Summer is upon the US, and with it peak airline travel: this July, more than 80 million passengers will cross American skies.
But what's good for vacation relaxation is terrible for the environment. According to the WWF, airline travel is the fastest growing source of global greenhouse gas emissions, leaving trucking and coal-burning power plants in their wake.
Already, the 3 billion flights each year produce about 5 per cent of the world's output from all sources. Every round-trip commercial flight between New York and LA generates the equivalent of two tons of carbon dioxide per passenger, as much as the average Nepalese family generates in five years. A Prius would have to travel 10,500 miles to have the same carbon impact.
Yet even those who want, urgently, to fight global warming have been slow to deal with the impact of air travel. Think of those high-profile do-gooders who flew 550 private jets to confab at Davos about carbon dioxide levels in January. The Environmental Protection Agency only just declared that air travel contributes to global warming and that it will begin regulating greenhouse emissions by carriers under the Clean Air Act.
Why, when car travel is perennially in the hot seat, has this issue gotten so little attention? We've got electric cars, smaller cars, cars with great gas mileage. Why haven't we seen the same technological evolution with planes?
The answer is that airplanes pose a fundamentally different engineering problem. They have to do work to carry their fuel through the air, so are limited by the energy density of the storage medium. Increasing efficiency of aircraft engines and wings will help, but only to a point.
And a lot of the "easy" fixes have been made. Companies like Boeing and Airbus have been working for five decades to build planes that burn less fuel, innovating materials and making computer-aided tweaks to aerodynamic design.
They've increased use of strong, lightweight materials like carbon fiber laminates that now make up more than 50 per cent of a modern passenger jet's airframe. They also developed sophisticated turbofan engines that are significantly more efficient than older models.
Their newest aircraft (released in 2000) use half as much fuel per mile as the jets of a half-century ago (Cars and SUVs are about 40 per cent more efficient today.) And engineers say there are few fixes left. "The present technology is already highly optimised," says writer and aircraft designer Peter Garrison. "The low-hanging fruit has been plucked."
An alternative idea would be to write off conventional technology as a dead-end, and instead invest in radical approaches like solar-powered aircraft. The topic has been in the news lately, thanks to the photovoltaic-encrusted experimental plane Solar Impulse. The plane, currently in Japan, is part of a 12-year project by Swiss adventurers Bertrand Piccard and André Borschberg, who want to fly around the world. If successful, the plane is expected to return to its starting point in Abu Dhabi in August.
If you squint, it's easy to see in Solar Impulse the dawn of the eco-friendly airliner. With four engines and wings as broad as a 747's, it physically resembles a commercial aircraft. But appearances deceive. To achieve its long-distance flights, Sunseeker not only uses advanced materials and design techniques, it also relies on extreme design choices.
It is extremely light: At just 5,000 pounds, the 70-foot-long aircraft weighs less than a Chevy Avalanche. A fully laden 737, on the small side for a commercial jet, weighs about 150,000 lb or 30 times more than Solar Impulse. In order to carry the additional weight of passengers and their baggage, a commercial airliner built using the same technology would have to be impractically enormous.
Slow and uncomfortable
It would also be bare of all familiar amenities. To save energy, Solar Impulse is neither pressurised nor climate controlled, so that temperatures in the tiny cockpit swing between 86 F and -4 F.
And it is slow. Solar Impulse cruises at a languid 56 mph, about one-tenth the speed of a typical commercial jet. The reason is simple aerodynamics. As it flies, a plane disrupts the air it moves through, and this takes power. The faster it goes, the more power it will consume.
Very efficient airplanes, like gliders or human-powered airplanes, have long, thin wings designed to move slowly through the air. That's appealing for aeronautical engineers, but not to travelers on tight schedules. To get from Hawaii to Arizona for the eighth stage of its round-the-world journey Solar Impulse will require more than four days of continuous flight.
Really, the only way for commercial aviation to become green would be for science to come up with a storage medium that's as energy dense as fossil fuel, but that doesn't release net carbon into the atmosphere. Someday, our descendants may find out a way to build super-batteries that can be charged using renewable energy sources or nuclear power.
At present, however, the only technology that fits the bill is bio fuel: a hydrocarbon energy source that we make in the here-and-now, whether from crops or from recycled cooking oil and animal fats.
Last year, aircraft manufacturers Boeing and Embraer teamed up to open a biofuel research centre in Brazil that will promote the development of sustainable aviation fuel. In December, Boeing carried out the first test flight of a 787 Dreamliner using "green diesel." So far, this kind effort represents baby steps: the Dreamliner flight used just a 15 per cent blend of green diesel with 85 per cent conventionally derived fuel, and that only in one of its two engines.
One problem with using higher percentages than that is that biofuel tends to freeze at typical airliner cruising altitudes. It's also more difficult to store, and can damage engine parts. We're a long way from running the world airline fleet on the stuff.
So as you sling your beach duffel into the overhead bin in the summer, know that your fun in the sun this year will unavoidably play a role in making next's years summer even summerier. If you want to minimise your carbon footprint, there's really only one option.
Just ask Slate writer Eric Holthaus. He's come up with an effective strategy - one that allowed him, he writes, to go "from having more than double the carbon footprint as the average American to about 30 per cent less than average." His trick? He gave up his 75,000-mile-a-year travel habit cold turkey. "This has to be the last flight I ever take," he tweeted as he boarded a plane last year. "I'm committing right now to stop flying."
Wise is a New York-based magazine writer and author of the The Plane That Wasn't There: Why We Haven't Found MH370 and Extreme Fear: The Science of Your Mind in Danger.