What atmospheric radiative effects, if any, do contrails cause?
We had a surprising opportunity to study this very question during the period of September 11-14, 2001, when all air traffic was grounded across the United States.

The research was inconclusive, but they found that there was a 1.8 degree celsius increase across the US during this time frame compared to the three days before and after that time frame.  However, this shift may have been coincidental.  It was found by others that this temperature shift was within the normal range for the time of year.

Ultimately, the question of whether these high-level clouds impact the weather and temperatures is still an open debate.



The source for this answer is from a Nature.com article.  However, they take their sources from the original research:


Travis, Carleton, &amp; Lauritsen showing contrails have an effect 
Hong, et al showing contrails do not have an effect