Over the next half century, international air travel is expected to as much as triple. But with more flights and more flyers comes a rising environmental and consumer cost to flying.

So what's being done to make air travel more efficient? In the short run, the answer is actually building larger aircraft. The Airbus 380, which is the largest passenger airliner in the world, has actually managed to bring down per passenger fuel use by 20 percent compared to a 747 simply by seating more flyers. The plane is also capable of running on a biofuel blend—which may or may not be impressive depending upon your opinions about biofuels.

In the longer term though, there are numerous hybrid and electric airplane concepts that could provide possible alternatives to traditional internal combustion aircraft. Boeing recently presented a design called the SUGAR Volt to NASA as part of the N+3 initiative, whose goal it is to "overcome significant performance and environmental challenges or the benefit of the general public."

The SUGAR Volt is in many ways similar to hybrid vehicle. Twin jet engines are used to power electric motors that in turn supplement those engines, yielding as much as a 70 percent increase in fuel economy. But the principle at play here isn't regenerative braking.

The electric motors in the SUGAR Volt aren't there to power the aircraft, but rather to run fans that mix cool air in with the hot air that is produced by the jet engines. Increasing the ratio of cool to hot air leads to greater efficiency because the hotter the air that passes through a turbine, the harder the engine has to work to propel the aircraft.

Right now, air travel is estimated to account for just about 2 percent of global emissions, but with the global push to decrease carbon levels in the coming decades, the aviation industry's footprint shouldn't be allowed to triple while others are being cut. Furthermore, flyers shouldn't be held hostage to fluctuating fuel prices. For example, for every dollar increase in the price of a barrel of crude, American Airlines' annual operating costs increase by $80 million—and those costs get passed on to consumers in the form of higher fares.

Reprinted with permission from Hybrid Cars