What was done
The author examined cloud data from land and ship stations throughout the world over the period 1982-1991 in an effort to establish a relationship between variations in the occurrence of cirrus clouds and airline traffic.

What was learned
The global amount of cirrus cloud was found to have increased over the period of study by 1.1% over land and 3.5% over the ocean.  In regions of high air traffic (principal flight corridors) cirrus cloud occurrence was found to be even higher.  The overall effect of the increased cloudiness was to provide an additional radiative forcing of about 0.7 W m^-2 between 1982 and 1991 in regions of high air traffic.  A search for other possible causes of the observed cirrus increase, such as volcanic aerosols or changes in humidity, revealed no viable explanation.

What it means
This study demonstrates another potential human link to climate change and may explain a significant portion of the 0.5°C observed global temperature increase of the past century.  The author notes, for example, that "a larger expected [than the reported 0.7 W m^-2] forcing would be if calculated from the beginning of commercial jet traffic until the present time."  In addition, he states that other climate processes might have "masked an even larger aviation impact on cirrus cloudiness."