Ausgeology. Years 3 to 10 lessons

Year 9 Science · AC9S9U03

The carbon cycle.

Every carbon atom in you has been somewhere else: in the air, in the sea, in a plant, in rock. Carbon keeps moving between Earth’s four spheres. In this lesson you will build a model of that cycle, test it against 50 years of measurements from a cliff in Tasmania, and improve it.

Illustration
The four spheres in one place: atmosphere (air), hydrosphere (the Southern Ocean), biosphere (woodland, soil life and plankton) and geosphere (rock, including a coal seam and limestone). On the cliff is the Kennaook / Cape Grim station. Not to scale.
Atmosphereair
Hydrospherewater
Biosphereliving things
Geosphererock

Learning intention

We are learning how carbon moves between Earth’s spheres, how combustion, photosynthesis and respiration drive that movement, and how scientists test their models of it against measurements.

Success criteria

  • 1I can represent the carbon cycle as stores and flows between the four spheres.
  • 2I can explain how combustion, photosynthesis and respiration move carbon and rely on more than one sphere.
  • 3I can use the Cape Grim record as evidence for how the carbon cycle is changing.
  • 4I can test a model against data and revise it.
  • 5I can explain how fire management in northern Australia changes the flow of carbon between the biosphere and the atmosphere.

Australian Curriculum v9, AC9S9U03: “represent the carbon cycle and examine how key processes including combustion, photosynthesis and respiration rely on interactions between Earth’s spheres (the geosphere, biosphere, hydrosphere and atmosphere)”

Printable activity

Plot the record

Plot 50 years of Cape Grim measurements by hand, work out how fast carbon dioxide is rising, and draw your revised model of the carbon cycle. The same links are at the end of the lesson.

Activity sheet (PDF) Editable (Word)

How it works

  1. Look at something that needs explaining: 50 years of carbon dioxide measurements.
  2. Build your first model of the carbon cycle, with what you already know.
  3. Predict what your model says the measurements should look like.
  4. Test the predictions against the data, and run a carbon model of your own.
  5. Revise your model, explain it, and compare it with the scientists’ model.

Scientists work this way too. A model is never finished; it gets better each time it meets new evidence. Your notes and answers are saved in this browser.

Step 2 · Something to explain

Fifty years of air.

On the north-west tip of Tasmania, the Kennaook / Cape Grim station has measured the air since April 1976. When the wind blows from the south-west it has crossed the Southern Ocean for days without touching land, so it is some of the cleanest air on Earth. That makes it baseline air: it shows what the whole southern atmosphere is doing.

Carbon dioxide in baseline air at Kennaook / Cape Grim

Data

Hover over or tap the line to read a month, or select the chart and use the arrow keys (left and right by a year, up and down by a month). ppm means parts per million.

Data: CSIRO and the Bureau of Meteorology, Kennaook / Cape Grim Baseline Air Pollution Station, monthly baseline means, May 1976 to September 2026. Mauna Loa: NOAA Global Monitoring Laboratory.

Two things to explain

The line climbs steadily, from 329 ppm to 425 ppm. And when you zoom in, it wobbles up and down once every year.

A good model of the carbon cycle has to explain both: the slow rise and the yearly wiggle. Keep both in mind as you build yours.

Notebook: notice and wonder

Step 3 · Your first model

Where does carbon go?

Build your first model of the carbon cycle with what you already know. For each process, decide which sphere the carbon leaves and which it enters. Nothing is marked yet. You will come back and revise this model once you have evidence.

Model builder

Illustration

Processes

Not sure what a process is?
  • Photosynthesis, respiration and decomposition are what living things do with carbon.
  • Combustion is burning: fossil fuels from rock, or plants in a bushfire.
  • The ocean both takes in and gives off carbon dioxide at its surface.
  • Weathering, shells and limestone, burial and volcanoes are the slow processes that move carbon into and out of rock.

Your model so far

    Notebook: first thoughts

    Step 4 · Zoom in to the atoms

    Three reactions.

