3. Studying the Earth: A Model of the Greenhouse Effect

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1 1 of 7 1/4/2008 1:54 πμ Download this file in: PDF ZIP 3. Studying the Earth: A Model of the Greenhouse Effect GLOBE Teacher's Key: After completing this activity, the student will be able to: Follow a protocol in setting up a simple physical model analogous to the earth's greenhouse effect. Explain how this model is like/unlike the earth and its atmosphere. Explain why and how variables are controlled in the experiment and will be able to record, graph and interpret data. Describe the greenhouse effect as a warming of the earth's atmosphere, and the mechanisms by which this occurs. Appropriate grade: Middle school and high school. Prerequisite skills: Students should have some experience with conducting simple controlled experiments, graphing, forming hypothesis and testing them, and concept mapping. Students should be familiar with the concepts of electromagnetic radiation, energy transfer processes and the layers and composition of the earth's atmosphere. Time needed: 2-4 classes Studying the Earth: - A Model of the Greenhouse Effect - Teachers Guide Comments for teachers regarding this activity 1. Engagement Use a large poster of the Earth and have students list what the physical conditions for life are that the Earth provides. Compile lists into a master class list and group the requirements. The list should include the following: a source of energy (sun), l iquid water, a protective non-toxic atmosphere, matter, and a temperature conducive to life. Describe how these factors interact, e.g., the average temperature ensures that most of the water is liquid and life itself influences the physical conditions, e. g., plant-like organisms produce oxygen. Liquid water and an amenable temperature are particularly necessary for life. Emphasize the Earth as a plant in balance, which provides conditions for life as we know it. This balance may be upset on a local or global scale by human activities. The simple physical model the students will construct will demonstrate the importance of the Earth's atmosphere in maintaining a temperature balance. 2. Explore Have students set up the model, conduct the experiment, record/graph the data. 1. Set up a complete model, and instruct students (in groups of 4) to follow instructions in the Student Handout for construction of their models. 2. Check the models. Be sure the thermometers can be read from the outside and that they are not covered by soil.

2 2 of 7 1/4/2008 1:54 πμ 3. Identify the open bottle as the "control" and the covered bottle as the "experimental" one. Explain that the two variables involved in the overall experiment are the temperature and time. Time is the independent variable and temperature is the dependen t variable because it will vary depending on the time. It may be suggested at this time that the uncovered container could be analogous to the moon and the covered container represents a "normal Earth." If the model is set up as in Student Instructions 4b, then the Saran covered container is analogous to the condition of global warming and the non-saran covered container represents "normal" Earth. Ask why we are being so careful with the details of setting up the experiment? "So that any temperature ch anges can be related to just one variable." 4. Ask if the two thermometers read the same. If not have the team monitoring one of the thermometers adjust each reading as they record it. 5. Pass out graph paper (2 sheets for each group of 4 students). 6. Ask students to write down what they think will happen to the temperature in each bottle on the back of the graph paper. This is their hypothesis. 7. Have two students monitor each thermometer; one should read and one should record temperatures on the graph paper every minute for minutes, or until the temperatures stop increasing and begin to level off. 8. After the experiment, have students swap data within their group; connect the data points, first in pencil, and then in pen (blue for the control and green for the experimental). Have them label the lines "control" and "experimental". 9. Ask each group of students to answer the questions on the back of the graph. 10. Ask the class to summarize the trends and discuss the answers to the questions. 11. Have students explain: 1. Why the temperature goes up in the two bottles? (Light/heat from the bulb passed through the plastic and warmed the air and soil.) Why the temperature went higher in the covered pop bottle? (Warm air can mix with the cold air in the open bottle, but the warm air is held inside the closed bottles by the plastic wrap.) Why the temperature in both bottles leveled off? (Heat from the light bulb can enter as well as exit from the container more easily.) To demonstrate the latter point turn off the light bulb, hold your hand behind the bottle and you should feel heat coming out. Heat moves from higher to lower temperatures, so as soon as the air/soil warmed-up higher than the outside temperature, heat beg an leaving the bottle. The point at which the temperature levels off is the equilibrium temperature. The energy entering the bottle equals the energy leaving the bottle at this temperature. What is the difference in the equilibrium temperature of the two bottles? This experiment works best if the room is cool. The temperature in the covered container should increase faster and level off at a higher equilibrium, but discuss whatever results students obtain. 3. Explain Describe the Earth's atmosphere as composed of layers; the lowest layer, the Troposphere, is about 4-7 miles thick; the next higher layer, the Stratosphere, extends up to about 30 miles. The ozone layer is spread out in the Stratosphere, but ozone concent rations are highest between miles. A part of a

