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[screen shot]

If clicking does not initiate a download, try right clicking or control clicking and choosing "Save" or "Download".(The run link is disabled for this model because it was made in a version prior to NetLogo 6.0, which NetLogo Web requires.)


This is a model of energy flow in the earth. It shows the earth as rose colored. On the earth surface is a green strip. Above that is a blue atmosphere and black space at the top. Clouds and CO2 molecules can be added to the atmosphere. The CO2 molecules represent greenhouse gasses that block infrared light that the earth emits.


Yellow arrowheads stream downward representing sunlight energy. Some of the sunlight reflects off clouds and more can reflect off the earth surface.

If sunlight is absorbed by the earth, it turns into a red dot, representing heat energy. Each dot represents the energy of one yellow sunlight arrowhead. The red dots randomly move around the earth. The temperature of the earth is related to the total number of red dots.

Sometimes the red dots transform into infrared (IR) light that heads toward space, carrying off energy. The probability of a red dot becoming IR light depends on the earth temperature. When the earth is cold, few red dots cause IR light; when it is hot, most do. The IR energy is represented by a magenta arrowhead. Each carries the same energy as a yellow arrowhead and as a red dot. The IR light goes through clouds but can bounce off CO2 molecules.


The "sun-brightness" slider controls how much sun energy enters the earth atmosphere. A value of 1.0 corresponds to our sun. Higher values allow you to see what would happen if the earth was closer to the sun, or if the sun got brighter.

The "albedo" slider controls how much of the sun energy hitting the earth is absorbed.
If the albedo is 1.0, the earth reflects all sunlight. This could happen if the earth froze and is indicated by a white surface. If the albedo is zero, the earth absorbs all sunlight. This is indicated as a black surface. The earth's albedo is about 0.6.

You can add and remove clouds with buttons. Clouds block sunlight but not IR.

You can add and remove greenhouse gasses, represented as CO2 molecules. CO2 blocks IR light but not sunlight. The buttons add and subtract molecules in groups of 25 up to 150.

The temperature of the earth is related to the amount of heat in the earth. The more red dots you see, the hotter it is.


Follow a single sunlight arrowhead. This easier if you slow down the model using the slider at the top of the model.

Here is a better way to follow an arrowhead. Stop the model and control-click on an arrowhead, select the last item "turle" followed by a number. This opens a sub-menu where you can select "watch" followed by a number. Now when you run the model, you will see a circle around that arrowhead.

What happens to the arrowhead when it hits the earth? Describe its later path. Does it escape the earth? What happens then? Do all arrowheads follow similar paths?


1. Play with model. Change the albedo and run the model.
Add clouds and CO2 to the model and then watch a single sunlight arrowhead.
What is the highest earth temperature you can produce?

2. Run the model with a bright sun but no clouds and no CO2. What happens to the temperature? It should rise quickly and then settle down around 50 degrees. Why does it stop rising? Why does the temperatuer continue to bounce around? Remember, the temperature reflects the number of red dots in the earth. When the temperature is constant, there about as many incoming yellow arrowheads as outgoing IR ones. Why?

3. Explore the effect of albedo holding everything else constant. Does increasing the albedo increase or decrease the earth temperature? When you experiment, be sure to run the model long enough for the temperature to settle down.

4. Explore the effect of clouds holding everything else constant.

5. Explore the effect of adding 100 CO2 molecules. What is the cause of the change you observe. Follow one sunlight arrowhead now.


There is a relation between the number of red dots in the earth and the temperature of the earth. This is because the earth temperature goes up as the total thermal energy is increased. Thermal energy is added by sunlight that reaches the earth as well as from infrared (IR) light reflected down to the earth. Thermal energy is removed by IR emitted by the earth. The balance of these determines the energy in the earth with is proportional to its temperature.

There are, of course, many simplifications in this model. The earth is not a single temperature, does not have a single albedo, and does not have a single heat capacity. Visible light is somewhat absorbed by CO2 and some IR light does bounce off clouds. No model is completely accurate. What is important, is that a model react in some ways like the system it is supposed to model. This model does that, showing how the greenhouse effect is caused by CO2 and other gases that absorb IR.


Created Nov 19, 2005 by Robert Tinker for the TELS project. Updated Jan 9, 2006.

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