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Environment & Climate
Planet Systems
Atmosphere, energy and the cycles that keep things running.
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Which gas is the most abundant in Earth's atmosphere?
How Carbon Moves Through the Planet
Carbon is constantly moving between four great stores, on timescales ranging from a single season to tens of millions of years.
About this topic
The carbon cycle is really two cycles running at different speeds. The fast one turns over in seasons: plants take carbon dioxide out of the air, respiration and decay put it back, and the two flows very nearly cancel. You can see the near-balance in the annual sawtooth of the Keeling curve, which has recorded atmospheric carbon dioxide at Mauna Loa since 1958 and dips every northern summer as forests leaf out. The slow cycle turns over in millions of years and involves far more carbon. Silicate rock weathers, consuming carbon dioxide and delivering dissolved carbonate to the sea, where organisms and chemistry lock it into limestone; volcanoes eventually return some of it. Because weathering speeds up in warm, wet conditions, that slow loop behaves like a thermostat over geological time. The questions here cover both cycles, the ocean's role and what makes a gas a greenhouse gas in the first place.
Questions & answers
How Carbon Moves Through the Planet: all 10 questions and answers
Every question in this quiz is listed below with its correct answer and the reasoning behind it. Play first if you would rather not see the answers — or read through as a study sheet.
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1Which pair of processes moves the largest amounts of carbon between the atmosphere and living things?
- A Evaporation and condensation
- B Weathering and sedimentation
- C Nitrogen fixation and denitrification
- D Photosynthesis and respiration Correct
Answer: D. Photosynthesis and respiration
Why: The two flows nearly cancel out each year, which is exactly why a comparatively small additional input accumulates so visibly in the atmosphere.
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2Where is most of the carbon near the Earth's surface stored?
- A In living plants and animals
- B In the top metre of soil
- C In rocks such as limestone Correct
- D In the atmosphere
Answer: C. In rocks such as limestone
Why: Carbonate rock holds vastly more than every other reservoir put together, and it exchanges carbon on timescales of millions of years rather than seasons.
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3When carbon dioxide dissolves in seawater, what happens?
- A It bonds with sodium chloride to form a stable compound
- B It forms carbonic acid, which lowers the water's pH Correct
- C It evaporates back out again almost immediately
- D It converts directly into solid limestone
Answer: B. It forms carbonic acid, which lowers the water's pH
Why: The extra hydrogen ions reduce the carbonate available for building shells, which is why corals and small molluscs feel the effect earliest.
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4The Keeling curve is a long-running record of what?
- A Atmospheric carbon dioxide concentration measured at Mauna Loa Correct
- B Global average sea level
- C The extent of Arctic sea ice
- D Ocean surface temperature in the tropics
Answer: A. Atmospheric carbon dioxide concentration measured at Mauna Loa
Why: Begun in 1958, it shows a steady climb overlaid with an annual sawtooth as northern hemisphere forests leaf out each spring and die back each autumn.
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5What property makes a gas a greenhouse gas?
- A It is denser than the surrounding air
- B It reflects incoming sunlight back into space
- C It reacts chemically with ozone
- D Its molecules absorb and re-emit infrared radiation Correct
Answer: D. Its molecules absorb and re-emit infrared radiation
Why: Nitrogen and oxygen make up most of the atmosphere yet do almost nothing here, because their symmetrical two-atom molecules cannot vibrate in the necessary way.
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6Compared with carbon dioxide, methane in the atmosphere is what?
- A Identical in effect but present in smaller amounts
- B Significant only in the upper atmosphere
- C A far stronger warming agent per molecule but much shorter-lived Correct
- D Weaker per molecule but far longer-lived
Answer: C. A far stronger warming agent per molecule but much shorter-lived
Why: It oxidises to carbon dioxide and water over roughly a decade, whereas a pulse of carbon dioxide takes centuries to be drawn back out.
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7Why does permafrost matter to the carbon cycle?
- A It stops rivers from carrying dissolved carbon to the sea
- B It holds large amounts of undecayed plant matter that breaks down if it thaws Correct
- C It reflects a large share of incoming sunlight back to space
- D It absorbs methane directly out of the atmosphere
Answer: B. It holds large amounts of undecayed plant matter that breaks down if it thaws
Why: That material has been piling up since the last glacial period precisely because the cold kept soil microbes from finishing the job.
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8The "biological pump" in the ocean refers to what?
- A Carbon fixed by surface plankton sinking into the deep ocean as they die Correct
- B Currents drawing cold water up to the surface
- C Fish carrying nutrients between different depths
- D Tides mixing the water column from top to bottom
Answer: A. Carbon fixed by surface plankton sinking into the deep ocean as they die
Why: Only a small share of it reaches the sea bed, but that trickle keeps the deep ocean carbon-rich and leaves the surface able to take up more from the air.
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9Chemical weathering of silicate rock affects the carbon cycle by doing what?
- A Releasing carbon dioxide locked inside the rock
- B Generating methane within waterlogged soils
- C Fixing carbon directly into plant tissue
- D Consuming carbon dioxide and delivering it to the sea as dissolved carbonate Correct
Answer: D. Consuming carbon dioxide and delivering it to the sea as dissolved carbonate
Why: It is slow but relentless, and over millions of years it acts as a thermostat, running faster when the climate is warm and wet and slower when it is cold.
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10What is a carbon sink?
- A A place where carbon is chemically destroyed
- B An underground cavity used to hold compressed gas
- C A reservoir that takes up more carbon than it gives back over a given period Correct
- D Any store of carbon, whatever the flows into and out of it
Answer: C. A reservoir that takes up more carbon than it gives back over a given period
Why: Forests and the ocean have both acted as sinks, but a sink can turn into a source: a burning or drought-stressed forest releases what it spent decades absorbing.
Frequently asked questions about How Carbon Moves Through the Planet
How many questions are in How Carbon Moves Through the Planet?
How Carbon Moves Through the Planet has 10 questions and takes about 7 minutes to finish. It is rated Medium and sits in our Environment & Climate category.
How is How Carbon Moves Through the Planet scored?
Each of the 10 correct answers adds 10 XP, up to 100 XP for a clean run. Wrong answers cost you nothing, and your full question-by-question review is shown at the end.
Are the answers to How Carbon Moves Through the Planet explained?
Yes. All 10 questions carry a written explanation of why the correct answer is right, so the quiz works as a study sheet as well as a test.
Can I replay How Carbon Moves Through the Planet?
Yes, as often as you like. The questions and the answer options are reshuffled on each run, and you can use Practice Mistakes afterwards to retry only the ones you missed. No account is needed to play.
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