Read: The Man Who Photographs Plane WrecksThis summer, a major domestic airline will start powering its jets with alternative fuels for the first time, cutting down carbon pollution in an industry that is one of the fastest growing contributors to global warming on Earth.The New York Times reports that United Airlines will begin testing fuel made partially from refined animal fats and farm waste—read: animal poop—on a flight from LA to San Francisco. For a two-week trial, United will run four to five California flights a day on a mix of 30-percent biofuel and 70-percent jet fuel. If all goes according to plan, United will then start to blend the biofuel into their overall supply.On Tuesday, United announced that they're investing $30 million in Fulcrum BioEnergy, one of the world's largest producers of sustainable jet fuel. That's the biggest investment an airline has ever made in the field of alternative fuels.Fulcrum told the New York Times that their biofuel can cut an airline's carbon emissions by 80 percent compared with standard jet fuel, which is a big deal—carbon is the largest contributor to greenhouse gas pollution, and carbon emissions lead to a rise in global temperatures, a rising sea level, and an uptick in natural disasters, heat waves, and droughts, according to the EPA.In 2013, United agreed to buy 15 million gallons of biofuels from AltAir fuels, the dudes who turn animal shit into gas. The first five million gallons, which AltAir will deliver to the airline this summer, are going to help power United's flights to San Francisco. While five million gallons might sound like a lot, it's really only a drop in the bucket for United, which used up 3.9 billion gallons of jet fuel last year.Photo via Flickr user Oliver Holzbauer