include interception (canopy), surface storage (puddles, lakes), soil moisture, groundwater and channel storage. Flows: precipitation, throughfall/stemflow, infiltration (capacity falls with saturation and soil compaction), percolation, throughflow, groundwater flow/baseflow, overland flow (Hortonian and saturation-excess) and channel flow. Water balance equation: P = E + Q ± ΔS. Seasonally this produces soil moisture recharge (autumn, when P > E), utilisation, surplus (winter) and deficit (summer, when E > P) - be ready to sketch the water balance graph for a UK basin (e.g. the River Exe at Thorverton).
Runoff variation: the storm hydrograph shows lag time (peak rainfall to peak discharge) and peak discharge. Flashy basins (small, circular, steep, impermeable, urbanised, saturated, low interception) have short lag and high peaks; subdued basins (large, elongated, permeable, forested) the reverse. Urbanisation and deforestation both increase flood risk by shortening lag - link forward to the Amazon feedback and flood management.
Water budget variation and security: the global budget is uneven in space and time. Only ~2.5% is freshwater and of that, less than 1% is accessible surface water; ~4 billion people experience severe water scarcity at least one month per year. Regional contrasts illustrate the budget: the UK (~1,100-1,600 mm/yr rain, water-surplus north-west vs deficit south-east) versus the Sahel or Australia's Murray-Darling Basin, where abstraction exceeds recharge and stores fall. Rising population, irrigation (~70% of global freshwater abstraction) and climate-change-driven shifts in the balance (more deficit months) turn the water balance into a management issue - demand-side (metering, efficiency) and supply-side (reservoirs, desalination, transfers) responses. In essays, use the equation to explain why deficits arise: when E > P, stores deplete unless inputs resume (recharge).
The carbon cycle: stores, fluxes and the fast/slow cycles (AQA 3.1.1.4; Edexcel 2.4; OCR 1.2.b)
Stores (sinks): sedimentary rocks (limestone, chalk) hold the overwhelming majority of carbon (~60-100 million Gt); oceans are the largest active near-surface store (~38,000-40,000 GtC as dissolved CO2, bicarbonate and carbonate); fossil fuels ~1,000 GtC; soils ~1,500-1,600 GtC; permafrost ~1,400-1,700 Gt of frozen organic carbon; vegetation ~560-600 GtC; atmosphere ~890 GtC (was ~590 GtC / 280 ppm pre-industrial).
Fluxes: photosynthesis fixes ~120 GtC/yr (gross primary production); respiration returns a similar amount; ocean-atmosphere exchange ~90 GtC/yr each way (net ocean sink ~2.5-3 GtC/yr, ~25-30% of anthropogenic emissions); volcanism degasses only ~0.1-0.3 GtC/yr - tiny compared with fossil fuel combustion ~10 GtC/yr (~36-37 Gt CO2). Use these numbers to show that the fast carbon cycle (biosphere-ocean-atmosphere, seconds to millennia) is being perturbed by transfers from the slow carbon cycle (silicate weathering, ocean burial of carbonate sediments, subduction and volcanic return, 100,000s of years). The slow cycle is Earth's long-term thermostat (negative feedback: warmer, wetter climates speed chemical weathering, drawing down CO2 over millions of years) but is far too slow to absorb current emissions.
Carbon budget and planetary temperature: the balance between sources and sinks sets atmospheric CO2, which drives the greenhouse effect (natural greenhouse lifts surface temperature from about -18°C to ~+15°C). Pre-industrial CO2 was ~280 ppm; it passed 420 ppm in the 2020s - higher than any point in at least 800,000 years of ice-core record. Human additions of ~10 GtC/yr are only ~3% of natural fluxes but overwhelm the net balance because sinks absorb only about half (land ~30%, ocean ~25%, airborne fraction ~45%).
Feedback loops and climate change links (AQA 3.1.1.5, 3.1.1.7-3.1.1.8; Edexcel 2.1, 2.7; OCR 1.3)
Examiners want chained feedbacks that cross between the two cycles. Learn these as causal sentences:
- Water vapour (positive): warming → more evaporation → more atmospheric water vapour (a greenhouse gas) → more warming. Roughly doubles the warming from CO2 alone.