3 circle (radius 4 feet) drawn on the blackboard can represent the Earth. The chalk line of the circumference which is about 1/4 inch thick, represents the lower 50 miles of the atmosphere; this layer con tains almost all of the gases in the atmosphere. Enlarge the chalk line to represent features at different elevations; i.e., Denver, Mt. Everest, airplane flight, Troposhere, Stratosphere, ozone layer. This information could be discussed as part of the en gagement. The atmosphere is thicker and more dense closer to the Earth; it is held by gravity. The atmosphere is composed of 78% nitrogen gas, 21% oxygen gas, 0.9% argron gas, 1-4% water vapor by volume and trace gases, including carbon dioxide 0.03%, methane, CFC' s and nitrous oxide. Explain how energy from the sun is filtered, reflected, scattered and absorbed by the Earth and its atmosphere; how the earth reradiates energy; how and why energy is absorbed by water vapor, and the trace gases; and how such absorption warms the Earth. We can track solar energy disposition through the atmosphere along a path that begins 100 miles from the surface of the Earth. Nearly all of the very short wavelengths (x-rays and gamma rays) are absorbed in the upper 50 miles of our transact; ultraviolet radiation is absorbed by the ozone layer. Of the energy entering the lower atmosphere, 21% is reflected by clouds, 6% is reflected by ground surfaces, and 5% is scattered, which yields a total depletion of 32%; this energy is removed from the Earth-Atmos phere system and plays no role in the warming of the lower atmosphere. The other 68% is absorbed; 50% by the earth's surface and 18% by dust, molecules and clouds. Some of the absorbed energy is reradiated at wavelengths back into the atmosphere, where it is absorbed by water vapor and trace gases. Absorption warms the lower atmosphere to an average temperature of 59.5 F. The warming is known as the "Greenhouse Effect", and it gives the Earth its "normal" temperature. Surface energy not reradiated is diss ipated through conduction, convention, evapotranspiration and latent heat transfer. Discuss how this model corresponds to and acts like the real Earth. "The bulb corresponds to the Sun, the air to the atmosphere, and the soil to the land. The air/atmosphere heats up because of the heat and light from the bulb (sun) and levels off at a n equilibrium temperature." Discuss how this model is different from the Earth. "Very simplified. There are no solid barriers in the Earth's atmosphere like the plastic of the bottle or plastic wrap. The plastic wrap is analogous to the trace gases." 4. Elaborate Have students complete the following chart. Variable to be in the model EXAMPLE: Possible effect on air temperature in the bottle Analogous to inchanged the real world Increase distance light bulb Decreases temperature Increase in Earth's distance from the Sun Have students do another experiment using one of the changes suggested in the above chart. 5. Evaluation Have students construct a concept map using the following terms: shortwave solar energy, Sun, Earth, atmosphere, equilibrium temperature, trace gases, transmitting, reflecting, absorbing, constructing models, collecting and analyzing data, longwave Earth energy, longwave atmospheric energy. Place these terms on cards and have students arrange the cards on a large sheet of paper using connecting phrases. Allow a few blank cards for students to add needed concepts. Activity: Studying the Earth - A Model of the Greenhouse Effect - Student Guide 3 of 7 1/4/2008 1:54 πμ

4 4 of 7 1/4/2008 1:54 πμ In this activity, you will conduct an experiment to learn about the greenhouse effect. The physical model you will construct out of pop bottles and a light bulb will represent the Sun, atmosphere and land. One bottle will be covered, and both will be expo sed to the same amount of light and heat from the light bulb. The results will demonstrate what happens in the Earth's atmosphere, but in different, analogous way. For each group of 4 students: 2 two-liter clear plastic pop bottles 2 thermometers 1 strip of thin cardboard, 1/2 X 3" 2 pieces of cardboard 2"X2" 1 piece plastic wrap approximately 6"x6" 2 marker pens (one blue, one green) 1 roll masking tape/duct tape watt light bulb 1 plastic 12 oz. cup 1 rubber band 3 cups of potting soil -- a 12 oz cup full for each bottle 1 clip-on lamp + block of wood or lamp base with bulb 1 extension cord (if needed to connect lamp to power outlet) 2 graphing data sheets entitled, "The Greenhouse Effect" Procedure (Groups of 4)

5 5 of 7 1/4/2008 1:54 πμ 1. Cut the 2-liter plastic pop bottles off at the point where they begin to narrow. (2 bottles for each group.) 2. Tape the thermometers and cardboard to the inside of the bottles as shown in the drawing, avoiding any labels on the bottle. Be sure you can read the thermometer from the outside. The bottom of the thermometer should be exactly 3" from the bottom of th e bottle or table top but should not touch the soil added in step Using a 12 oz. cup, add 12 oz. of dry potting soil to each bottle. 4a. Place plastic wrap, (non-saran Wrap) across the top of one bottle and secure it with a rubber band. 4b. An alternative would be to cover one bottle with 1 layer of non-saran plastic Wrap and the other with 2 layers of Saran Wrap. This would represent the normal Earth and the Earth with enhanced greenhouse gases. 5. Arrange the bottles exactly 1/2 inch from the light bulb with the thermometer facing outward. (Use the cardboard to measure.) 6. The final set-up should look like the diagram below: 7. Obtain two sheets of graph paper for each group of 4 students. Two students will be responsible for each bottle; one should observe the temperature, and the other should record the temperature. Record temperature readings directly on the graph paper. 8. Describe what you think will happen to the temperature in the two bottles on the back of your graph

6 6 of 7 1/4/2008 1:54 πμ paper. This is your hypothesis. 9. When all groups are ready, a timer will call out the time every minute for minutes. Record the temperature each minute directly on the graph paper. 10. After the experiment, exchange data with the other two people in your group, graph the date, use different colored markers to connect the dots, and label the lines "control" (uncovered) and experimental (covered). 11. Share your results with the class, and discuss the class results in a paragraph on the back of your graph. Answer the following questions: Why did the temperature go up in the two bottles? Why did the temperature go higher in the covered bottle? Why did the temperature in the two bottles level off? PART II Complete the following chart; provide as many examples as you can. Variable to be changed in the model EXAMPLE: Possible effect on air temperature in the bottle Analogous to in the real world Increase distance from the light bulb Decreases temperature Increase in Earth's distance from the Sun References GEMS - "Global Warming and the Greenhouse Effect" pp Lawrence Hall of Science, University of California, Berkley, CA Penn State - College of Education, toc.html Global Atmospheric Change - Investigation Lesson 7 Activities of the Changing Earth System, Ohio State University Research Foundation , ISBN

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