MadSci Network: Earth Sciences |
Carbon Footprint is not a well-defined thing. The idea is to estimate the impact that a person's way of life has on the composition of the atmosphere. As you note, we add carbon dioxide to the atmosphere simply by exhaling. However, the magnitude of CO2 added by our breathing is very small: less than 1/10 of human-generated co2. So we can't all reduce our carbon impact by not breathing as much!
No, the real impact on the atmosphere comes from fuel burning, especially from fossil fuels. A gallon of gasoline has about 31,000 food calories-worth of energy, which is as much energy as nearly 10 pounds of human adipose tissue (fat)! So we can't reduce our carbon impact by replacing fossil fuels with sweat! (People can, for instance, walk to work if it's only a couple of miles-- however, very few people can push their cars to work!)
Some uses of fuels are harder to control. Insulation and energy-smart appliances are nice, but you will always need a lot of cooling in the summer in Texas, and folks in Minnesota will always need a lot of heating in the winter. The most direct choices that people make that influences their carbon footprint are 1) living alone vs. living with other people, and 2)how far they drive to get to work.
Here is a fairly good calculator, although, as you probably figured out, many of the questions don't apply directly to most 9th-graders!
Hope this helps!
Try the links in the MadSci Library for more information on Earth Sciences.