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Teacher guide: Year 9 Earth and space

Earth's four spheres and the carbon that moves between them: photosynthesis, respiration, decomposition and burning fossil fuels. Concept tutorials with a landscape of the spheres and a live carbon cycle you can run for decades.

Open the student course

Each lesson works two ways. Students can work through the online tutorial at their own pace (it checks their answers and saves their progress in their browser), or you can teach it to the whole class from the slides, then send students into the tutorial for the Modify and Make steps.

Every lesson guide has plain-language notes, the answers to every check, a solution for every task, and slides and a worksheet for Waypoints Prepare. These guide pages are not linked from student pages, but they are not secret either: keep printed Teacher editions for answers you do not want students to find.

The carbon cycle

Earth's spheres and the processes that move carbon between them (AC9S9U03).

The carbon cycle in 5 minutes (read before teaching this unit)

The idea in plain words

  • Earth has four spheres: the atmosphere (air), biosphere (living things), hydrosphere (water) and geosphere (rock, soil, magma). They overlap and interact: soil is rock and living things; a cloud is water in the air.
  • Carbon is stored in every sphere: CO₂ in the air, living things and soil, dissolved in the oceans, and rocks and fossil fuels in the geosphere. It moves between stores.
  • Photosynthesis takes CO₂ from the air into living things. Respiration (by plants, animals and decomposers) returns it. These two flows are huge (about 120 billion tonnes of carbon a year each way) and nearly balance.
  • Combustion of fossil fuels moves carbon from the geosphere, where it was stored over millions of years, into the air in a few years. The air's CO₂ has risen from about 280 ppm (before 1800) to about 420 ppm.
  • When flows in and out of a store balance, the store stays steady. When they don't, it grows or shrinks.

Where the picture breaks

  • The board's model is simplified: four stores, a few flows, and simple rules for how plants and oceans respond. Real numbers vary between sources.
  • The geosphere store shows only fossil fuels; rocks such as limestone hold far more carbon but move it very slowly.
  • In the board, switching off a whole process is a "what if": real processes never stop entirely.

Common misconceptions

  • Plants get their mass from the soil (most of a plant's mass is carbon taken from the air).
  • Only animals respire (plants and decomposers respire too).
  • Carbon is used up or destroyed when fuel burns (it moves to the air as CO₂).
  • Humans' extra CO₂ is small next to natural flows, so it can't matter (the natural flows balance; the extra doesn't).

Carbon stores (node 2)

Carbon is stored in all four spheres (CO₂ in air, dissolved in oceans, living things and soil, fossil fuels and rocks) and flows between them without being made or destroyed. A store is an amount; a flow is an amount a year.

Investigate in class

Optional ideas if you run a practical for this unit: pick one, adapt it to your equipment, or use your own.

  • Does light change how fast a water plant takes up CO₂? Change: light (lamp on, or the tube in a dark cupboard). Measure: the time for bromothymol blue indicator, turned yellow by breathing out through a straw, to turn back to blue. Keep the same: the length of pondweed, the volume, the temperature.
  • Does yeast give off more CO₂ when it is warmer? Change: water temperature (20, 30, 40 °C). Measure: the balloon's circumference on a bottle of yeast, sugar and water after 20 minutes. Keep the same: the amounts of yeast, sugar and water.
  • Is the CO₂ in the air still rising? Secondary data: CSIRO's Cape Grim CO₂ records. Change: the year. Measure: CO₂ in ppm. Keep the same: the station, the month you read.

Safety and ethics: blow out through straws, never suck; bromothymol blue stains. Return the pondweed to its tank afterwards.