Besides using up fuel and emitting carbon dioxide exhaust, airplanes also harm the environment by creating artificial contrail clouds of condensed water vapor in their wakes. The clouds act as atmospheric insulation, trapping heat underneath them on the planet. To combat the problem, some scientists have proposed requiring planes to fly at lower altitudes, where contrails are less likely to form. But to fly lower, planes would have to travel longer to reach their destinations. This would also use up more fuel, but advocates say the drop in fuel efficiency would be offset by the gain in reducing contrails. Plus, aircraft engineers could focus on making planes more fuel-efficient at lower altitudes.