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Human Energy Systems | Lesson 4 - Fossil Fuels and Carbon Pools

Lesson 4: Fossil Fuels and Carbon Pools

Students are introduced the Four Questions at a large scale to examine the major pools of carbon (with special attention to the fossil fuel pool). Then they look more closely at the Keeling Curve and examine how fossil fuels are formed and how we use them.

Guiding Question

How did fossil fuels form, how do we use them, and how does that impact Earth systems?

Activities in this Lesson (3 hrs 35 min)

  • Activity 4.1: Questions for this Lesson (30 min)
  • Activity 4.2: Carbon Pools and Fossil Fuels (35 min)
  • Activity 4.3: Tiny World Modeling (50 min)
  • Activity 4.4: Global Computer Model (50 min)
  • Optional Activity 4.5: Effects of Seasons and Oceans (50 min)


  1. Explain the origins and composition of fossil fuels.
  2. Explain what happens when fossil fuels are burned.
  3. Explain what causes CO2 concentrations in Hawaii to go down every summer and up every winter.
  4. Explain what causes CO2 concentrations to go up every year.
  5. Predict what will happen to CO2 concentrations in the future for different scenarios.
  6. Explain how annual cycles of CO2 concentrations are different for the Northern and Southern hemispheres.

NGSS Performance Expectations

Middle school

  • Waves and Electronic Radiation. MS-PS4-2. Develop and use a model to describe that waves are reflected, absorbed, or transmitted through various materials.
  • MS-ESS2-1. Develop a model to describe the cycling of the Earth’s materials and the flow of energy that drives this process.
  • Human Impacts. MS-ESS3-4. Construct an argument supported by evidence for how increases in human population and per-capital consumption of natural resources impact Earth's systems.
  • Earth and Human Activity. MS-ESS3-5. Ask questions to clarify evidence of the factors that have caused the rise in global temperatures over the past century.

High school

  • Ecosystems: Interactions, Energy, and Dynamics. HS-LS2-5. Develop a model to illustrate the role of photosynthesis and cellular respiration in the cycling of carbon among the biosphere, atmosphere, hydrosphere, and geosphere.
  • Earth’s Systems. HS-ESS2-2. Analyze geoscience data to make the claim that one change to Earth’s surface can create feedbacks that cause changes to other Earth systems.
  • Weather and Climate. HS-ESS2-4. Use a model to describe how variations in the flow of energy into and out of Earth’s systems result in changes in climate.
  • Earth and Human Activity. HS-ESS3-5. Analyze geoscience data and the results from global climate models to make an evidence-based forecast of the current rate of global or regional climate change and associated future impacts to Earth systems.
  • Earth and Human Activity. HS-ESS3-6. Use a computational representation to illustrate the relationships among Earth systems and how those relationships are being modified due to human activity.

Background Information

The Keeling Curve is often presented as easily interpretable evidence that the concentration of CO2 in the Earth’s atmosphere is increasing, but our research shows that interpreting this graph presents many challenges for students. In particular:

  • The variable measured—concentration of CO2 in parts per million—is not easy for students to understand.
  • It is not at all clear to students how measurements of CO2 concentration on a mountain in Hawaii might be related to CO2 concentrations in other parts of the world.

There are two patterns evident in the Keeling Curve: an annual cycle caused primarily by changing rates of photosynthesis in the Northern Hemisphere and a long-term increase caused primarily by burning of fossil fuels and land-use changes that release carbon from biomass or soil carbon into the atmosphere.

We assume that students studying this Unit will be familiar with carbon-transforming processes (photosynthesis, cellular respiration, combustion, digestion, biosynthesis) in individual plants and possibly animals and decomposers. In this Lesson they consider how these processes affect carbon pools on a global scale.

Activity 4.1 Introduces students to the two patterns in the Keeling Curve (the short-term seasonal fluctuation and the long-term trend) and asks them to document their initial ideas about these two key patterns. Because increasing atmospheric CO2 is the driver of all other Earth Systems explored in this unit, we spend more time on this pattern than others. In previous lessons they collected evidence that shows that this phenomenon is happening; in this activity they go one step further and explain why they think this is happening. Because this is an initial ideas stage, students should not be penalized for incorrect ideas.

Activity 4.2 takes a step back from the regular progression of the unit to examine fossil fuels. In the organismal units, this would be the equivalent of the “foundational knowledge” activity. Students examine fossil fuels in three different ways. The first is through introduction to the Carbon Pools Question, where they examine the different Carbon Pools in the unit. They then zoom into the fossil fuels pool specifically to learn about (a) the molecular structure of fossil fuels, and (b) how fossil fuels were formed. This provides the foundational information for understanding why fossil fuels burn (they are constructed from organic molecules).

