It's long been known that they are useful indicators of weather to come. Now, it seems, they may affect the climate in a more long-term way. A report released yesterday says these clouds of ice, which often drift into forms indistinguishable from naturally occurring cirrus clouds, already cause about one percent of all manmade greenhouse effects and will increase enough in the next 50 years to contribute significantly to global warming by adding to the insulating blanket of moisture and gases in Earth's atmosphere. There is always water vapor in the air. When air at a given temperature reaches a critical level of moisture content, the moisture will condense, or turn from a gas to a liquid state .
Cold air is less capable of holding moisture before it reaches the saturation point.
How readily a contrail forms, and how long it persists, indicates how much moisture was already in the air. Meteorologists value the information that contrails can provide, because they form in a region of the atmosphere where there aren't many monitoring devices. What researchers expect The new report, to be published in the July 1 issue of the journal Geophysical Research Letters, concludes that contrails already contribute to a warming of Earth s atmosphere, although their impact is currently small compared to other greenhouse effects, accounting for about 1 percent of the total. But the research team of American and German scientists, headed by NASA's Patrick Minnis, says the contrails will increase sixfold by 2050. Minnis and his colleagues say global air traffic rose by more than 7 percent each year from 1994 to 1997, as measured by passenger miles flown.