IS THE EARTH WARMING?
A TEST OF THE EFFECT OF INCREASED CARBON DIOXIDE ON THE
THERMAL PROPERTIES OF AIR

David J. Davies, Department of Geology
Ernest W. Blakeney, Department of Chemistry
Centenary College of Louisiana
Shreveport, LA 71134

Level: Senior high (and junior high for Part B)

 

Purpose

The purpose of this demonstration/experiment is to show the environmental and economic effects of increased carbon dioxide (CO2) content in the atmosphere. It has been claimed that the Earth's climate is warming due to changes in the relative concentration of gases in the atmosphere. The gases involved are:

Of these, CO2 is building up in the atmosphere at a much greater rate. Several factors contribute to this build up. The use of fossil fuels in internal combustion engines and in the generation of electricity has greatly increased the amount of CO2 released to the air. Green plants remove CO2 from the air during photosynthesis and turn it into O2. Deforestation of large areas, especially the rain forests in tropical regions of the world, has decreased the amount of CO2 removed from the atmosphere by plants.

The question must be asked as to how the increased amounts of these gases in the atmosphere could contribute to the hypothesized climate warming or "Greenhouse" effect. One possibility is that the changes in gas composition of the atmosphere increase the heat capacity of the atmosphere. The gases of the atmosphere are heated by the sun during the day. At night, some of this heat is radiated away from the Earth into space. The heat capacity of a substance can be regarded as the relative tendency to retain heat. Some materials lose heat to surroundings quickly, while others tend to retain heat longer. If the changes in the composition of the atmosphere allow its heat capacity to increase, less heat is lost to space during the night. The long-term effect of this decreased loss of heat would be a gradual warming of the atmosphere.

Scientists have documented that the concentrations of these "Greenhouse" gases are increasing in the atmosphere today; for example, today the concentration of CO2 is approximately 350 parts per million (ppm), or 0.035%, versus the pre-Industrial Revolution concentration of 200-280 ppm (0.020 - 0.028%). We must determine the effect of the future increases in concentrations of these gases on air temperature. Could doubling the concentration of carbon dioxide, as is expected to occur in the next few decades, significantly warm the atmosphere?

In this experiment/demonstration air with increased concentrations of CO2 is produced and the rate of heat loss of that air is compared to today's "normal" air.

Equipment and Materials

Procedure

You will treat the air in the large glass vessel as a model of the atmosphere. The vessel will be covered and heated by the "sun" (the heat lamp) until the temperature rises to 15 degrees C above the ambient (surrounding) temperature. The heat lamp is then turned off ("nighttime"), and the air allowed to cool naturally. Temperature readings are taken at definite time intervals and plotted on graph paper. This will show the rate of heat loss from "normal" air. Then, the CO2 concentration of the air will be increased and the experiment repeated. The results will demonstrate the effects of increased CO2 on the rate of heat loss from air. You will then evaluate claims that increasing carbon dioxide will significantly increase the temperature of the atmosphere, and thus lead to climate change, in your lifetime.


PART A: DETERMINING CO2 CONCENTRATIONS

When Alka-Seltzer dissolves in water, a gas is released. This gas is CO2, so Alka-Seltzer tablets serve as a convenient source of this gas. The determination of the amount of CO2 released is an interesting laboratory procedure in itself, and we recommend that high school students be involved in this portion of the experiment .

The first step is to determine the volume of the glass vessel. If a gallon jar is used, use the conversion factor

1 gallon = 3.785 liters.

If some other sized vessel, such as an aquarium, is used its volume can be measured by filling with water using a pint jar as a measuring vessel. The relationship of

1 gallon = 8 pints

can be combined with the conversion factor above to calculate the volume of the vessel in liters. The remainder of this procedure will be described assuming you use a gallon jar.

The next step is to calculate the mass of the air in the vessel. The true density of air is dependent on the amount of moisture present in the air (the humidity) and on the barometric pressure. There are tables and equations which allow a precise computation of the density of air under any set of conditions. These tables can be found in the CRC Handbook of Chemistry and Physics. This calculation is rather complicated; thus for the purposes of this demonstration/experiment, the density of dry air at a pressure of 760 mm of Hg will be assumed. This density is:


0.0012 gm/milliliter.

Since there are 1000 milliliters in 1 liter, the gallon jar contains

3.785 liters X (1000 milliliters/liter) = 3785 milliliters.

