A round-trip flight from San Francisco to New York melts 54 square feet of Arctic sea ice per person, when you account for the warming effect of carbon emissions. Take one less flight at that distance, and your personal carbon footprint falls to the equivalent of driving 18 fewer miles per day and eating all local food, combined.

The good news is that flights might not always be so carbon-intensive. A June 9 report from the International Council of Clean Transportation (ICCT)—called “Vision 2050: Aligning Aviation With the Paris Agreement”—finds that it’s technically possible to reduce flight-related carbon dioxide (CO2) emissions to nearly zero by 2050. 

The bad news? Technology won’t save us if the number of flights keeps skyrocketing.

That’s the lesson of a June 15 blog post by Dan Rutherford, director of the ICCT’s aviation and marine programs and a co-author of Vision 2050. In “The Tortoise and the Hare? The Race to Decarbonize Aviation” Rutherford explains that for the next three decades, technological change will be in a race with demand growth to keep aviation’s emissions in check. If growth in traffic slows, or better yet stalls altogether, adopting alternative technologies might keep aviation’s CO2 emissions to manageable levels. But if global traffic growth continues at the same breakneck rate as it did before COVID—around 7 percent per year—aviation will soar past its carbon budget with no landing in sight.

What exactly is a carbon budget? In 2021, the Intergovernmental Panel on Climate Change (IPCC) calculated how much more carbon dioxide the world could emit by 2050 while giving us a two-thirds chance of keeping temperature rise below 1.5 degrees Celsius (2.7 degrees Fahrenheit). This number, 400 gigatons, is known as the global 1.5-degree “carbon budget.” In Vision 2050, the ICCT delegates 2.9 percent of this budget to aviation, based on flight’s current share of emissions. So, 2.9 percent of 400 gigatons is 11.6 gigatons: aviation’s 1.5-degree carbon budget. (A similar process will give you the 1.75-degree budget, 2-degree budget, etc.)

The biggest takeaway of Vision 2050 is that they couldn't reach the 1.5-degree goal. Even with every assumption about alternative fuels, new technologies, and efficiency made as optimistic as possible, the ICCT’s most ambitious scenario—called “Breakthrough”—predicted cumulative aviation emissions of 22.5 gigatons, almost twice the 1.5-degree budget. 

Still, Breakthrough is impressive. It brings emissions in line with a goal of 1.75 degrees Celsius warming, or 3.15 degrees Fahrenheit. This is not as low as many frontline communities are fighting for, but still well under the commonly cited 2-degree goal (3.6 degrees Fahrenheit). To do this, it calls for aviation emissions to peak as soon as 2025—a “very scary” number, concedes report co-author Sola Zheng, but one that “says a lot about the urgency of taking action.” In the short term, emissions begin to fall due to efficiency improvements and an immediate uptake in biofuel use. A notable longer-term efficiency improvement was “formation flying,” in which planes draft off one another to use less fuel.

But efficiency improvements aren’t enough, and neither are biofuels: growing crops for fuel often either displaces food production and/or requires clearing new land. This creates carbon emissions of its own while posing other sustainability concerns, so biofuel production has its limits. For this reason, the report calls for huge amounts of synthetic “e-fuels.”

These e-fuels have two main ingredients. The first is carbon, either captured from another source of emissions or sucked directly from the atmosphere. Both techniques use expensive technologies unproven at scale and direct air capture in particular requires large amounts of electricity. The second is hydrogen—for this to be clean, it must be derived from water (rather than methane, as most hydrogen today is), a process that also requires significant electricity. All of this electricity, of course, must be renewable or the tech does not sufficiently reduce emissions. E-fuels are currently far more expensive than conventional jet fuel (estimates range from two to eight times), so the report suggests taxing the latter to help even the playing field.

