What is a carbon footprint? Is it true that one gallon of gasoline produces 19 pounds of carbon dioxide?

The term carbon footprint describes an action's impact on the environment. All human activities produce varying amounts of the element carbon, the primary chemical building block of all living things.

The act of burning something, such as fuel for a power plant or gasoline for a car, emits carbon as a by-product. So does manufacturing a house, harvesting a bushel of corn or mining stone for cement. Much of this carbon ends up in the atmosphere as greenhouse gases or other compounds that can harm the earth.

The term embodied energy relates to carbon footprint. The greater the quantity of energy inputs needed to grow, harvest, extract or manufacture an object, the higher its embodied energy. For example, straw for straw bale homes has a low embodied energy, while brick and steel must be processed intensively and thus have a high embodied energy.

And yes, one gallon of gasoline produces approximately 19 pounds of carbon dioxide — actually, 19.4 pounds. See below for the underlying calculations.

Calculating carbon dioxide emissions
The Intergovernmental Panel on Climate Change (www.ipcc.ch/) guidelines for calculating emissions inventories require that an oxidation factor be applied to the carbon content to account for a small portion of the fuel that is not oxidized into carbon dioxide. For all oil and oil products, the oxidation factor used is 0.99 (99 percent of the carbon in the fuel is eventually oxidized, while one percent remains unoxidized.)

To calculate the carbon dioxide emissions from a gallon of fuel, the carbon emissions are multiplied by the ratio of the molecular weight of carbon dioxide (m.w. 44) to the molecular weight of carbon (m.w.12): 44/12.

Carbon dioxide emissions from a gallon of gasoline = 2,421 grams x 0.99 x (44/12) = 8,788 grams = 8.8 kg per gallon or 19.4 pounds per gallon.
Carbon dioxide emissions from a gallon of diesel = 2,778 grams x 0.99 x (44/12) = 10,084 grams = 10.1 kg per gallon or 22.2 pounds per gallon. Calculations are based on information from this Web site: http://www.epa.gov/otaq/climate/420f05001.htm