For those who fly regularly, boarding airplanes for far-flung vacation destinations and distant family gatherings, flights could account for a large and growing share of their carbon footprint. A single round-trip between New York and Los Angeles produces more than half a ton of CO2 per person, or 4 percent of the total amount attributable to a typical U.S. resident in a year. U.S. residents who fly make an average of four round-trip flights each year. Fifteen percent of such people make at least 9 round trips a year.

Aviation officials point out that planes are becoming increasingly efficient. But the volume of air travel, in the U.S. and globally, is growing at a faster pace, overwhelming these gains, much as vehicle miles travelled (VMT) can offset automotive technology advances. The amount of CO2 in airplane exhaust grew by 80 percent between 1990 and 2010. CO2 output from all human activities during the same period grew by 50 percent. Each year passenger planes account for an expanding fraction of the carbon dioxide and other warming gasses that people release into the atmosphere.

Today, airplane engines spew about 1.5 percent of the CO2 that we Earthlings create burning fossil fuels – as much as Canada’s carbon footprint. A 2015 report by the European Parliament on the climate impacts of aviation and maritime travel estimates that flying could account for 22 percent of the CO2 let off by 2050. Some experts warn that the goal of keeping the planet from warming more than 2 degrees Celsius – the less stringent of the two commitments nations agreed to in 2015 in Paris – could become virtually unachievable if air travel continues growing exponentially, as it has in recent decades.

Challenges confront social sciences

But social scientists who study air travel warn that cutting back on air travel will be difficult. In the U.S. and Europe, long-distance flying for many has become inextricably woven into modern life. In some of the largest less-developed countries, such as Indonesia and India, discount airlines are attracting a mushrooming demand for the fast, long-distance mobility that developed countries have long enjoyed.

Comparing the per capita carbon footprint of a flight with an equivalent surface trip, such as in a car or bus ride, is complicated. The calculation depends on indeterminate factors such as whether the flight or ground vehicle is packed or only partially full. Moreover, unlike cars and other surface vehicles, airplanes inject other climate-warming gases – including oxides of nitrogen and sulfur – directly into the upper atmosphere. Planes also increase cirrus cloud cover that, like CO2, prevents heat from escaping the planet into space. Scientists have difficulty calculating the magnitude of these effects. Policy discussions of flying often cope with this uncertainty by ignoring non-CO2 warming. But this response gives flying an undeserved benefit of the doubt.

Regardless of its precise amplitude, atmospheric scientists agree that the non-CO2 impact of flying is likely large. The European Parliament aviation report concludes that airplanes warm the planet twice as much when non-CO2 effects are taken into account.

What is unquestionable is that jets travel much faster than surface transportation. Kevin Anderson, deputy director of the Tyndall Center for Climate Research in England, argues that this speed – not an excessive per-mile carbon-footprint of flying – is what makes airplanes problematic. Planes travel so fast they make possible otherwise inconceivably-long trips. Anderson says that without flying, many long-distance trips would never take place: Weekend cross-country jaunts would be impossible. Several years ago Anderson imposed a personal flying moratorium on himself. When invited to make a presentation in China in 2011, he insisted on going from England by train – a three-week round trip that he says he’s not likely to repeat again soon.

The airline industry contends that technological innovations and substitution of biofuels such as vegetable oil for fossil fuels will dramatically reduce the adverse environmental impacts of flying in coming decades. The industry has pledged to increase airplane fuel efficiency by 1.5 percent each year and – by 2050 – to reduce aviation emissions of CO2 to 50 percent of the 2005 level.

But many researchers say there’s little evidence airlines are accomplishing – or can accomplish – these goals. A scathing analysis in Transportation Research Part D Transport and Environment, published in 2016, points out that continuous efficiency improvements since 1960 have failed to substantially bend aviation’s upward curve of CO2 output. Recent industry promises to make air travel consistent with the Paris commitments are dangerous “myths,” the paper says. They “induce inaction that continues to delay much-needed progress in climate policy for aviation.”

