This activity uses a computer model to explore the energy balance between incoming and outgoing radiation on the earth.

What happens when sunlight strikes the earth?

Most of the energy that warms the earth comes from sunlight. A majority of this energy arrives at the earth as visible radiation. This energy passes through our atmosphere and is absorbed by oceans and land. Some of it is reflected by snow and cloud cover.

At the same time, the earth’s warm surface radiates energy to space, but this is infrared radiation and is not visible. It is partly absorbed by the atmosphere. The earth’s temperature changes until the incoming and outgoing energy are equal.

When water vapor is present, the atmosphere absorbs more infrared. For this reason water vapor is called a greenhouse gas, because it traps heat. Water vapor plays the largest role, but there are other greenhouse gases, such as carbon dioxide and methane.

Make a list of all of the physical features of the atmosphere and the earth’s surface that might change this energy balance. For each feature, explain why it would either increase or decrease the trapping of heat.

This activity addresses NSES standards for earth and space science and inquiry at grades 5-8
(http://books.nap.edu/readingroom/books/nses/6d.html#es).

1. Below is a model of how the earth’s atmosphere and surface affect the energy balance between incoming and outgoing radiation. In this model, the earth is pink, the earth’s surface is green, the atmosphere is blue, and outer space is black. Yellow arrows represent sunlight (solar radiation). Red arrows represent heat energy (infrared radiation) emitted by the earth. Red dots represent heat energy trapped in the earth. (See screenshot.)

2. To run the model, always click the SETUP button first, and then click the GO button. You can pause the model by clicking the GO button again. Watch the model run and observe what happens to the sunlight (yellow arrows). You can slow down the model with the slider above the graphics window. What happens when a ray hits the earth’s surface? Name two things that happened to the sunlight.
3. Let the model run until the temperature stops changing very much. At this point the energy arriving on the earth is roughly equal to the energy leaving the earth. What is the shape of the temperature graph? Why?
4. How long does it take to reach equilibrium?

5. Move the SUN-BRIGHTNESS slider up to 5. This might correspond to sunlight in the middle of the day compared to early morning. Let the temperature settle down again. What happens to the temperature? Why?
6. Move the SUN-BRIGHTNESS slider down to 0. This might correspond to the earth’s surface at night. Let the temperature settle down again. What happens to the temperature? Why?
7. See if you can imitate the day-night cycle by changing the SUN-BRIGHTNESS slider while the model is running. Describe the resulting temperature graph.

For more information on NetLogo, go to the NetLogo Home Page. You can download NetLogo and build your own models or modify the ones in the Models Library.