I have recently become intrigued by contrails. It started in October, when my dad and I were out to lunch for a belated birthday celebration and we got into a conversation about geoengineering. The ethical implications aside, geoengineering (intentional, man-made altering of the environment to influence climate) is a tricky subject. My dad began explaining to me that geoengineering is no new phenomena; that governments have been experimenting since at least the Cold War era. I quickly realized that we were discussing two different (but the same) things: contrails versus chemtrails.
As a side note, chemtrails are associated with weather manipulation conspiracies and contrails are widely accepted and scientifically proven occurrences due to aviation. Chemtrails include crop dusting, military spraying like that during the Vietnam War (agent orange), and even cloud seeding trials. Adversely, contrails are an unintentional physical by-product of air travel. Contrails or "condensation trail" occur at high altitudes where water vapor from the plane's engine exhaust condenses into ice crystals leaving relatively persistent clouds. The potential for both though are weather modification.
Scientists have established that the water vapor emissions from planes in the form of persistent contrails have a warming effect on (see IPCC Assessment Report 4 Working Group I and 5 Working Group II). A study out of the University of Reading suggests that simply by changing flight paths the warming impact can be reduced. Although this may entail longer routes (avoiding areas where contrails will be formed) and more fuel being burned, the fuel emissions could account for less warming than that of the water vapor left in the skies. Read more about this proposal and the science behind it here.
And another thing...
Did you know that water vapor is a greenhouse gas (or GHG)? Here is more info on GHGs
Learn more about contrails and how they have a higher warming potential than actual aviation emissions from burning fuel
And a quick overview of our current knowledge of how water vapor influences Earth's Climate and a 2011 NASA project that seeks to understand more