Air travel: from extraordinary to routine event

Study coauthor Scott Cohen, head of the Department of Tourism and Events, at the University of Surrey in England, says that technological fixes alone won’t reverse the rising trajectory of warming caused by aviation. It will be hard to curb the public’s appetite for air travel, but he says people have to fly less

Cohen and a handful of other scientists who research consumption of – and attitudes toward – flying have noted that in industrialized countries traveling by air has changed from an “extraordinary event,” a few decades ago to the “domain of the everyday.” Cohen has focused mostly on leisure travel, including tourism and visits to friends and relatives. But he says business travelers and air freight raise similar concerns. Long-distance trips to celebrate events such as birthdays, anniversaries and stag parties, Cohen says, have become common, even socially expected. And in recent years exotic travel to far-flung destinations has become democratized. He says a large fraction of the people in industrialized countries have become “hypermobile.”

Social media influence on travel trends

This trend has been amplified in the past decade by social media, including Facebook and Instagram, which give travelers a larger audience for sharing their adventures – whetting appetites and lengthening the bucket lists of “friends” – with photos and videos. “The potential for travel to act as a marker of social status is amplified,” Cohen said in a recent phone call.

A decade ago Cohen began investigating how to reduce discretionary flying. He had concluded that no western government was ready to impose meaningful flight taxes or other policies for curtailing flying: Air travel is too popular. He hoped that travelers could be induced to voluntarily make fewer and shorter trips.

Cohen and a small community of travel researchers have looked into whether people would wean themselves off flying if they knew more about its adverse effects. The answer, they’ve learned, is no. The challenge involves a stubborn unwillingness of people familiar with the adverse impacts of flying to let climate concerns influence travel plans, an example of what social scientists call an “attitude-behavior gap.”

A 2017 study of British citizens published in Global Environmental Change illustrates the obstacles to convincing travelers to fly less. Researchers compared environmentally concerned respondents in two large public opinion surveys with people who had no green leanings. Environmental awareness was identified by self-reported behaviors such as turning off lights before leaving a room, avoiding excess packaging, and recycling.

In “Green on the Ground Not in the Air,” the authors report that people concerned about the environment fly just as much as everyone else. The result conformed with a phenomenon that’s been dubbed “green at home not on holiday.” The authors conclude that a policy for curbing air travel that relies on “improving pro-environmental attitudes, awareness of climate change outcomes and encouraging routine pro-environmental household behaviors” overlooks the evidence.

Recently Cohen has reluctantly concluded that for the foreseeable future there’s no way to induce people to voluntarily fly less. “There is no appetite for restricting mobility,” he said. Aviation emissions are likely to keep growing until, “top-down regulation,” discourages flying. But he says there’s no likelihood of that on the horizon either. For the now, “it is an impasse.”

An ‘ecocrite’? Author’s own air travel practices and philosophy

Am I an “ecocrite”?

In the past year I’ve taken four long trips from my home in Boston: to Peru, Indonesia, Panama and the Democratic Republic of Congo. According to the carbon footprint calculator of the UN’s International Civil Aviation Organization, my flights produced four metric tons (4.4 tons) of carbon dioxide, twenty-five percent of an average U.S. resident’s carbon footprint for a year.

I have replaced all the light bulbs in my house with LEDs. My wife and I have exchanged our second car for an electrical bicycle. And we forgo air conditioning. But the jet fuel burned carrying me on these flights has overwhelmed all my conservation efforts and sacrifices. Does that make me unfit to write about the climate damage inflicted by flying? Does it invalidate, or compromise, my reporting?

Readers will have to make their own judgments. In my defense I’ll say that slowing the growth of flying doesn’t mean eliminating all flights. In my case, I weigh the climate consequences – not just the cost – of flying before I book any ticket. I fly for personal reasons with reluctance.

My recent international flights all took me to climate research sites. I reported on how future warming and drying could wither the Congo rainforest; how increased vine growth could reduce the carbon uptake of tropical forests; and about one whacky experiment to investigate whether climate change will destroy cloud forests. I hope and trust that the benefits of informing the public about climate change and how scientists study it outweigh the damages resulting from my travels to get these stories.

You decide.