Researchers have found that nighttime airline flights add to global warming. While the overall effect is small, this finding raises concerns about the growing volume of air travel.

Cars are not the only form of transportation that contribute to global warming. Researchers say jets leave behind condensation trails that also have an impact.

Much like high altitude clouds, these contrails of tiny ice crystals trap heat from the Earth's surface, preventing it from radiating into space. The contrails also have a cooling effect during daytime, because they reflect some of the sun's rays. Although these are opposite effects, the warming is the stronger one.

British scientists, led by Nicola Stuber of the University of Reading sought to understand the impact of the phenomenon by analyzing Northern Hemisphere flight maps and weather balloon data.

Stuber reports in the journal, Nature, that the time and season of flights affect contrail behavior. Their impact is greater at night and in winter, because the opposite cooling effect is present only when the sun is up.

"We discovered that flights during the nighttime are responsible for at least 60 percent of the climate warming associated with contrails over the U.K. That's despite the fact that they amount only to 25 percent of the daily total of flights. The second thing we found is that flights between December and February cause about half the climate warming associated with contrails over the U.K., and so we get one half of the climate effect from just one quarter of the year and, actually, less than a quarter of the annual air traffic," he said.

Stuber says that contrails make up less than five percent of the overall global warming scientists say is caused by humans. Nevertheless, with aviation volume increasing worldwide, she says the effects of contrails should not be ignored. Stuber concedes that restricting flights in the winter is unrealistic, but points out that limiting night travel would minimize their impact on warming.

"Rescheduling flights would certainly be one measure to think about, if policy makers decided to reduce aviation - reduce climate change. It's not the only measure we could do. You could actually try to avoid forming contrails. So, I think it's a combination of different measures, but rescheduling flights would have an impact really," he said.

These measures could include charting flight paths below contrail forming regions, and designing aircraft that eliminate contrails.