Science Daily(1) recently reported: "NASA scientists have found that cirrus clouds, formed by contrails from aircraft engine exhaust, are capable of increasing average surface temperatures enough to account for a warming trend in the United States that occurred between 1975 and 1994."
Photo: Contrails over the southeast U.S. on an average day
1. http://www.sciencedaily.com/releases/2004/04/040428061056.htm