AQA Specification focus:
‘Case study of a tropical rainforest to illustrate water and carbon cycles, environmental change, and human activity.’
Tropical rainforests offer dynamic insights into the interactions between water and carbon cycles, environmental change, and human activity through a specific local or regional context.
The Amazon Rainforest: An Overview
Geographic Context
The Amazon Rainforest spans over 5.5 million km², across nine countries in South America, with most of it located in Brazil. It is the largest tropical rainforest in the world and plays a crucial role in global hydrological and carbon systems.
Climate and Vegetation
Hot, humid climate with high annual rainfall (over 2,000 mm)
Dense, multi-layered vegetation with year-round photosynthesis
A key carbon sink, absorbing CO₂ through plant growth
The Water Cycle in the Amazon
Inputs and Outputs
Input: High levels of precipitation from convective rainfall due to intense heating and evapotranspiration
Outputs: Include evapotranspiration, runoff into the Amazon River system, and eventual discharge into the Atlantic Ocean
Evapotranspiration: The combined process of water loss from soil through evaporation and from plants through transpiration.
Stores and Flows
Stores:
Atmosphere: Moisture held in tropical air masses
Soil moisture: Retained in porous tropical soils
Vegetation: Stores moisture in leaves and stems
Aquifers and surface water: Rivers, lakes, and groundwater
Flows:
Infiltration into soils
Percolation to groundwater
Surface runoff during heavy rainfall
Throughflow via soil layers
Water Balance
Rainforest water cycles exhibit positive water balances, meaning inputs exceed outputs, supporting lush vegetation and year-round biomass growth.
This diagram illustrates the hydrological cycle in the Amazon, showing natural processes—precipitation, evapotranspiration, infiltration, and runoff—with added labels for human activities like water abstraction. Extra annotations on water management interventions extend beyond the core syllabus but help visualise anthropogenic influences on water stores. Source
The Carbon Cycle in the Amazon
Major Carbon Stores
Biosphere: Massive quantities of carbon in trees and understorey vegetation
Soils: Store carbon from decayed organic matter
Atmosphere: Source of CO₂ absorbed by photosynthesising plants
Key Processes
Photosynthesis absorbs atmospheric CO₂
Respiration by plants and animals releases CO₂
Decomposition of leaf litter returns carbon to the soil and atmosphere
Carbon sequestration occurs in woody biomass and soil organic matter
Carbon sequestration: The long-term storage of carbon in plants, soils, geologic formations, and the ocean.
Carbon Budget
The Amazon has historically operated as a net carbon sink, removing more CO₂ than it emits.
The diagram shows carbon fluxes between reservoirs, with yellow arrows for natural exchanges and red for human-driven fluxes, and white numbers indicating stored carbon. Additional emphasis on slow geological processes (e.g. sediment formation, volcanic release) provides broader context beyond the regional rainforest focus. Source
Environmental Change in the Amazon
Natural Change
Droughts: Linked to El Niño cycles, reducing water availability and slowing photosynthesis
Forest dieback: In severe drought years, vegetation loss leads to reduced carbon uptake
Human-Induced Change
Deforestation: Large-scale logging, agriculture (e.g., soy farming), and cattle ranching are the biggest threats
Slash-and-burn techniques: Release stored carbon, reduce soil moisture retention, and decrease regional rainfall
Soil degradation: Removal of canopy exposes soils, reducing carbon storage and increasing erosion
Feedback Mechanisms
Deforestation reduces evapotranspiration, leading to less precipitation and further drying
Loss of vegetation leads to lower carbon sequestration, increasing atmospheric CO₂ levels
This results in positive feedback contributing to global warming
Positive feedback: A process in which the effects of a change in a system amplify that change further.
