Carbon Dioxide Exercise

This exercise enables students to:

• Estimate changes in global carbon dioxide concentrations over a 7-year span
• Learn about variation in the carbon cycle driven by photosynthesis
• Understand how important sampling interval can be when studying changes over time
• Practice basic quantitative skills

This exercise will take at least one lecture period in a unit dealing with the carbon cycle, the atmosphere, or human impact on the environment.

Divide the class into eight groups and distribute an overhead pen and a blank carbon dioxide overhead to each group. Here's the graph paper to use for the overhead:

• Blank Carbon Dioxide Graph Paper 2000-2006 (PowerPoint 21kB May21 07)

Cut the data set below into strips and put them in a hat or bucket. Have each group draw 10 or 11 of them (however many it takes to get all of them distributed; there are 84 data points in the list below).

• 2000-2006 Carbon Dioxide Data - Monthly (Microsoft Word 29kB May21 07)

If you need more data for more groups, here is a larger data set.

• 1958-2007 Carbon Dioxide Data - Annual (Microsoft Word 38kB May21 07)

Give each student a copy of your version of the following handout detailing the assignment:

• Carbon Dioxide Assignment (Microsoft Word 43kB May21 07)

Tell each group to plot their data on the overhead. Use the overhead below to emphasize how big each data point should be:

• Carbon Dioxide Graph Paper 2000-2006 with 1 Data Point (PowerPoint 19kB May21 07)

Have each group draw a line through their data and estimate the slope and how long it will take carbon dioxide to increase to 200% of its original value. You'll need to explain that a best-fit line does not simply connect the first and last point. Have each group write their estimates on the board. There will be considerable variation. Collect the overheads and stack them up on a projector (no more than 8 at a time). Now ask them why the estimates varied so much. The following lecture should deal with the interannual and intrannual variation in carbon dioxide levels. You may wish to assign homework on this topic (some is provided on the handout).

For the benefit of the instructor, below is a spreadsheet with two graphs, Keeling's data in its original format and formatted for graphing:

• Keeling data spreadsheet and graphs (Excel 72kB May21 07)

Some groups may need a little help with the quantitative elements of this exercise, so it's nice to be able to do initial plots in class, then to reinforce those skills with homework. Misplotted points will be hard to see on the individual overheads, but will stand out sharply when eight of them are stacked together. You may wish to discuss outliers with the class.

You will be able to do informal assessment of the student's understanding in class. The homework (part II of the exercise) provides an opportunity to reinforce skills and to formally assess the students' learning.

For more carbon dioxide data:

• The Ice Core Gateway: Vostok Ice Core CO2 Data (more info) has an even longer record of global carbon dioxide concentrations, with variations caused by factors other than photosynthesis and human activity.
• Atmospheric carbon dioxide record from Mauna Loa: "Atmospheric CO2 records from sites in the SIO air sampling network" (more info) has fresh data and a bibliography, along with a short description of the project.