NetLogo User Community Models
by Aaron Unger (Submitted: 01/08/2008)
WHAT IS IT?
This is a simplified model of the Greenhouse Effect. Greenhouse gas molecules, represented by CO2 in this model, absorb infrared light but not visible light. These molecules then re-emit some of the absorbed energy towards the earth's surface.
This model shows the earth as rose colored. On the earth surface is a green strip. Above that is a blue atmosphere, with black space at the top. Clouds and CO2 molecules can be added to the atmosphere. The CO2 molecules represent the many different greenhouse gases, such as methane.
HOW IT WORKS
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.
HOW TO USE IT
The "Reset" button sets the model to a reasonable approximation of the situation in the year 2000, if "Use-My-Start-Values" is switched off. If "Use-My-Start-Values" is switched on, "Reset" uses the values in the "year," and "temp" sliders. The "Go" button runs the model and the "Stop" button stops it.
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.
You can add and remove clouds with the FORM CLOUDS and REMOVE CLOUDS 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.
The graph and brown box on the left display the Global Temperature over time as the model runs.
The brown box on the top-right simply displays the current value for CO2 Level.
THINGS TO NOTICE
Follow a single sunlight arrowhead using the WATCH SUNRAY button. This easier if you slow down the model using the slider at the top of the model.
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 model. Change the albedo and run the model.
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.
DETAILS ABOUT THE MODEL
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.
USAGE DATA OUTPUT
This model saves data about how it is used, intended solely for educational research purposes. The model must be run from a location where it will have write access to the directory it is in. Because of this, it doesn't currently work as a standalone Java applet. It also saves teh datat files in the same folder that it is in, rather than a subfolder. Trying to fix this.
The output consists mainly of one tab-delimited text file, "GCCpop_data.txt", and screenshots of the model.
There are three general times when new data is written to this file:
In all of the above cases, the current values of all the major variables are written to the file, along with other helpful stuff. When Stop is clicked, a screen capture of the window is saved also.
CREDITS AND REFERENCES
Created Nov 19, 2005 by Robert Tinker for the TELS project. Updated Jan 9, 2006.
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