In mid-August, to much fanfare, sixteen-year-old climate activist Greta Thunberg set sail from England on the Malizia II, a solar-powered yacht. With a small crew, she embarked on a journey across the Atlantic in order to attend the UN Climate Action summit in New York in September. Conditions on the vessel were austere, The New York Times reported. While aboard, Thunberg drank seawater made potable by a desalination machine; in lieu of a toilet, she used a bucket. Last Wednesday, after two weeks at sea, the yacht pulled into the North Cove Marina in Lower Manhattan, where it was greeted by a jubilant crowd.

Thunberg is the most celebrated of a small but expanding tribe of environmentalists who eschew air travel: “non-flyers,” as some of them call themselves. Non-flyers are not typically afraid of flying—at least not in the usual sense. They do not fear that a plane will malfunction but that it will function exactly as intended. If Thunberg had flown from London to New York and back, her share of the flight’s CO2 emissions would have amounted to roughly a ton: more than the average annual per capita emissions in fifty-six of the world’s countries, according to an analysis by The Guardian. In Thunberg’s native Sweden, two women have launched a campaign to encourage people to give up flying for a year. Reflecting these attitudes is a Swedish neologism: flygskam, or flight-shame.

Here in the US, Thunberg has given new life and luster to a crusade that has been building slowly for some years, mainly centered in academia. A growing number of environmentalist academics have pledged to cut back on flying or stop altogether, and a few are trying to persuade their colleagues and institutions to follow their lead. In 2015, Parke Wilde, a food economist at Tufts University, and Joseph Nevins, a geographer at Vassar, established a petition, asking universities to take measures to reduce flying by faculty, staff, and students “commensurate with the cuts suggested by climate science.”

Aviation accounts for a relatively small fraction of global greenhouse gas emissions: a commonly cited figure is 2 percent, although some estimates are higher. But air travel is projected to rise sharply in the next few decades, and aviation is one of the sectors of economic activity least susceptible to greening.

In 2017, Peter Kalmus, an earth scientist at the UCLA Joint Institute for Regional Earth System Science & Engineering, founded a website, No Fly Climate Sci. The site gathers stories of scholars who have chosen to fly less and to push for systemic changes in higher education. “Academics are expected to attend conferences, workshops, and meetings,” the site notes. “Many academics, including Earth scientists, have large climate footprints dominated by flying. Meanwhile, colleges and universities ostensibly exist to make a better future, especially for young people. We want our institutions to live up to that promise.”

Aviation has long enjoyed a kind of exceptionalism. Many people who take pride in their green lifestyles—perhaps they bike to work and always carry a travel mug—also happen to be frequent flyers. This incongruity grows in part out of cultural factors. A certain type (and I count myself in this category) aspires to be both worldly and socially conscious. We would never think of driving an SUV, say, but we’ve been known to drop the names of far-flung capitals we’ve visited. To be sure, our portable bamboo utensil sets and canvas grocery bags accord with our principles, but they also accord with our self-image, our aesthetics, our personal brands.

In other words, those choices are not sacrifices. Opting out of flying, by contrast, requires actual renunciation.

Air travel also elicits a particularly pronounced version of the common feeling that individual actions don’t matter. If you walk to work instead of driving, the amount of carbon emissions averted is trivial, but at least you know you averted them. If you choose not to buy a plane ticket for a given flight, though, that flight won’t be canceled. The act of flying feels extremely remote from its consequences, even though the associated emissions exceed those from almost any other single activity that we might personally engage in.

In 2012, Kalmus, the earth scientist, was on his way to a meeting in Rome. He was sitting in his seat on the plane in Los Angeles, waiting to take off. When the doors shut, he was overcome by a visceral sense that he didn’t belong there. It felt “gross,” he told me, “kind of like committing a crime.” Since then, he has not set foot on a plane, except in nightmares. Kalmus has absorbed the reality of what we are doing when we fly; very few of the rest of us have done the same.

For all of these reasons, aviation has until recently remained largely off the radar in discussions of emissions reductions. But the paradoxes and contradictions of this situation, as they become increasingly clear, might ultimately represent opportunities to confront the problem.

