Commercial aviation is now a$700 billion industry that sees a flight take off nearly every second and moves more than 8 million people around the world every day. The FAA hasforecasted airline passenger travel will nearly double within two decades. Yet as more passengers are flying longer distances, there are unavoidable unintended consequences, particularly for the Earth.

"If aviation carries on as it is doing now, it will account for 10 to 20 percent of global CO2 emissions by the end of the century," says Paul Peeters, an associate professor of sustainable transport and tourism at the NHTV Breda University of Applied Sciences in the Netherlands. "There's nothing that shows it will slow down. In the future the problem will be very difficult to deal with and will be a very significant driver of climate change."

Governments have started to recognize the environmental dilemma posed by air travel, albeit slowly. The Environmental Protection Agency, for instance, took its "first steps" toward addressing the regulation of aviation's climate impact just last year. When world leaders convened in Paris last November for the COP21 climate conference, aviation was conspicuously absent from the dialogue and resulting agreements. Finally, after years of negotiation, 190 countries just this month signed a plan to reduce aviation emissions from international air travel put forth at an International Civil Aviation Organization meeting.

Even so, aviation emissions are growing at such a rate that the problem can't simply be legislated out of existence. An analysis of the agreement by the International Council on Clean Transportation found that it would fall short of the aviation industry's professed goal of having carbon neutral growth by 2020.

"All the other sectors have to move away from oil and aviation can use the last bit."

Is there any way to turn things around and make flying greener? People have put forth a host of ideas with varying degrees of likelihood, some based in social engineering and some in futuristic technology.

Abolish First Class

Here's the thinking behind the idea: Because every flight is different, the most accurate way to measure emissions is on a per-passenger basis. But not all seats are created equal. The larger the first- and business-class sections of an aircraft, the more economy seats are displaced. The fewer people on the aircraft, the higher the per-person carbon contribution.

Of course, it's not fair to count that against the people in coach—they're not the ones taking up more room. When it studied this idea, the World Bank adjusted an individual's carbon contribution relative to their seats. The result: Business-class and first class-passengers can have carbon footprints three to nine times higher than people in coach.

Airbus A380 BriYYZ Flickr

Those differences add up. The International Council on Clean Transport recently studied the A380, which is marketed as the "green giant," and found its environmentally friendly label was calculated based on the aircraft's ability to seat 850 economy passengers. In reality, a typical A380 carries 525 seats. When this discrepancy is accounted for, "the green giant" isn't so green after all.

Smash Frequent Flier Programs

Cramming even more bodies on a plane is unlikely to win over many people for the "greening aviation" cause, though. So how about doing away with frequent flyer programs, which encourage people to fly more?

It would be an all-or-nothing deal requiring airlines around the world all agreeing to quit. Good luck with that.

With nearly 150 airlines and roughly 200 million around the world participating in frequent flyer programs, calling for their abolition might sound absurd, but it's not unprecedented. In Denmark, frequent flier programs were banned until 1992, when the law was repealed because it harmed the competitiveness of Danish airlines. Norway banned domestic frequent flyer programs in 2002, a prohibition which lasted until it was challenged by SAS Airlines in 2013 and ultimately repealed on grounds similar to those in Denmark.

While a wealth of research has illustrated the negative side effects of such programs, implementing a ban on them would be a gargantuan task. A Canadian study on the topic pointed out that a unilateral stop to an airline's frequent flier program would make no sense competitively. It would be an all-or-nothing deal requiring airlines around the world all agreeing to quit. Good luck with that.

Tax It

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Then there's the economist's solution: tax the things you don't like. The U.K. recently implemented itsAir Passenger Duty, which makes passengers pay a tax in accordance with their distance of travel and their seating class on the aircraft. A passenger flying economy on a flight of less than 2,000 miles is liable to pay a duty of £13, whereas a passenger flying more than 2,000 miles in business or first class could pay 10 times as much.

"We know which citizens participate in these flights and it would be very easy to justify that these citizens should also be responsible for the impact they are causing irrespective of where they are flying," said Stefan Gössling, a professor at Sweden's Lund University and co-editor of Climate Change and Aviation.

While Gössling sees a green tax on flying as an effective way to offset the carbon footprint, the idea has received a lot of pushback from most airlines and organizations like the International Air Travel Association. Economic analyses have argued that such duties shrink national GDPs, destroy jobs, and hurt airlines by making air travel less appealing because of higher ticket prices. Moreover, there isn't even a consensus on how effective such tax schemes actually are once they're implemented. For example, a 2007 study from the University of Oxford cast doubt on the idea by showing that those with the highest carbon footprint from flying are wealthy enough to absorb the tax without changing their flying habits.

Whether taxation on air travel will actually offset air travel's carbon footprint is being put to the test once again with the latest ICAO aviation plan unveiled earlier this month. The plan will require airlines to improve their fuel economy and likely lead to airlines replacing old aircraft sooner than they otherwise would have. The cost of these measures will almost certainly be passed on to the passengers through increased ticket prices. Other airlines, like Delta and United, have created voluntary programs that allow passengers to pay a fee when they purchase their tickets to offset their personal carbon footprint from the flight.

