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PREPARATION
- Ask each student to bring a 2-liter bottle to class. You could also
try to obtain them from a recycling center. You can cut the bottles
before class or have the students cut them. Cut half of the bottles
near the shoulder and the other half about 4 inches from the bottom.
The short bottles will be used as controls for the experiment, because
they are shorter they will not trap air as well.
- If you plan to do this activity on a cloudy day, you will need a light/heat
source. You can use a 100+ watt bulb in a clip-on lamp attached to a
stand.
- It should take about 15 minutes to set up the greenhouses. You will
need more time if the students cut their own bottles. You must let the
bottles sit in direct sunlight for 30 to 50 minutes, depending on weather
conditions. Allow 50 minutes if you are using the light bulb indoors.
Plan on 30 minutes to record and discuss observations.
- The students can create any set-up that will test their hypothesis.
You might want to have a sample set-up to show the students. It can
look something like the image at bottom.
- There are several suggestions for experiments. For all of the experiments,
make sure the bottles are in a place where they will remain in the direct
sunlight for the duration of the experiment. Also make sure that the
thermometers are facing the same direction relative to the light source.
- For testing the effect with damp soil, make sure to use the same type
and amount of soil in both the experimental and control bottle.
- For testing the effect on water temperature, make sure to use the
same amount of water in the bottom of each bottle. The thermometer bulb
must extend into the water to the same depth.
- For testing the effect on ice water, start with about 3 inches of
water in the bottom of each bottle. Mark the water level on the outside
of each bottle with a permanent marker. Add 6 - 12 ice cubes to each
bottle. In this experiment change in the water level is more important
than the temperature difference. Continue the experiment until all the
ice is melted.
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PROCEDURE
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Engagement
- The Earth's greenhouse effect is a natural consequence of the
chemical makeup of its atmosphere. If it were not for the greenhouse
effect, life as we know it could not exist on Earth: it would
be too cold. Our atmosphere is made mostly
of nitrogen and oxygen, but also contains several other gases.
These include carbon dioxide, methane, nitrous oxide, water vapor
and ozone, which are known as greenhouse gases. They absorb very
little visible light, so most sunlight passes through these gases
to Earth's surface. However, Earth radiates heat in the form of
infrared radiation, which greenhouse gases absorb. The gases then
re-radiate infrared radiation, and some of it heads back toward
the ground. In this way the greenhouse gases
act like a blanket, insulating the earth.
- Greenhouse gases are released naturally through geologic processes
and the metabolic activity of living things. However, there is
an ongoing debate over human effects on Earth's global climate.
Human industrial activity on the planet has increased the quantities
of greenhouse gases through agriculture, manufacturing, power
generation, and engines used for transportation. Key questions
include: Is the average temperature of Earth increasing? Does
the presence of human beings contribute to global warming? If
the temperature of Earth does become warmer, how will it affect
us?
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Activity
- After discussing the greenhouse effect and explaining the model
that the students will set up, spend several minutes brainstorming
for ways to test the effects of the greenhouse effect on the following:
air temperature alone; air temperature above soil; soil temperature;
water temperature; and polar ice caps.
- Give each team of students an equipment tray with the materials
listed above. Have teams of students select one of the effects
from the list in Step 1 to investigate, and design an experiment
using the equipment on the tray. Each team should create a control
experiment using the shorter bottle without the plastic cover.
The student-designed experiment should include a hypothesis, a
procedure (including a sketch) and a data recording table, including
appropriate units of measure.
- When the experiments have been approved, the students will set-up
and complete their experiment. They should let the bottles sit
in direct sunlight for 30 to 50 minutes and record their observations,
before, during and after the period the bottles are in the sunlight.
- Have each team report on their set-up and findings. Record their
data on the board or on an overhead projector transparency. List
their hypotheses and whether their hypotheses were confirmed by
the experiment. Were there any experimental errors? What other
experiments do the data suggest?
- What might happen if we continue to release greenhouse gases
into the atmosphere?
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Explanation
- In this activity, the bottles are greenhouses. The temperature
reflects the radiative properties and effective insulating ability
of the plastic wrap that covers the top of the bottles. This is
a simple model of the greenhouse effect that is analogous to the
atmosphere's ability to trap heat.
- When the Sun's energy reaches Earth's atmosphere, some of it
is reflected back into space by the denser layers of the atmosphere,
particulate matter, cloud tops, and the reflective properties
of Earth's surface, oceans and ice caps. The remaining energy
passes through the layer of greenhouse gases and is absorbed by
the surface of Earth (land and water). The land and water eventually
release some of that energy as infrared radiation. When infrared
radiation encounters greenhouse gases, much of it is absorbed
and re-radiated. Some of it is radiated back to the ground and
oceans which absorb the re-radiated energy. If it were not for
the insulating properties of the atmospheric greenhouse effect,
life as we know it would not be possible on Earth.
- Global warming refers to the gradual warming of Earth's overall
(average) temperature. Scientists are using computer models to
predict increases in overall temperature based on levels of carbon
dioxide in the atmosphere. Data from monitoring stations show
that there has been an increase in the levels of carbon dioxide,
as well as other greenhouse gases in recent years. One challenge
in understanding this data is that we only have a few decades
worth of reliable direct data, and natural changes in the past
have been very large, so sorting out human from natural changes
can be difficult. Scientists wonder whether we are in the process
of creating a catastrophic condition or are merely experiencing
part of a long, natural, self-regulating cycle.
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EXTENSION
- You may wish to try a few variations of the experiment to extend the
lesson. The students might test other variables, such as wet versus
dry soil, different kinds or colors of soil, evaporation rates of water
from soil, and salt water versus fresh water. The students can also
test the effect of using green rather than clear bottles or of changing
the amount of light entering the bottle.
- You can also have your students research and debate the data surrounding
the issue of global warming, as well as conduct a simulated World Climate
Conference to discuss the issue of global warming: is it occurring,
is it caused by natural events or human activities, and what, if anything
should be done about it.
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VOCABULARY
- atmosphere: gaseous layer surrounding a planet; the whole mass
of air surrounding the earth.
- global warming: increase in average global temperature. Some
of the increases are due to build-up of gases such as CO2
, NO2, and chlorofluorocarbons in the atmosphere
as a result of industrialization.
- greenhouse: glass- or plastic-paned structure with a wood
or metal frame. Temperature and humidity can be controlled for growing
plants out of season.
- greenhouse effect: the heating of Earth's atmosphere that results
from the absorption of infrared radiation from Earth's surface by atmospheric
gases such as water vapor and carbon dioxide.
- greenhouse gas: atmospheric gas (water vapor, carbon dioxide,
methane) that absorb energy radiated from Earth's surface.
- hypothesis: an assumption made to account for or relate known
facts.
- infrared radiation: the portion of the electromagnetic spectrum
that extends from the long wavelength, or red, end of the visible-light
range to the microwave range.
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