Contrails on their own are relatively harmless. It's only when the cool, crystalized water begins to fall and turn into water vapour that it begins to contribute to the green house effect. But natural vapour from the earth's atmosphere vastly eclipses any vapour created by contrails. Just check out that quote morgan provided from the IPCC. Humans contribute - oh I'm sorry, I meant to say influence - very little directly toward condensation.
After September 11th, there was that Travis study in 2002 where they claimed contrails were the reason for climate change. That was later debunked in 2004 because the weather conditions were abnormal for that period, so the temperature change was greatly overstated.
Travis et al. (2002) produced a calculation suggesting
that the lack of contrails during the airline shutdown
following the attacks of 11 September 2001
resulted in a substantial rise in the diurnal temperature
range across the US. The obvious interpretation is that
contrails have a strong impact on surface temperature
patterns at the regional scale. However, Travis et al.
(2002) did not control for the air-mass conditions
across the US that may have been responsible for the
observed increase in diurnal temperature range
immediately following the attacks.
By controlling for the air masses present across the
US, we found that the unusual temperatures on 11 and
12 September were a result of a particularly clear
weather pattern, not a lack of jet contrails. Furthermore,
the average diurnal temperature range for 11 to 14
September 2001 was only slightly above average,
but well below what should have occurred based upon
air masses present across the country. There is no
doubt that contrails have some effect on the regional,
hemispheric, and global climate system (Penner et al.
1999), but we conclude that the magnitude of this
effect was overestimated by the Travis et al. (2002)
study.
http://www.int-res.com/articles/cr2004/26/c026p001.pdf
