AQA A-level Geography · 3.1.1

The carbon cycle

50 exam-style questions with answers for 3.1.1 The carbon cycle in AQA A-level Geography (spec 7037). Practise with flashcards, a topic quiz, a random quiz across the course, 5 crosswords, or read every question and answer below. Not official AQA material.

All 50 questions and answers

Question 1 · Spec 3.1.1.3 · Global carbon stores
Name the five main global stores of carbon. [5 marks]
Answer:
Lithosphere, such as sedimentary rocks and fossil fuels (1); hydrosphere, the oceans (1); cryosphere, such as permafrost (1); biosphere, living organisms and soils (1); atmosphere, mainly carbon dioxide and methane (1).
Question 2 · Spec 3.1.1.3 · Global carbon stores
Identify the largest store of carbon and the forms in which it is held. [2 marks]
Answer:
The lithosphere (1); in sedimentary rocks such as limestone and in fossil fuels (1).
Question 3 · Spec 3.1.1.3 · Global carbon stores
Compare the size of the carbon store in the oceans with that in the atmosphere. [2 marks]
Answer:
The oceans hold many times more carbon than the atmosphere (1); mostly as dissolved inorganic carbon, whereas atmospheric carbon is mainly carbon dioxide gas (1).
Question 4 · Spec 3.1.1.3 · Global carbon stores
Explain why soils are important stores of carbon. [2 marks]
Answer:
Soils contain organic matter from dead plants and animals (1); in cold or waterlogged soils decomposition is slow, so carbon accumulates, for example in peat (1).
Question 5 · Spec 3.1.1.3 · Global carbon stores
Explain why permafrost is an important carbon store. [2 marks]
Answer:
Plant material has been frozen before it could decompose (1); permafrost therefore holds very large amounts of organic carbon, which could be released if it thaws (1).
Question 6 · Spec 3.1.1.3 · Global carbon stores
Distinguish between a carbon sink and a carbon source. [2 marks]
Answer:
A carbon sink absorbs more carbon than it releases, so it increases the store (1); a carbon source releases more carbon than it absorbs (1).
Question 7 · Spec 3.1.1.3 · Global carbon stores
Distinguish between the fast (biological) and slow (geological) carbon cycles. [3 marks]
Answer:
The fast cycle moves carbon between the atmosphere, oceans, vegetation and soils (1); over years to centuries (1); the slow cycle moves carbon into and out of rocks over millions of years (1).
Question 8 · Spec 3.1.1.3 · Global carbon stores
Compare the residence time of carbon in the atmosphere with that in sedimentary rocks. [2 marks]
Answer:
In the atmosphere, carbon stays for only a few years (1); in sedimentary rocks it can stay for millions of years (1).
Question 9 · Spec 3.1.1.3 · Global carbon stores
Explain why the carbon cycle is a closed system at the global scale. [2 marks]
Answer:
Almost no carbon enters or leaves the Earth (1); so the total amount of carbon is fixed and only moves between stores, while energy from the Sun flows through (1).
Question 10 · Spec 3.1.1.3 · Carbon flows
Define photosynthesis and state its overall effect on carbon. [2 marks]
Answer:
The process by which plants use sunlight to make glucose from carbon dioxide and water (1); it removes carbon dioxide from the atmosphere and stores carbon in plant biomass (1).
Question 11 · Spec 3.1.1.3 · Carbon flows
Define respiration. [1 mark]
Answer:
The release of energy from glucose by living organisms, which releases carbon dioxide.
Question 12 · Spec 3.1.1.3 · Carbon flows
Explain the role of decomposition in the carbon cycle. [2 marks]
Answer:
Bacteria and fungi break down dead organic matter, releasing carbon dioxide to the atmosphere (1); in waterlogged, oxygen-poor conditions decomposition is anaerobic and releases methane (1).
Question 13 · Spec 3.1.1.3 · Carbon flows
Define combustion in the carbon cycle. [1 mark]
Answer:
The burning of organic material such as wood, peat and fossil fuels, which releases carbon dioxide rapidly.
