This graph shows recent trends in atmospheric carbon dioxide starting in 1958. These data are gathered from air samples taken at the Mauna Loa Observatory, Hawaii near the summit of Mauna Loa. The x-axis shows time in years and the y-axis shows carbon dioxide concentration as ppmv which stands for, parts per million volts. This unit is derived from the process in which the CO2 is measured. Air samples are collected in small vials referred to as cells which are placed into an analyzer. The analyzer passes infrared light through the cell. Carbon dioxide has the ability to absorb infrared light so the more carbon dioxide in the sample less infrared light will pass through. On the far side of the cell is a detector which is able to measure the amount of light passing through and this quantity of light is measured in volts. Using a series of standards the measured volts can be translated into the amount of carbon dioxide present in the sample. (Put another way: volts and carbon dioxide concentration are inversely related.).
Student will likely notice two things:
1. The overall large positive trend of atmospheric carbon dioxide concentration over the last few decades.
2.Annual variation (cycle) in carbon dioxide concentration.
This is a good place to have students hypothesize about the mechanisms behind the annual cycle. If they examine the small inset graph they will notice a decrease in carbon dioxide concentration during the months of approximately April through September. This is the uptake of carbon dioxide as plants begin photosynthesizing during the warmer months. The subsequent increase (i.e. October through March) is due to respiration especially from soils. This annual dynamic can provide a nice opportunity to review with students the processes of photosynthesis and respiration including the respiration of microorganisms in the soils.
Questions for the teacher to use:
- Why is there an annual cycle in CO2 levels
- What is the difference between temperature and climate during the crest and trough in the annual cycle?
- Why are these data collected at Mauna Loa?
- Do you accept these data as scientific? Why or why not?
- Have students brainstorm multiple reasons CO2 is fluctuating during the annual cycle.
- Have students brainstorm multiple reasons for the overall increase in CO2 concentration.
- Have students hypothesize reasons for observing at Mauna Loa. Teacher could have students hypothesize other places in the world that would be good for recording atmospheric data and then research to verify.
*** It is worthwhile to note to students that this pattern of increasing atmospheric carbon dioxide has been confirmed by many other laboratories around the world. ***