Outside: “We are the world’s first and only zero-emissions airline,” says Nature Air spokes-man Alexi Huntley, who, along with his father, brother, and a group of investors, has turned a fleet of six DeHavilland Twin Otter planes and two twin-engine Britten Norman Islanders into a Central American shuttle system. The small airline makes 74 daily flights to 17 adventure travel destinations in Costa Rica, Nicaragua, and Panama.So how does an airplane dial its CO2 output back to zero? Nature Air first uses its yearly fuel consumption to calculate emissions—some 4,650 tons in 2004. Then, working with the Costa Rican government, it assigns that number a monetary value. Last December, when the company started the project, Nature Air offset its 2004 emissions with a $12,322 investment in reforestation and habitat-protection projects on the Osa Peninsula, one of Central America’s most biologically diverse rainforests.