Question 14 · Spec 3.1.1.3 · Carbon flows
Explain how carbonation weathering links the atmosphere and the oceans. [3 marks]
Answer:
Carbon dioxide dissolves in rainwater to form weak carbonic acid (1); this reacts with rocks such as limestone, dissolving them into bicarbonate (1); rivers carry the dissolved carbon to the oceans, where it is used by organisms or stored in sediments (1).
Question 15 · Spec 3.1.1.3 · Carbon flows
Explain how the physical (solubility) pump transfers carbon into the deep ocean. [3 marks]
Answer:
Carbon dioxide dissolves into cold surface water at high latitudes (1); cold, dense water sinks, carrying dissolved carbon downwards (1); deep currents store it in the deep ocean for hundreds or thousands of years (1).
Question 16 · Spec 3.1.1.3 · Carbon flows
Explain how the biological pump transfers carbon into the deep ocean. [3 marks]
Answer:
Phytoplankton take in carbon dioxide by photosynthesis (1); they are eaten or die, and organic material and shells sink to the deep ocean (1); some is buried in sediments, removing carbon from the surface cycle (1).
Question 17 · Spec 3.1.1.3 · Carbon flows
Explain how carbon becomes stored in sedimentary rocks. [3 marks]
Answer:
Shells and skeletons of marine organisms sink to the sea floor (1); layers are buried, compressed and cemented to form rocks such as limestone (1); buried organic matter can form fossil fuels over millions of years (1).
Question 18 · Spec 3.1.1.3 · Carbon flows
What is meant by carbon sequestration? [1 mark]
Answer:
The long-term capture and storage of carbon, for example in forests, soils, oceans and rocks.
Question 19 · Spec 3.1.1.3 · Carbon flows
Explain why warming reduces the ability of the oceans to absorb carbon dioxide. [2 marks]
Answer:
Warm water holds less dissolved gas than cold water (1); so warmer oceans take up less carbon dioxide, leaving more in the atmosphere (1).
Question 20 · Spec 3.1.1.3 · Scale
Describe the carbon flows at the scale of a single plant. [3 marks]
Answer:
Photosynthesis takes carbon dioxide in and stores carbon in plant tissue (1); respiration releases carbon dioxide (1); leaf fall and death transfer carbon to the soil (1).
Question 21 · Spec 3.1.1.3 · Scale
Explain how carbon storage changes as a sere develops. [3 marks]
Answer:
Biomass increases as pioneer species give way to larger plants (1); soil organic carbon accumulates (1); at climax, uptake by photosynthesis roughly balances losses by respiration and decay (1).
Question 22 · Spec 3.1.1.3 · Scale
Describe carbon flows at a continental scale. [3 marks]
Answer:
Forests and grasslands absorb carbon dioxide as sinks (1); rivers carry dissolved and particulate carbon towards the oceans (1); industrial and urban regions are sources of carbon dioxide from fossil fuel burning (1).
Question 23 · Spec 3.1.1.3 · Natural variation
Explain how wildfires affect the carbon cycle. [3 marks]
Answer:
Burning releases carbon dioxide rapidly from vegetation and soil (1); regrowth of vegetation afterwards absorbs carbon dioxide again over years (1); more frequent fires linked to warming can leave a net release to the atmosphere (1).
Question 24 · Spec 3.1.1.3 · Natural variation
Explain the role of volcanic activity in the carbon cycle. [2 marks]
Answer:
Volcanoes release carbon dioxide from the lithosphere into the atmosphere (1); natural volcanic emissions are small compared with emissions from human activity (1).
Question 25 · Spec 3.1.1.3 · Natural variation
Describe the seasonal pattern shown by atmospheric carbon dioxide measured at Mauna Loa. [2 marks]
Answer:
Carbon dioxide falls in the northern hemisphere spring and summer as plant growth absorbs it (1); it rises in autumn and winter as photosynthesis declines and decomposition continues (1).
