Looking into Surface Albedo
|This activity will demonstrate how the color of materials that cover the Earth affects the amounts of solar energy absorbed.||Materials:
For each group of 4 students:
Worksheet: Albedo data page
|Lisa Gardiner, UCAR Education and Outreach|
|20 minutes for activity and short discussion, or 40 minutes to include more thorough data interpretation.|
Student Learning Outcomes:
- Ask students if they have ever noticed that wearing a black shirt on a warm sunny day will make you hotter than wearing a white shirt. Discuss how light is absorbed and transformed into heat energy.
- Show students a picture of the Earth from space. What colors do they see? Which are the lightest colors? Which are the darkest? Where do they expect that most sunlight will be absorbed? Where do they expect that the least sunlight will be absorbed?
- Have each group of students look at one of the three satellite photos (listed above). Ask students what they see in the picture (land, rock, trees, snow and ice, or ocean).
- Ask students to make a hypothesis about which areas of the photograph they think would absorb the most solar energy and which would absorb the least. Write the hypothesis into the top of the data collection worksheet.
- Instruct students to fix their thermometers to the back of the picture using tape. One thermometer bulb should be under a section of light colored ice and the other thermometer bulb under a section of dark land or ocean. Remember to place the thermometers so that when you lay the picture down on a table, the thermometers are right side up and can be read.
- Place the light directly above the picture, about a foot above. Do not turn it on yet!
- Ask students to decide who in their group is going to record the data, who is going to read the ice thermometer, who is going to read the land/ocean thermometer and who is going to be the time keeper.
- The two students read their thermometers before the light is turned on and give the numbers to the data collector. The thermometer readings should be approximately the same.
- Once they have the initial readings, groups should turn on their light and begin timing. Temperature readings will be taken every two minute (for 6 or 8 minutes total). Advise students to read the thermometers without shading the light.
- Discuss results (or do a more throughout analysis as described in Assessment section below). Point out how this model is different than the real world. This model shows relative differences based on the color of the surface but does not take into account the type of material. Explain that ice is melting. How would less ice affect the system?
- Once each group has collected their data, compile all data on computer, blackboard, or overhead transparency so that groups can compare data. Ask students to make a graph of the data that shows ice shelf and ocean with different colors. Ask students if there a difference in the amount of heat the differently-colored surfaces absorbed.
What Is Albedo?
The amount of energy retained by the Earth is strongly dependent on the albedo of Earth surfaces. Scientists use the term albedo to define the percentage of solar energy reflected back by a surface. Forests, grasslands, ocean surfaces, ice caps, deserts, and cities all absorb, reflect, and radiate solar energy differently. Light colored ice and snow are very weakly absorptive, reflecting 80-90% of incoming solar energy. Dark-colored land surfaces, are strongly absorptive, reflecting only 10-20% of the incoming solar energy, and contributing to significant heating of the Earth's surface and lower atmosphere.
Understanding local, regional, and global albedo is critical to predicting global climate change. As global temperatures increase and snow and ice cover shrink, the exposed darker surfaces underneath absorb more solar radiation, causing further warming. This is called ice-albedo feedback. The magnitude of the effect is a subject of current scientific research.
How Much Ice Is Melting?
In most areas of the world, the ice of the cryosphere melting. Between 1961 and 1998 mountain glaciers lost an average of 7 meters of ice thickness and glaciers in mountainous areas near the equator have been particularly hard-hit. According to global climate models, all of the glaciers in Glacier National Park in Montana will be gone by the year 2030. Larger glaciers and the ice sheet that caps Greenland are also melting as the Arctic warms rapidly. Snow and ice cover near the North Pole is currently decreasing at approximately 0.4% per year. Arctic sea ice has been decreasing at about 2.9% per decade. And in the South Polar Region, seven ice shelves, most in Antarctica have retreated by a total of approximately 13,500 square kilometers.
About the Images
Bhutan Land and Glacier: A satellite image taken by the ASTER instrument aboard NASA's Terra satellite, this picture shows retreating mountain glaciers in the Himalayan mountains of Bhutan. The glaciers have been melting over the past few decades, and lakes have formed on the surfaces and near the termini of many of the glaciers. Some of the glaciers are white as the ice is covered with snow pack. Other parts are rocky and have the same color as the surrounding land.
Sea Ice and Ocean: This image, from the MODIS instrument on the Terra satellite, shows Antarctica's ice-covered Ross Island (lower left), Ross Ice Shelf, sea ice, and the Southern Ocean (upper right). Only the outer edge of an ice shelf floats in the ocean water; most of it is suspended over the water because it is connected to the ice sheet on land. Sea ice, on the other hand, floats in the water and is made of frozen ocean water. Sea ice and ice shelves are both very light in color and, thus, reflect most energy.
Greenland Land and Glacier: This image of Petermann Glacier in northwestern Greenland was taken on July 5, 2003 with the MODIS instrument on NASA’s Terra satellite. During summer, turquoise blue pools of melt water form on the glacier surface and snow melt at low elevations allows a some rocky land to be exposed. Scientists have observed widespread thinning around the edges of Greenland's ice since at least the late 1990s.
RELATED SECTIONS OF THE WINDOWS TO THE UNIVERSE WEBSITE:
- The National Snow and Ice Data Center (a source of pictures of glaciers)
- NASA Earth Observatory (a source of other satellite pictures)