- Ice-albedo (positive): warming → melting sea ice/snow (a water-cycle store change) → lower albedo → more absorption → more warming. Arctic amplification.
- Permafrost carbon (positive): warming → thawing permafrost → microbial decomposition releases CO2 and CH4 → more greenhouse warming → more thaw. Store of ~1,400-1,700 GtC at risk; large-scale methane release is a potential tipping point.
- Forest dieback / transpiration recycling (positive): warming and drought → reduced vegetation and transpiration → less moisture recycled to the atmosphere → less rainfall → further vegetation loss → carbon store released as CO2. The key Amazon feedback (below).
- CO2 fertilisation (negative): higher CO2 → enhanced photosynthesis → greater biomass uptake → partial removal of CO2. Land sink absorbs ~30% of emissions; efficiency may decline as nutrient and water limits bite.
- Water-use efficiency (negative): higher CO2 lets stomata partially close, cutting transpiration per unit carbon fixed, helping plants survive drought - observed as greening of semi-arid regions.
- Cloud feedback (uncertain): low cloud cools (high albedo), high cirrus warms; net sign contested - citing uncertainty earns evaluation marks.
Essay technique: state the loop, name the stores/fluxes, give the sign, then evaluate - e.g. negative feedbacks currently offset about half of anthropogenic carbon, but their capacity is finite while positive feedbacks accelerate with warming.
Both cycles jointly regulate planetary temperature. The water cycle responds to warming via: intensified evaporation/precipitation (~7% more water vapour per °C - Clausius-Clapeyron); an enhanced hydrological cycle (wetter wet seasons, deeper droughts elsewhere); shrinking cryosphere (glacier retreat; Greenland losing ~150 Gt/yr ice); and eustatic sea-level rise from thermal expansion and meltwater (~3.3-3.7 mm/yr since the 1990s), complicated regionally by isostatic adjustment.
The carbon cycle's perturbation is the cause: fossil fuel emissions (~36-37 Gt CO2/yr) plus land-use change (~4-6 Gt CO2/yr) raise atmospheric CO2 and warm the planet; warming then feeds back into the water cycle (above) and weakens carbon sinks - Amazon dieback, permafrost thaw, and reduced ocean uptake as surface waters warm and stratify (warmer water holds less CO2; a weaker biological pump). Ocean acidification (pH down ~0.1 since 1750, ~30% more H+ ions) links carbon uptake to marine ecosystems - coral bleaching plus reduced calcification.
Synthesis sentence for essays: the two cycles are coupled - carbon change alters the water cycle, and water-cycle change (vapour, ice, forests) feeds back onto carbon stores - so management must target both.
Amazon case study: deforestation and cycle change (AQA 3.1.1.6; Edexcel 2.5-2.6; OCR 1.3)
The Amazon rainforest (~5.5 million km2, ~60% in Brazil) is the world's largest tropical carbon store: vegetation ~120-150 GtC; ~150-200 GtC with soils. Basin rainfall is high (~2,000-2,300 mm/yr at Manaus) and deeply coupled to carbon: 50-75% of rainfall is recycled via evapotranspiration and transpiration, generating 'flying rivers' of vapour that water agriculture further south (La Plata basin). Trees anchor a tight fast carbon cycle: high NPP, deep roots, rapid decomposition.
Deforestation data: ~17-20% of the original forest cleared by the 2020s (Brazilian Amazon); annual clearing peaked at ~27,700 km2 in 2004, fell to ~4,500 km2 by 2012 under enforcement (PPCDAm, soy moratorium), rose again after 2019 to ~11,600 km2 in 2022, then fell ~50% in 2023-24. Drivers: cattle ranching (~70-80% of cleared land), soya, roads (BR-319, Trans-Amazonian), mining, fires.