Question 26 · Spec 3.1.1.3 · Natural variation
Describe the long-term trend in atmospheric carbon dioxide since 1958. [2 marks]
Answer:
A continuous rise (1); from about 315 ppm in 1958 to over 420 ppm today (1).
Question 27 · Spec 3.1.1.3 · Natural variation
Describe how atmospheric carbon dioxide has varied with glacial cycles over the last 800,000 years. [2 marks]
Answer:
Concentrations were lower, around 180 ppm, in cold glacial periods (1); and higher, around 280 ppm, in warm interglacials (1).
Question 28 · Spec 3.1.1.3 · Human impact
Explain how burning fossil fuels changes the carbon cycle. [3 marks]
Answer:
Fossil fuels are carbon stores in the lithosphere formed over millions of years (1); extraction and combustion transfer this carbon to the atmosphere within decades (1); this is much faster than natural processes can remove it, so atmospheric carbon increases (1).
Question 29 · Spec 3.1.1.3 · Human impact
Explain how farming practices can alter the carbon cycle. [3 marks]
Answer:
Any three from: ploughing exposes soil organic matter to oxidation, releasing carbon dioxide (1); livestock and rice paddies release methane (1); clearing land for crops removes carbon stored in vegetation (1); drainage of peat soils releases stored carbon (1); cover crops and reduced tillage can store more carbon (1).
Question 30 · Spec 3.1.1.3 · Human impact
Explain how deforestation affects the carbon cycle. [3 marks]
Answer:
Removing trees reduces carbon uptake by photosynthesis (1); burning or decay of cleared vegetation releases carbon dioxide (1); soil erosion and disturbance release stored soil carbon (1).
Question 31 · Spec 3.1.1.3 · Human impact
Explain how land use change can turn a carbon sink into a carbon source. [2 marks]
Answer:
Converting forest, grassland or peat to cropland, pasture or urban land removes biomass and exposes soil carbon (1); carbon dioxide released by burning, decay and oxidation outweighs any new uptake (1).
Question 32 · Spec 3.1.1.3 · Human impact
Explain why wetlands and rice paddies are important sources of methane. [2 marks]
Answer:
Waterlogged soils lack oxygen, so microbes decompose organic matter anaerobically (1); this releases methane, a potent greenhouse gas (1).
Question 33 · Spec 3.1.1.3 · Carbon budget
Define the global carbon budget. [2 marks]
Answer:
The balance between carbon added to and removed from the atmosphere, oceans and land (1); showing how much emitted carbon stays in the atmosphere compared with what is absorbed by sinks (1).
Question 34 · Spec 3.1.1.3 · Carbon budget
Describe what happens to carbon dioxide emitted by human activity. [3 marks]
Answer:
About half stays in the atmosphere, raising concentrations (1); about 30% is absorbed by land ecosystems such as forests (1); about 25% is absorbed by the oceans (1).
Question 35 · Spec 3.1.1.3 · Carbon budget
In one year, human emissions are 10 GtC. The atmosphere increases by 4.5 GtC and the oceans absorb 2.5 GtC. Calculate the land uptake. [2 marks]
Answer:
Land uptake = 10 − (4.5 + 2.5) (1); = 3 GtC (1).
Question 36 · Spec 3.1.1.3 · Carbon budget
One GtC equals 3.67 GtCO₂. Convert 10 GtC into GtCO₂. [2 marks]
Answer:
10 × 3.67 (1); = 36.7 GtCO₂ (1).
Question 37 · Spec 3.1.1.3 · Carbon budget
A forest holds 12,000 tonnes of carbon. It absorbs 400 tonnes a year and loses 150 tonnes a year to respiration and decay. Calculate the carbon stored after 10 years. [3 marks]
Answer:
Net annual gain = 400 − 150 = 250 tonnes (1); gain over 10 years = 2,500 tonnes (1); store = 12,000 + 2,500 = 14,500 tonnes (1).
Question 38 · Spec 3.1.1.3 · Carbon budget
What does ppm stand for when measuring atmospheric carbon dioxide? [1 mark]
Answer:
Parts per million: the number of carbon dioxide molecules per million molecules of air.