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An airplane defies gravity and effectively shrinks vast distances. This defiance of nature makes it a marvelous achievement, but also an ecological disaster. The high cruising altitudes of jetliners exacerbate their climate impact. The contrails can form clouds that trap thermal radiation, and at that height, other emissions, such as nitrogen oxides, also contribute to warming. As a result, the total warming caused by aviation is estimated to be at least double the amount caused by the CO2 emissions alone. The supernatural speed of planes also amplifies their environmental harm: they make feasible trips that would otherwise not occur. In other words, flying from JFK to LAX not only emits far more greenhouse gases than, say, taking a bus; if we had to travel by ground, we would be far less likely to make the trip at all.

It’s hard to know exactly when the idea of flight abstinence was born, but one important milestone cited by non-flyers was the British writer George Monbiot’s 2006 book Heat: How We Can Stop the Planet Burning. One of the chapters (excerpted at the time in The Guardian, where Monbiot is a columnist) covered the climate hazards of air travel. After reviewing the literature, Monbiot concluded that there was, alas, “no technofix. The growth in aviation and the need to address climate change cannot be reconciled.” Most airplanes would need to be grounded, he wrote, which he realized was “not a popular message.” He went on: “But I urge you to remember that these privations affect only a tiny proportion of the world’s people. The reason they seem so harsh is that this tiny proportion almost certainly includes you.”

The idea of carbon offsets emerged at around the same time, amid a general rise in concern about climate change (Al Gore’s documentary film An Inconvenient Truth had appeared in 2006). Some airlines began offering the option to add a small fee to the price of the ticket, which would then help underwrite projects like planting trees or building wind farms. Offset programs, however, have been widely scorned—likened to “indulgences” in the Middle Ages, which allowed sinners to pay off the Church in this world in order to escape punishment in the next.

To some environmentalists, particularly in the UK, there was a simpler, albeit not especially appealing, answer: If you want to prevent the damage caused by flying, you shouldn’t fly. Beyond Flying, an anthology that appeared in 2014, included essays mainly from British writers and activists, all of whom had changed their flying behavior as a result of climate concerns.

That book was pivotal for Parke Wilde, the Tufts food economist. Starting about a dozen years ago, he had begun to consciously reduce the number of flights he took. Then, inspired by the anthology and a couple of other non-flying academics—Kevin Anderson, a climate scientist in the UK, and Nevins, the Vassar geographer—he decided in 2014 to stop flying altogether. Wilde and Nevins run a blog titled Flying Less.

Non-flying academics can’t help but notice a conspicuous tension between, on the one hand, the espoused values of universities and professors, and, on the other, the flying behavior that is condoned, incentivized, and relished at their institutions. Professors are not especially highly paid, considering their educational credentials, and getting flown out to give talks and hobnob at conferences in destinations such as Berlin, Bangkok, or Johannesburg is a major perk of the job. At the same time, even if they would prefer to stay put, junior faculty members feel pressure to travel, in order to schmooze with colleagues and promote their work.

With their petition, which currently has signatures from more than 600 academics, Wilde and Nevins ask both universities and professional associations to take steps to modify this system. One idea they propose is the “regional hub” conference model, in which academics would congregate in their respective regions for personal connections and use video-conferencing to interact with other hubs. A few of these associations have begun to consider experiments with the conference model, which, after all, has remained static for decades—why shouldn’t it change in the face of both new technological options and new environmental imperatives?

The pioneers include those parties you might expect—notably, the American Geophysical Union, which includes a good many climate scientists—and those with a less obvious connection to climate change. In April 2018, the Society for Cultural Anthropology held a virtual conference titled “Displacements,” which it advertised as “an international experiment in carbon-conscious conferencing and radically distributed access.” The American Anthropological Association has articulated, as perhaps only anthropologists could, the need to rethink longstanding ways of doing things: In a report, they wrote, “Reshaping the relationship between people and their carbon-intensive lifeways entails a shift in habitus.”

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Changing our lifeways means reassessing not only conference design but also our personal customs. It might mean some amount of sacrifice, which has become a taboo “ask” of people. Wilde objects to the assumption that “it’s naïve to make sacrifices,” he told me. “People make sacrifices for the common good all the time.” He does not think it’s viable to send the message that our efforts to stave off climate catastrophe can be sacrifice-free. “The message has to be, ‘If we made this change, we would still be doing great.’ Zero sacrifice is beyond what I can promise.”

