OxfordAQA International A-level Geography (9635): 3.3.1 Water, carbon and life on earth – Revision Notes

Read the Water, carbon and life on earth study guide first; these notes condense it for revision.

These notes cover section 3.3.1, Water, carbon and life on earth (3.3.1.1 to 3.3.1.6), of the OxfordAQA International AS and A-level Geography (9635) specification, Version 3.2. The section is Section A of Unit 3: Physical geography 2, a 1 hour 30 minute written exam; you answer all questions in Section A (40 marks) and Section B (40 marks).

Test yourself afterwards with the practice questions for this unit. The 9635 geography hub and the 9635 checklist show where this section fits. Section B of the same unit is covered in the Ecosystems under stress guide, and catchment fieldwork methods link to the Geographical and fieldwork skills guide.

What the specification lists

Section	Content as printed (summarised)
3.3.1.1 Systems	Inputs/outputs, energy, stores/components, flows/transfers, positive/negative feedback, dynamic equilibrium
3.3.1.2 Water cycle	Size and distribution of stores; evaporation, condensation, cloud formation, causes of precipitation, cryospheric processes at hill slope, basin and global scales; drainage basins as open systems; water balance; flood hydrograph; natural and human change
3.3.1.3 Carbon cycle	Size and distribution of stores; photosynthesis, respiration, decomposition, combustion, sequestration in oceans and sediments, weathering at plant, community and continental scales; natural and human change; carbon budget and its effects, including on global climate
3.3.1.4 Climate and life	Role of the cycles in supporting life; links between the cycles in the atmosphere; feedbacks and climate change; human interventions to mitigate climate change
3.3.1.5 Skills	Simple mass balance, unit conversions, analysis and presentation of field data
3.3.1.6 Case studies	A tropical rainforest; a local river catchment with field data, covering water supply and/or flooding

Key terms

Term	Definition
Open system	Energy and matter cross the boundary (a drainage basin)
Closed system	Only energy crosses the boundary (close to true for the global cycles)
Positive feedback	A change sets off effects that amplify it
Negative feedback	A change sets off effects that reduce it
Dynamic equilibrium	Inputs and outputs balance, so stores stay roughly steady
Interception	Precipitation caught by vegetation before it reaches the ground
Stemflow / throughfall	Water running down stems / dripping from leaves to the ground
Infiltration / percolation	Water entering soil / moving down into rock
Throughflow	Water moving sideways through soil
Overland flow	Water moving across the surface when soil cannot absorb it
Baseflow	River flow fed slowly by groundwater
Water balance	P = Q + E ± ΔS
Lag time	Time from peak rainfall to peak discharge
Ablation / accumulation	Loss of ice by melting, sublimation and calving / gain of ice from snowfall
Carbon sequestration	Long-term capture of carbon in a store such as ocean sediment or soil
Biological pump	Plankton fix carbon; their remains sink to the deep ocean
Carbon budget	The balance of carbon entering and leaving stores
Albedo	The share of incoming sunlight a surface reflects

Store sizes to quote

Water (USGS)	Share of all water
Oceans	about 96.5%
Ice caps, glaciers, permanent snow	about 1.74%
Groundwater	about 1.69%
Atmosphere	about 0.001%

Carbon (year 2000 estimates)	GtC
Sedimentary rocks	tens of millions
Oceans	about 38,400
Fossil fuels	about 4,130
Soils	about 1,500
Living biomass	about 600 to 1,000
Atmosphere	about 720 then; now nearer 890

The 890 figure comes from 421 ppm x 2.12 GtC per ppm. Round store sizes in exams and say they are estimates.

Case studies and evidence at a glance

Example	What	Key facts	One evaluation point
Amazon rainforest	Tropical rainforest water and carbon cycles	About 5.5 million km² of forest, 60% in Brazil; about half of rainfall recycled from the forest; about 110 billion tonnes of carbon	Store sizes are estimates with wide uncertainty
Amazon deforestation	Human change	27,772 km² cleared in 2004, 4,571 km² in 2012; about 80% linked to cattle; about 20% of the forest lost by 2022	The fall after 2004 shows policy can work, but it was not sustained
Gatti and colleagues (2021)	Aircraft carbon dioxide measurements over Amazonia	The south-eastern Amazon had become a net carbon source	Based on aircraft profiles at four sites, not the whole forest
River Eden, Cumbria	Local catchment, flooding and supply	January 2005: up to 180 mm upstream in a day, about 2,700 Carlisle homes flooded; December 2015, Storm Desmond: defences overwhelmed	Carlisle’s defences were breached in 2015, so defences alone are not enough
Haweswater Reservoir	Water supply in the Eden basin	About 25% of North West England’s water; filled by 1935	Storage secures supply but moves water out of the basin
Sleipner, Norway	Carbon capture and storage	About 1 million tonnes of carbon dioxide a year into the Utsira formation since 1996	Proven, but tiny next to global emissions
Mauna Loa record	Atmospheric carbon dioxide	Annual mean 315.98 ppm (1959) to 421.08 ppm (2023)	One site, but matches global records

