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

This guide covers 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. It is Section A of Unit 3: Physical geography 2, a 1 hour 30 minute written exam in which you answer all questions in Section A (40 marks) and all questions in Section B (40 marks). Unit 3 is one of the units taken for the full International A-level, and the specification gives it 20% of the A-level.

Use the revision notes for this unit and the practice questions for this unit alongside this guide. The 9635 geography hub and the 9635 checklist show where this section fits. Systems ideas first appear in Unit 1, so see the Physical Geography 1 guide too.

Coverage table

Syllabus reference	What you must know or be able to do (from the document)	Where assessed
3.3.1.1	Systems concepts applied to the water and carbon cycles: inputs/outputs, energy, stores/components, flows/transfers, positive/negative feedback, dynamic equilibrium	Unit 3, Section A
3.3.1.2	Global distribution and size of water stores; processes changing them at hill slope, drainage basin and global scales; drainage basins as open systems; water balance; runoff variation and the flood hydrograph; natural and human changes	Unit 3, Section A
3.3.1.3	Global distribution and size of carbon stores; flows at plant, vegetation community and continental scales; natural and human changes; the carbon budget and its effects on land, ocean, atmosphere and climate	Unit 3, Section A
3.3.1.4	Role of water and carbon in supporting life; links between the cycles in the atmosphere; feedbacks and climate change; human interventions to mitigate climate change	Unit 3, Section A
3.3.1.5	Simple mass balance, unit conversions, analysis and presentation of field data	Unit 3, Section A
3.3.1.6	Case study of a tropical rainforest; case study of a local river catchment, using field data	Unit 3, Section A

3.3.1.1 Water and carbon cycles as systems

A system is a set of linked stores and flows. In an open system energy and matter cross the boundary; a drainage basin gains precipitation and loses runoff and evapotranspiration. In a closed system only energy crosses the boundary. The global water and carbon cycles are close to closed systems, driven by solar energy and gravity.

Key terms

• Store (component): where water or carbon is held, such as an ocean or a forest.
• Flow (transfer): movement between stores, such as infiltration or photosynthesis.
• Positive feedback: a change triggers effects that amplify the original change.
• Negative feedback: a change triggers effects that reduce it and restore balance.
• Dynamic equilibrium: inputs and outputs balance over time, so stores stay roughly constant despite constant flows.

Use these terms: “infiltration transfers water to the soil store” beats “water goes into the soil”.

3.3.1.2 The water cycle

Global stores. USGS figures put about 96.5% of Earth’s water in the oceans (hydrosphere). Ice caps, glaciers and permanent snow (cryosphere) hold about 1.74%, and groundwater (lithosphere) about 1.69%. The atmosphere holds only about 0.001%, yet it moves water fastest.

Processes. Evaporation needs energy, so it is greatest over warm oceans. Rising air cools (adiabatic cooling); at its dew point, water vapour condenses on tiny particles (condensation nuclei) to form clouds. Precipitation follows when droplets or ice crystals grow heavy enough to fall. There are three main causes of uplift:

• Convectional: surface heating makes air rise, as in daily afternoon storms over rainforests.
• Frontal: warm air is forced over cold air at a depression’s front.
• Orographic (relief): air is forced over hills and mountains.

Cryospheric processes are accumulation (snowfall adding to ice) and ablation (melting, sublimation and calving). At the global scale, during the last glacial maximum, so much water was stored as ice that sea level was about 120 to 125 m lower than today. At the drainage basin scale, snowmelt in spring can raise river flow for weeks. On hill slopes, frozen ground stops infiltration.

Drainage basins as open systems.

• Inputs: precipitation.
• Stores: interception (on leaves), surface storage (puddles, lakes), soil water, groundwater and channel storage.
• Flows: throughfall and stemflow (water reaching the ground from leaves and down stems), infiltration (into soil), percolation (into rock), throughflow (sideways in soil), overland flow (across the surface), groundwater flow and channel flow.
• Outputs: evapotranspiration and runoff (river discharge leaving the basin).

The water balance is P = Q + E ± ΔS: precipitation equals runoff plus evapotranspiration plus or minus change in storage.

Runoff variation and the flood hydrograph. A storm hydrograph plots discharge against time with rainfall shown as bars. Key parts are the rising limb, peak discharge, falling (recession) limb, baseflow and stormflow. Lag time is the time between peak rainfall and peak discharge.

Gives a flashy hydrograph (short lag, high peak)	Gives a subdued hydrograph (long lag, low peak)
Small, round basin with steep slopes	Large, elongated basin with gentle slopes
High drainage density	Low drainage density
Impermeable rock, thin or saturated soil	Permeable rock, deep dry soil
Little vegetation, urban surfaces, drains	Forest, high interception
Intense rain or rapid snowmelt	Light, steady rain

Changes over time. Natural changes include storm events and seasonal patterns. Human impacts include farming practices (ploughing downslope, soil compaction by livestock and machinery, field drains, irrigation), land use change (deforestation, urbanisation) and water abstraction. Abstraction lowers groundwater stores and can reduce baseflow in dry seasons.

3.3.1.3 The carbon cycle

Global stores (year 2000 estimates quoted on the Wikipedia carbon cycle page, in gigatonnes of carbon, GtC):

Store	Size (GtC)
Lithosphere: sedimentary carbonates and kerogens	more than 60,000,000 and about 15,000,000
Lithosphere: fossil fuels	about 4,130
Hydrosphere: oceans (mostly deep ocean)	about 38,400
Biosphere: soils	about 1,500
Biosphere: living biomass	about 600 to 1,000
Atmosphere	about 720 in 2000, rising since

The cryosphere also stores carbon, mainly as frozen organic matter in permafrost.

Flows at three scales.

• Plant: photosynthesis takes in carbon dioxide; respiration releases it. Decomposition by microbes returns carbon from dead matter to the soil and atmosphere.
• Vegetation community: a forest stores more carbon as it grows through succession, then reaches a balance between uptake and loss. Fire (combustion) releases carbon quickly.
• Continental: Northern Hemisphere forests take up carbon in summer and release it in winter. This produces the yearly zig-zag in the carbon dioxide record at Mauna Loa.

Ocean and rock processes. Carbon dioxide dissolves in cold, high-latitude water, which sinks (the physical pump). Plankton fix carbon, and their remains and shells sink (the biological pump). Shells collect as sediments that can become limestone. Chemical weathering of rocks by weak carbonic acid in rain removes carbon dioxide from the atmosphere over very long timescales.

Changes over time.

• Natural: wildfires release carbon quickly, but regrowth can take it back over decades. Volcanic activity releases carbon from the lithosphere.
• Human: extracting and burning hydrocarbon fuels moves carbon from a slow store (the lithosphere) to a fast one (the atmosphere). Farming practices matter too: ploughing exposes soil carbon to decomposition, and cattle and flooded rice fields produce methane. Deforestation and land use changes, such as draining peat or urban growth, also release carbon.

The carbon budget compares carbon entering and leaving each store. The Global Carbon Project projected 2018 fossil emissions at 37.1 billion tonnes of carbon dioxide, about 10.1 GtC. Somewhat under half of human emissions stays in the atmosphere; oceans and land take up the rest. Atmospheric carbon dioxide at Mauna Loa rose from an annual mean of 315.98 ppm in 1959 to 421.08 ppm in 2023, compared with about 280 ppm before industrialisation. Effects include an enhanced greenhouse effect and warming (atmosphere), ocean acidification (ocean) and changes to plant growth and soil carbon (land).

3.3.1.4 Water, carbon, climate and life on Earth

Carbon is the basis of all organic molecules, and liquid water is needed by every living cell. Water vapour and carbon dioxide are both greenhouse gases. Without the natural greenhouse effect, Earth’s mean surface temperature would be about −18 °C rather than about 15 °C.

Links in the atmosphere. Warming from extra carbon dioxide increases evaporation. Warmer air holds more water vapour, which traps more heat. Plants link the cycles directly: when stomata open to take in carbon dioxide, water is transpired.

Feedbacks.

• Positive: less ice lowers albedo, so more heat is absorbed (ice-albedo feedback). Thawing permafrost releases carbon dioxide and methane. Drying rainforest burns more easily, releasing carbon.
• Negative: higher carbon dioxide can increase photosynthesis (carbon dioxide fertilisation). Faster chemical weathering in a warmer, wetter climate draws down carbon dioxide, but only over very long timescales.

Implications for life include shifting rainfall belts, drought, flood risk and coral stress from warmer, more acidic oceans.

Human interventions aim to reduce carbon transfers to the atmosphere or increase them out of it:

• Afforestation and reforestation; protecting forests through schemes such as the UN’s REDD+ (Reducing Emissions from Deforestation and forest Degradation).
• Restoring peatlands and wetlands; farming changes such as reduced tillage and cover crops.
• Carbon capture and storage. At Sleipner in the Norwegian North Sea, about one million tonnes of carbon dioxide a year have been injected into the Utsira formation since 1996.
• Cutting fossil fuel use, supported by international agreements such as the Paris Agreement (2015).

Evaluate each by scale, cost and permanence. Trees store carbon only while they survive; carbon capture and storage works at single sites but is small next to global emissions.

3.3.1.5 Quantitative skills

Unit conversions.

