Air travel is a huge contributor to climate change. A new global movement wants you to be ashamed to fly.

It was 2007, during a trip to visit her sister in Norway’s pristine Lofoten Islands, when Maja Rosén had an unsettling thought.

As she took in the breathtaking archipelago north of the Arctic Circle that is dotted with mountains, carved with fjords, and circled by sea eagles, she remembered she was looking at one of the fastest-warming regions of the planet.

And she realized that how she got there was part of the problem.

She’d carpooled with friends to Oslo from her home in Gothenburg, Sweden. The final leg was a short boat ride to the islands. And in between was a 500-mile flight from Oslo to Bodø.

For the distance, short flights produce a larger amount of greenhouse gas emissions per passenger compared to longer routes. That fact wasn’t something that struck her on her previous jaunts, like her flights to the United Kingdom to visit friends.

But upon basking in the fragile and sublime wonders of Lofoten, Rosén began to consider how her own actions might be threatening the region. The contradiction between her admiration for the scenery and her pollution from getting there, she decided, was too much to bear.

“It felt so wrong that my flight there was contributing to destroying that place,” Rosén, now 38, said. Soon after, she drastically curbed her flying, but in 2008, she concluded it wasn’t enough. “That’s when I decided not to fly again, and I have not regretted that decision,” she said.

Rosén has only become more alarmed and more determined to reduce emissions from air travel since then. Last year, she gave up her spot in medical school to focus on convincing other people to join her.

She founded a group called We Stay on the Ground in 2018 to recruit people to pledge to give up flying for one year. But the pledge only kicks in once 100,000 people in a given country have committed to doing the same. The threshold is a way to show participants that they’re not alone.

“For most people, it’s to know that others have made this decision. That’s really the most powerful way to make people change their minds,” Rosén said. So far, more than 8,000 people around the world have made the pledge.

Her effort may now be getting a boost from another Swede, 16-year-old climate change activist Greta Thunberg. She gained recognition when she went on strike from school last year to protest her government’s inaction on climate change, sparking a series of worldwide demonstrations, most recently the September 20 strike that drew an estimated 4 million people around the world.

But even after becoming a global celebrity, Thunberg has led by example, traveling to events around Europe mainly by train. She’s currently sailing from the US to Portugal to attend the UN climate meeting in Madrid in December.

Some Swedish airports have now reported a decline in travelers, which some activists attribute to the “Greta effect,” a newfound awareness of humanity’s impacts on the planet and a desire to make a difference.

The Swedes have even coined a word for the shame that travelers are beginning to feel about flying: flygskam, pronounced “fleeg-skahm.”

Rosén is trying to use flygskam to her advantage. She resolved last year to swallow her squeamishness about making her friends reckon with their own travel “because I sort of got fed up with being more scared of being socially inconvenient than climate collapse,” she said.

It’s not just Sweden; environmental activists, scientists who study the climate, and ordinary people in other countries like Switzerland, the United Kingdom, Germany, and the United States are curbing their air travel, if not giving it up outright.

However, the growing global alarm about the environmental impacts of aviation comes as air travel continues to rise. A record 31.6 million passengers are expected travel on US airlines this week for the Thanksgiving holiday, CNN reported. Our global economy is tightly interwoven with aviation as it carries goods and facilitates commerce. Leisure flights are also increasing, and growing demand for services like two-day and overnight shipping has led some companies like Amazon to invest more in cargo aircraft.

All this demand is expected to soar higher, particularly as prices for flights decline and wealth grows in emerging economies.

For regular flyers, air travel is often the dominant contributor to their greenhouse gas footprints. With the window rapidly closing to limit global warming to a bearable level — scientists warn that the planet has as little as 12 years to halve global emissions to restrict warming to 1.5 degrees this century — it is more critical than ever to find a way to shrink aviation’s carbon footprint. Every bit of carbon dioxide we emit now will linger in the atmosphere and warm the planet for decades, but completely decarbonizing aircraft will likely require technologies that are decades away. Reducing the number of flights is one of the few surefire ways to curb emissions in the meantime.

But unlike many other activities that contribute to climate change, air travel serves a valuable social function. It gives remote towns a lifeline to critical fuels, food, and medicines. It helps families stay connected across continents. It opens the door to life-changing experiences.

So reducing air travel demands a difficult moral reckoning, even if we make the decision solely for ourselves. But activists like Rosén say these actions have consequences for the whole world, so we cannot afford to make them without forethought.

If you’re a regular flyer, odds are that your biggest single source of greenhouse gas emissions each year is air travel. It likely dwarfs the footprint of all the lights in your home, your commute to work, your hobbies, and maybe even your diet.

