On Dec. 17, 1903, Orville and Wilbur Wright made history when they piloted a heavier-than air, gasoline-powered biplane for 12 seconds over a distance of 120 feet. More than a century after that first flight, air travel is a routine mode of transportation for millions of people each day. A result of all these people flying may be a discernable bump in global temperatures, according to scientists who study the impact of so-called contrails on the global climate.
Contrails, or condensation trails, form when the hot, humid air from a jet engine mixes with the colder, drier air in the surrounding environment. These streaks can spread out into thin and wispy cirrus clouds. In heavy air-traffic regions, this can increase cloud cover. This image from the Moderate Resolution Imaging Spectrometer (MODIS) on NASA's Terra satellite shows a contrail streaked sky over the Midwestern U.S. Mingled contrails in the top of the image generate cloud cover. Distinct tracks are visible in the southern portion of the image.
Clouds can have variable effects on the global climate, depending on their extent, thickness and altitude among other factors. They can cool the climate by blocking incoming sunlight, for example, but they can also warm the planet by absorbing energy radiated from the Earth's surface. Thin cirrus clouds are said to have more of a warming effect: their thinness makes them a poor shield against incoming sunlight, but they absorb outgoing radiation that would otherwise escape to space.
In 2004, a NASA-led study published in Journal of Climate concluded that contrail-generated cirrus clouds could be responsible for much of the warming of surface temperatures over the U.S. from 1975 to 1994. This warming, noted Patrick Minnis, a senior research scientist at NASA's Langley Research Center and a co-author of the paper, is an addition to any effect attributed to increasing greenhouse gases.
"This study demonstrates that human activity has a visible and significant impact on cloud cover and, therefore, on climate," he said in a news release about the study. "It indicates that contrails should be included in climate change scenarios."
The effect of contrails on climate change continues to be debated. A widely cited 2002 study published by David Travis at the University of Wisconsin-Whitewater and colleagues in Nature found that the daily temperature range increased by a few degrees in the days following the 2001 terrorist attacks when all air travel was grounded. The finding was seen as support for the idea that contrails warm the Earth. Other papers published more recently, however, suggest that the 20001 effect was due to a shift in low clouds.
Whatever the effect of contrails on climate, the International Ecotourism Society calculates that greenhouse gas emissions from air traffic generate about 10 percent of all greenhouse gases. That's quite a change from the Wright brothers inaugural flight on this day in 1903.
Terra's big-picture view of the contrails over the Midwest is part of the Cosmic Log Space Advent Calendar, which is highlighting images of Earth from space every day until Christmas. For more Advent calendar goodies, check out the Web links below:
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