• 1 mm of rain over 1 km² is 1,000 m³. So 30 mm on the fictional 25 km² Holmby Water catchment is 750,000 m³.
• Multiply GtC by 44/12 (about 3.67) to get gigatonnes of carbon dioxide.
• 1 ppm of atmospheric carbon dioxide is about 2.12 GtC, so 421 ppm is about 893 GtC.

Simple mass balance. Change in store = inputs − outputs. A fictional glacier gaining 2.4 million m³ water equivalent from accumulation and losing 3.1 million m³ to ablation has a balance of −0.7 million m³, so it is shrinking.

Field data. Catchment fieldwork often measures infiltration and river discharge (cross-sectional area x velocity). Use bar charts to compare sites, scatter graphs for relationships and hydrographs for discharge over time; analyse with means and ranges.

3.3.1.6 Case studies

Tropical rainforest: the Amazon

Water cycle. The rainforest covers about 5.5 million km² of a basin of about 7 million km², and about 60% is in Brazil. A tall, multi-layered canopy gives high interception and evapotranspiration. Moist air rises in daily convectional storms. About half of the region’s rainfall is produced by the forest itself, and moisture from the forest affects rainfall in Brazil, Paraguay and Argentina.

Carbon cycle. Amazonian forests are estimated to hold about 110 billion tonnes of carbon. Most is in the trees, not the soil, because warm, wet conditions speed up decomposition.

Change and human activity. Deforestation in the Brazilian Amazon reached 27,772 km² in 2004, then fell to 4,571 km² in 2012, an 84% drop. About 80% of deforestation is linked to cattle ranching. Responses included the Soy Moratorium (2006) and the Amazon Fund, to which Norway pledged US$1 billion in 2008. By 2022 about 20% of the Amazon had been deforested and a further 6% was highly degraded. Some research suggests a tipping point to degraded savanna at about 20 to 25% loss. Gatti and colleagues (2021), using aircraft measurements of carbon dioxide, reported that the south-eastern Amazon had become a net carbon source, linked to deforestation and a warmer, drier dry season.

Evaluation. The 2004 to 2012 fall shows policy can work, but it was not sustained.

Local river catchment: the River Eden, Cumbria

The specification asks for a local-scale catchment with field data, so your teacher will confirm which catchment you study. The Eden is a well-documented example.

The Eden rises at Black Fell Moss, Mallerstang. It flows through Kirkby Stephen, Appleby-in-Westmorland and Carlisle to the Solway Firth, joined by the Eamont, Caldew, Petteril and Irthing.

• Flooding, 7 January 2005: up to 180 mm of rain fell upstream in a day. The Eden, Caldew and Petteril flooded about 2,700 homes in Carlisle and three people died.
• Flooding, December 2015 (Storm Desmond): Honister Pass in the Lake District recorded 341.4 mm on 5 December. About 36 hours of rain overwhelmed Carlisle’s defences, and about 5,200 homes flooded across Cumbria and Lancashire. After days of rain, soil stores were close to full, so more rain became overland flow and reached channels quickly.
• Water supply: Haweswater Reservoir, filled by 1935 for Manchester and now run by United Utilities, supplies about 25% of North West England’s water. It drains via Haweswater Beck towards the Lowther, Eamont and Eden. Storage can protect supply in dry spells, but it moves water out of the Eden basin.

Evaluation. The Eden shows the same basin can face both too much water and pressure on supply. Higher defences protect Carlisle but can pass water downstream. Upland land use change, such as tree planting, could increase interception, but is unproven at the scale of a 2015-type storm.

Worked example

Assess how deforestation changes the water cycle in a tropical rainforest. (extract)

Clearing the Amazon removes the multi-layered canopy, so interception falls and more rain reaches bare ground (knowledge). Where cattle compact the soil, infiltration falls and overland flow rises, carrying soil into rivers (application). Because about half of Amazon rainfall is recycled from the forest, less evapotranspiration means less moisture for rain downwind, which can lengthen the dry season and make the remaining forest more likely to burn: a positive feedback (analysis). However, the effect depends on scale: small clearings may change little, whereas loss nearing the suggested 20 to 25% tipping point could shift whole regions towards savanna, so the scale of clearance matters more than any single clearing (evaluation).

Common errors

• Calling the drainage basin a closed system. It is open.
• Defining lag time from the start of rainfall, not from peak rainfall.
• Saying the oceans absorb “all” human carbon dioxide; somewhat under half stays in the atmosphere.
• Confusing ppm, GtC and gigatonnes of carbon dioxide in calculations.
• Treating feedback as always negative. Several key climate feedbacks, such as ice-albedo, are positive.
• Using rainfall figures from Honister Pass as if they were measured in the Eden basin.

For exam technique across all units, see OxfordAQA A-level Geography exam preparation. Revise hazards links in the revision notes for Physical Geography 1.

Official syllabus

This guide follows 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.