During the three-day commercial flight hiatus, when the artificial clouds known as contrails all but disappeared, the variations in high and low temperatures increased by 1.1 degrees Celsius (2 degrees Fahrenheit) each day, said meteorological researchers.
http://www.cnn.com/2002/TECH/science/08/07/contrails.climate/index.html
So they didn't reduce the overall temperature by that much, they reduced the daily temperature variation by that much. Contrails like clouds (which they essentially are for the most part) both reduce daytime highs by blocking/reflecting sunlight but increase nighttime lows by trapping heat. That article didn't make clear which is affected more.