Which pair of processes moves the largest amounts of carbon between the atmosphere and living things?

Nitrogen fixation and denitrification

When carbon dioxide dissolves in seawater, what happens?

It forms carbonic acid, which lowers the water's pH

It evaporates back out again almost immediately

It converts directly into solid limestone

It bonds with sodium chloride to form a stable compound

The Keeling curve is a long-running record of what?

Atmospheric carbon dioxide concentration measured at Mauna Loa

Ocean surface temperature in the tropics

What property makes a gas a greenhouse gas?

Its molecules absorb and re-emit infrared radiation

It is denser than the surrounding air

It reflects incoming sunlight back into space

It reacts chemically with ozone

Compared with carbon dioxide, methane in the atmosphere is what?

A far stronger warming agent per molecule but much shorter-lived

Weaker per molecule but far longer-lived

Identical in effect but present in smaller amounts

Significant only in the upper atmosphere

Why does permafrost matter to the carbon cycle?

It holds large amounts of undecayed plant matter that breaks down if it thaws

It reflects a large share of incoming sunlight back to space

It absorbs methane directly out of the atmosphere

It stops rivers from carrying dissolved carbon to the sea

The "biological pump" in the ocean refers to what?

Carbon fixed by surface plankton sinking into the deep ocean as they die

Currents drawing cold water up to the surface

Fish carrying nutrients between different depths

Tides mixing the water column from top to bottom

Chemical weathering of silicate rock affects the carbon cycle by doing what?

Consuming carbon dioxide and delivering it to the sea as dissolved carbonate

Releasing carbon dioxide locked inside the rock

Generating methane within waterlogged soils

Fixing carbon directly into plant tissue

What is a carbon sink?

A reservoir that takes up more carbon than it gives back over a given period

Any store of carbon, whatever the flows into and out of it

A place where carbon is chemically destroyed

An underground cavity used to hold compressed gas

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Environment & Climate Medium ⚡ 10 Questions ⏱️ ~7 Mins +100 XP

How Carbon Moves Through the Planet

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Carbon is constantly moving between four great stores, on timescales ranging from a single season to tens of millions of years.

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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.

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How Carbon Moves Through the Planet: all 10 questions and answers

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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.
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.
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.
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.
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.
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.
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.
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.
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.
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.

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