Memorial Day weekend is the unofficial start of the summer travel season and that means a lot of driving and flying. When discussing greenhouse gas emissions, the focus is usually on cars and trucks since they generate the most. But the aviation sector is under pressure to reduce its carbon footprint. Science correspondent Miles O'Brien looks at efforts to create greener fuels for the skies. PBS NewsHour is supported by - https://www.pbs.org/newshour/about/funders