Hey Mr. Green,

In Sierra's September/October issue, you said that "burning a gallon of gas emits almost 20 pounds of carbon dioxide." What? Since matter can't be created or destroyed but only changes form, how do we get 20 pounds of carbon dioxide out of just 6 pounds of liquid? All I can think is that the extra weight comes from the air used to burn the gas.

—Eunice in Menifee, California

Only God can make something out of nothing. Or, as we say in Latin, Ex nihilo nihil fit.

Your hunch about the weight coming from the air is right. A gallon of gasoline weighs 6.3 pounds—5.5 pounds of which is carbon. When a carbon atom burns, it hitches up with two oxygen atoms, producing CO₂. And oxygen atoms weigh 1.33 times more than carbon atoms do, which means those two oxygen atoms in CO₂ are 2.66 times as heavy as the carbon atom. So your original 5.5 pounds of carbon combine with 14.6 pounds of oxygen, theoretically totaling 20.1 pounds. The actual yield is 19.4 pounds, because some carbon turns into carbon monoxide and some doesn't combust.

When it's burned, coal emits about a third more CO₂ than gasoline, and gasoline emits about a third more CO₂ than natural gas. But given the collateral damage caused by hydraulic fracturing to extract it, natural gas may not be safer than coal. One thing, however, is certain: Through our heating, cooling, and transportation choices, Americans emit 19 tons of CO₂ per capita a year—around twice the rate of Italy, Japan and the United Kingdom. —Bob Schildgen

Got a question? Ask Mr. Green!

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 --illustration by Little Friends of Printmaking