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Earth’s reflection: Albedo by Brandon Gillette and Cheri Hamilton

H

ave you ever walked outside on a sunny winter morning to look at the freshly fallen snow? If so, I hope you had your sunglasses on! When viewing objects of different colors, you might notice that some appear brighter than others. This is because light is reflected differently from various surfaces, depending on their physical properties. Different surfaces of Earth also reflect light in different ways. The word albedo is used to describe how reflective a surface is. An ideal white body has an albedo of 100%, total reflection, and an ideal black body has 0% albedo, or total absorption. The
Earth-atmosphere system has a combined albedo of about 30%, a number that is highly dependent on local surface makeup, ground cover, angle of incidence, and cloud distribution (Budikova and Hogan 2010). Figure 1 shows the range in albedo of a variety of common surfaces, ranging from about 5% for dark objects such as forests and asphalt, to as much as 90% for light objects, such as fresh snow.
Because Earth’s albedo affects the amount of sunlight the planet absorbs, it has a direct effect on Earth’s energy budget and, therefore, global temperatures.
Over the next few decades, effects of climate change, including decreasing areas of highly reflective snow and ice, will decrease Earth’s albedo. This will accelerate the rate of global temperature rise, creating what is known as a positive feedback loop. Snow cover reflects the majority of sunlight back into space, helping keep the planet cool. When snow is absent, much more sunlight is absorbed, leading to further warming. As

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temperatures continue to increase, more snow and sea ice melt, leaving the less reflective ocean water behind. This in turn absorbs more sunlight, increasing temperatures, and so on. Water is very effective at storing heat, and a season of above-average melting could add enough heat to the system to limit ice formation the next season. Figure 2 shows how this process continues to compound on itself and provides a formula for calculating albedo.
On a smaller, local scale, the term u rban heat island is used to describe large metropolitan areas where forests and green space are being transformed into asphalt parks. Heat islands are created when growing cities substitute asphalt roads, tar roofs, and other low-albedo features for forest growth. Despite their low albedo at the top of the forest, trees provide shade for the surface and cool the air through evaporation. Hard, dark surfaces such as pavement store heat from the Sun during the day, which is then released at night, keeping cities hotter for longer periods of time. The island heating effect is characterized by city centers commonly having annual mean temperatures 1°–2°C higher than those of their surrounding environment (AMS 2009). Much of this temperature increase is due to the natural landscape having been paved over with darker materials that tend to absorb energy from sunlight and radiate energy back in the form of heat.
Begin the series of activities described here using the information provided in this background to discuss the very general concept and definition of albedo.

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FIGURE 1

Percentage of reflected sunlight from common surfaces

CReSIS gRaphIC CReated by Cat CoquIllette.

Materials
Activity 1: Measuring albedo (per group of two to three students)
• 1 photo of sea ice and ocean water (Figure 3)
• 2 aquarium LCD tape thermometers ($1.25 each); alcohol thermometers will work, but not as quickly ($49 for 100)
• desk lamp (40W, $30)
• timer/stopwatch ($3), or use a wall clock
• safety glasses ($3.50 )
• Activity Worksheet 1

Activity 2: Data collection (per group)
• Handheld photographic light meter ($30) or a camera with built-in light meter; probeware light sensor may be used in place of the handheld light meter
• Activity Worksheet 2

Activity 1: Measuring albedo
Activity 1 is designed as an engagement activity to provoke student thinking about the concept of albedo. In it, students will measure temperature changes in surfaces with different albedos using the materi-

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FIGURE 2

Ice albedo feedback

FIGURE 3

Sea ice

CReSIS gRaphIC CReated by Cat CoquIllette.

als listed above. Before starting this lesson, students should know the definition of albedo. They should also check to make sure the thermometers are reading the same temperature and make a note of the initial temperature to ensure the change in temperature is accurately computed. Calibration is a difficult issue, and students will learn to appreciate that thermometers don’t all read the same.
A great way to get started on a topic as unfamiliar as albedo is to also discuss experimental design.
Provide the class with the list of materials for Activity 1 and 5 to 10 minutes of discussion time to brainstorm ideas for how to set up an experiment to measure albedo of the sea-ice image. Have students also think about what variables are in play in this experiment. Some examples they may come up with include the lightbulb wattage, the height of the lamp above the surface, or the actual colors on their sea-ice image.
Depending on your comfort level with the scientific process, you may expand this activity to have groups of students select a variable to change and then compare results with the class. There is a section on the Activity 1 worksheet for students to record experiment notes.
Students’ experiments should measure differences in temperature between sea ice and water. The variables they change are up to them (different wattage of bulb, distance of lamp above surface, etc.).

