Biofuels have been around longer than cars have, but cheap gasoline and diesel have long kept them on the fringe. Spikes in oil prices, and now global efforts to stave off the worst effects of climate change, have lent new urgency to the search for clean, renewable fuels.

Our road travel, flights, and shipping account for nearly a quarter of the world's greenhouse gas emissions, and transportation today remains heavily dependent on fossil fuels. The idea behind biofuel is to replace traditional fuels with those made from plant material or other feedstocks that are renewable.

But the concept of using farmland to produce fuel instead of food comes with its own challenges, and solutions that rely on waste