By Sid Perkins
Immediately after four hijacked airliners slammed into New York’s World Trade Center, the Pentagon, and a field in southwestern Pennsylvania, the Federal Aviation Administration shut down all U.S. commercial air traffic for 3 days. The unprecedented grounding of airliners enabled airports to step up security measures. At the same time, scientists stepped up to a unique opportunity to study the influence of high-flying aircraft on Earth’s climate.
One way that aircraft may affect climate is through their cloud like contrails, which appear behind jets flying at high altitude. Contrails are made of ice crystals that form within seconds around the small particles present in aircraft exhaust, says David J. Travis, an atmospheric scientist at the University of Wisconsin-Whitewater. Although jet fuel produces water vapor as it burns, more than 90 percent of the ice in long-lived contrails comes from water vapor already present in the air, says Travis.