    Three chemical reactions do most of the moving: combustion, photosynthesis and respiration. In each one the atoms are rearranged into new molecules, but none are made or lost. Count the carbon atoms before and after.

    Combustion

    Reaction lens

    Before After

    Where and how:

    Which spheres does it use?

    Key idea

    Atoms are never used up. Six carbon atoms go into photosynthesis and six come out, just in a different molecule and a different sphere.

    That is the law of conservation of mass at work. It is why carbon cycles: the same atoms move round and round. It is also why the carbon in the air cannot just disappear. If it leaves the air, it has gone somewhere else.

    A cell respires one molecule of glucose, C6H12O6. How many carbon atoms end up in carbon dioxide?

    Which pair of processes has the same overall chemical equation?

    Notebook: a carbon atom’s journey

    Step 5 · Predict

    What does your model say?

    A model earns its keep by predicting. Use your model of the carbon cycle to predict three things about the Cape Grim data. You will check each one on the next screens. Guessing wrong is fine; that is how models get better.

    Notebook: three predictions

    1. Over one year at Cape Grim, when should carbon dioxide be lowest?

    Cape Grim is in the southern hemisphere. Think about when plants there grow most.

    2. Mauna Loa in Hawaii is in the northern hemisphere, which has about two-thirds of the world’s land. Compared with Cape Grim, how big should its yearly wiggle be?
    3. Which process in your model explains most of the 50-year rise?

    Step 6 · Evidence: the yearly wiggle

    The planet breathes.

    Each hemisphere has a growing season. When its plants are growing fast, photosynthesis pulls carbon dioxide out of the air faster than respiration and decomposition put it back. In the dormant months the balance flips. Step through the year and watch.

    A year of growth

    3D globe
    Jan Dec

    • Kennaook / Cape Grim, 41°S
    • Mauna Loa, Hawaii, 19°N

    Model, not a photo. The land comes from map data. The greening is simplified: it follows the seasons by hemisphere and keeps the tropics green. Real vegetation also depends on rain and soil.

    The yearly cycle at the two stations

    Data

    Average month by month, 2000 to 2025, with the slow rise removed so only the yearly pattern is left. Zero is the yearly average. Tap a month to move the globe to it.

      The data show Cape Grim is lowest around March and April. What does that tell you about the southern summer?

      Mauna Loa’s wiggle is about six times bigger than Cape Grim’s. Why?

      Notebook: the wiggle

      Step 7 · Evidence: the rise

      Run the model.

      Here is a working model of the carbon cycle as a set of yearly flows. Each year the air gains what burning and land clearing add, and loses what the ocean and plants take up. Try different explanations for the rise and see which one reproduces the record. Before 1976, the comparison is with air trapped in Antarctic ice.

      Carbon box model, 1850 to 2026

      Model

      Try an explanation

      Or set the dials yourself

      Flows in the year 2026

      What the model leaves out. It treats the whole atmosphere as one box and the ocean and plants as one uptake that grows with the extra carbon in the air. It leaves out volcanoes and weathering, which roughly cancel, and it cannot show the yearly wiggle. Fossil fuel and land clearing figures: Global Carbon Budget. Ice: Law Dome, Antarctica (CSIRO).

      Which run of the model matched the Cape Grim record?

      People add about 12 billion tonnes of carbon to the air each year, but the air gains only about 5 or 6 on average. Where does the rest go?

      What the evidence says

      Only the runs that include burning fossil fuels climb like the record. Volcanoes cannot do it; they add about one hundredth as much carbon as people do.

      And the air holds only about half of what is added. The ocean and plants take up the rest, which is why they are called carbon sinks.

      Notebook: the rise

      Step 8 · Biosphere and atmosphere

      Fire and Country.

      Northern Australia’s tropical savannas burn almost every year. When they burn, and how hot, changes how much carbon goes from the biosphere into the atmosphere. First Nations people have managed that with fire for tens of thousands of years, and today Indigenous ranger groups do it on a huge scale.