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Activity 2: Data collection
The main goal of this activity is to bring the concepts of albedo and climate change to a more local level. After discussing the concept of albedo with students in
Activity 1, explain that they will be measuring the albedo of a variety of surfaces in and around the school.
Show students the light meter and explain how it works. The light meter is measuring reflected light in the visible spectrum, rather than infrared or ultraviolet, so the results may not be 100% accurate.
As described in Activity 1, before collecting data, discuss the variables that might affect the results. Some examples may include the angle of the light shining on the surface (angle of incidence), the distance, and the angle of the light meter when readings are taken
(Figure 4). Encourage students to measure a variety of surfaces, such as pavement of different colors, sand, soil, and grass or other vegetation.
Measuring the reflectance alone will only provide a relative number, giving students a general idea about how much light is being reflected from each surface; the numbers will not have scientific meaning. Should you choose to have students measure incident light and calculate the albedo, emphasize that they are not to look directly at the Sun. Getting an incident reading from the Sun can help quantify students’ results, but they must exercise extreme caution when measuring. It might help to demonstrate this technique before having students measure the incoming light. Using the equation

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FIGURE 4

Angle of incidence

FIGURE 5

Screenshot of NetLogo climatechange model

CReSIS gRaphIC CReated by Cat CoquIllette.

in Figure 2, it is possible to measure albedo. This activity may also be done in the classroom using the overhead lighting as your light source and measuring the reflected light of different-colored surfaces around the room.

Activity 3: Climate modeling
For this activity, students will need access to the NetLogo website from Northwestern University (http:// ccl.northwestern.edu/netlogo/models/ClimateChange), which includes a model of how the Earth’s atmosphere and surface affect the energy balance between incoming and outgoing radiation. Students use this model to further explore the interactions of incoming and outgoing solar radiation and the surface albedo. Using the model, they are manipulating changes in albedo to illustrate the connections between surface reflectivity and temperature changes. Access is free and does not require any registration information. Figure 5 shows a screenshot from the model.
If access to technology is an issue, this activity can be done as a class with one computer. Simply run the activity as directed in the Activity Worksheet 3 and have students fill in the data as you move through the steps.

Discussion and assessment
There are a number of discussions that can stem from these activities, on topics that range from the local to the global scale. Pose the following questions for students to consider and have a class discussion on the three activities.

CouRteSy of Netlogo.

In this model, the earth is pink, the earth’s surface is green, the atmosphere is blue, and outer space is black. yellow arrows represent sunlight (solar radiation). Red arrows represent heat energy (infrared radiation) emitted by earth. Red dots represent heat energy trapped on earth (in the soil, rocks, ocean, etc.).

1. As the Arctic continues to warm, trees are starting to grow farther north, replacing tundra vegetation. How might this affect albedo?
2. Are there changes taking place locally that might be affecting albedo in your area?
The assessment for this topic is related to an article published by the NASA Earth Observatory. Students answer a series of questions formulated from a world map showing relative surface albedo. These questions are designed to assess a student’s understanding of the practical applications of the concept of albedo and are graded on accuracy (see Figure 6).
See Resources for more information from the NASA
Earth Observatory.

Conclusion
Climate science is a very hot topic in current events, with the melting Arctic sea ice being a major area of interest.
This lesson helps to link current events with the science behind the melting ice. Changes in albedo are a large fac-

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FIGURE 6

Assessment

Activity Worksheet 1: Measuring albedo

Name: ________________

How does ice or water affect temperature?

Earth’s albedo in decline

Safety note: Safety glasses must be worn during this activity. Review safety guidelines for use of thermometers and electrical devices.

1. What color on this map reflects the most energy?
______________________________________________
______________________________________________
2. Which geographic features absorb the most energy?
______________________________________________
FIGURE 6
______________________________________________
3. What happens to the energy that is not reflected from the surface of the object?
______________________________________________
______________________________________________
4. how might decreasing sea-ice extent in the arctic affect earth’s albedo?
______________________________________________
______________________________________________
Name:

5. how might clouds affect earth’s albedo?
EARTH’S ALBEDO IN DECLINE
______________________________________________
1. What color on this map reflects the most energy?
______________________________________________
2.

Which geographic features absorb the most energy?

6. looking at the energy that is notwhy is the surface of the object? higher here than point a, reflected from the albedo
3. What happens to other parts of africa?
4. How might decreasing sea ice extent in the Arctic affect Earth’s albedo?
______________________________________________
5. How might clouds affect Earth’s albedo?
______________________________________________
6.

Looking at point A, why is the albedo higher here than other parts of Africa?

