Air and ocean transport make up only 3% of global greenhouse gas emissions. But projections spelled out in a new report reviewing the issue  by the Pew Center on Global Climate Change suggests that by 2050 the total amount of carbon pollution from these sources could increase tenfold, depending on population, economics, and technology trends. Were that to happen, emissions would be as high as the entire transportation sector, which takes up 14% of global greenhouse emissions, currently dominated by pollution from cars and trucks. "It's startling what the potential for growth in this field is," says report co-author David Greene of Oak Ridge National Laboratory in Tennessee.
A number of trends suggest that the industry may be able to forge a more sustainable future. When Greene began analyzing emissions from the aviation sector in the 1980s, "the consensus was there really wasn't a role for biofuels in aircraft—let alone hydrogen," he says. But as the report lays out, the industry has begun looking into both alternatives to jet fuel. "They see the way that the industry is moving." Last October, the International Air Transport Association set up a target of improving its fuel efficiency by 1.5% a year through 2020, which is a "fairly aggressive" goal, Greene says.