Global Equilibrium Energy Balance Interactive Tinker Toy (GEEBITT)
Using a Spreadsheet Climate Model in the High School Classroom
In the third section of this activity students will be faced with the question of what does the model need in order to correct the average temperature of the earth. What are we missing? With little prompting, the students should realize that we need to include the atmosphere of the planet. Each group is then asked to fill out a chart for atmospheric design to include the gases and clouds that would be found in the atmosphere. (See figure 7). After some discussion, the students complete these tables and then condense their choices into nitrogen, oxygen and other. These condensed choices are then entered into the second set of gray boxes on page 3 of the spreadsheet (Fig. 3). The groups must also enter the atmospheric pressure (in mb) at the planet's surface in the first gray box at the top of the page. The reflectivity and absorption characteristics of these gases are not adjustable in the original version of the model. The spreadsheet then calculates the resulting temperature of the earth surface which again appears in a line at the bottom right hand side of the page (not shown in figure 3). The main idea that students should come away with on this page is that even though oxygen and nitrogen are the main components in the earth's atmosphere, they have little effect on the incoming solar radiation. Almost all of the effects come from the small amount of "other gases" that are present in the atmosphere.
The fourth page of the ICP Spreadsheet Climate Model has no gray boxes and nothing for the students to input. On this page, the various gases of the atmosphere are simply reorganized as "absorbing" or non-absorbing" with respect to the incoming solar radiation. This occasionally produces a slight change in the average surface temperature depending upon the original amounts of gases.
Students are now ready to input their cloud characteristics as decided in the handout (figure 7). Page 5 of the ICP Spreadsheet model allows the user to input the amount of cloud coverage. This can be determined by giving each group another set of white index cards that can be placed over the surface clouds. Any where from 0 to 10 cards can be placed. This number is entered in the top gray box of page 5 as shown in figure 5. The groups must then decide how reflective (as seen from space) on average these clouds are and can choose albedos from 0 to 100%. The values shown in figure 5 correspond to the average values currently found on the earth. The spreadsheet model then calculates the temperature at the earth's surface taking the effect of the clouds into account. This result appears in the last line under the yellow boxes at the bottom right side of page 5. The previously obtained values also appear in this table in order to ease the making of comparisons. In Figure 5 it can be seen that the directions of the changes in temperature reflect the decreasing temperatures one would expect by increasing the amount of energy being reflected from the earth. Since less energy is being absorbed, the temperature should be lower. The ICP Excel Spreadsheet Climate Model tends to exaggerate the magnitude of these changes, but this limitation should be used to have the students suggest possible changes that should be made in this model. The instructor should make an extra effort to emphasize that this is only a model and that its accuracy has yet to be proven. Students should interpret the results in terms of what they would expect to happen given certain changes.
The final page of the spreadsheet is another page on which there is nothing to be inputted by the student. This page is a rough attempt to take into account the long wave radiation (infrared/heat) that is remitted by the earth and the atmosphere after they have absorbed some of the incoming solar radiation. The calculations done on this page are very rough and will be treated in more detail in the completed Excel Version of GEEBITT. That version will also allow more variety in the parameters that can be manipulated in the model. Even though the calculations are rough, they give the correct direction of the change in the temperature, and for the "default" current values used in these examples, results in a final average temperature of the earth (seen at the bottom of figure 5) of 11.6°C - not far from the expected value of 15°C.
The final phase of this activity involves giving each group one or more scenarios (depending on the time available) related to possible climate changes. Possible scenarios include: (1) What if human industrialization adds more greenhouse gases to the atmosphere, causing your surface temperature to warm in the future? (2) What if we were faced with another ice age? (3) What if the earth were hit by a large asteroid? (4) What if a large volcanic explosion occurred in southeast Asia? For each scenario, the groups would explore which their box model could respond to the "what if" scenario. For each response they would demonstrate that response by manipulating the cards in their box model and then explain the reasoning for their response. Care should be taken in the event that some of the responses initiate a "chain reaction" of responses. If so, the group should describe this chain of responses and try to explain each step.
The groups can then use the second page of the Table of Results (figure 7) to record new values for surface and cloud percentages that result from their "what if" scenario. They then recalculate the new surface temperature using the ICP Excel Spreadsheet Climate Model. Each group should then explain the significance of their resultant surface temperature. These analyses can then be compared between the groups and this would lead to a discussion as to the strengths and weaknesses of the model. At a later date students could then be introduced to the Excel Version of GEEBITT and perform more accurate experiments with given and self-generated scenarios.