The mass of air in the jar can be calculated from the density:

3785 milliliters X (0.0012 gm/milliliter) = 4.542 gm

At this point, assume you wish to determine the rate of heat loss from an atmosphere in which the CO2 concentration has doubled, e.g. which contains 700 ppm CO2. The air in the gallon jar should contain:

4.542 grams X (700/1,000,000) = 0.0032 gm CO2

Thus, we will need to add an additional 0.0016 gm of CO2 to the air in the vessel.

The CO2 will be obtained from the solution of Alka-Seltzer in water. We have performed the following simple test to determine how much CO2 is released from an Alka-Seltzer tablet:

  1. A glass vessel containing 75 milliliters of water is weighed to the nearest milligram.
  2. An Alka-Seltzer tablet is weighed to the nearest milligram.
  3. The tablet is dropped into the water and allowed to dissolve.
  4. After 10 minutes, the vessel, water and dissolved Alka-Seltzer is reweighed.
  5. The initial weight of the vessel plus water is added to the weight of the Alka-Seltzer tablet (Add values from steps 1 and 2). Then the weight of the vessel, water and dissolved Alka-Seltzer is subtracted from this sum (Subtract value of step 4 from the sum of 1 and 2). The difference is the weight of the CO2 that was released from the Alka-Seltzer.

A typical determination gave the following values:

 Weight of vessel plus water  122.37 gm
 Weight of Alka-Seltzer tablet  3.35 gm
 Total weight  125.72 gm
 Weight of vessel, water, and Alka-Seltzer  125.09 gm
 Weight of CO2 released  0.63 gm

This information can now be used to determine the amount of Alka-Seltzer necessary to give the 0.0016 gm of CO2 which must be added to the air in the jar to give a concentration of 700 ppm CO2:

(3.25 gm Alka-Seltzer/0.63 gm CO2) X 0.0016 gm CO2

= 0.0083 gm Alka-Seltzer

This is equal to only 0.25% of one Alka-Seltzer tablet (a quarter of 1%); this is a very small amount.


PART B: EFFECT OF CO2 CONCENTRATION ON HEAT LOSS OF AIR

I. Measurement of heat loss for today's "normal" air:

  1. Place the small glass vessel filled with water along with the thermometer in the gallon jar and cover the jar loosely with aluminum foil.
  2. Heat the air in the jar using the heat lamp "sun" until the temperature is raised by 15 degrees C.
  3. Remove the heat lamps to simulate "night". Record the temperature each minute until the temperature returns to ambient.
  4. Using graph paper, plot temperature on the Y-axis versus elapsed cooling time on the X-axis.

II. Measurement of heat loss from tomorrow's air containing increased CO2:

  1. Crush up an Alka-Seltzer tablet and weigh out the amount needed to increase the CO2 concentration to the desired value. Remember, this will be a very small amount of Alka-Seltzer.
  2. Drop the Alka-Seltzer into the water in the small glass vessel inside the large jar. Immediately cover loosely with foil.
  3. Allow 10 minutes for all of the CO2 to be released.
  4. Repeat steps I-2, I-3, and I-4 above. Plot your results on the same piece of graph paper as above.

Interpretation of your results:

Using the outcome of your experiment, answer the following questions.

  1. Which stayed warm longer, the "normal" air, or the air with increased CO2? After cooling for five minutes, what was the difference in their temperatures?
  2. Did increasing the CO2 content increase the heat capacity of the air in the jar?
  3. Does this experiment indicate that increasing the CO2 content in the atmosphere would cause the climate to get warmer?
  4. Explain your answer to question number 3.
  5. If the climate does get warmer, explain what may happen to the ice in the glaciers of Antarctica and Greenland. Most of the fresh water on Earth is in these ice sheets.
  6. Nearly all of the large cities of the United States are seaports; most U.S. citizens live within 75 miles of the ocean. Using your answer to question number 5, explain the possible economic impacts on the inhabitants of these coastal cities.
  7. Corn and wheat, like all plants, need the proper temperatures and amounts of water to grow. Explain the possible economic impact on U.S. farmers and consumers if rainfall should decrease in the Midwest due to a temperature increase. This scenario has been hypothesized by many scientists.
  8. List, and explain, three other impacts on your life that an increase in atmospheric CO2 could cause.
  9. List at least three ways that we can decrease the amount of CO2 that people add to the atmosphere.

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