Across the globe, there are scenes in airports not seen since the dawn of the jet age. With global demand for air travel down 80 percent, the industry is in freefall. The International Air Transport Association predicts half of all airlines will be bankrupt by June if governments don't step in with loans and bailouts. Climate campaigners, however, believe the pause in aviation due to Covid-19 is an opportunity to build a cleaner air transport system.