Activity 4.3 uses a hands-on activity (the Tiny World Modeling game) to help students figure out explanations for both the annual cycle and the long-term trend in the Keeling Curve. The Tiny World has three carbon pools: (a) atmospheric CO2, (b) organic carbon in living systems and soils, and (c) fossil fuels. Carbon atoms move among these pools in three carbon fluxes associated with carbon-transforming processes that students have studied before:

  • Photosynthesis moves carbon atoms from the atmosphere to organic matter in plants, animals, and soil.
  • Cellular respiration moves carbon atoms from organic matter in plants, animals, and soil to the atmosphere
  • Combustion moves carbon atoms from the fossil fuels pool to the atmosphere.

In playing the game students see how the balance among fluxes determines changes in pools, and how seasonal variations in fluxes and combustion of fossil fuels leads to patterns like those in the Keeling Curve.

In Activity 4.4 students use a computer model to make quantitative predictions about effects of changes in pools and fluxes. The computer model has the same pools and fluxes as the Tiny World model (Activity 4.3), but the sizes of pools and fluxes are based on current data. Students can control the size and timing of changes in fluxes and see projections of the long-term effects (50 years) for those changes.

Activity 4.5 is an optional (two turtle) activity that dives into the seasonal fluctuation of carbon dioxide in the atmosphere pool and into other pools and fluxes in the global system. Students identify photosynthesis as the specific flux driving seasonal variations in CO2 concentrations. They view videos with animations of data to show how variations in sunlight in the hemispheres drive different yearly patterns of concentration. They also use a global carbon cycling diagram to discuss other pools and fluxes (associated with oceans and land use change) and make predictions about the effects of decreasing use of fossil fuels. These explanations and predictions are the most complex in the unit.

More information on the Pumphandle Video.
[1] This video provides a visual representation of the data that has been collected of carbon dioxide concentrations in the atmosphere and how it has changed over time. Tell students that when they are watching the video, to see if the same seasonal and upward trends that are present in the Keeling Curve are present in other places on the planet where data about carbon dioxide are collected. Watch the video, which is 3 minutes and 35 seconds long.

  • 0:00-0:30—Point out to students the various pieces of information in this image. Each colorful dot represents a location on the planet where data about atmospheric carbon dioxide has been collected over time. The data are aligned by latitude, with the right side of the graph showing the most northern points. The bright red dot represents Mauna Loa. The bright blue dot represents the South Pole in Antarctica. They can see where these locations are on the small map on the right side of the image.
  • 0:30-1:00—Point out that the graph on the right side of the images is charting data from both Mauna Loa (northern hemisphere) and also Antarctica (southern hemisphere). Ask them if they notice the difference between the data from the northern and southern hemispheres. What might be the reason for the larger flux in the northern hemisphere, and the smaller flux in the southern hemisphere? (There is more land and more plants in the middle of the Northern Hemisphere compared to the South Pole.) Point out the small circle on the right side of the image that shows the time of year.
  • 1:00 – Around this time in the video, students will notice that some of the CO2 levels recorded at different places on the planet fluctuate much more dramatically than at Mauna Loa. Ask them why they think this might be happening. Point out that the Mauna Loa data does not fluctuate as much because the readings are taken on top of a mountain that is surrounded by ocean, so the signals from the plants and human emissions (both releasing and taking in CO2) do not impact the reading as dramatically.

This video is a nice opportunity to point out that even though there are short-term variations in the temperature and CO2 levels, that the overall trend is still increasing. Students may ask “If global warming is happening why was it so cold this winter?” Even as the global temperatures continue to increase, we will still see unusually cold winters and even summers. These short-term cold periods (e.g., one season or month) are due to local weather, short-term changes in the movement of polar winds, and ocean circulations, and do not reflect the overall warming trend. However, it is predicted that climate change might make some of these local, extreme weather events more severe over a long-time scale.

  • 1:15 – Pause the video and ask students to notice during which months of the year the CO2 levels are highest in the northern hemisphere. Ask them to make connections to what they see on the screen and their image of the Keeling Curve. Point out that this is the seasonal cycle. Then, ask students to notice that the line on the left is continuously rising. Ask them which line on the Keeling Curve image on their worksheet corresponds with this increase. Point out that this is the upward trend.
  • 2:00 – Remind students that in addition to having data about CO2 in the atmosphere from the past 60 years from Keeling’s and others’ experiments, we also have information about how much carbon was in the atmosphere from many years in the past. We get these data from studying carbon isotopes in the ice cores in Antarctica.
  • 3:10 – With the long-term carbon dioxide levels on display in the graph, ask the students what they think this graph is showing. Point out that although the levels of carbon dioxide on the planet have fluctuated over time, in millions of years they have never reached the levels that they are today: nearly 400 ppm

Key Carbon-Transforming Processes: Combustion, Photosynthesis, Cellular Respiration

Unit Map

Unit Map for Lesson 4