When looking for evidence of humanity's hand in climate change, it's easy to spot the belchers of greenhouse gases sitting at eye level: snaking rivers of bumper-to-bumper cars, say, or vapor-shrouded smokestacks at power plants.
Few people tend to look straight up, though, which if you had done it last Friday would've yielded a jittering mess of billowy streaks.
Contrails are fascinating to look at, especially if you're a UFO conspiracy theorist who is certain that joyriding aliens are responsible. They're also a fascinating player in the warming of the planet, though to what degree is still a matter of debate.
Scientists have known that contrails influence the climate since at least the late 1960s, when two meteorologists named Nicodemus and McQuigg suggested deliberately creating a blanket of contrails over the U.S. Midwest to reduce sun stress on corn crops. In the '80s, others proposed a similar weather modification above cities so people wouldn't have to spend so much on their heating and cooling bills.
A few years ago, a team from NASA’s Langley Research Center dug up evidence that increasing masses of jet-born clouds could help account for a long period of warming in the U.S. from 1975 to 1994. It looked like the jet exhaust had added to the power of the greenhouse-gas effect, the researchers noted, suggesting that future generations will find it necessary to include airplanes in their climate models.