Question 39 · Spec 3.1.1.3 · Impacts of the cycle
Describe how atmospheric carbon dioxide has changed since the industrial revolution. [2 marks]
Answer:
It has risen from about 280 ppm before 1750 (1); to over 420 ppm in the 2020s, the highest for at least 800,000 years (1).
Question 40 · Spec 3.1.1.3 · Impacts of the cycle
Explain how increased carbon dioxide affects global climate. [3 marks]
Answer:
Carbon dioxide absorbs and re-emits long-wave (infrared) radiation from the Earth (1); more carbon dioxide traps more heat, enhancing the greenhouse effect (1); global average temperature has risen by about 1.1 °C since pre-industrial times (1).
Question 41 · Spec 3.1.1.3 · Impacts of the cycle
Explain how rising carbon dioxide causes ocean acidification. [3 marks]
Answer:
Oceans absorb carbon dioxide from the atmosphere (1); it reacts with water to form carbonic acid, lowering pH (1); fewer carbonate ions are available, harming shell-forming organisms and corals (1).
Question 42 · Spec 3.1.1.3 · Impacts of the cycle
Explain how higher atmospheric carbon dioxide may affect vegetation growth. [2 marks]
Answer:
Carbon dioxide fertilisation can increase photosynthesis and plant growth (1); but the effect is limited by water, nutrient supply and heat stress, so may not persist (1).
Question 43 · Spec 3.1.1.3 · Feedbacks
Explain how thawing permafrost may create a positive feedback in the carbon cycle. [3 marks]
Answer:
Warming thaws permafrost, allowing frozen organic matter to decompose (1); carbon dioxide and methane are released to the atmosphere (1); this strengthens the greenhouse effect, causing further warming and thaw (1).
Question 44 · Spec 3.1.1.3 · Feedbacks
Explain how drought and forest dieback could produce a positive feedback. [3 marks]
Answer:
Climate change brings more frequent drought and fire to forests (1); trees die and burn, releasing stored carbon and reducing photosynthesis (1); atmospheric carbon dioxide rises further, increasing warming (1).
Question 45 · Spec 3.1.1.3 · Feedbacks
Explain how increased plant growth could act as a negative feedback on rising carbon dioxide. [2 marks]
Answer:
Higher carbon dioxide and warmer, longer growing seasons can increase photosynthesis (1); more carbon is stored in plants and soil, reducing the rate of increase in the atmosphere (1).
Question 46 · Spec 3.1.1.3 · Feedbacks
Suggest why the carbon cycle is no longer in dynamic equilibrium. [2 marks]
Answer:
Human emissions add carbon to the atmosphere faster than natural sinks can absorb it (1); so atmospheric carbon has increased steadily instead of staying balanced (1).
Question 47 · Spec 3.1.1.3 · Global carbon stores
Explain why peatlands are important carbon sinks. [2 marks]
Answer:
Waterlogged, acidic and cold conditions slow decomposition (1); so dead plant material accumulates over thousands of years as peat, storing large amounts of carbon (1).
Question 48 · Spec 3.1.1.3 · Global carbon stores
Explain why tropical rainforests are significant stores of carbon. [2 marks]
Answer:
They have very large biomass in trees and other vegetation (1); high temperatures and rainfall give rapid growth and continuous photosynthesis, so large amounts of carbon are taken up (1).
Question 49 · Spec 3.1.1.3 · Carbon flows
Name the two main greenhouse gases in the carbon cycle and give one source of each. [2 marks]
Answer:
Carbon dioxide, from combustion or respiration (1); methane, from anaerobic decomposition in wetlands, rice paddies or livestock (1).
Question 50 · Spec 3.1.1.3 · Carbon flows
Describe how marine organisms transfer carbon from the oceans to the sea floor. [2 marks]
Answer:
Organisms such as shellfish and corals take dissolved carbon to build calcium carbonate shells and skeletons (1); when they die these sink and accumulate as sediment, which may form limestone (1).

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The carbon cycle

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