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

Lesson 4: Fossil Fuels and Carbon Pools

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

Guiding Question

How did fossil fuels form, and how do we use them?

Activities in this Lesson

  • Activity 4.1: Your Ideas about the Keeling Curve (30 min)
  • Activity 4.2: Carbon Pools and Fossil Fuels (35 min)
  • Activity 4.3: Carbon Movement (40 min)
  • Activity 4.4: The Seasonal Cycle (45 min)
  • Activity 4.5: The Upward Trend (30 min)


  1. Locate organic and inorganic carbon pools near the Earth’s surface (atmosphere, biomass, soil, fossil fuels, oceanic carbon).
  2. Explain changes in atmospheric CO2 between seasons in terms of fluxes associated with carbon-transforming processes: photosynthesis and cellular respiration.
  3. Describe carbon cycling within Earth and Human systems.
  4. Explain changes in atmospheric CO2 in terms of fluxes associated with carbon-transforming processes: photosynthesis and cellular respiration.
  5. Describe the atmosphere and biomass pools as changing in size over time.
  6. Explain the seasonal cycle in the Keeling Curve.
  7. Locate organic and inorganic carbon pools near the Earth’s surface (atmosphere, biosphere, soil organic carbon, fossil fuels, and ocean pools)
  8. Identify the molecular structure of gasoline as an organic molecule, and all fossil fuels as originating as organic plant materials.
  9. Trace energy associated with human lifestyles to its sources, particularly combustion of fossil fuels.
  10. Describe energy as flowing through Earth systems, from sunlight to chemical energy to heat that is radiated into space.
  11. Describe the processes involved in fossil fuel formation and identify the time associated with each process.

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.
  • Earth's Systems. 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 somewhat 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 gives students an introduction to the second of the Three Questions: the Carbon Movement Question. This activity is key for understanding which processes cause “fluxes,” or movement of carbon over time, from one pool to another. In this activity we focus specifically on two carbon-transforming processes that move carbon between the atmosphere and biomass carbon pools: photosynthesis (atmosphere to biomass) and cellular respiration (biomass to atmosphere). This sets students up for being able to explain the seasonal cycle in the following activity.

Activity 4.4 dives into the seasonal fluctuation of carbon dioxide in the atmosphere pool. Building on foundational knowledge from the previous activity, students develop scientific explanations for why carbon dioxide levels in the atmosphere goes up in the winter and down in the summer. They watch a video that shows how most of the Earth’s biomass (trees, grass, etc.) is concentrated in the Northern hemisphere. They also examine how changes in plant growth in the northern hemisphere drive the increase and decrease of CO2 in the Keeling curve.

Activity 4.5 serves as a culminating activity for examination of the Keeling Curve. Now that the students have developed scientific explanations for the seasonal cycle (previous activity), they look at the cause of the overall trend: that carbon dioxide is increasing in the atmosphere globally. Using the “pumphandle video,” students also examine atmospheric CO2 levels over the past 800,000 years, gathering evidence that CO2 levels are higher than they have been during this time scale.

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