Better Engines

People don't want to fly less. People don't want to pay more. People don't want to lose their airline miles. Airlines don't want to anger their customers. No surprise, then, that the plans we've discussed so far, which want to solve aviation's climate problem through social engineering, haven't taken off. Instead, many in the industry are hoping technology will save them.

Let's start with the engines. When commercial aviation began a century ago, all aircraft were powered by piston engines. By the 1960s, jets came on the scene. The new jet engines came in two basic flavors, turboprop and turbofan. What the early jet engines lacked in fuel efficiency they made up for in speeds that blew their piston-powered forerunners out of the water (or air, rather).

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According to the Intergovernmental Panel on Climate Change (the group that makes those big U.N. reports on global warming), turbofan engines were about 70 percent more efficient in the year 2000 than the original jet engines were. But, as adamning 2005 report from the Netherlands National Aerospace Laboratory points out, this statistic ignores emission records from the pre-jet era. In fact, the jet engines of today are just about as efficient as the piston-powered engines of 50-plus years ago.

Jet engines are getting better—industry insiders they are improving fuel efficiency by about 1 percent annually. But jets are quickly approaching the upper limit for improvement. "Engine efficiency improvement gets slower and slower every year, yet the industry is set on sticking to a number like 1 or 2 percent efficiency improvement for year," Peeters says. "Engine efficiency hinges on mechanical efficiency, but that's already been at 99 percent efficiency for decades. There's not much to win there."

Better Fuels

United Airlines

If better engines aren't going to solve this, then what about improving the fuel in these engines? Airlines and aircraft manufacturers have praised biojetfuel as a miracle cure for years—Virgin Atlantic flew a test flight from London to Amsterdam using a mixture of 20 percent biofuel to 80 percent jet fuel in 2008, and then there was United Airlines' recent move to a 70/30 jet/biofuel blend for flights between L.A. and San Francisco. You need only look at cars, though, to see how the realities of physics and economics rein in the promised revolution.

There has been pushback those worried that the production of biofuels could lead to deforestation and thereby exacerbate climate change, a point brought up by Greenpeace in the aftermath of Virgin's 2008 biofuel stunt. Moreover, flying is an energy-intensive process. Biofuels, which have less energy density than gasoline or jet fuel, just aren't cut out for it. This is the same reason why solar-powered airplanes, another favorite industry miracle, are not likely to show up at your local airport any time soon (unless the Solar Impulse concept plane is passing through town).

"Microalgae biofuels are very efficient in space use, per square meter you can get quite a lot of product but then there's the energy problem," Peeters says. "Most of the other biofuels are using 5 to 100 times as much space and would need like 20 percent of the United States' land area to produce the fuel needed to feed the current fleet of aircraft. Furthermore, the whole process to make the fuel costs an enormous amount of energy. It's not a viable solution."

Better Planes

The last and perhaps most radical technological solution is to redesign the plane itself. This is the most promising area of technological advancement, because this is where there is the most to be gained.

In February, NASA announced it would revamp its experimental X-plane program, which dates back to 1945 as a way of exploring new aviation technologies and concepts. NASA has seen steadily decreasing it budget for aeronautics for years now, but director Charles Bolden says the agency is interested in reviving its flagging X-plane program to increase fuel efficiency and reduce noise and pollution from commercial aircraft. Maybe it will even lead to new supersonic commercial aircraft someday.

As Gössling and Peeters say, dealing with environmental costs of aviation will ultimately take both technical and policy decisions. Perhaps we'll have to resort to ultra-radical measures like Stanford engineer Mark Jacobson's call for aircraft powered by liquid hydrogen (which has proven technically, but not economically, viable). Although new innovations surely will play a role, on a shorter timeline it will be the implementation of environmentally conscious aviation policy that will have the greatest effect on curbing emissions.

Something? Anything?

Even if NASA or Boeing could design a new, hyper-efficient commercial aircraft, it may not come soon enough. It can take up to a decade for a new aircraft to be put into action, and due to the huge cost of manufacturing aircraft, planes tend to fly as long as possible, sometimes 30 years or more.

As much as we want to engineer our way out of this problem, Gössling says, the base problem with commercial aviation is just that too many people are flying. "We've seen all of these promising technologies—electric flight, blended wing bodies, different types of biofuels—but they are never the breakthrough that they are sold for," Gössling says. "We have never seen anyone that has made a major contribution to solving the problem."

Peeters, too, says we are unlikely to see an aviation industry that's truly sustainable, and that it's a better bet to offset aviation's impact in other areas of our society. "It's very difficult for aviation to get rid of oil, so let's face it: all the other sectors have to move away from oil and aviation can use the last bit," Peeters says. "It's very difficult to envision, but even then the long term prospects of aviation are still no good. You can make a [sustainable] system, but that system has to be much wider than aviation alone."

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