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C4.2.20—Keeling Curve analysis

Keeling Curve analysis explains how energy flow, matter cycling, trophic transfer or carbon movement is represented, measured or limited within ecosystems.

Syllabus
First assessment 2025
Objective
C4.2.20
Level
HL

Exam analysis

Chance of appearing2%of analysed past papers
Latest appearanceMay 2025
Most common paperPaper1
Typical marks1

Common command terms

  • Identify

Scoring notes

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

Recent exam appearances

May 2025Paper1A ["HL"] · TZ336[ 1 ]C4.2.20—Keeling Curve analysis
November 2024Paper1 ["HL"] · TZ017[ 1 ]C4.2.20—Keeling Curve analysis
Practice this objective

Coverage 2024–2025 · Updated 16 Jul 2026

The Keeling Curve combines trend and seasonality

The Keeling Curve shows both a long-term rise in atmospheric CO₂ and repeated annual fluctuations.

Seasonal photosynthesis and respiration, especially across Northern Hemisphere land ecosystems, create the annual oscillation: growing-season uptake lowers CO₂, while reduced photosynthesis and continuing respiration raise it later.

Over many years, CO₂ peaks and troughs both shift upward because combustion adds carbon faster than global sinks remove the additional amount.

When reading the curve, compare equivalent points in successive years to identify the long-term trend; do not mistake one seasonal decline for a reversal of the multi-year rise.

Photosynthesis, respiration and combustion explain different components of the pattern. Axis units and time scale must be checked before interpreting magnitude or rate.

Keeling Curve analysis

Assessment in practice

1 marks
How it is assessed

This objective is assessed through multiple choice, commonly using Identify.

Command terms

Identify

What earns marks

Build the answer around this relationship: Keeling Curve analysis 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: 1]

The graph shows data collected at Mauna Loa, USA, for monthly mean carbon dioxide concentration.

What causes the decreases in monthly mean carbon dioxide concentration each year?

A

Combustion increases.

B

Respiration increases.

C

Decomposition increases.

D

Photosynthesis increases.

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

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