What was done
The authors analyzed surface-based measurements of cirrus coverage (CC) for different parts of the world for the period 1971-1995, while employing similar measurements obtained from the International Satellite Cloud Climatology Project (ISCCP) for 1984-1996 as a consistency check on them.  The linear trends derived from the data were then input to a relationship between changes in ciris amount and surface temperature (derived from a general circulation model of the atmosphere) in order to calculate their climatic impact over the United States.

What was learned
Minnis et al. report that "values of CC increased over the United States, the North Atlantic and Pacific, and Japan, but dropped over most of Asia, Europe, Africa, and South America," making particular note of the fact that "the largest concentrated increases occurred over the northern Pacific and Atlantic and roughly correspond to the major air traffic routes."  Their U.S. temperature assessment additionally indicated that "the cirrus trends over the United States are estimated to cause a tropospheric warming of 0.2°-0.3°C per decade, a range that includes the observed tropospheric temperature trend of 0.27°C per decade between 1975 and 1994."

What it means
According to the researchers' results, nearly all of the surface warming observed over the United States between 1975 and 1994, which they report to be 0.54°C, may well be explained by aircraft-induced increases in cirrus cloud coverage over that period. If true, this result would imply that little to none of the observed U.S. warming over that period could be attributed to the concomitant increase in the air's CO2 content.