“Euro for euro, hour for hour, flying is the quickest and cheapest way to warm the planet,” said Andrew Murphy, aviation manager at Transport & Environment, a think tank in Brussels.

That’s alarming because humanity can only emit so much more carbon dioxide to limit warming this century to 1.5 degrees Celsius, the more ambitious goal under the 2015 Paris climate agreement. An international team of researchers last year reported that meeting this target would require halving global emissions by as soon as 2030, reaching net-zero emissions by 2050, and even getting to negative emissions thereafter.

Right now, the world is flying in the opposite direction. Global emissions reached a record high last year, and so did atmospheric concentrations of carbon dioxide.

Air travel is a big reason why. A one-way flight across the Atlantic from New York City to London emits one ton of carbon dioxide per passenger. There are upward of 2,500 flights over the North Atlantic every day.

And that’s just one air corridor. Around the world, aviation emits about 860 million metric tons of carbon dioxide every year, or about 2 percent of total global greenhouse gas emissions. Those numbers are poised to soar. The International Civil Aviation Organization projects that emissions from air travel will grow between 300 and 700 percent by 2050 compared to 2005 levels.

Those emissions in turn stand to have a devastating impact. The planet has already warmed by 1 degree Celsius since the dawn of the Industrial Revolution, which has caused rising seas and more frequent and intense heat waves. Every metric ton of carbon dioxide emitted leads to 3 square meters of Arctic sea ice loss. Aircraft also emit several other pollutants at altitude, like particulates, sulfur compounds, and nitrogen compounds, which have an additional warming effect. In some parts of the Arctic under busy air routes, these pollutants combined contribute one-fifth of the warming.

So the environmental costs of air travel are huge and growing, and the worst impacts will befall future generations. At the same time, there are very few options to limit those emissions except to not fly.

But that’s if you fly to begin with. In the United States, fewer than half of travelers in 2017 took a trip by air, according to an industry survey. Globally, less than one-fifth of the population has ever buckled in for a flight. That means a minority of frequent flyers contribute a disproportionate share of emissions. So reducing air travel is one of the most effective things individuals can do to shrink their carbon footprints.

The fundamental problem behind decarbonizing air travel is the physics. To fly, you need an energy source that crams a lot of power into a small space, and right now, there is nothing as energy-dense as jet fuel, which has a specific energy of 11,890 watt-hours per kilogram.

Batteries aren’t even in the same airport. The best lithium-ion batteries top out at 265 watt-hours per kilogram, which is nowhere near enough get an airliner across the Pacific. The technology is improving, but one estimate shows that electrification of airliners will only start to make a dent in air travel emissions by midcentury.

At the same time, there is very little room left for making air travel more efficient. The current generation of jet engines is already closing in on its maximum efficiency. Fuel is also often the largest single expense for airlines, so they already face intense pressure to go farther with less.

One strategy to deal with aircraft emissions is to purchase credits or offsets. Many websites will calculate the emissions of your flight and sell you means to offset them, whether through planting trees that take up a given quantity of carbon dioxide or financing renewable energy projects to displace fossil fuels. But these offsetting programs are only as good as the accounting behind them, and for some, their effectiveness so far in limiting greenhouse gas emissions is questionable.

“The research shows that three-quarters of the offsets don’t deliver the reductions they claim to deliver,” said Anja Kollmuss, a policy analyst in Zurich who studies emissions trading.

Another option is to use a carbon-neutral fuel. Airlines are experimenting with biofuels derived from plants. Since plants recycle carbon that’s already in the atmosphere rather than introducing new carbon into the air, in theory, fuels derived from these crops have no net effect on the climate. In practice, it can be tricky to manage the energy balance of growing biofuels such that you aren’t expending more energy than you get out of them. Fuel crops also require land, and it’s not clear where all the land needed to sustain a wholesale shift of the global aviation industry will come from. Right now, biofuels are also expensive.

Yet another possibility is electrofuels. That’s where you use electricity to power a mechanism that stitches carbon dioxide from the air into longer molecules that can serve as fuels. However, it requires gobs of zero-emissions energy, and the technology is still in a gestational phase.

While there may be technology solutions for cutting the emissions for aviation in the future, there are few options available today beyond simply flying less. “We see this as individuals taking this into their own hands after governments have failed to act,” Murphy said.

It takes a lot of energy to get a fully loaded airliner 6 miles into the air. On short flights, upward of 25 percent of the fuel used is consumed during takeoff.

