What Factors Impact a Greenhouse?
Modified with permission from Global Climates - Past,
Present, and Future, S. Henderson, S. Holman, and L. Mortensen (Eds.). EPA Report
No. EPA/600/R-93/126, U.S. Environmental Protection Agency, Office of Research
and Development, Washington, DC 47 - 52.
The earth's atmospheric "greenhouse effect" is much more complex
than the simple greenhouse experiment described in Activity 12. While the earth's
temperature is dependent upon the greenhouse-like action of the atmosphere,
the amount of heating and cooling are strongly influenced by several factors.
The type of surface that sunlight first encounters is the most important factor.
Forests, grasslands, ocean surfaces, ice caps, deserts, and cities all absorb,
reflect, and radiate radiation differently. Sunlight falling on a white glacier
surface strongly reflects back into space, resulting in minimal heating of the
surface and lower atmosphere. Sunlight falling on a dark desert soil is strongly
absorbed, on the other hand, and contributes to significant heating of the surface
and lower atmosphere. Cloud cover also affects greenhouse warming by both reducing
the amount of solar radiation reaching the earth's surface and by reducing the
amount of radiation energy emitted into space.
Scientists use the term albedo to define the percentage of solar energy
reflected back by a surface. Understanding local, regional, and global albedo
effects is critical to predicting global climate change. The following are some
of the factors that influence the earth's albedo.
- Clouds: On a hot, sunny day, we usually welcome a big fluffy cumulus
cloud passing overhead because we feel cooler immediately. That's because
the top of the cloud reflects sunlight back into space before it ever reaches
earth. Depending on their altitude and optical properties, clouds either cool
or warm the earth. Large, thick, relatively low-altitude clouds, such as cumulus
and cumulonimbus, reflect incoming solar radiation and thereby reduce warming
of the surface. The whitewash on plant greenhouses
has the same effect on a smaller scale. High-altitude, thinner clouds, such
as cirrus clouds, absorb longwave radiation reflected from the earth's surface,
causing increased warming.
- Surface albedo: Just as some clouds reflect solar energy into space,
so do light-colored land surfaces. This surface albedo effect strongly influences
the absorption of sunlight. Snow and ice cover are highly reflective, as are
light-colored deserts. Large expanses of reflective surfaces can significantly
reduce solar warming. Dark-colored land surfaces, in contrast, are strongly
absorptive and contribute to warming. If global temperatures increase, snow
and ice cover may shrink. The exposed darker surfaces underneath may absorb
more solar radiation, causing further warming. The magnitude of the effect
is currently a matter of serious scientific study and debate.
- Oceans: From space, oceans look much different than adjacent land
areas - they often appear darker, suggesting that they should be absorbing
far more sunlight. But unlike dry land, water absorbs energy in a dynamic
fashion. Some of the solar energy contacting the surface may be carried away
by currents, some may go into producing water vapor, and some may penetrate
the surface and be mixed meters deep into the water column. These factors
combine to make the influence of the ocean surface an extremely complex and
difficult phenomenon to predict.
Water also has the capacity to store heat and transport large amounts of heat
energy. In addition, oceans are an important sink (storage site) for atmospheric
, and their ability to absorb is strongly related to ocean temperature.
Because of their enormous size and depth, oceans are extremely important in
determining global climate and the future rate of global temperature change.
- Forested areas: Like the oceans, the interaction of forests and sunlight
is complex. The amount of solar radiation absorbed by forest vegetation depends
upon the type and color of vegetation, the time of year, and how well watered
and healthy the plants are. In general, plants provide a dark surface, so
you might expect high solar absorption. A significant fraction of the solar
radiation is captured by the plants and used to make food through photosynthesis
(and thus it doesn't re-radiate as heat); some of the energy is dissipated
as water evaporates from plant leaves; and some is absorbed and distributed
deep within the forest canopy. These complexities make a simple definition
of forest influences impossible.
To a lesser extent, the same complexities apply to any relatively continuous-cover
ecosystem (for example, grasslands and farmlands).
In this exercise, students will form their own conclusions as to how different
surface and cover types affect heating using the model bottle systems introduced
in Activity 12.