      One year on the savanna, two ways

      Model
      Jan Dec

      Simplified. Real burning follows the Traditional Owners’ knowledge of their own Country: which patches, in what order, in which weeks. The strip shows the principle, not a real place.

      What the science says

      Early dry season fires are cool and patchy. The grass is still part green, the air is humid and the nights are cool, so fires go out by themselves and leave a mosaic of burnt and unburnt country. Late dry season fires are hot and large. They burn more fuel more completely and release more greenhouse gases, including methane and nitrous oxide.

      Since 2012 the Australian Government has counted the reductions. Across 24 million hectares of savanna, early burning avoids about 1.2 million tonnes of carbon dioxide equivalent a year, and 74 per cent of the carbon credits from this work go to Indigenous carbon businesses.

      Source: Clean Energy Regulator. The method counts methane and nitrous oxide because the carbon dioxide from burnt grass is taken back up as the grass regrows.

      • WhoIndigenous ranger groups such as the Djelk, Mimal and Arafura Swamp rangers run the West Arnhem Land Fire Abatement project. ALFA (NT), an entirely Aboriginal-owned not-for-profit, supports Traditional Owners to run five fire projects across more than 80,000 square kilometres of Arnhem Land. Projects like these cover much of northern Australia.
      • KnowledgeEarly burning in patches is traditional practice. It was disrupted after colonisation, when large late-season wildfires became more common. Today the practice combines Traditional Owners’ knowledge of Country with satellite fire mapping.
      • The spheresFire moves carbon from the biosphere to the atmosphere. Managing when and how the savanna burns changes that flow, and leaves more carbon stored in logs, litter and soil.

      How does early dry season burning reduce the greenhouse gases released from the savanna each year?

      Notebook: fire in your model

      Step 9 · Revise your model

      Second model.

      You have seen the atoms, the yearly wiggle, the 50-year rise and what fire does. Your first model is copied below. Change anything the evidence tells you to change: add arrows, remove them, or turn them round. Then check it against the scientists’ model.

      Model builder: revised

      Illustration

      Processes

      Your revised model

        Show the scientists’ model, with how much carbon moves each year
        Illustration

          Numbers are billions of tonnes of carbon a year (Gt C), rounded, for the 2010s and 2020s. Stores are in billions of tonnes. Sources: IPCC and the Global Carbon Budget.

          Read it carefully

          The natural flows are huge and nearly balanced. The fossil fuel arrow is small by comparison, but it only goes one way, and it has been going one way for 200 years.

          About 120 billion tonnes of carbon go into plants each year and about 120 come back out. Burning adds about 10 on top, every year, with nothing natural to match it. That is enough to tip the balance, and the Cape Grim record is the result.

          Notebook: explain your model

          Name the processes and the spheres they connect. Use at least two numbers from the lesson.

          Check your explanation

          Step 10 · Exit check

          Show what you know.

          One try for each question. Your results show which parts of the lesson to look at again.

          In a model of the carbon cycle, which arrow is drawn correctly?

          Why does burning coal in a power station rely on more than one of Earth’s spheres?

          Carbon dioxide at Cape Grim dips every southern autumn, and the dip at Mauna Loa in the north is six times deeper. What best explains both facts?

          A student’s model says volcanoes cause the 50-year rise in carbon dioxide. Which result would test that model fairly?

          How does early dry season burning in northern Australia change the flow of carbon between spheres?

          Offline activity

          Plot the record

          Plot 50 years of Cape Grim measurements by hand, work out how fast carbon dioxide is rising, and draw your revised model of the carbon cycle.

          Activity sheet (PDF) Editable (Word)
          For teachers

          Curriculum

          Australian Curriculum: Science, Version 9. Year 9, Science Understanding, Earth and space sciences.

