To understand climate change, scientists must understand a seemingly infinite number of variables. Depending on their shape, size and altitude, clouds can have both a cooling and warming effect on the atmosphere.

When skies are clear, clouds made by airplanes and the effluent they expel into the sky -- called contrails -- aren't thought to have much of an effect on the atmosphere. But new research suggests contrails magnify the reflectivity of an already cloudy sky.

Researchers used highly sensitive satellite-based laser instruments to measure the optical thickness of clouds along popular air travel corridors from Honolulu to Los Angeles, Seattle and San Francisco. Optical thickness was greater near the corridor than farther away, suggesting contrails make clouds more reflective -- or brighter.

"Imaging sensors conventionally used for this kind of research are not able to resolve what is going on inside the clouds," Matthias Tesche, a physics lecturer at the University of Hertfordshire in England, said in a news release. "We were excited to see that overcoming this limitation would immediately lead to a new discovery."

The researchers published their latest findings on contrails in the journal Nature Communications.

Scientists say contrails aren't likely to have much of a brightening effect near the equator or the poles.

"The most important areas are in the Northern and Southern mid-latitudes," added Kevin Noone, a professor of environmental science at Stockholm University. "Work is in progress to calculate the climatic effects of the changes we've observed."