But what about the argument that the plane will take off anyway? When I raised this question—with perhaps a touch of desperation creeping into my voice—Wilde was having none of it. “Think about any time you see a new route announced for airplanes. Isn’t it sort of obvious to you that it’s because of the consumer demand for it?” he asked. “I kind of want to give you a hard time about this one.”

Every time you book a flight, non-flyers argue, you are sending a signal to the airline industry about the appetite for its service; you are also sending a signal to friends, family, anyone who knows about your travel plans. If you choose to stop flying, that decision also sends a signal. “There’s this silly debate about individual versus collective action,” says Peter Kalmus. “The only thing we have is this stream of choices we make every day. And all of those choices influence other people.”

And if refraining from flying is a significant sacrifice, some non-flyers consider that a point in its favor. Renunciation conveys to those around us that the situation is serious. Especially in the case of climate change—where the effects are dispersed, gradual, and tenuously linked to the causes—this kind of social cue is crucial.

Nobody would claim that one person’s airplane avoidance will forestall any heat waves or keep any glaciers intact. Non-flyers see their choice as part of a larger commitment—a way of living with integrity by declining to take part in an activity that feels wrong to them. All of the non-flyers I have mentioned are also deeply engaged in campaigning for more systemic reform.

The prime example, of course, is Greta Thunberg, who has inspired millions with her unyielding moral clarity. And yet, Thunberg’s heart-stirring voyage across the Atlantic also reveals the pitfalls of environmental purism. As her detractors were quick to point out, the journey did end up involving flying, after all—just not for her. The manager of Team Malizia acknowledged that two more crew members would fly across the Atlantic to return the boat to Europe.

Abjuring fossil fuels in a society powered by them is tricky. Even with the sincerest of intentions, strict rules about personal behavior can lead to absurdities and hypocrisies.

I for one do not begrudge Thunberg her adventure, which also illustrates another favorite argument advanced by non-flyers: that abstaining from air travel can be a blessing, that slower modes of transit offer richer experiences. Thunberg’s trip certainly seemed more exhilarating than any transatlantic flight I’ve taken. (She was able to document the trip by using a satellite phone to send photos and messages to friends, who posted them on Twitter. “Some dolphins showed up and swam along the boat last night!” she wrote on Day 2.) But it is also worth noting the limitations of her trip as a model: most of us do not get offered free rides across the ocean on a solar-powered yacht.

A more fruitful alternative might be the concept of “flying less,” which is analogous to “reducetarianism”—the practice of cutting back on animal products, without renouncing them altogether. Flying less seems both more feasible on a large scale and focused less on personal purity than on achieving a broad impact.

In Sweden, there is some evidence that public awareness of this issue may have shifted consumer choices, with a recent dip in air travel and a rise in train travel. (The counterpart to flygskam is tågskryt, or train-bragging.) But the global picture looks quite different. A decade after George Monbiot wrote that most planes would need to be grounded, the International Civil Aviation Organization (ICAO) announced that in 2017, the industry had carried a record-breaking 4.1 billion passengers, a 7.1 percent increase over 2016. By 2037, the International Air Transport Association projects that passenger numbers could double, to 8.2 billion. And it’s not just personal travel that’s on the rise; air cargo has increased globally, with freight traffic up 9.5% in 2017.

As Wilde points out, aviation exceptionalism extends far beyond the realm of individual consumer choice; it is systemic, influencing public policy on a transnational scale—with air travel winning exemptions from international agreements, including the Paris accords, and tax breaks on jet fuel. In France, it is widely known that the gilets jaunes protesters have loudly opposed a proposed tax on diesel fuel for their cars; less publicized is that they have proposed a fuel tax on airplanes as an alternative. They see it as an issue of social justice: Why should working-class folks bear more of a burden than affluent jet-setters? Other activists, such as the British environmental group Plane Stupid, have protested airport expansions, sometimes by blocking taxiways with their bodies.

While our personal travel habits can play a role in shifting the culture, more systemic and institutional changes will obviously be needed if we are to have any hope of curbing emissions from aviation. Universities that boast about their LEED-certified buildings but encourage excessive flying among their faculties; governments that neither tax jet fuel nor invest in low-carbon ground travel infrastructure—the people behind these decisions are the ones who really ought to get acquainted with flygskam.