Hydrological impacts: clearing removes interception and transpiration → less vapour recycling and less convective rainfall; runoff and overland flow increase → higher flood peaks, soil erosion and river siltation; deep rooting lost, so dry-season baseflow falls over time. Model studies suggest clearing beyond ~20-25% could trigger large-scale 'savannisation' of eastern/southern Amazonia - a positive feedback (less forest → less rain → less forest), with observed dry-season lengthening in Rondônia and Mato Grosso.
Carbon impacts: deforestation converts a sink toward a source; land-use change is Brazil's largest emissions source in most years; fires release stored carbon immediately while remaining biomass decays over decades. Southeastern Amazonia monitoring (flux towers, AmazonFACE) already shows some plots switching from net sink to net source under drought and fire stress.
Management (AQA 3.1.1.9; Edexcel 2.8; OCR 1.4): a ladder of strategies - protected areas and indigenous territories (lowest deforestation rates), enforcement and satellite monitoring (DETER/INPE), market levers (soy and beef moratoria, REDD+ / Amazon Fund), restoration (Brazil's 12 Mha pledge), demand-side regulation (EU deforestation regulation). Global carbon levers: Paris Agreement NDCs, carbon pricing, CCS, afforestation. Weigh feasibility, cost and sovereignty (Brazil's development needs) - evaluation, not description, earns Level 4/5 marks.
💡 Worked Example
Exam-style: 'Cycles and feedbacks within the water and carbon cycles maintain dynamic equilibrium at a global scale.' To what extent do you agree with this view? (20 marks) - AQA-style synoptic question (AQA 3.1.1.1, 3.1.1.5, 3.1.1.7; Edexcel 2.1, 2.7; OCR 1.3)
Mark-scheme logic: <p><strong>Task:</strong> 'To what extent' demands a supported judgement. Plan: (1) evidence FOR equilibrium - closed global water cycle, balanced pre-industrial carbon cycle, negative feedbacks; (2) evidence AGAINST - anthropogenic perturbation, positive feedbacks, tipping points; (3) judgement. <strong>AO1 (knowledge, ~6 marks):</strong> define dynamic equilibrium; water cycle as closed system with evaporation approximating precipitation globally; pre-industrial carbon balance (atmosphere ~280 ppm stable for millennia per ice cores); negative feedbacks - CO2 fertilisation, silicate weathering thermostat, water-use efficiency. <strong>AO2 (application/analysis, ~8 marks):</strong> apply feedback chains to stores/fluxes with data: fossil fuel flux ~10 GtC/yr vs volcanic ~0.1-0.3 GtC/yr; sinks absorb ~50% of emissions, so equilibrium is broken - CO2 rose from 280 to >420 ppm; positive feedbacks (water vapour roughly doubling CO2 forcing, ice-albedo, permafrost 1,400-1,700 GtC, Amazon dieback beyond ~20-25% clearing) push the system toward a new equilibrium, not the old one. Worked example: Amazon deforestation → less transpiration → less recycled rainfall → further dieback → carbon release. <strong>AO3 (evaluation, ~6 marks):</strong> equilibrium is a useful model at geological timescales (slow carbon cycle thermostat) but not on human timescales; the system has been out of dynamic equilibrium since industrialisation and may be approaching tipping points; caveat - negative feedbacks still absorb ~half of emissions, so the system is disturbed but not yet runaway; cloud-feedback uncertainty adds error bars. Conclude: agree at long timescales, disagree at the scale that matters for 21st-century climate policy.</p>
❓ Practice Questions & Mark Schemes
Outline the stores and fluxes of the global water cycle. (4 marks) - AQA-style short question
Answer: Stores: ocean (96.5% of all water), ice caps/glaciers (~68.7% of freshwater), groundwater (~30.1% of freshwater), plus small atmospheric (~0.001% at any time), soil and biosphere stores. Fluxes: evaporation (~505,000 km3/yr, ~86% from oceans), precipitation, net vapour transfer of ~37,000-40,000 km3/yr ocean to land returned as runoff; drainage-basin fluxes include interception, infiltration, throughflow, percolation, baseflow and channel flow. One mark per accurate store/flux, up to 4.
