Name:
PS101 Lab: The Greenhouse Effect
Simulation Website:
http://phet.colorado.edu/en/simulation/greenhouse
Introduction: The “Greenhouse Effect” refers to the manner in which certain gases in the atmosphere help keep the Earth warmer than it would be merely based on its distance from the Sun.
Much like in a glass greenhouse, the Earth’s surface is heated by the Sun during the day. At night
that heat cannot readily escape, helping keep the Earth warm. Thus, the more greenhouse gases in
the atmosphere, the warmer the Earth.
The amount of greenhouse gases in the atmosphere has changed over geologic time. Other factors
are also important, such as the amount of clouds. Overall this is a complicated system with many
interacting factors. However, it has been clearly shown that the amount of greenhouses gases in the
atmosphere directly determines the average temperature, and that these gases can remain in the
atmosphere for a very long time. In this lab you will see how the basic Greenhouse Effect operates.
1) Initial Screen
If you have not already done so, start the simulation. When it first starts it will look something
like this:
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If the simulation does not start up “running” click on the arrow button on the bottom. You can
pause it or change the speed anytime you wish.
The thermometer on the right displays the temperature in Kelvins on the top and either Fahrenheit
or Celsius on the bottom (depending on the button setting to the right.)
The “Reset All” button to the right can be used to reset the simulation to its initial settings.
Notice the yellow and red “stars”. These represent incoming visible and outgoing infrared photons,
respectively. Only a small number are shown. If you click the “View all photons” button (just
above Reset All) you will see a lot more photons. Try it, but then unclick the button if you are
having trouble watching them all. Let the simulation run for a bit until the temperature is roughly
stable. This is called “thermodynamic equilibrium.”
1. At what temperature does the Earth reach equilibrium?
2. Watch the paths of the red and yellow photons. Do either of them seem to be affected by the
atmosphere? If so, which of them and how are they affected?
2) A Greenhouse
Click on the “Glass Layers” tab. Note that under “Options” to the right you can change the number
of glass layers in your greenhouse. (If you have been experimenting with the simulation, be sure to
hit the “Reset All” button before preceding.)
Start with 0 layers and let the greenhouse reach thermodynamic equilibrium (stable temperature).
Also note the path of the photons. Where do they start and end? How do they travel? Are there
any changes in direction? Then add 1 layer at a time, again letting it reach equilibrium. Record
your findings in the table below.
Note: Using “View all photons” might help with some of these questions.
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Number
of Glass
Panes
Temp.
(◦ F)
Path/Behavior of Sunlight
Photons
Path/Behavior of Infrared
Photons
0
1
2
3
3. What do the sunlight photons do when they hit the glass panes from the top?
4. What do the infrared photons do when they hit the glass panes from the bottom? Be specific.
5. Based on these observations, why does the change in temperature make sense?
6. What happens to the temperature as additional glass panes are added?
7. Explain why this happens.
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8. How might what you have just discovered regarding greenhouses apply to the Earth and its
atmosphere?
3) Greenhouse Gases
Which gases in the atmosphere are “greenhouse gases”? You will investigate this question in the
next section. Click on the “Photon Absorption” tab. You will see a light that can switch between
emitting visible and infrared photons. The slider on the lamp adjusts its intensity - how many
photons it emits. Turn this all the way up (to the right).
Try each molecule in turn and see if it affects (scatters) visible or infrared photons.
9. Which molecules affect visible light?
10. Which molecules affect infrared light?
11. Which of the ones that affect either visible or infrared light are commonly found in our atmosphere?
4) Earth’s Atmosphere
Click on the “Greenhouse Effect” tab again and hit the “Reset All” button. The slider to the
right can control the amount of greenhouse gases in the atmosphere. Set it to “None” and let the
temperature come to equilibrium. Then try it on “Lots.”
12. What would happen to the Earth if there were no greenhouse gases?
13. What would happen if were there a lot?
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Notice that you can select atmospheres representing different time periods using the buttons on the
right. Select each of these in turn and let the temperature come to equilibrium. Fill in the table
below.
Time
H2 O
CO2
CH4
N2
Temp.
◦
F
Ice Age
1750
Today
14. Which gas or gases seem to affect the temperature the most? Give a reason for your choice.
15. Based on the rate of change you see in your table, what do you think the average temperature
will be in 100 years? What about the amount of greenhouse gases?
Hit the “Reset All” button. You may also want to select “View All Photons.” Let the temperature
come to equilibrium, then add 1 cloud. Let the simulation run for about a minute, then add another
cloud. Repeat this one more time.
16. What effect do clouds have on visible (sunlight) photons?
17. What effect do clouds have on infrared photons?
18. Do clouds tend to warm or cool the Earth?
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