The holidays often mean traveling, which frequently means flying. For many who are hyper-aware of their carbon footprints, flying can bring on a guilt trip. Commercial flights account for about 2.5% of global CO2 emissions. Now, though, there are a variety of sustainable aviation fuels designed to lower the emissions that are generated by air travel. Marketplace’s Jed Kim speaks with Evan Sherwin, a postdoctoral researcher in energy and resources engineering at Stanford University, about the kinds of sustainable jet fuels out there.