Airplanes are a big contributor to the atmosphere's load of carbon dioxide, which contributes to global warming. A study in Nature Climate Change from the Institute for Atmospheric Physics at the German Aerospace Centre may show that the contrails have just as large an effect.

Contrails are the white clouds you see behind a jet airplane. They happen because the air from the plane (especially the engines) is hot and moist, while the ambient air is at temperatures of 50 degrees below zero Fahrenheit and usually quite dry. That makes ice crystals and the characteristic trails.

The contrails behave just like cirrus clouds, and can look just like them after a few hours. They found that just like cirrus clouds, the trails reflect sunlight from above, and hold heat from below. Their calculations show that the contrails alter the amount of energy radiated from the Earth by about 31 milliwatts per square meter. Carbon dioxide emissions from air travel cause a change of 28 milliwatts per square meter.

For a long time scientists thought the carbon dioxide would be the bigger effect. The CO2 emissions from airplanes account for about three percent of the total emissions globally. (Most is from burning coal and automobiles).

There were intimations that contrails might be more important than previously thought. In 2006 a study from the University of Reading in England found that the timing of flights can affect whether contrails form and how long they persist, and that contrails warm the Earth more if they occur at night. After the Sept. 11 attacks, atmospheric scientist David Travis of the University of Wisconsin-Whitewater found that the absence of planes - and contrails - from the skies for three days caused slightly warmer days and cooler nights.

While the immediate effect of the contrails is larger than that of the CO2, one big difference is the persistence in the atmosphere. A contrail might last hours. CO2 stays in the atmosphere for decades. That means the warming effect of CO2 lasts much longer.