Mr. Green is busy on his world-wide publicity tour for his new book. In the meantime, here's a Mr. Green classic column from March 2007.

Hey Mr. Green,
Please explain how carbon dioxide emissions are measured. I find it hard to believe that 2,000 pounds of CO₂ are emitted per passenger for a round-trip flight from Los Angeles to New York City. —Heath in Calabasas, California

Your perplexity probably stems from the way the units are stated. As I explain below, it only takes 100 gallons of fossil fuel to make 2,000 pounds of CO₂. So for the approximately 5,000-mile cross-country round-trip, an airliner averages about 50 miles per passenger per gallon--way better than you'd get driving all by your lonesome in most cars. Plus, you're only subjected to two airline meals, as opposed to a dozen fast-food stops.

Now for the math. Atomic behavior isn't always intuitive, as the chemistry of combustion reveals. A gallon of gas or jet fuel weighs 6 to 6.5 pounds, about 5.5 pounds of which is carbon. When an atom of carbon burns, it joins up with two atoms of oxygen to produce CO₂. Although carbon seems like the heavier part of this equation--it's a hefty ingredient in everything from coal to tennis rackets--oxygen has 1.33 times the atomic weight. Therefore, the two oxygen atoms in carbon dioxide are 2.66 times heavier than the original carbon atom. So your initial 5.5 pounds of carbon combine with 14.6 pounds of oxygen to give you 20 pounds of CO₂. Call it intelligent design or confusing design or unfathomable design, but that's the way it is.