CHICAGO -- In its annual world outlook for commercial airplanes, Boeing predicts a 6.1% annual increase in air cargo traffic over the next 20 years. Coupled with higher fuel costs, that suggests cargo rates will continue to fly higher in coming years.

The world's largest maker of planes also predicts passenger traffic to increase 4.9% a year through 2025.

The company said worldwide air freighter fleet will nearly double over the next 20 years to 3,560 airplanes, from 1,790.