If the global aviation industry were a country, it would be the seventh-biggest carbon dioxide polluter in the world, falling right between two industrial powerhouses—Germany and South Korea—at around 777 million tons.

So can a world that takes climate change seriously continue to travel by air, perhaps the most energy-intensive form of mass transportation invented?

It’s not impossible, say two authors of a new study on aviation and climate policy, published Friday in the journal Transportation Research Part D.

But the current crop of theoretical fixes—such as advanced biofuels and solar-powered flight—amount to “technology myths,” they believe, that get more positive attention in the media than warranted given they are likely to contribute to lowering airplane emissions in the next several decades.

“The idea that aviation can be made sustainable [is] slowing down policy makers in terms of making effective policy for aviation emissions,” said study co-author Scott Cohen of the University of Surrey in the United Kingdom. “Our key finding is that that technological silver bullet is not forthcoming.”

But the “aviation industry continues to fly under radar,” he added, “because they give the impression, with the help of media, that the problem will be solved by technology.”

The aviation industry has pledged to reduce carbon emissions 50 percent compared to 2005 levels by 2020, and to improve fuel efficiency by 1.5 percent a year, according to the Air Transport Action Group, a trade group.

Improvements such as hotter-burning jet engines, super-streamlined airframe designs, and development of ultra-lightweight materials have lowered carbon emissions while reducing fuel costs.

“Aviation engineers have already done a very good job at improving fuel efficiency,” said Paul Peeters of the University of Applied Sciences in the Netherlands, the study’s lead author. “But now they are reaching the edge of the physical laws of nature.”

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“We need a completely new concept, but we don’t know that that will be,” he added.

Flights currently account for about 2 percent of the total carbon emissions and 12 percent of emissions from transportation.

About 80 percent of emissions come from transoceanic flights, which were not included in carbon reduction targets set by the 2015 Paris climate accord.

Those emissions could triple by mid-century. The International Aviation Transportation Agency, a trade association, expects airline passenger numbers to reach 7.4 billion by 2034, more than double the 3.3 billion that flew in 2014. Most of that growth is happening in industrializing nations such as China, India, Indonesia, and Brazil.

The IATA expects U.S. passenger numbers to grow by about a half million in the next two decades, to 1.2 billion a year by 2034.

“Efficiency will not solve this problem,” Peeters said.

Andreas Schafer, a systems engineer at Stanford University and University College London who studies carbon emissions from aviation, agreed that most of the low-carbon aviation technologies that Peeters, Cohen, and their colleagues evaluated remain decades from widespread use.

The U.S. Navy is striving to reduce traditional jet fuel use in the name of energy security, and commercial airlines have been experimenting with jet fuel-biofuel mixtures.

But “if we look back at the history of air transportation, I would not be surprised if we are looking at several decades at a minimum for these technologies to get off the ground, literally,” Schafer said.

Some experimental alternatives, such as solar energy, are wholly unrealistic, he added. “If you look at the low energy density of sunlight and the large surfaces you would need to catch enough solar radiation, it’s clearly a no brainer that this is not going to be a replacement for jet fuel.”

Of the current low- or no-carbon options being researched and developed, hydrogen-powered flight could most readily be commercialized, said Schafer.

“It’s been looked at since the 1970s, and no technological challenge whatsoever,” he said. “But the capital costs for putting up the infrastructure will be high.”

Without a mandate from policy makers, he said, the industry won’t spend the money.

“Without a price on carbon to make the cost of oil more predictable over the long haul, there is no incentive to spend the money to shift aviation infrastructure to hydrogen,” said Schafer. “And the price has to be high.”

Reducing demand for air flight is an unrealistic option, Shafer believes.

“This would mean reducing all the long-term trends that have shaped our transportation industry today,” said Schafer. “The richer people are, the more they shift to faster modes of transportation.”

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Original article from TakePart