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:
This image from the MODIS instrument aboard NASA's Terra satellite shows skies over Spain and Portugal scored again and again by the hot engines of passenger jets. (Large version.) The planes blow out plumes of tiny particulate matter that serve as seed beds for ice crystals. The resulting trails of flash-frozen vapor can linger in the atmosphere for as long as 14 hours, drifting off their original courses by thousands of kilometers.
Here's another view of a massive contrail battle, this time over the English Channel in 2003:
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.
A couple things have prevented scientists from obtaining a full picture of contrails' climate-bending antics. They tend to bloat into amorphous shapes that blend into herds of cirrus clouds that Mother Nature herself puffs out. They also exert contradictory effects – a fat one hovering in a cloudless sky will cut down on the amount of solar radiation that makes it to the ground, but will increase the amount of radiation sucked up by the atmosphere. It just adds to the complexity of analyzing these things.
Recently, a group of Virginia scientists forged a new process of nailing down the output of gassy airplanes. Here's NASA explaining the general thrust of their discovery, which originated from the study of 2006 data from the Aqua satellite:
They have developed a computer algorithm that searches through data from MODIS and automatically distinguishes between natural cirrus clouds and young- to medium-aged contrails. This has made it easier to estimate how much contrails contribute to overall cirrus and cloud coverage. In a study published in 2013, the group estimated that contrails cover between 0.07 percent to 0.40 percent of the sky in a given year [in the Northern Hemisphere]. They also concluded that contrails produce a slight net warming effect on the Earth.
Up to 0.4 percent of the sky: That may not sound like it, but it is huge. Think about it the next time you and your sweetheart are hunting for clouds shaped like puppies and flowers, and instead encounter something like a field of ghost snakes: