Title: The Greenhouse Effect

Overview & Outcomes:

This lesson helps learners:

• define the greenhouse effect.
• identify naturally occurring greenhouse gases.
• identify other common examples of the greenhouse effect.
• explain the limitations of the term greenhouse effect as an explanation of the phenomena responsible for keeping the planet warm.

The concept map shows model relationships among concepts this lesson seeks to develop. Concepts introduced in this lesson are bolded on that concept map and concepts from other lessons are in plain text (not bolded).

Background Notes for the Teacher:

To begin with Earth's equilibrium temperature can be calculated using a series of equations generated from important physical laws, including Plank's Law, the Stefan-Boltzmann Law, and Wen's Law. The equilibrium temperature is the temperature of the Earth resulting from the balance of incoming short wave and outgoing long wave radiation, and is a function of the distance the earth is from the sun (which affect the intensity of the solar radiation reaching the Earth) and the Earth's albedo. Earth's equilibrium temperature, as solved by the equations is 0oF. That is, 32 degrees below freezing. However, we know from measurements that the actual temperature of the Earth is 60oF! When you think about it, we know the calculated equilibrium temperature can't be "right" because if it were, then most of the surface of the earth would be frozen.

The question then is, Why is there a discrepancy between the calculated and observed temperatures? The answer is: the Greenhouse Effect. Remember from Investigations Lesson 5 that some of the incoming short wave radiation is absorbed by the Earth's surface and re-radiated as long wave radiation. The atmosphere is not very transparent to these long waves. Atmospheric gases such as CO2, water vapor, methane, ozone and N2O are known as greenhouse gases because they absorb and re-emit long-wavelength energy, causing much of it to go back to the earth's surface. Thus this long-wavelength energy that cannot escape the earth's atmosphere warms the earth. This is the greenhouse effect, which is very important in that if this phenomena did not occur, the Earth would be a frigid 0oF (or -18° C), otherwise. The intensity of this effect depends on the concentration of absorbing gases in the atmosphere. Although CO2 makes up only about .03% (pre-industrial concentration - years 1750 to 1800 - was about 280 parts per million by volume) of the atmosphere, it is vital in maintaining the temperature balance because it strongly absorbs infrared energy. Water vapor (ranging in quantities from 0-4% by volume) also absorbs considerable infrared energy. The other naturally occurring greenhouse gases, methane, ozone and N20, play a part in this as well. Another example of the greenhouse effect-We have all had the experience of getting into our car on a sunny day only to find that the temperature of the air is warmer inside the car than outside. This is especially notable if the car has a dark interior. This common experience provides a good example of the greenhouse effect.

The greenhouse effect...a misnomer-While this discussion may seem unnecessary, we must be careful not to promote any alternative conceptions to students. The term "greenhouse effect" came to be used as an analogy to describe the atmosphere's role in the warming of the planet because of the similarities between the glass of a greenhouse and the atmosphere. They are both "clear" and allow many wavelengths of electromagnetic energy to pass through. They then both seem to prevent heat (long-wavelength infrared energy) from escaping. Here is where the analogy breaks down. A greenhouse stays warm because the glass acts as a solid barrier that effectively prevents air inside the greenhouse from mixing with air on the outside. The greenhouse gases do not function similarly. First of all, these gases are not a solid boundary. Secondly, as previously described in Investigations Lesson 5, these gases absorb and re-emit long-wavelength energy and do so in all directions, not just back to the earth. This is why it is the concentration of greenhouse gases that determines the degree of warming.

Additional Teacher References. Both earth science and physics (physical science books) are useful references.

Materials:

For class.

1. Materials to represent greenhouse gases so that they may be added to the bulletin/storyboard.

Preparation:

Prior to day of lesson.

Instructional Procedures: (1 Day, 40 minutes)

1. Using information from Lessons 1, 2 and 5 and this lesson's Background Notes for Teacher, organize the lesson in such a way as to most effectively present the information. It is suggested that the bulletin/storyboard continue to be used, depending on how it was originally set up. For example, if the gases discussed in this lesson (CO2, methane, ozone and N20) have already been represented on the bulletin/storyboard, perhaps they could now be distinguished from the other atmospheric gases using color or a border. It is important that students understand that not all gases in the atmosphere are greenhouse gases. It is also important that the gases be seen as separate molecules, not as a solid layer or boundary.
2. Ask students if they have ever been to a greenhouse. Explore what distinguishes a greenhouse from other buildings and why it is particularly well suited for growing plants. Ask students if they can think of another situation that they personally have experienced that is similar to a greenhouse (i.e., a car interior on a sunny day). Relate this discussion not only to the greenhouse, but also to the previous discussion of albedo (do darker car interiors produce warmer temperatures?) and reflection of incoming radiation by clouds (why do people put sun shields in the windshield? what color are the sun shields?).
3. Expand on this discussion by defining the greenhouse effect and discussing how certain gases in the atmosphere are both similar and different from the glass of a greenhouse or a car. (Refer to Background Notes.) Explain to students that a car with the windows down slightly is a better analogy of our atmosphere. The car interior typically remains warmer than the outside temperature, but some infra-red wavelengths do "escape."

Assessment/Portfolio Items:

Journal Entry.

Project.

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