And that is the core of the greenhouse-gas problem: growth. The global market forecast from Airbus, Boeing's only major competitor, predicts that the number of passenger aircraft in service will more than double by 2025, from 12,676 at the end of 2005 to 27,307. Based on increased air traffic, the FAA estimates that aircraft greenhouse-gas emissions in the U.S. will increase by 60 percent in that time. (That might not be the worst part. Recent studies have concluded that water vapor from airplane contrails may have three to four times the warming impact of the carbon dioxide in the exhaust.) The extra demand means that just 4,500 of the airplanes currently in operation, about a third, will be retired; the other 8,200 will remain in service or be converted into freighter aircraft. Even biofuel blends, which appear to be at least a decade off, might not be enough to slow the increase in emissions.