Re: Global Cooling
You're right that there was an observed warming over the US after 9/11, indicating that the contrails had a cooling impact (e.g. http://www.nature.com/news/2008/081231/full/news.2008.1335.html#B1 ).
It's worth pointing out that this is not inconsistent with the results of this study, or convential climate science.
As explained in the article, the impact of contrails can be either a cooling by reducing the amount of sunlight (SW radiation) reaching the ground, or a warming due to the interception of infrared (LW) radiation leaving the Earth.
The SW effect is strongest during the day, especially in the Tropics or in Northern/Southern hemisphere in their respective Summers.
The LW impact is occurring all the time, all day and throughout the year. It is stronger over warm surfaces where there is more upwelling LW to start with. It dominates over the SW effect at night, and in Winter.
The impact of both the LW and SW effects are reducing by the precence of other natural clouds. This is hard to determine though, as some contrails grow into large sheets of cirrus clouds, which wouldn't have formed without an initial contrail to get them started (because ice clouds need to an ice nuclei to get them started).
The balance of the impact of contrails depends very strongly on their spatial and temporal distribution, which means that adding up these two impacts is quite hard to do.
The researchers in this case have tried to do this, and shown that the net impact is a warming. This is consistent with previous studies.
September, over the US, is only just in the Autumn/Fall, so there's still a lot of sunlight coming in and the SW cooling dominated over the LW warming. If the same thing had happened in January or Febuary then it's very likely that there would have been a warming.
Because of the seasonality there could be a more nuanced message than just 'avoid contrails', which is what comes across in the Reg(+other) article. That would be to seriously avoid contrails in Winter in mid-latitudes, but that contrails in Summer or the Tropics are OK.
The paper in question doesn't actually say 'avoid all contrails' but instead "We have developed a simple framework to enable the trade-off between contrail and CO climate impacts to be estimated for a single flight.". So they've developed a methodology to see which contrails should be avoided, and which ones aren't.