Doug says,
“Some time back, one of the Boingers posted an article about the environmental effects of airplane traffic. To further support this article that may or may not exist, I submit today’s Astonomy Picture of the Day that shows plane contrails like scars on the land. Link.
And a “contrail count for kids” program over the next two days: Link.”
From an April, 2004 NASA press release about the environmental effects of plane exhaust:
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.