          AC9S9U03: represent the carbon cycle and examine how key processes including combustion, photosynthesis and respiration rely on interactions between Earth’s spheres (the geosphere, biosphere, hydrosphere and atmosphere)

          Screen 8 addresses elaboration AC9S9U03_E7, on the interactions between the biosphere and the atmosphere in the context of bushfire management and First Nations Australians’ fire practices. Screen 4 connects to AC9S9U07 (modelling the rearrangement of atoms and conservation of mass). The lesson also works through the Science Inquiry strand: predicting from a model, testing against secondary data, and evaluating the model.

          How the lesson is taught

          The lesson is a model-based inquiry, after Windschitl, Thompson and Braaten (2008). Students build a model first, from what they already know, and the lesson treats it as a serious scientific object: it makes predictions, it meets evidence, it gets revised.

          • Phenomenon (screen 2): the Cape Grim record, with two features to explain.
          • Initial model (screen 3): built with no marking. Resist correcting it. The misconceptions it contains (respiration drawn backwards, volcanoes as the big source, carbon “used up”) are what the evidence screens are for.
          • Mechanism (screen 4): the three reactions at the atomic scale, with carbon counted in and out.
          • Predictions (screen 5): three testable predictions drawn from each student’s own model.
          • Evidence (screens 6 to 8): the seasonal cycle, the long-term rise through a runnable box model, and fire as a managed flow.
          • Revision (screen 9): the first model is copied forward and revised, then checked against the scientists’ model. The written explanation is the main assessment piece.
          • Exit check (screen 10): five questions mapped to the success criteria.

          Allow three lessons: screens 1 to 4, then 5 to 8, then 9 and 10. The whole-class conversations matter more than the clicking, especially after screens 3, 7 and 9.

          About the data and models

          • Cape Grim monthly baseline means are from CSIRO and the Bureau of Meteorology, May 1976 to September 2026 (the most recent year is provisional). Mauna Loa monthly means are from NOAA. Both are baked into the lesson, so it works offline.
          • The seasonal cycle chart removes the long-term trend with a 12-month running mean and averages 2000 to 2025. Cape Grim swings about 1 ppm; Mauna Loa about 6 ppm, roughly six months out of step.
          • The box model is a one-box atmosphere: each year the air gains fossil fuel and land-use emissions (Global Carbon Budget, via Our World in Data) and loses an uptake proportional to the excess over 278 ppm raised to the power 0.7, fitted to the Cape Grim record. It starts in 1850 at the Law Dome ice core value. It runs a few ppm low between 1880 and 1960, mostly because early land clearing emissions are uncertain, and its “stop in 2000” recovery is too fast, because the real surface ocean saturates. Both are worth discussing as limits of a model.
          • The globe’s greening is a seasonal rule by hemisphere, not satellite vegetation data. The land outline is Natural Earth. Latitudes above 62 degrees are shown as ice.
          • Flux and store sizes in the scientists’ model are rounded from IPCC assessments and the Global Carbon Budget 2024 (fossil 10.1 Gt C in 2023; ocean sink 2.9; land sink 2.3; atmospheric growth 5.9).
          • The savanna fire strip shows the principle of mosaic burning, not a real property or season. National figures are from the Clean Energy Regulator; ALFA (NT) describes itself as an entirely Aboriginal-owned not-for-profit.

          Answers at a glance

          • Scientists’ model arrows: photosynthesis atmosphere to biosphere; respiration, decomposition and fire biosphere to atmosphere; burning fossil fuels and volcanoes geosphere to atmosphere; ocean uptake atmosphere to hydrosphere and release the reverse; weathering atmosphere (or geosphere) to hydrosphere; shells and limestone hydrosphere to geosphere; burial biosphere to geosphere.
          • Predictions: Cape Grim is lowest around March to April; the northern wiggle is bigger; only runs with fossil fuels reproduce the rise.
          • Box model in 2026: as it happened about 423 ppm (measured 425); no fossil fuels about 295; volcanoes only about 278; no uptake over 600; twice the burning about 550.

          Saved progress

          Answers, models and notes are saved only in this browser on this device. Nothing is sent anywhere.

          Sources

          Glossary