GreenFL Y (greenfly.ucdavis.edu) is an airline flight search website that prominently displays greenhouse gas emissions estimates along with the other important flight information, such as price and times, for each possible flight itinerary. We describe its software components and graphic design principles. Then we present a discrete choice experiment in which we asked participants to choose between itineraries presented in the GreenFL Y format. Results suggest that consumers are willing to pay a significant amount for lower-emissions flights in the context of online flight search, especially when lower emissions are combined with fewer layovers.