Planes go greener by shifting altitude

The streams of water vapour and ice particles that form behind an aircraft, called contrails, are known to create cirrus clouds. These clouds can trap heat radiating from the Earth's surface and thus add to global warming.
Until now the only strategy to avoid contrails was for planes to reduce their altitudes from about 33,000 feet to as low as 24,000 feet. At this height the air is not supersaturated with ice, so contrails cannot form. But it is not ideal. "If you lower the altitude substantially you place a heavy load on air traffic control, and the engines don't operate as efficiently," says Hermann Mannstein at the German Aerospace Center in Oberpfaffenhofen. This is because aircraft engines are optimised to fly at higher altitudes.