Flying from Hawaii, where I live, to just about anywhere in the world requires at least one 2,400-mile leg of cross-ocean air travel—that’s roughly the width of the continental US. Not an ideal situation for someone trying to minimize his climate-change impact while still seeing far-off friends and family once a year. Air travel produces up to 3% of global carbon dioxide emissions each year and 10% of transportation-related emissions in the US—and the problem’s not going away any time soon. Shorter flights have the potential to go electric; efforts take place in the margins where the complicated math of battery weight versus power output begin to make sense. But pound for combustible pound, nothing comes close to good old-fashioned jet fuel for keeping the big planes in the sky.