Measurement Units

The industry uses a mix of English and metric units, along with needlessly large, obscure, units. For example

Teragrams carbon per quadrillion Btu means grams per thousand BTU.
Metric tonnes or mega-grams per million dollars means grams per dollar.
Kilotonnes of CO2e per million US$ means kilos per US$

PREFIXES are defined at http://en.wikipedia.org/wiki/SI_prefix, such as

k, kilo-, thousand, 1,000
M, mega-, million, 1,000,000
(However MBTU usually means thousand BTU, rather than million BTU)
G, giga-, billion, 1,000,000,000
T, tera-, trillion, 1,000,000,000,000
P, peta-, quadrillion, 1,000,000,000,000,000

Our footprint calculator converts all these to your choice of pounds or kilos.

Our spreadsheet converts all these to pounds of CO2e, since the site was originally directed at a United States audience. We will provide metric units when possible.

CARBON OR CO2e?

CO2e is reported by weight (actually mass), which is consistent from place to place. Volume (cubic foot or meter, gallon, liter) is not consistent or meaningful, since the volume of a fixed amount of CO2e varies with temperature and pressure. CO2e could be reported by the number of atoms, but the numbers would be so large they would be hard to work with.

The weight of CO2e is usually estimated, not by capturing & weighing it, but by estimating (a) how much weight of Carbon was present to start with (in gasoline, wood, coal, etc.), and (b) what percent of the Carbon combines with oxygen to make CO2e. This is typically around 99%. Each 12 pounds of Carbon becomes 44 pounds of CO2e, because of the relative weights of Carbon and oxygen atoms.

The main exception happens when oxygen is scarce. If water is also scarce, then some Carbon may remain as soot. If water is present (such as an animal's stomach, and the bottom of a reservoir or landfill), then some Carbon may combine with water to make Methane: 2C + 2H2O => CH4 + CO2. Since a pound of Methane warms the earth 25 times as much as a pound of CO2, it is important to account for any creation of Methane.

Weights of Methane and other greenhouse gases, such as N2O created when bacteria break down fertilizer, are usually multiplied by standard factors to reflect the amount of CO2 which would warm the earth just as much over the next 100 years. For example pounds of Methane are multiplied by 25. Then all the gases are added up and reported as a weight of CO2 equivalent. This has been done in our figures for meat, dairy, hydroelectric power, airplanes, and other topics.

Some websites report data on CO2e, and some report Carbon. The weight of Carbon in CO2 is 12/44 of the weight of the CO2 (because of chemical formulas) so either works if you're consistent.

This website reports CO2e, because it is CO2e which warms the earth. The US Energy Information Administration (EIA) explains why they sometimes use Carbon:

"Because most fossil fuels are 75 percent to 90 percent carbon by weight, it is easy and convenient to compare the weight of carbon emissions (in carbon units) with the weight of the fuel burned. Similarly, carbon sequestration in forests and soils is always measured in tons of carbon, and the use of carbon units makes it simple to compare sequestration with emissions."

http://tonto.eia.doe.gov/FTPROOT/environment/057398.pdf p.3 (p.16 of pdf)