For some, blue skies crisscrossed with airplane contrails elicit worries about global warming more than wistful dreams of faraway lands. Now there may be further cause for concern: major U.S. air carriers, including United and American Airlines, are using jet fuel derived from Canada's tar sands region, the extraction of which is both environmentally ruinous and highly energy intensive (see "Canada's Highway to Hell," Fall 2007). Similarly, JetBlue is promoting a type of fuel made from coal. Producing these fuels emits up to five times more greenhouse gas pollution than producing conventional oil, and global warming emissions from airplanes are already expected to climb 60 percent by 2025. Through its Cool Fuels pledge, NRDC is asking airlines to invest in cleaner, renewable sources, such as biofuels, and to make improvements in efficiency that will save fuel and money.