Once at cruising altitude, though, the aircraft becomes much more fuel-efficient. That means longer, direct journeys have a smaller carbon footprint than shorter connecting hops. But only to a point.

For extremely long hauls, the extra fuel needed for the journey adds enough weight that the flight’s fuel efficiency is reduced, thereby increasing its carbon footprint per mile.

Depending on the aircraft and the route, there is an optimal distance for an air route that minimizes carbon dioxide emissions per passenger per mile — it follows a bathtub curve. One estimate from the Worldwatch Institute pegged the most fuel-efficient flight length at 2,600 miles, a bit longer than the distance between New York and Los Angeles.

But short-haul flights are increasing as countries like China, India, and Brazil open new routes to accommodate a voracious demand for domestic air travel.

Flying first-class also carries a larger carbon footprint, upward of three times larger than passengers in coach — partly because first-class seats are heavier and take up more floor space than cheaper sections of the aircraft.

Sweden is a somewhat odd place to emerge as the leader in flying shame and staying on the ground: It’s not the country with the most air travel or the highest per capita emissions. But in recent years, Swedish celebrities started pushing the idea into the mainstream. In 2015, Swedish Olympic biathlon gold medalist Björn Ferry committed to stop flying. Then in the fall of 2017, 10 Swedish celebrities published an article about deciding to no longer fly.

In 2018, the Swedish government began debating a tax on flying, and more national celebrities began to weigh in against air travel; the renowned Swedish writer Jens Liljestrand published a well-read article with the memorable title “I’m fed up with showing my child a dying world.”

The potential impacts of climate change also became startlingly vivid to many Swedes last year as an oppressive heat wave baked the country and dried out its forests. That heat helped fuel wildfires, with several igniting north of the Arctic Circle.

“It’s the first time Swedish people felt the consequences of climate change themselves,” Rosén said. “[L]ast summer was so dry and things were just looking yellow, and we were lacking water.”

Then in August 2018, Thunberg began her strike outside the Swedish parliament building, an action that soon launched her message worldwide.

Birgitta Frejhagen, 76, was so inspired by Thunberg that she founded a group called “Gretas Gamlingar” (Greta’s oldies). Her goal is to encourage older people to get involved in climate activism. She is currently aiming to recruit 10,000 Swedish seniors to participate in the World Action Day for the Climate on September 27 to coincide with a global youth climate strike.

Frejhagen noted that despite the alarm about the climate, flying is hard for Swedes like her to avoid. Many have family spread out over the large, sparsely populated country. Frejhagen broke her hip earlier this year, so long train or bus journeys are a painful ordeal.

“There is a shame of flying, but sometimes you have to fly,” she said.

Rosén said there isn’t anything unique in the Swedish soul that has made so many across the country so concerned about flying. “This could have happened anywhere,” she said. “We’ve had some good coincidences that have worked together to create this discussion.”

Nonetheless, the movement to reduce flying has created a subculture in Sweden, complete with its own hashtags on social media. Beyond flygskam, there’s flygfritt (flight free), and vi stannar på marken (we stay on the ground).

Rosén said that judging by all the organizing she’s seen in other countries, she thinks Sweden won’t long hold the lead in forgoing flying. “I wouldn’t be surprised if the Germans would follow us soon,” she said.

Kim Cobb, a climate scientist at the Georgia Institute of Technology, has curbed her air travel by 75 percent.

“I really started thinking about my carbon footprint after Trump was elected,” she said. “Doing my climate science and donating to the right candidates was never going to be enough, even if you took that to scale.”

She created a spreadsheet to track her personal carbon footprint and found that flying formed the dominant share of her emissions. “By the end of 2017, 85 percent of my carbon footprint was related to flying,” she said.

Much of Cobb’s research — examining geochemical signals in coral to reconstruct historical climate variability — required her to travel to field sites in the equatorial Pacific.

While she doesn’t anticipate giving up those visits entirely, Cobb has taken on more research projects closer to home, including an experiment tracking sea level rise in Georgia. She has drastically reduced her attendance at academic conferences and this year plans to give a keynote address remotely for an event in Sydney.

Cobb is just one of a growing number of academics, particularly those who study the earth, who have made efforts in recent years to cut their air travel.

While she doesn’t anticipate making a dent in the 2.6 million pounds per second of greenhouse gases that all of humanity emits, Cobb said her goal is to send a signal to airlines and policymakers that there is a demand for cleaner aviation.

But she noted that her family is spread out across the country and that her husband’s family lives in Italy. She wants her children to stay close to her relatives, and that’s harder to do without visiting them. “The personal calculus is much, much harder,” she said.

