What does burning fossil fuels do to the carbon cycle?
Fossil fuels are carbon taken out of the cycle and stored in the geosphere over millions of years. Burning them returns it to the atmosphere in years, faster than plants and oceans can take it back, so CO₂ in the air rises.
Coal is a forest from 300 million years ago. Burning it takes seconds.
Photosynthesis and respiration balance. This lesson: what happens when a new flow joins in.
Engage: Commit to a prediction before you test anything. A wrong guess is fine: it shows you what to look for.
Explore: Try it on the board and compare what happens with your prediction.
Explain: Use the particle model to explain what you saw.
Elaborate: Use the same idea in a new context.
Evaluate: Look back at your first ideas and what changed.
WidthMore contexts, same idea.
Make your prediction first. Help opens once you've locked it in.
EngageCommit to a prediction before you test anything. A wrong guess is fine: it shows you what to look for.
Engage: small next to big?
ExploreTry it on the board and compare what happens with your prediction.
Explore: burn for 50 years
Set Burning to about today's rate (10), then Run 50 years. Watch the fossil fuel store, the air, and the oceans and living things.
Try it yourselfYour edits are saved in this browser.
Burn fossil fuels and run the years until CO₂ in the air passes 330 ppm.
Use the board. This ticks when you get there.
ExplainUse the particle model to explain what you saw.
Explain: from the geosphere to the air
In your book: add a fossil fuels box and a burning arrow to your carbon cycle diagram, and write one sentence about the timescales.
On the board, about what fraction of the carbon burned ended up staying in the air? (Enter a percentage.)
Elaborate TypicalUse the same idea in a new context.
Elaborate: starting from today
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Starting from today, find a burning rate that keeps CO₂ in the air steady (changing by less than 2.5 ppm in 10 years).
Use the board. This ticks when you get there.
Elaborate WiderUse the same idea in a new context.
Elaborate: a cool burn and a coal fire
EvaluateLook back at your first ideas and what changed.
Evaluate: look back
Write one sentence: "At first I thought… Now I think… because…"
Revision summary
Fossil fuels are carbon stored in the geosphere over millions of years.
Combustion moves it to the atmosphere in years: a new flow added to a balanced cycle.
Oceans and plants take up part of it (in the real world a bit over half); the rest builds up, so CO₂ in the air rises (280 ppm before 1800, about 420 ppm today).
For teachers
Students explain that burning fossil fuels moves carbon from the geosphere to the atmosphere and why this is a net addition to the air's carbon store (AC9S9U03, node 4). About 22 minutes.
I can explain where the carbon in fossil fuels comes from.
I can explain why burning them raises CO₂ in the air even though natural flows are bigger.
I can describe where the extra carbon goes.
Before class
A concept tutorial for introducing or revising the idea. Set the width with ?width=minimum, typical (default) or wider.
The common reasoning error is comparing human emissions (about 10 GtC a year) with gross natural flows (about 120) and concluding they can't matter; the board shows the natural flows balance, so the extra accumulates.
Board notes: the model starts balanced at 280 ppm (or at 420 ppm with start: today, burning at 10). Oceans and plants take up extra CO₂ in proportion to how far the air is above 280 ppm, so in the model about 60% of what is burned stays in the air over 50 years (about 45% in the real world). Numbers are rounded; the model leaves out land-use change and slow rock processes. The typical tier asks for a burning rate that holds today's CO₂ steady (about 4 GtC a year in the model: well under half of today's emissions, and the uptake would shrink over time in reality).
Running the lesson
Engage: count how many chose 'nothing: 10 is tiny'.
Explore: ask where the burned carbon went (the stores show it).
Explain: the 'why it adds up' question is the checkpoint.
The hard part (on purpose)
Seeing why a small extra flow matters when the big ones balance. When a student says human emissions are too small to matter, ask what happens to the extra each year.
When students get stuck
A student says the burned carbon is destroyed. Ask: What does burning make? Fix: CO₂: the carbon moves to the air.
A student thinks CO₂ rises by exactly the amount burned. Ask: Did the oceans and living things change too? Fix: They took up part of it (about 40% in the model).
Stretch and support
Stretch: Calculate how long it would take the oceans and plants to remove 10 GtC at the board's uptake rate.
Support: Give the carbon cycle diagram with the burning arrow and fossil fuel box to add.
Where it leads
Next: Year 10, the greenhouse effect and climate change (AC9S10U04).
What each step is for
Engage: small next to big?: Students who compare human emissions (about 10) with natural flows (about 120) conclude they can't matter; the board shows the natural flows balance and the extra accumulates.
Explore: burn for 50 years: Running decades of combustion against a balanced cycle shows the air's store growing, while oceans and plants take up only part.
Explain: from the geosphere to the air: Connecting the timescale of storage with the speed of release explains why combustion is a net addition.
Elaborate: starting from today: Trying lower burning rates from today's 420 ppm shows what would steady the air's CO₂.
Elaborate: a cool burn and a coal fire: Comparing a fire whose carbon regrows with one whose carbon was locked away for millions of years sharpens the idea of a net addition.
Now you can
Explain where the carbon in fossil fuels comes from.
Explain why burning them raises CO₂ in the air even though natural flows are bigger.