The evidence is not all in, but so far it suggests that, while contrails may have some significant effects in heavily travelled regions, the global impact is likely to be small.
by HENRY HENGEVELD
If you look up at the sky on a clear day, odds are you will see a number of white streaks across the sky, like pencil strokes against the blue backdrop of endless space. Here and there, the lines are broken or have broadened into thin, almost transparent wisps. Often, you can see the line still being formed as you watch, a silver dart glistening slightly ahead of its leading point. Another jet leaving its contrail!