Explain one way in which changes in the water cycle can affect the carbon cycle. (6 marks) - AQA-style
Answer: Levels-based; full marks need one developed, specific mechanism. Example: warming increases evaporation and atmospheric water vapour (water cycle); water vapour is a greenhouse gas, amplifying warming (positive feedback); warming thaws permafrost, releasing stored organic carbon (~1,400-1,700 GtC) as CO2 and CH4 via microbial decomposition, raising the atmospheric carbon flux. Alternative: drought reduces transpiration and NPP (e.g. Amazon), weakening the photosynthetic sink so more CO2 stays airborne. One mechanism, chained causally, with named stores/fluxes = 5-6.
'Deforestation of the tropical rainforest has a greater impact on the water cycle than on the carbon cycle.' Discuss. (9 marks) - Edexcel-style
Answer: Agree: local hydrology changes rapidly and measurably - interception loss raises overland flow and flood peaks, causes soil erosion; reduced transpiration cuts rainfall recycling (50-75% of Amazon rainfall is transpiration-recycled), so regional water-cycle impact is immediate. However: carbon impacts are globally significant - cleared biomass releases ~100+ tC/ha, land-use change is Brazil's largest emissions source, and the sink-to-source transition threatens the ~120-150 GtC store. Judgement: water-cycle impacts dominate locally and trigger the dieback feedback, but carbon impacts dominate globally because CO2 mixes through the whole atmosphere - the statement is only partly true; scale matters (AO3).
Evaluate the effectiveness of management strategies in protecting the Amazon's water and carbon cycles. (20 marks)
Answer: AO1: describe strategies - protected areas/indigenous reserves (deforestation several times lower inside boundaries), INPE satellite enforcement (DETER), PPCDAm policy (2004-2012: clearing cut from ~27,700 to ~4,500 km2/yr), soy moratorium (2006), beef agreements, REDD+ / Amazon Fund (Norway pledges >$1bn), restoration targets (12 Mha), demand-side rules (EU deforestation regulation). AO2: apply effectiveness to both cycles - enforcement protects transpiration recycling (water) and avoids carbon store loss; effectiveness is measurable through deforestation rates (2023-24: ~50% fall). AO3: reversal after 2019 (clearing ~11,600 km2 in 2022) shows dependence on national political will; sovereignty vs conditionality tension; leakage into other biomes/countries; restoration slow, enforcement cheaper; global beef/soya demand is the root driver, so local management alone is insufficient. Conclude: most effective = enforced protected areas + demand-side levers combined; single-strategy answers cap at Level 3.
Assess the relative importance of stores of carbon in regulating planetary temperature. (9 marks) - AQA/OCR style
Answer: Geological store is by far the largest (sedimentary rock ~99.9% of carbon) but its fluxes (weathering, burial, volcanism ~0.1-0.3 GtC/yr) operate over 100,000s of years - a long-term thermostat, irrelevant to decade-scale change. Ocean is the key active store: ~38,000-40,000 GtC, absorbing ~25-30% of anthropogenic emissions, though warming weakens solubility and stratification. Soils (~1,500-1,600 GtC) and permafrost (~1,400-1,700 GtC) are potential sources under warming; atmosphere (~890 GtC) is small but directly sets the greenhouse effect. Judgement: atmospheric concentration is what matters for temperature, but the ocean store is the most important regulator this century because of its uptake; the geological store dominates only at million-year scales.
Using Figure X (storm hydrograph), explain how urbanisation changes a drainage basin's water balance and flows. (6 marks) - OCR/AQA skills question
Answer: Impervious surfaces reduce infiltration and soil moisture recharge; pipes and gullies route water rapidly to the channel, so overland flow and quickflow rise; lag time shortens (often halved), peak discharge rises and stormflow volume increases - a flashy hydrograph; baseflow falls between storms because groundwater recharge is reduced. Effect on P = E + Q ± ΔS: higher Q, lower ΔS (soil/groundwater storage), often lower E (less evapotranspiration from vegetation). Link each clause to the figure (steeper rising limb, higher peak, shorter lag) for full marks.