While biofuels and e-fuels—together known as sustainable aviation fuels (SAFs)—account for most Breakthrough emissions savings and efficiency makes up most of the rest, two other fledgling technologies help: battery-powered electric planes and planes that directly burn liquid hydrogen. However, the heavy weight of batteries means electric planes likely won’t be able to go very far. And using hydrogen requires new airplane designs, so many experts don’t think it will be a factor for decades to come (although one report suggested 60 percent of planes might be hydrogen by 2050). In Breakthrough, hydrogen contributes about 5 percent of emissions savings.

Despite its technological ambition, the ICCT is more cautious when it comes to decreasing demand. For all scenarios, the ICCT assumed a baseline of 3 percent traffic growth per year, measured in “passenger-miles.” (If 5 people fly 100 miles, that is 500 passenger-miles.) This growth rate will dip slightly, the report predicts, but not by much, as more expensive fuels result in higher ticket prices and a few people shift flights to high-speed rail.

This lack of ambition helps explain why Breakthrough can only get to 1.75 degrees. As the report notes, “To get to 1.5°C, out-of-sector action [such as increased carbon capture from the air] and/or significant direct curbs to traffic growth would be needed.” Vision 2050 did not explore these options, however, preferring to emphasize technological shifts rather than anything that “would cost airlines,” the conclusion explains. But Rutherford’s follow-up analysis shows that dampening traffic growth is just as critical as technology.

The Vision 2050 assumption of 3 percent growth per year is already a reduction from the pre-COVID norm of 7 percent per year. If the 7 percent rate returns, Rutherford explains, even with Breakthrough technology, aviation blows by the 1.5-degree budget within a few years and by 2050 reaches emissions consistent with well over 2 degrees of warming. This scenario is even more devastating for the climate than just keeping 3 percent growth and making no technology changes at all.

By contrast, if “reductions in business and leisure travel … cap aviation traffic at 2019 levels,” Breakthrough technology can bring emissions in line with less than 1.6 degrees warming, reducing total CO2 emissions by roughly a third compared to the 3 percent-growth scenario. Reaching 1.5 degrees would require flying even less, especially in the short-term when alternative fuels are in short supply.

Even if global air traffic doesn’t shrink, the rich still need to fly less. The United States emits more each year from aviation than the next 10 countries combined, primarily driven by those who fly several times a year. In fact, about half of flight-related emissions can be attributed to the top 1 percent of flyers, and only 11 percent of the world has ever been on an airplane. If people in other countries begin to fly more, as is expected, this should be offset by the wealthy flying less.

Trains, too, are part of the solution. Vision 2050 posits that after 2030, about 20 percent of passengers on short flights (466 miles or less) between cities will opt for high-speed rail instead. This is fairly conservative: In a March 29 blog postZheng explained that in China, high-speed rail had helped offset flights up to 1,400 miles, three times farther than Vision 2050’s prediction (although, she tells Sierra, rail’s effects may be less reliable at longer distances). And short flights might be entirely replaced by rail with government ambition. In France, flights are now banned if there is a rail route that takes less than 2.5 hours. 

In the end, flying less may be better for the planet even after fossil fuels. Vision 2050 calculates that the amount of electricity needed to power flights in 2050, primarily used to make e-fuels and hydrogen, is nearly equal to all renewable electricity produced today (including hydropower and bioenergy). If we wanted to power 2050’s flights purely on solar and wind, we would need to quadruple current global capacity—and that is on top of all the solar and wind we will need to power our homes, businesses, ground vehicles, and more. 

Solar and wind are wonderful, far better than fossil fuels, but they are not infinite. They require metals, create waste, disrupt ecosystems and threaten endangered species from the desert tortoise to the whooping crane. This does not mean we should stop using electricity, or even stop flying, but it does mean we should think twice before putting so much energy toward planes when trains are far more energy efficient.

Vision 2050 is a useful document, as much for what it doesn't show as for what it does. Technologies like e-fuels and hydrogen, as well as improvements in aircraft efficiency, no doubt have their uses. But there is only one solution that is already available today, with no need to scale up new fuels or introduce new plane designs, and that’s flying less.