Copyright Information - 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, 39 - 44.
This activity is designed to have students become familiar with how a greenhouse retains heat by building simple models. Through discussion, you can explain how the atmospheric 'greenhouse effect' retains heat.
Greenhouses are used extensively by botanists, commercial plant growers, and dedicated gardeners. Particularly in cool climates, greenhouses are useful for growing and propagating plants because they both allow sunlight to enter and prevent heat from escaping. The transparent covering of the greenhouse allows visible light to enter unhindered, where it warms the interior as it is absorbed by the material within. The transparent covering also prevents the heat from leaving by reflecting the energy back into the interior and preventing outside winds from carrying it away.
Like the greenhouse covering, our atmosphere also serves to retain heat at the surface of the earth. Much of the sun's energy reaches earth as visible light. Of the visible light that enters the atmosphere, about 30% is reflected back out into space by clouds, snow and ice-covered land, sea surfaces, and atmospheric dust. The rest is absorbed by the liquids, solids, and gases that constitute our planet. The energy absorbed is eventually reemitted, but not as visible light (only very hot objects such as the sun can emit visible light). Instead, it's emitted as longer-wavelength light called infrared radiation. This is also called "heat" radiation, because although we cannot see in infrared, we can feel its presence as heat. This is what you feel when you put your hand near the surface of a hot skillet. Certain gases in our atmosphere (known as "trace" gases because they make up only a tiny fraction of the atmosphere) can absorb this outgoing infrared radiation, in effect trapping the heat energy. This trapped heat energy makes the earth warmer than it would be without these trace gases.
The ability of certain trace gases to be relatively transparent to incoming visible light from the sun yet opaque to the energy radiated from earth is one of the best-understood processes in atmospheric science. This phenomenon has been called the "greenhouse effect" because the trace gases trap heat similar to the way that a greenhouse's transparent covering traps heat. Without our atmospheric greenhouse effect, earth's surface temperature would be far below freezing. On the other hand, an increase in atmospheric trace gases could result in increased trapped heat and rising global temperatures.
Alignment to National Standards
National Science Education Standards
Benchmarks for Science Literacy, Project 2061, AAAS
For each team of four students:
Experimental chamber construction
For each chamber, you will need a two-liter plastic soda bottle (with cap) and a 14- to 16-oz. plastic container for the base.
Observations and Questions
Cautionary Note: The analogy between the plastic cover and the atmosphere
is not a perfect one. Greenhouse covers prevent heat losses from convection
(air movement carrying away the heat) as well as by radiation (direct transfer
of heat energy). The atmosphere prevents only heat loss by radiation. The greenhouses
used in this activity serve as a crude model of the actual atmospheric process
and are only of limited use in understanding the nature and scope of the actual
Modifications for Alternative Learners
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 menu.