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C4.2.18—Carbon sinks and sources

Carbon sinks and sources explains how energy flow, matter cycling or carbon transfer is represented, measured or limited within ecosystems.

Syllabus
First assessment 2025
Objective
C4.2.18
Level
SL

Exam analysis

Chance of appearing4%of analysed past papers
Latest appearanceNovember 2022
Most common paperPaper1
Typical marks1

Common command terms

  • Identify
  • Explain
  • Describe

Scoring notes

Common mistake
Reversing photosynthesis and respiration arrows in carbon-cycle diagrams.

Recent exam appearances

November 2022Paper1 ["SL"] · TZ019[ 1 ]C4.2.18—Carbon sinks and sources
May 2018Paper1 ["SL"] · TZ118[ 1 ]C4.2.18—Carbon sinks and sources
November 2017Paper2 ["SL"] · TZ07(c)[ 4 ]C4.2.18—Carbon sinks and sources
November 2014Paper1 ["SL"] · TZ020[ 1 ]C4.2.18—Carbon sinks and sources
May 2013Paper1 ["SL"] · TZ220[ 1 ]C4.2.18—Carbon sinks and sources
Practice this objective

Coverage 2013–2022 · Updated 16 Jul 2026

A carbon sink gains carbon over a chosen interval

An ecosystem is a carbon sink when photosynthesis removes more CO₂ than respiration releases, and a carbon source when respiration releases more CO₂ than photosynthesis removes.

Classification depends on the net balance across a stated ecosystem boundary and time interval, not on whether both fluxes occur—both photosynthesis and respiration normally continue.

Photosynthesis > respiration → net CO₂ uptake → sink. Respiration > photosynthesis → net CO₂ release → source. Equal fluxes → no net exchange from these two processes.

A growing forest can be a sink while carbon accumulation exceeds respiratory release; after disturbance, high respiration and decomposition can make the same area a source.

Sink/source status is not permanent and changes with season, disturbance, ecosystem age, boundary and measurement interval.

Carbon sinks and sources

Assessment in practice

1–2 marks
How it is assessed

This objective is assessed through structured response, commonly using Identify / Explain / Describe.

Command terms

Identify / Explain / Describe

What earns marks

Build the answer around this relationship: Carbon sinks and sources must be linked to the correct source, store, transfer or loss process.

Watch for

Reversing photosynthesis and respiration arrows in carbon-cycle diagrams.

Representative question

Question 1

[Maximum number: 2]

The graph shows long-term fluctuations in atmospheric CO2\mathrm{CO}_{2} concentration of about 80 ppm . The decreases in CO2\mathrm{CO}_{2} concentration are probably due to oceans acting as a sink by holding large quantities of dissolved CO2\mathrm{CO}_{2}.

Identify two other examples of natural sinks that can remove carbon from the carbon cycle and reduce atmospheric CO2\mathrm{CO}_{2} concentration.

Energy and Matter

  • Ecosystems are open systems: energy flows through them and leaves as heat, while matter is recycled and may enter or leave.
  • Photoautotrophs capture light; chemoautotrophs oxidize inorganic substances. Both build biomass from inorganic carbon. Heterotrophs obtain organic carbon from other organisms.
  • Food-web arrows show energy and biomass transfer. Energy decreases between trophic levels through respiration, heat, egestion, excretion and uneaten material, limiting chain length.
  • Gross primary production minus producer respiration gives net primary production; secondary production is heterotroph biomass gain.
  • Decomposers obtain energy from detritus and return inorganic nutrients to producers.
  • Carbon-cycle diagrams distinguish stores and fluxes. Photosynthesis removes CO2; respiration, decomposition and combustion release it.
  • A sink absorbs more carbon than it releases; a source does the reverse. The Keeling Curve shows a long-term atmospheric CO2 rise with seasonal oscillation.

Concept essentials

  • Carbon sinks and sources must be linked to the correct source, store, transfer or loss process.
  • The evidence for carbon sinks and sources depends on distinguishing energy flow from matter cycling.
  • Carbon sinks and sources is clearer when arrows, units and trophic positions match the biological process.
  • Data or diagrams for carbon sinks and sources need interpretation as ecosystem transfer evidence.