That's a tiny fraction of the approximately 900 million metric tons spewed into the atmosphere by air travel in 2018, but it's also, the Guardian helpfully points out, more than the average person emits in an entire year in 68 countries around the world. I'm sure there are other things I do that are bad for the planet merely by existing as a relatively well-off human being in the U.S., but those trips are unquestionably the biggest component of my carbon footprint, my biggest personal contribution to the slow death of the world as we know it.

About once a year, my wife and I fly back to Ireland to visit her family. In doing so, we emit more than our fair share of carbon dioxide, a small act that pushes the climate a little closer to the brink . According to an online calculator from the Guardian, the emissions generated by two people flying round-trip from Los Angeles to Dublin work out to around 3,000 kilograms of carbon.

Though there are efforts to make flying more climate-friendly (through more efficient jet fuel, for example), there's little prospect of zero-emission aircraft anytime soon. Planes are much heavier than cars or trucks, and harder to make electric. "It is going to be one of the hardest and last forms of transportation to decarbonize," said Brandon Graver, who authored the ICCT report. He added that though one Swedish company, Heart Aerospace , has been working on building electric commuter planes, larger aircraft used for longer flights was a trickier prospect. "Electrifying a 300-passenger aircraft is going to be very difficult."

And while air travel emissions are a collective problem, they're caused by a relatively tiny number of people, largely because the majority of the world's population can't afford to fly. Flights in the U.S. produce about a quarter of worldwide air travel emissions, but according to the ICCT most American adults didn't even fly once in 2017. A whopping 68 percent of all air-based trips were taken by the 12 percent of American adults who fly six or more times a year. If you are part of this demographic, you are producing far more than your share of carbon. If you are trying to lessen your personal culpability for climate change, your frequent flyer status is standing in the way. And if you're thinking about how to reduce emissions on a large scale, frequent flyers should be thought of as a problem to be solved.

Air travel makes up a tiny fraction of the world's carbon emissions—estimates range from 2.5 percent to above 5 percent . But as flying becomes a more common mode of transportation around the world, it's going to become a larger factor in climate change. The United Nations predicts air travel emissions will triple by 2050, and a recent report from the International Council on Clean Transportation says that estimate is low .

As society comes to grips with what climate change really means, and what curbing it really requires of us, there are two questions hovering over every takeoff and landing: How guilty should I feel about this? and Should the world step in to make these flights less common?

A ProPublica investigation from earlier this year into the effectiveness of carbon offsets delivered bleak news : "In case after case," reporter Lisa Song wrote, "I found that carbon credits hadn’t offset the amount of pollution they were supposed to, or they had brought gains that were quickly reversed or that couldn’t be accurately measured to begin with." Offsets are a nice idea, but frequent flyers relying on them to clean their footprints are either being fooled or fooling themselves.

"I flew about 75k miles in 2018 and the cost to offset that was only about $400 if I remember correctly—an amount that's just a small fraction of what the travel costs," Cameron said. "Subsequently I did a little bit of research into the carbon offsets markets and found out that they're not all necessarily as altruistic as I would have hoped."

Cameron, who did not want to publish his last name for privacy reasons, is one of these frequent flyers. The 37-year-old from Chicago takes between five and ten flights a month, mostly domestic, which he says is necessary for his work as the owner of a video production company. "In the last year I've started to be really conscious about my flying but I have a small business to run which I can't entirely do from my office," he said. As he's become more concerned about his carbon footprint, he has looked into offsets, the practice of donating money to organizations that do climate-friendly activities like planting trees to make up for the damage your air travel does—sort of a modern spin on buying "indulgences" from the Catholic Church to wipe away sins.

Parke Wilde, a professor at Tufts who studies food and nutrition, was years ahead of the curve when it comes to flygskam. In 2015, he started FlyingLess , a campaign to persuade universities to take a hard look at how often their academics fly and do what they can to cut down on air travel. He also is personally a "non-flyer," which he said is "an experiment to prove feasibility, just so people don't think we're asking for an impossibility."

But even so, his attempt to stay on the ground is a sign of how unfashionable flying has become among the climate-conscious. The Swedes have a word for it: flygskam, or "flight shame," which goes hand-in-hand with tågskryt, or "train bragging." When famed activists Greta Thunberg traveled by boat to the United Nations climate summit this year, it wasn't technically tågskryt, but it was certainly in the same spirit.

Francesco, who is from Italy and now lives in London, has read enough to know that offsets aren't as good as actually cutting down on his carbon footprint. (Like Cameron, he didn't want his last name used for privacy reasons.) He flies back to visit his family six to eight times a year, and his one effort to switch to a more eco-friendly mode of transportation didn't go well. "Last summer I traveled for the first time from Rome to London by bus. It took me 30 hours and it felt terrible at times," he said. "I don't think I want to do that again."

That makes him an outlier among academics, who are expected to travel routinely to conferences and other speaking engagements. But he makes it work by looking for opportunities on the East Coast he can reach without flying, and a couple times a year he'll take a longer overland trip and call up schools near his itinerary to see if he can give a seminar—a gauche and slightly odd thing for a full professor to do, but he doesn't mind.

"If you decide you want to live in a way that is consistent with principles of protecting the climate, you start thinking about reducing unnecessary flying," he said. "My goal initially wasn't to become a non-flyer. I just wanted to reduce unnecessary flying and use an honest definition of 'necessary.' I was going to tell myself that I ought to be able to willing to put up with a bit of hassle, and I ought to be willing to make at least small career sacrifices."

If you believe that fighting climate change will entail some sacrifice, it only seems fair to ask frequent flyers—who practically by definition are all relatively wealthy—to chip in.

As with other aspects of carbon reduction, individual choices are not enough to make a real difference. Institutions both public and private will need to step up to reduce air travel. Wilde hopes that if universities work to cut back on flying, it will serve as a model to large corporations and other major actors. Politicians in France and Germany have proposed eliminating domestic flights and relying solely on trains (if either adopted that policy, it would be a country-wide tågskryt). Less radically, there's a push in Europe to end tax exemptions on jet fuel and plane tickets; France has already introduced a plan to tax flights from its airports. Last month, a U.K. climate commission proposed banning frequent flyer miles programs and levying additional taxes on those who flew the most.