• What were the temperature readings?
______________________________________________
• Which one was the warmest? Coldest?
______________________________________________
• Why was one cooler than the other?
______________________________________________
• Can you think of a time or scenario where you might pay attention to the color of your clothes?
______________________________________________
• Why are scientists concerned that if some of the ice on earth melts, global warming might accelerate?
______________________________________________
experiment notes:
______________________________________________
______________________________________________
Vocabulary
Albedo: the amount of sunlight that is reflected from a particular surface.
Angle of incidence: the angle that a light ray or electromagnetic wave striking a surface makes with a line perpendicular to the reflecting surface.
Urban heat island: a metropolitan area that is significantly warmer than its surrounding rural areas.

A.

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1. Check the thermometers for the same temperature reading on both. If they are not the same, make sure to record each temperature for comparison purposes.
2. place the picture of the glacier face up under the lamp.
3. Slide one thermometer face up under the glacier picture under the white ice.
4. Slide the other thermometer under the dark land area.
5. Set the timer for five minutes.
6. turn on the lamp and wait for the timer to go off to read the thermometers.

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Activity Worksheet 2: Data collection

Activity Worksheet 3: Climate modeling

How does the color of a surface affect its albedo?

1. go to http://ccl.northwestern.edu/netlogo/models/ClimateChange. 2. to run the model, always click the “setup” button first, and then click the “go” button. you can pause the model by clicking the “go” button again. Watch the model run and observe what happens to the sunlight (yellow arrows) and radiated infrared (red arrows). you can slow down the model with the slider above the picture. are any of the red arrows absorbed by the atmosphere, or do they all escape into space?
3. let the model run until the temperature becomes nearly stable. at this point, the energy arriving on the earth is roughly equal to the energy leaving the earth.
4. Note the albedo slider, which ranges from 0.00 (0%) to 1.00 (100%). Change the albedo. What does it do to the earth’s surface in the model? take note of the color changes. 5. try different values of albedo, including 0 and 1. Run the model long enough for the temperature to stabilize. Record the average temperature for three different values of albedo.
6. Set the albedo to 0.4 and reset the model. try clicking the
“add clouds” button. What effects do clouds have on the incoming sunlight and the outgoing infrared?
7. try different amounts of clouds and record how the temperature changes. after each change, run the model until the temperature stabilizes.

1. predict which of the colored papers will reflect the most light and which will reflect the least. Record your predictions in the form of a hypothesis. hypothesis _______________________________
____________________________________________
2. use the light meter or camera to determine the relative amount of light being reflected off the different colors. first, determine your independent and dependent variables, and what things need to be kept constant to minimize the chance of other factors affecting the results of the experiment.
Independent variable: dependent variable:
Constants:
3. Record the results in the table below:
Sample color

Light meter reading trial 1

trial 2

trial 3

trial 4

albedo

4. discuss your results. What happened and why do you think it happened?
______________________________________________
______________________________________________
5. What happened to the energy that was not reflected from the surface?
______________________________________________
______________________________________________
6. If arctic sea ice continues to disappear, how might this affect the earth’s albedo and the temperature in the region?
______________________________________________
______________________________________________

Clouds
(yes/no)

Color of surface temperature

describe the relationship you found between temperature and albedo in the model.
______________________________________________
______________________________________________ describe the relationship you found between temperature and cloud cover in the model.
______________________________________________
______________________________________________

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TRIED AND TRUE tor in the rapid alterations taking place in the Arctic. Most students hear stories in the news about the endangerment of polar bears and other wildlife populations that occupy the Arctic. This lesson helps to explain why these changes are taking place. For more information and other activities related to sea-level rise, glacier dynamics, and climate change, visit the Center for Remote Sensing of Ice Sheets education pages at www.cresis.ku.edu/education. n

References

American Meteorological Society (AMS). 2009. Glossary of meteorology: Urban heat island. http://amsglossary.al lenpress.com/glossary/search?id=urban-heat-island1. Budikova, D., and C.M. Hogan. 2010. Albedo. The Encyclopedia of Earth. www.eoearth.org/article/Albedo.

Resources

Albedo—www.arcticice.org/albedo.htm
Earth’s albedo in decline—http://earthobservatory.nasa. gov/IOTD/view.php?id=5484 Brandon Gillette (bgillette@ku.edu) is a graduate student in geology at the Center for Remote Sensing of Ice
Sheets (CReSIS) at the University of Kansas in Lawrence,
Kansas. Cheri Hamilton (chamilton@creis.ku.edu) is the education outreach coordinator at CReSIS.

Copyright © 2011, National Science Teachers Association (NSTA).
Reprinted with permission from Science Scope, Vol. 34, No. 6, February 2011.

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