Human Activity and Management Responses
Impacts of Human Activity
Agricultural expansion drives habitat loss and alters regional hydrological and carbon cycles
Hydroelectric dams (e.g., Belo Monte) impact river discharge, sediment transport, and aquatic ecosystems
Infrastructure development (e.g., roads) fragments forest areas, reducing biodiversity and carbon storage
Sustainable Strategies
To counteract these pressures, several management approaches are in use:
REDD+ Program (Reducing Emissions from Deforestation and Forest Degradation):
Encourages developing countries to preserve forests in exchange for financial incentives
Agroforestry:
Integrates trees and crops, maintaining carbon storage while allowing sustainable land use
Ecotourism and conservation schemes:
Provide alternative income sources while promoting rainforest protection
Agroforestry: A land-use system combining trees with crops or livestock to increase biodiversity and sustainability.
Monitoring and Enforcement
Satellite imaging and GIS used to track deforestation
Government legislation, such as Brazil’s Forest Code, aims to regulate land clearance
NGO involvement (e.g., WWF, Rainforest Alliance) supports local education, reforestation, and advocacy
Conclusion of Case Study Alignment
This case study of the Amazon Rainforest illustrates:
Complex interactions between water and carbon cycles
The influence of natural events and human activity on environmental change
Examples of mitigation and management that aim to stabilise both local ecosystems and global processes.
FAQ
The tallest emergent and canopy trees in the Amazon are the most significant contributors to carbon sequestration due to their large biomass. These species include hardwoods like mahogany and Brazil nut trees.
They absorb vast quantities of CO₂ during photosynthesis and store it in trunks, branches, and roots over decades.
Understorey vegetation and epiphytes also play a role, though their contribution is comparatively smaller due to lower biomass.
Road construction leads to vegetation clearance, compaction of soils, and altered drainage patterns.
Impacts include:
Increased surface runoff due to reduced infiltration
Greater soil erosion from exposed ground
Disruption of throughflow and groundwater recharge
Roads can also fragment the forest, reducing regional evapotranspiration and lowering local rainfall levels over time.
The Amazon is called a 'carbon time bomb' because of the risk that continued deforestation and climate stress could turn it from a carbon sink into a carbon source.
If tree mortality increases or fires become more frequent:
Stored carbon may be released rapidly into the atmosphere
The feedback loop could accelerate global warming. This tipping point would undermine climate regulation at a global scale.
Indigenous communities typically use small-scale, rotational practices that maintain forest cover and biodiversity.
Key differences include:
Minimal long-term deforestation
Use of polyculture and agroforestry systems
Lower carbon emissions and soil disturbance
In contrast, commercial land uses like cattle ranching or soy farming involve large-scale, permanent clearance, leading to major disruptions of both the water and carbon cycles.
Rainforest soils store carbon in organic matter from leaf litter and decomposed organisms.
However, they are:
Often nutrient-poor and thin, relying on the canopy for constant organic input
Vulnerable to erosion when vegetation is removed
Deforestation exposes soils to rain and sunlight, accelerating decomposition and reducing their carbon storage capacity over time.
Practice Questions
Identify three ways in which deforestation in tropical rainforests such as the Amazon impacts the carbon cycle. (3 marks)
1 mark for each correct impact, up to a maximum of 3 marks.
Accept any of the following:
Reduces carbon sequestration by decreasing biomass.
Increases atmospheric CO₂ due to burning vegetation.
Disturbs soil carbon stores through erosion or exposure.
Reduces photosynthesis rates due to loss of canopy.
Increases carbon emissions from decomposing cleared vegetation.
Explain how human activity in the Amazon rainforest affects both the water and carbon cycles. (6 marks)
Level 1 (1–2 marks): Basic statements with limited understanding, likely descriptive. e.g. “Deforestation releases carbon and affects rainfall.”
Level 2 (3–4 marks): Clear understanding of how human activities (e.g. agriculture, infrastructure) impact one or both cycles. Begins to link processes.
e.g. “Deforestation leads to reduced evapotranspiration, lowering rainfall. It also reduces carbon storage and increases emissions from burning.”Level 3 (5–6 marks): Detailed explanation with clear links between human activities and impacts on both cycles. May include examples such as farming, road-building or hydroelectric projects.
e.g. “Cattle ranching reduces vegetation cover, lowering evapotranspiration and rainfall, which disrupts the water cycle. At the same time, burning trees for land clearance emits stored CO₂, weakening the forest’s role as a carbon sink.”
