At any given moment in time, there are about 5,000 non-military aircraft plying the skies over the United States. It takes a tremendous amount of fuel to keep all these transports aloft, and the burned fuel results in exhaust similar to that coming from car engines: nitrogen oxides, sulfur dioxide, carbon monoxide, and soot. In addition, water vapors are released, which form ice crystals at high elevations commonly called contrails. Meteorologists believe that these contrails increase the formation of cirrus clouds, which may contribute to global warming.