She also acknowledged that it might be harder for other researchers to follow in her footsteps, particularly those just starting out. As a world-renowned climate scientist with tenure at her university, Cobb said she has the clout to turn down conference invitations or request video conferences. Younger scientists still building their careers may need in-person meetings and events to make a name for themselves. So she sees it as her responsibility to be careful with her air travel. “People like me have to be even more choosy,” she said.

Activists and diplomats who work on international climate issues are also struggling to reconcile their travel habits with their worries about warming. There is even a crowdfunding campaign for activists in Europe to sail to the United Nations climate conference in Chile later this year.

But perhaps the most difficult aspect of limiting air travel is the issue of justice. A minority of individuals, companies, and countries have contributed to the bulk of greenhouse gas emissions from flights and profited handsomely from it. Is it now fair to ask a new generation of travelers to fly less too?

At least one airline is beginning to acknowledge the concern around flying. KLM CEO Pieter Elbers wrote in a letter in June that “we invite all air travellers to make responsible decisions about flying.” The letter showed no sign of the airline itself changing its ways, but the fact that KLM was even hinting at shaming its own passengers shows that climate concerns are difficult to ignore.

Cultural changes could become a big part of reshaping demand for air travel. Shifting tastes away from impressing friends with distant, Instagram-perfect destinations and more staycations could eventually yield some reductions in greenhouse gases from aircraft.

Transport & Environment’s Murphy also noted that for a long time, aviation fuels in many countries weren’t taxed, nor were their greenhouse gas emissions, so the aviation sector hasn’t faced the same pressure to decarbonize as the automotive industry. In fact, many countries directly and indirectly subsidize air travel, whether through tax breaks for aircraft manufacturers or government ownership of airlines. While this is slowly changing — France is set to introduce a new tax on airlines, for example — much more drastic policy action is needed to curb emissions from air travel.

However, targeting the consumers of goods and services rather than just their producers is a much more fraught political debate. It’s a more direct way of changing behavior and it shifts some of the costs directly to buyers, making the costs of curbing emissions much more visible, and contentious. Cutting consumption also brings up concerns about justice. Many activists argue that the heaviest burden of fighting climate change should be borne by large institutions rather than individuals. So while some airlines would prefer to embarrass their customers, climate campaigners say it’s the airlines themselves that should feel most ashamed.

“Travel is fatal to prejudice, bigotry and narrow-mindedness, and many of our people need it sorely on these accounts,” wrote Mark Twain in The Innocents Abroad. “Broad, wholesome, charitable views of men and things can not be acquired by vegetating in one little corner of the earth all one’s lifetime.”

Air travel has yielded immense benefits to humanity. Movement is the story of human civilization, and as mobility has increased, so too has prosperity. Airplanes, the fastest way to cross continents and oceans, have facilitated this. And while some countries have recently retreated from the world stage amid nationalist fervor, the ease of air travel has created a strong countercurrent of travelers looking to learn from other cultures.

Compared to other personal concessions for the sake of the environment, reducing air travel has a disproportionately high social cost. Give up meat and you eat from a different menu. Give up flying and you may never see some members of your family again.

So it’s hard to make a categorical judgment about who should fly and under what circumstances.

But if you’re weighing a plane ticket for yourself, Paul Thompson, a professor of philosophy who studies environmental ethics at Michigan State University, said there are several factors to consider.

First, think about where you can have the most meaningful impact on climate change as an individual — and it might not be changing how you are personally getting around. If advocacy is your thing, you could push for more research and development in cleaner aviation, building high-speed rail systems, or pricing the greenhouse gas emissions of dirty fuels. “That’s the first thing that I think I would be focused on, as opposed to things that would necessarily discourage air travel,” Thompson said. Voting for leaders who make fighting climate change a priority would also help.

If you end up on a booking site, think about why you’re flying and if your flight could be replaced with a video call.

Next, consider what method of travel has the smallest impact on the world, within your budget and time constraints. If you are hoping to come up with a numerical threshold, be aware that the math can get tricky. Online carbon footprint calculators can help.

And if you do choose to fly and feel shame about it, well, it can be a good thing. “I think it’s actually appropriate to have some sense of either grieving or at least concern about the loss you experience that way,” Thompson said. Thinking carefully about the trade-offs you’re making can push you toward many actions that are more beneficial for the climate, whether that’s flying less, offsetting emissions, or advocating for more aggressive climate policies.

Nonetheless, shame is not a great feeling, and it’s hard to convince people they need more of it. But Rosén says forgoing flying is a point of pride, and she’s optimistic that the movement to stay grounded will continue to take off.