For many people reading this, air travel is their most serious environmental sin. One round-trip flight from New York to Europe or to San Francisco creates a warming effect equivalent to 2 or 3 tons of carbon dioxide per person. The average American generates about 19 tons of carbon dioxide a year; the average European, 10.
Though air travel emissions now account for only about 5 percent of warming, that fraction is projected to rise significantly, since the volume of air travel is increasing much faster than gains in flight fuel efficiency. (Also, emissions from most other sectors are falling.)