Lufthansa has climate-friendly fuel on its flights—for a fee. Air travel is bad for the planet, which is why climate activist Greta Thunberg is sitting on a boat, sailing across the Atlantic in a zero-emissions yacht to speak at the UN’s climate summit. That is also why France joined other European countries in adding an eco-tax to airline tickets, United Airlines is testing a biofuel, and JetBlue is investing in hybrid planes and ecofuels.