For a long while it was thought that the net effect of contrails was cooling - they do indeed reflect back incoming radiation during the day (they increase Earth's albedo).
See, for example, Airbourne observations of contrail effects on the radiation budget - Peter Kuhn, 1970
Pat Minnis etal published a much quoted paper Contrails, Cirrus Trends and Climate in 2003 in which it was concluded:
Using results from a general circulation model simulation of contrails, the cirrus trends over the United States are estimated to cause a tropospheric warming of 0.28–0.38C decade, a range that includes the observed tropospheric temperature trend of 0.278C decade between 1975 and 1994.
It should be noted though that some more recent studies have called into question the effect on diurnal temperature range noted in the post 911 studies. For example, Contrails, natural clouds and diurnal temperature range - Dietmuller etal 2007
Research into this continues, but the current consensus is that persistent contrails produce an overall net increase in temperature due to the reflection back of outgoing radiation at night and in winter. This effect is most noticeabe at higher latitudes and it may be one reason why Arctic regions are warming greater than other parts of the planet - a lot of tranatlantic flights cross Greenland, for example.
The IPCC 4AR only briefly touches on this issue and highlights the current uncertainties, but indicates that this is a likely radiative forcing (warming) of at least +0.010 W m–2 as a result of aircraft contrails.
Doesn't sound a lot, but it well may be higher and will of course be greater in areas of heavy aircraft traffic - such as Europe and N America.
There is also the simple fact that increased air traffic means more contrails meaning that on some days when, without air traffic, we'd have clear blue skies, we instead have an in increasing pall of high cirrus cloud. This was very noticeable in the Scottish Highlands on the 17th October this year when there was not a natural cloud in the sky, yet by midday it was looking like this:
[edit on 25-10-2009 by Essan]
