One and a half century ago birds and insects were the only airborne creatures. The largest movements in the sky were, for example, swarms of starlings, migrating geese or a lone circling hawk. From the twentieth century a new entrant made its rise: aircrafts. Today 8.3 million people are held aloft by airplanes everyday. Commercial air traffic accounts for the 4% of the total global output of greenhouse gasses. Indeed, the impact on the environment is clearly visible and widespread. Near urban areas a clear blue sky without contrails is close to non-existent. Not only airplanes have become part of the visible and audible horizon, they also directly transform our landscape by creating cirrus cloud formations.
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