- Students will be able to identify at least three factors affecting the
heat-trapping ability of a greenhouse, including the transparency of the greenhouse
cover, color of the surfaces inside the greenhouse, and type of surfaces inside.
- Students will be able to explain the factors important in the atmosphere's
heat trapping ability.
- Students will understand the influence of albedo on earth's temperature.
Alignment to National Standards
National Science Education Standards
- Unifying Concepts and Processes, Grades K to 12, pg. 117: "Models are tentative
schemes or structures that correspond to real objects, events, or classes
of events and that have explanatory power."
- Physical Science, Transfer of Energy, Grades 5 to 8, pg. 155, Item #2: "Heat
moves in predictable ways flowing from warmer objects to cooler ones, until
both reach the same temperature."
- Earth and Space Science, Grades 9 to 12, pg. 189, Item #3: "Heating of earth's
surface and atmosphere by the sun drives convection within the atmosphere
and oceans, producing winds and ocean currents."
Benchmarks for Science Literacy, Project 2061, AAAS
- Common Themes, Models, Grades 6 to 8, pg. 269, Item #1: "Models are often
used to think about processes that happen too slowly, too quickly, or on too
small a scale to observe directly, or that are too vast to be changed deliberately,
or that are potentially dangerous."
- The Physical Setting, Energy Transformations, Grades 6 to 8, pg. 85, Item
#3: "Heat can be transferred through materials by the collisions of atoms
or across space by radiation. If the material is fluid, currents will set
up in it that aid the transfer of heat."
- Grade level: 5 to 9
- Introduction by teacher: 15 minutes
- Student activity (assuming bottle construction already done): 60 minutes
Materials for Each Team of Four Students
- Six soda bottle experimental chambers (see materials in Activity 12)
- Six thermometers
- Tape (transparent or light-colored)
- White paint
- Three cups of dark soil (garden or potting soil)
- Three cups of white sand or perlite
- Water and dump buckets
- One 150-watt floodlight bulb
- Portable reflector lamp
- Stand to support lamp set-up
- Graph paper
- To save time, you (or your students) should prepare the model greenhouses
prior to class. For each team of four students, you will need six experimental
chambers. Paint the upper third of three of the bottles white.
- Label the bottles A, B, C, D, E, and F with bottles B, D, and F having
the white paint.
- Fill the base of bottles A and B with dark soil, bottles C and D with white
sand, and bottles E and F with room-temperature water.
- Tape a thermometer (using transparent tape or light-colored masking tape)
to the inside of each bottle (facing out).
- Place the bottle tops in the bases. Make sure the bottles are capped.
- Make sure the bulbs of the thermometers are just above the top of the bases.
If the bulbs are below the base, the thermometer may record the heat absorbed
directly by the soil or water, complicating the results.
- Ask students to predict which bottle will get hotter. Why? Record predictions.
- Have each team set up a graph of time (in minutes) vs. temperature to record
- Each student should have a specific responsibility during the experiment,
either keeping track of the time or recording the temperature for the different
- Place the bottles approximately six inches away from the lamp with the
thermometer facing away from the light. Record the baseline temperatures.
- Turn on the light and begin recording the temperatures every two minutes.
Continue for at least 20 minutes.
Cautionary Note: If your lamp is not big enough, six bottles may be too many
to have under the light at the same time. The ones further from the light may
not get the same intensity of heat as the bottles closer to the light thereby
compromising the experiment. You may have the students use a sub-set of the
bottles at one time. If you make changes in the experiment, make sure you also
change the student guide.
Observations and Questions
- Compare the graphed information from the different bottles.
- Discuss the results and propose some possible explanations.
- Relate the factors affecting the model greenhouses to the factors affecting
the "global greenhouse." Which factors are the same? Which are different?
- After discussing their findings, ask students to sketch and explain how
to set up a model greenhouse (with the light on) with the absolutely coolest
possible temperatures. Where might such a condition be found on earth?
- Now ask them to sketch and explain a greenhouse designed to generate the
maximum possible heat. Where might such a condition really exist?
Modifications for Alternative Learners
- Team English Language Limited students with more proficient students.
When you're finished with the activity, click on To Student
Guide or Back to Activities List at the top of the page to return to the activity