This activity uses a computer model to explore how the earth’s atmosphere affects the energy balance between incoming and outgoing radiation.

How does the earth’s atmosphere affect the radiation energy balance?

Carbon dioxide is one of several “greenhouse gases.” List the sources of carbon dioxide. Which ones are natural, and which ones are produced by human activity?

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. Here 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) and radiated infrared (red arrows). You can slow down the model with the slider above the picture. Are any of the red arrows absorbed by the atmosphere, or do they all escape into space?

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.

4. Click the ADD CO2 button three times to create a CO₂ level of 75 ppm in the atmosphere.

5. Follow a single light ray and a single infrared ray. Can you figure out what happens when these rays hit a CO₂ molecule?

6. Let the model run until the temperature stops changing. What happens to the temperature?

7. Click the ADD CO2 button three more times to create a CO₂ level of 150 ppm in the atmosphere. Let the model run until the temperature stops changing. What happens to the temperature?

8. Click the REMOVE CO2 button until all of the CO₂ molecules are removed. Let the model run until the temperature stops changing. What happens to the temperature?

1. In the answer box below, re-create the table below and fill it in, using data from your graph. Measure the temperature with no CO₂ twice, once at the beginning and once at the end. If you bring the cursor over the graph in the model, the numerical temperature value (Y-axis) is displayed.

	Amount of CO2	Temperature
None
75 ppm
150 ppm
None

2. As you may have noticed as you watched the model, incoming rays of sunlight are not affected by the CO₂ molecules. On the other hand, infrared radiation is strongly absorbed by CO₂ molecules. Explain why this would cause the earth’s temperature to rise.

3. Ordinary glass will transmit light but absorb infrared radiation. How would this help sunlight heat up a greenhouse?

4. How does this explain why CO₂ is called a “greenhouse gas”? Do you think this is a good analogy?

Greenhouse gas quantities can affect temperature, and then temperature can affect greenhouse gas quantities. For example, rising temperatures tend to melt large areas of permafrost, which then decay and release methane and carbon dioxide. On the other hand, more water evaporates from warmer oceans, raising the level of water vapor.

Describe how you would change this model to represent each of these effects.

There are many sources of greenhouse gases, some natural and some man-made. Do some research on all the sources of carbon dioxide, methane, and water. What is the relative importance of each source? What natural or man-made events could change how much greenhouse gas was put into the atmosphere?

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.