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## WHAT IS IT?

This is a model of energy flow in the earth, particularly heat energy. It shows the earth as rose colored, and the surface of the planet is represented by a black strip. Above the strip there is a blue atmosphere and black space at the top. Clouds and carbon dioxide (CO2) molecules can be added to the atmosphere. The CO2 molecules represent greenhouse gases that block infrared light that is emitted by the earth. Clouds block incoming or outgoing sun rays, influencing the heating up or cooling down of the planet.

## HOW IT WORKS

Yellow arrowheads stream downward representing sunlight energy. Some of the sunlight reflects off clouds and more can reflect off the earth's 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, and its temperature is related to the total number of red dots.

Sometimes the red dots transform themselves into infrared (IR) light that heads toward space, carrying off energy. The probability of a red dot becoming IR light depends on the earth's temperature. When the earth is cold, few red dots generate 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.

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, which 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 reacts 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.

## HOW TO USE IT

The SUN-BRIGHTNESS slider controls how much sun energy enters the earth's 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 it 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 gases, 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.

## THINGS TO NOTICE

Watch a single sunlight arrowhead. This is easier if you slow down the model using the slider at the top of the model. You can also use the WATCH A RAY button.

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?

## THINGS TO TRY

1. Play with the 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 37 degrees. Why does it stop rising? Why does the temperature continue to bounce around? Remember, the temperature reflects the number of red dots in the earth. When the temperature is constant, there are 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.

## EXTENDING THE MODEL

Try to add some other factors influencing the earth's temperature. For example, you could add patches of vegetation and then see what happens as they are consumed for human occupation. Also, you could try to add variable albedo to the model, instead of having one value for the whole planet. You could have glaciers with high albedo, and seas with low albedo, and then evaluate what happens when the glaciers melt into the seas.

## NETLOGO FEATURES

Note that clouds are actually made up of lots of small circular turtles.

## RELATED MODELS

Daisyworld

## CREDITS AND REFERENCES

This model is based on an earlier version created in 2005 by Robert Tinker for the TELS project.

## HOW TO CITE

If you mention this model or the NetLogo software in a publication, we ask that you include the citations below.

For the model itself:

* Tinker, R. and Wilensky, U. (2007). NetLogo Climate Change model. http://ccl.northwestern.edu/netlogo/models/ClimateChange. Center for Connected Learning and Computer-Based Modeling, Northwestern University, Evanston, IL.

Please cite the NetLogo software as:

* Wilensky, U. (1999). NetLogo. http://ccl.northwestern.edu/netlogo/. Center for Connected Learning and Computer-Based Modeling, Northwestern University, Evanston, IL.

## COPYRIGHT AND LICENSE

Copyright 2007 Uri Wilensky.


This work is licensed under the Creative Commons Attribution-NonCommercial-ShareAlike 3.0 License. To view a copy of this license, visit https://creativecommons.org/licenses/by-nc-sa/3.0/ or send a letter to Creative Commons, 559 Nathan Abbott Way, Stanford, California 94305, USA.

Commercial licenses are also available. To inquire about commercial licenses, please contact Uri Wilensky at uri@northwestern.edu.

<!-- 2007 Cite: Tinker, R. -->