HookThe Amazon has rivers in the sky — and it just started leaking carbon
A single large tree in the Amazon can transpire around 1,000 litres of water on a hot day, and the forest as a whole lifts roughly 20 billion tonnes of water vapour into the atmosphere every 24 hours — more water than the Amazon River itself pours into the Atlantic. Blown south-west by the trade winds and steered by the Andes, these rios voadores — 'flying rivers' — fall as rain over São Paulo and northern Argentina. The same forest holds an estimated 150–200 billion tonnes of carbon in its wood, leaves and soils. Water and carbon are not two separate stories here; they are two flows running through the same coupled machine.
In 2021 a nine-year aircraft study led by Luciana Gatti, published in Nature, found that the south-eastern Amazon had flipped from a carbon sink to a carbon source — deforestation, drought and fire now release more carbon than the surviving trees absorb. That single finding is this entire topic compressed into one sentence: water and carbon move through natural systems held in dynamic equilibrium, and when a store or a flow is disturbed, feedback loops either damp the change back down or amplify it into something new. Every question in this unit is a version of the same interrogation — what are the stores and flows, what has disturbed them, and does the system self-correct or spiral?
ModelEverything here is a system — learn the vocabulary first
AQA builds this whole unit on systems theory, so the marks begin with the language. A system has inputs, outputs, stores (or components) and flows/transfers that move mass and energy between the stores. An open system exchanges both energy and matter across its boundary — a drainage basin is the classic example, gaining water as precipitation and losing it as river discharge and evaporation. A closed system exchanges energy but not matter; the planet's water as a whole is effectively closed, which is why the total volume of water on Earth barely changes while it is endlessly redistributed.
Most of the time a system sits in dynamic equilibrium — inputs and outputs are balanced, so the stores stay roughly constant even though matter is constantly moving through. Disturb one variable and the system responds through feedback. Negative feedback damps the disturbance and nudges the system back towards equilibrium; it is self-correcting. Positive feedback amplifies the disturbance and drives the system further from where it started. Examiners love this distinction because students reflexively read 'positive' as good and 'negative' as bad, which is exactly wrong. Finally, systems are cascading: the output of one becomes the input of the next, which is how a single drainage basin links to the global cycle.
Take the Arctic permafrost feedback as a worked loop. Disturbance: warming thaws frozen soil. Response: microbes decompose the newly-available organic matter and release carbon dioxide and methane. Consequence: those greenhouse gases trap more heat, which thaws more permafrost, which releases more carbon. The output (warming) feeds back to increase the input, so the change is amplified — that is positive feedback, and it is destabilising. Contrast it with a negative-feedback loop: warmer air holds more water vapour, more cloud can raise albedo and reflect incoming radiation, cooling the surface and offsetting the original warming. Being able to name the loop, state its direction, and say whether it stabilises or destabilises the system is a clean route to AO1 and AO2 marks.
MechanismThe water cycle: the drainage basin as an open system
Globally, about 97% of water sits in the oceans and only around 2.5% is freshwater — and of that freshwater roughly two-thirds is locked in ice sheets and glaciers, most of the rest is groundwater, and less than 1% is the accessible surface water in rivers, lakes and the atmosphere. These are the global stores; the flows between them are driven by solar energy and gravity.
At the scale AQA examines most closely, the drainage basin is an open system with one input — precipitation — and two main outputs: evapotranspiration and runoff (river discharge leaving at the mouth). In between sit the flows and stores you must be able to sequence: interception by vegetation, then infiltration into the soil, overland flow across the surface when rain exceeds infiltration capacity, throughflow laterally through the soil, percolation down to the water table, and groundwater flow through the rock. The water balance ties it together as an equation: precipitation = evapotranspiration + runoff ± change in storage. Land use, geology and antecedent conditions decide how that budget splits, and the split is what a flood hydrograph reveals — a short lag time and high peak discharge signal a flashy basin (steep, impermeable, urbanised) where water reaches the channel as fast overland flow rather than slow groundwater flow.
A catchment receives 1,200 mm of precipitation in a year. Evapotranspiration is measured at 450 mm and, because soil and groundwater stores end the year roughly where they began, the change in storage is treated as zero. Rearranging the water balance P = E + Q ± ΔS gives runoff Q = 1,200 − 450 = 750 mm. The runoff ratio is 750 ÷ 1,200 = 0.63, meaning 63% of incoming rainfall leaves as river flow. Interpret it, don't just state it: a ratio that high points to limited infiltration and storage — impermeable rock, saturated or urbanised surfaces — which is precisely the profile of a basin prone to a flashy hydrograph and flooding. One equation, one ratio, one sentence of interpretation is a full AO3-plus-AO2 response.
MechanismThe carbon cycle: a fast lane and a slow lane
Carbon is held in four great stores: the atmosphere (about 880 gigatonnes of carbon as carbon dioxide and methane), the hydrosphere — mainly the oceans, which hold roughly 38,000 gigatonnes and are the largest active store — the biosphere (vegetation and soils, together well over 2,000 gigatonnes), and the lithosphere, where sedimentary rocks and fossil fuels lock away an enormous reservoir measured in tens of millions of gigatonnes. What matters for the exam is not just the sizes but the speeds.
