This fact sheet begins a series on commercial aviation, by examining the impact the growth of air travel and freight will have on greenhouse gas emissions. In 1960, 100 million passengers traveled by air, at the time a relatively expensive mode of transportation available only to a small fraction of the public. By 2017, the total annual world-wide passenger count was 4 billion. The "hypermobility" of air travel is available to greater numbers of people worldwide, with rapid growth in aviation projected for developing nations and sustained growth in the large established aviation markets of developed countries. While our collective use of automobiles, our production of electricity, and the industrial and agricultural sectors each exceed the climate change impact of commercial aviation, passenger air travel is producing the highest and fastest growth of individual emissions, despite a significant improvement in efficiency of aircraft and flight operations over the last 60 years.