Must-know distinctions

• Store vs flow: groundwater is a store; groundwater flow is a transfer.
• Open vs closed system: a drainage basin is open; the global water cycle is close to closed.
• Infiltration-excess vs saturation-excess overland flow: rain falls faster than soil can absorb it, versus soil already full of water.
• Peak discharge vs bankfull discharge: the highest flow in a storm versus the flow that just fills the channel; flooding happens only when discharge exceeds bankfull.
• Fast vs slow carbon cycle: photosynthesis and respiration take days to decades; rock weathering and sedimentation take millions of years. Burning fossil fuels moves carbon from slow to fast.
• GtC vs gigatonnes of carbon dioxide: multiply GtC by 44/12 (about 3.67) to get carbon dioxide.
• Natural vs enhanced greenhouse effect: the natural effect keeps Earth habitable; the enhanced effect is the extra warming from human emissions.
• Mitigation vs adaptation: reducing carbon transfers to the atmosphere versus coping with the effects. The specification’s human interventions point is about mitigation.

Feedback chains to learn

• Ice-albedo (positive): warming → ice melts → darker surface → more heat absorbed → more warming.
• Water vapour (positive): warming → more evaporation → more water vapour, a greenhouse gas → more warming.
• Permafrost (positive): warming → thaw → decomposition releases carbon dioxide and methane → more warming.
• Rainforest drying (positive): deforestation → less evapotranspiration → less rain → more fire → carbon released.
• Carbon dioxide fertilisation (negative): more carbon dioxide → faster photosynthesis → more uptake.
• Weathering (negative, very slow): warmer and wetter → faster chemical weathering → carbon dioxide drawn down.

Mini essay plan

“Human activity is now the main driver of change in the carbon cycle.” To what extent do you agree?

1. Introduction: define the carbon cycle as linked stores and flows; separate natural and human drivers; state your line (agree for the fast cycle over recent decades).
2. Human drivers: burning hydrocarbons moves carbon from the lithosphere to the atmosphere; deforestation, as in the Amazon, and farming release biosphere carbon. Use the Mauna Loa rise from 315.98 to 421.08 ppm.
3. Scale and speed: human transfers happen in decades; natural slow-cycle transfers take millions of years.
4. Natural drivers still matter: wildfires and volcanic activity release carbon; seasonal photosynthesis creates the yearly Mauna Loa cycle.
5. Interaction: human warming can trigger natural feedbacks, such as permafrost thaw and forest drying, so the line between natural and human is blurred.
6. Conclusion: agree largely for the last century and for the atmosphere store; over geological time natural processes dominate. Judge by timescale and store.

Quick self-test

1. Name the four water stores listed in the specification.
2. Give the water balance equation.
3. What is stemflow?
4. Define lag time.
5. Give two drainage basin features that make a hydrograph flashy.
6. Name two ways carbon is sequestered in oceans.
7. Convert 5 GtC into gigatonnes of carbon dioxide.
8. A glacier gains 1.8 million m³ and loses 2.2 million m³ of water equivalent in a year. What is its mass balance?
9. Why does most carbon in a tropical rainforest sit in the vegetation rather than the soil?
10. Give one positive and one negative feedback in the carbon cycle.
11. Name one human intervention designed to reduce carbon transfers to the atmosphere.
12. How many cubic metres of water fall when 12 mm of rain falls on 3 km²?

Answers

1. Lithosphere, hydrosphere, cryosphere and atmosphere.
2. P = Q + E ± ΔS (precipitation = runoff + evapotranspiration ± change in storage).
3. Intercepted water running down plant stems and trunks to the ground.
4. The time between peak rainfall and peak discharge.
5. Any two: steep slopes, impermeable rock, high drainage density, small round basin, urban surfaces, saturated soil.
6. The physical pump (carbon dioxide dissolving in cold water that sinks) and the biological pump (plankton remains sinking); also shells forming sediments.
7. 5 x 44/12 = about 18.3 gigatonnes of carbon dioxide.
8. 1.8 − 2.2 = −0.4 million m³: a negative balance, so the glacier shrinks.
9. Warm, wet conditions speed up decomposition, so dead matter is broken down fast and nutrients are taken back into plants quickly.
10. Positive: permafrost thaw releasing carbon. Negative: carbon dioxide fertilisation of plants.
11. Any one: afforestation, carbon capture and storage, peatland restoration, reduced tillage, forest protection such as REDD+.
12. 12 x 3 x 1,000 = 36,000 m³.

Where marks are usually lost

• Calling a drainage basin a closed system.
• Listing stores and flows without naming the direction of the transfer.
• Quoting store sizes with no units, or mixing GtC and gigatonnes of carbon dioxide.
• Explaining hydrograph shape with one factor only, with no link to infiltration or throughflow.
• Treating Honister Pass rainfall as if it fell in the Eden basin.
• Rainforest answers that describe deforestation but never say what happens to interception, evapotranspiration or carbon stores.
• Evaluating human interventions as simply “good” without judging scale, cost or permanence.
• Essays with no judgement on “to what extent”.

Official syllabus

These notes follow the OxfordAQA International AS and A-level Geography (9635) specification, Version 3.2, section 3.3.1 (Water, carbon and life on earth), sections 3.3.1.1 to 3.3.1.6, assessed in Unit 3: Physical geography 2, Section A.