HES Lesson 3 Background Information

Three-dimensional Learning Progression

Lesson 3 focuses on students examining key foundational knowledge about how CO2 interacts with energy in the atmosphere. In order to understand the longer chain of events that leads to Arctic sea ice decrease and sea level rise, students must first understand that (a) we have evidence that CO2 is increasing in the atmosphere, and that (b) greenhouse gases interact with long-wave radiation coming from the Earth’s surface in ways that increase temperatures.

Lesson 3 focuses on how CO2 and other greenhouse gases affect the Earth’s temperature. Your students may be familiar with the metaphor that greenhouse gases act like a blanket to keep the Earth warm. That’s not a terrible metaphor, in that both blankets and greenhouse gases slow down heat transfer, but it has significant limitations. The mechanisms are different: blankets reduce heat transfer by reducing convection, while greenhouse gases reduce heat transfer by reducing radiation.

More importantly, a key goal for the Human Energy Systems unit is to help students explain changes by tracing pools and fluxes of energy and matter. Lessons 4 and 5 focus on matter; this lesson focuses on energy. Students need to understand that the Earth’s temperature depends on the balance between two different energy fluxes:

  • The sun’s radiation coming in (mostly in the form of visible light)
  • Infrared radiation going out (mostly in the form of infrared light)

During a typical day there is more radiation coming in, so the Earth’s pool of thermal energy gets larger and the temperature rises. During a typical night there is more radiation going out, so the Earth’s pool of thermal energy gets smaller and the temperature falls.

Greenhouse gases have their effects because they absorb infrared but not visible light, so they slow down only the outgoing energy flux, causing the Earth’s temperature to gradually rise.

In Lessons 4 and 5 students will explain carbon pools and fluxes in multiple Earth Systems. The goal of this lesson is to helps students identify the causal relationship between atmospheric CO2 concentrations and temperature (the Greenhouse Effect), and to establish the problem for a deeper study of the Keeling Curve.

Key Ideas and Practices for Each Activity

In Activity 3.1 students learn about a scientist named Charles David Keeling by reading a short story and listening to a radio broadcast. These stories explain how Keeling’s research alerted the global community to the fact that atmospheric CO2 is increasing, and what work lead to the discovery of this important information. Previously, students used the graphs but did not yet know how the data were collected.

In Activity 3.2, students examine the Greenhouse Effect. This gives them the first piece of evidence they need to eventually explain that increasing atmospheric CO2 is the driving factor among the multiple Earth systems they examined in the previous lesson. This causal chain of events includes the Greenhouse Effect, or the interaction between greenhouse gases and long-wave radiation coming from Earth that traps heat in the atmosphere. Upon completing the reading, students will utilize a simulation to investigate the interactions between different atmospheric gases and different forms of radiation.

Note that the analogy between the atmospheric Greenhouse Effect and actual greenhouses (or the closed car that is used as an analogy in the reading) is not perfect. Greenhouse gases are like glass in that they absorb infrared radiation, but unlike greenhouse gases, glass also blocks heat loss by convection.

In Activity 3.3, students use a concept map to document their ideas about the relationships between different Earth systems at this stage in the unit. With their new knowledge of the greenhouse effect, this should begin to resemble a scientific explanation for the relationship between increases in atmospheric CO2 and temperature rise.

Content Boundaries and Extensions

This lesson focuses on how greenhouse gases affect energy fluxes, and how that makes CO2 concentrations the driver of other changes in Earth systems. Students may be interested in how molecular vibrations lead to absorption of infrared radiation (discussed briefly in The Greenhouse Effect Reading, but this is not a primary focus for this lesson.