The fast carbon cycle moves carbon between the atmosphere, biosphere and surface ocean over days to centuries, driven by biological processes: photosynthesis draws carbon dioxide down into plants, respiration and decomposition return it, and the oceanic carbon pump dissolves and biologically sinks carbon into deeper water. The slow carbon cycle operates over millions of years through chemical weathering of rock, carbonate deposition on the sea floor, burial and sequestration into sedimentary rock, and release through volcanic outgassing. The reason atmospheric carbon dioxide is rising is a mismatch of speeds: combustion of fossil fuels moves roughly 10 gigatonnes of carbon a year out of the slow store and into the fast cycle far quicker than weathering and sequestration can move it back. Human activity has, in effect, opened a valve between the two lanes.
DataHow the two cycles run the climate — and how you measure them
The cycles are coupled through climate. Carbon dioxide and water vapour are the two dominant greenhouse gases, so the carbon cycle sets the atmosphere's heat-trapping capacity while the water cycle both responds to and reinforces it — warmer air evaporates more water, more water vapour traps more heat, and the balance of resulting cloud can either amplify or offset the warming. This is why the topic insists you think in budgets and feedback rather than isolated facts: a change in one store propagates through the whole coupled system, and life on Earth — the carbon locked in the biosphere, the freshwater that ecosystems depend on — sits inside these balances.
AO3 is examined directly through quantitative and qualitative skills. You should be able to read and interpret a flood hydrograph (lag time, rising limb, peak discharge, baseflow), calculate a water balance and runoff ratio, work with a carbon-flux diagram in gigatonnes per year, and handle change over time from proxy and monitoring data such as the Mauna Loa carbon-dioxide record. The examiner is testing whether you can convert numbers into geographical meaning, not whether you can recite them.
A carbon-flux diagram shows a temperate forest fixing 12 gigatonnes of carbon a year through photosynthesis while releasing 9 through plant respiration and 2 through soil decomposition. Net ecosystem exchange = 12 − (9 + 2) = +1 gigatonne a year, so the forest is a net sink. Now add a disturbance: a severe drought year cuts photosynthesis to 8 while a wildfire adds 3 gigatonnes of combustion. The balance becomes 8 − (9 + 2 + 3) = −6 gigatonnes — the same forest is now a net source. That flip, worked from the fluxes, is exactly the Gatti finding for the south-eastern Amazon, and showing it numerically is far stronger than asserting 'forests can release carbon'.
CaseThe Amazon and a Yorkshire river: the cycles on the ground
AQA requires a tropical rainforest case study and a river catchment case study, and rewards field-scale specifics over generic labels. In the Amazon Basin, the intact forest tightly couples the two cycles: dense interception and rapid evapotranspiration recycle up to half of regional rainfall through the flying rivers, while photosynthesis holds the vast carbon store. Deforestation and fire break both flows at once — clearance reduces transpiration and interception, so more rainfall becomes overland flow and less is recycled, drying the regional climate, while burning and decomposition shift carbon from store to atmosphere. The result is the documented tipping from sink towards source in the degraded south-east, a textbook illustration of a disturbed system in danger of crossing into positive feedback.
For the river catchment, a UK basin lets you connect processes to real hydrographs. In the Boxing Day 2015 floods, storm Eva dropped extreme rainfall onto the already-saturated, steep and partly-urbanised catchments of the Rivers Calder and Aire in West Yorkshire; short lag times and high peak discharge overwhelmed Hebden Bridge and Mytholmroyd. Upland peat degradation, grazing and channelisation had reduced storage and speeded runoff — the human fingerprint on the water balance. Set against it, catchment interventions such as upstream tree planting, peatland restoration and leaky dams work by lengthening lag time and rebuilding storage. Naming the storm, the rivers, the towns and the mechanism turns a vague answer into an AO2 one.
VocabularyKey terms the mark scheme pays for
TrapsMisconceptions that cost marks
ExamWhat examiners want
AQA marks this unit against AO1 (knowledge of the systems, stores, flows and processes), AO2 (applying and evaluating that knowledge, especially on the 6-, 9- and 20-mark items) and AO3 (the quantitative and skills questions). On short 'using the figure' items, quote the data — a value, a gradient, a lag time — because AO3 credit is for reading the resource, not describing it in general terms. Always convert a number into a geographical statement: not '63% runoff' but '63% runoff, indicating limited infiltration and a flashy regime'.
On the 20-mark essays, the command word decides the shape. 'Assess', 'Evaluate' and 'To what extent' demand a running judgement, not a conclusion bolted on at the end — the top level of the mark scheme wants a coherent, evidenced line of reasoning and a substantiated conclusion. Anchor every claim in a named store, flow, feedback loop or case study (the Amazon flip, the 2015 Calder floods) rather than abstract theory. A reliable structure is to lead with the systems framework, apply it to a specific disturbance, weigh whether feedback stabilises or amplifies, and finish with a clear, defended verdict on the question actually asked.