What was done
Based on what is known of the radiative properties of jet aircraft contrails, the authors conducted a model study of their consequences for climate at the earth's surface.

What was learned
The model employed by the authors predicted that the presence of contrails tends to cool the Earth's surface during daylight hours and warm it at night.  It was also noted that aircraft emissions may cause additional indirect climate forcing by changing the particle size of natural cirrus clouds and that "this indirect forcing may be comparable to the direct forcing due to additional contrail cloud cover."

What it means
The authors state that the current instantaneous climatic effects of jet aircraft contrails are "of the same order of magnitude as the radiative forcing due to all previous carbon dioxide emissions by aviation."  Although both of these phenomena are of relatively minor significance to ongoing climate change, this exercise nevertheless demonstrates that a number of things in addition to CO2 emissions may be impacting the climate in both positive and negative ways, and that we have yet to sort them all out.