We're not entirely sure, it turns out. For a long time now, climatologists have believed that the contrails–those white vapor trails left by airplanes you see criss-crossing the sky–had a net warming effect.They believed that the amount of heat the makeshift clouds trapped down towards the Earth's surface was greater than the amount of warming they prevented by blocking out the sun's rays. But one researcher looked at data collected in the three days after September 11th, when the skies above the U.S. were empty, and found that the average high temperatures during those days were higher than normal.The only problem is that those three days were also abnormally clear and cloud-free, so it might be natural variation at play. That there is some warming or cooling impact is certain; we need some more research to determine which it is.Image courtesy of Wikimedia (cc) and NASAThis post originally appeared on www.refresheverything.com, as part of GOOD's collaboration with the Pepsi Refresh Project, a catalyst for world-changing ideas. Find out more about the Refresh campaign, or to submit your own idea today.