How much does air travel warm the planet? New study gives a figure

When Milan Klöwer spoke to Mongabay, he was fresh off the train in Germany. As a postdoctoral researcher and climate scientist at the University of Oxford, Klöwer said he does his best to avoid flying in planes — one of the worst climate change culprits.

“We’re climate scientists and we try to also lower our own carbon footprint,” Klöwer said, “taking an airplane is the one activity which probably unites all of science in terms of where we are really high emitters.”

Airplanes emit around 100 times more CO₂ than a shared bus or train ride, and the emissions of global aviation are around 1 billion tons of CO₂ per year — more than the emissions of most countries, including Germany. Aviation contributes an estimated 2.4{6932ee47e64f4ce8eedbbd5224581f6531cba18a35225771c06e4f1b3f0d9667} of global annual CO₂ emissions, most of it from commercial travel.

However, Klöwer said, “Most people think of warming in terms of degrees, not tons of carbon emitted, so we wanted to calculate that.”

In a recent study published in the journal Environmental Research Letters, Klöwer and his colleagues calculated that aviation contributes around 4{6932ee47e64f4ce8eedbbd5224581f6531cba18a35225771c06e4f1b3f0d9667} to human-induced global warming and is projected to cause about 0.1° Celsius (0.2° Fahrenheit) of warming by 2050 if aviation continues growing at pre-pandemic rates.

“These numbers don’t seem high,” Klöwer said, “but again, remember that is more than most countries emit.”  If aviation were a country, it would be the world’s sixth-biggest emitter, falling after China, the US, India, Russia, and Japan.

“Why the focus on flying? Hour for hour, there’s no better way to burn fossil fuel and heat the planet,” Peter Kalmus, climate scientist and author writes on the site No Fly Climate Sci, a website for scientists, academics, and individuals committed to flying less.

In 2018, there were 4.3 billion passenger journeys recorded. The COVID-19 pandemic halted global travel and reduced aviation by 45{6932ee47e64f4ce8eedbbd5224581f6531cba18a35225771c06e4f1b3f0d9667} in 2020, but CO₂ emissions persist for hundreds of years, so all emissions from all past flights are still at play. Recent disruptions may have slowed warming by about five years, Klöwer said, but they’re not all that significant to aviation’s overall climate impacts.

“The biggest contribution from this paper to my mind was essentially a reminder that emissions last,” Sebastian Eastham, a research scientist in the laboratory for aviation and the environment at the Massachusetts Institute of Technology (MIT), who was not involved in the study, told Mongabay. “Aviation is contributing significantly to global climate change as well as to global air pollution. It’s this large amount of fossil fuel combustion that’s contributing most of the problem.”

When jet fuel burns, it produces CO₂ as well as non-CO₂ emissions, including nitrogen oxides (NO_x), soot, water vapor and sulfate aerosols. All of these interact with the atmosphere and have an effect on the climate in different ways and at different time scales, making them complicated to calculate.

Condensation trails (contrails) are estimated to be the largest contributor to aviation-related climate change after CO2, but they are short-lived. Contrails are clouds of ice that form when the soot from engine exhaust mixes with the cold, humid air of the atmosphere. These white streaks left across the sky function like clouds, reflecting the radiation of the sun skyward by day and trapping heat radiated from the earth at night.

Nitrous oxides, another of the planes’ emissions, interact with the atmosphere and form ozone, which lasts on the order of weeks. Ozone is a warming agent and can also have health consequences.

Air pollution from planes (ozone and fine particulate matter or soot) is responsible for an estimated 16 ,000 premature deaths per year.

The most effective solution to reducing both the climate and health impacts of aviation would be to fly less. Klöwer and colleagues calculated that a sustained annual decrease in air traffic by 2.5{6932ee47e64f4ce8eedbbd5224581f6531cba18a35225771c06e4f1b3f0d9667}, or a transition to a 90{6932ee47e64f4ce8eedbbd5224581f6531cba18a35225771c06e4f1b3f0d9667} carbon-neutral fuel mix by 2050, would halt aviation’s contribution to further warming.

However, despite the recent lull in travel, the demand for aviation is expected to return to normal once pandemic-related restrictions are lifted. Still, Klöwer said he still thinks this 2.5{6932ee47e64f4ce8eedbbd5224581f6531cba18a35225771c06e4f1b3f0d9667} annual decrease is possible and called on individuals, companies and governments to ask, “Which flight is actually necessary for me?”

“I’ll just stick to the ones that are that are really important, and replace the other ones with virtual meetings or holidays closer to home,” he added.

A criticism of some environmental messaging is that it places the onus of systemic change on individual consumer choices. Klöwer agreed that much of the decarbonizing power is in the hands of industry and government, but examining your own carbon footprint, reducing unnecessary travel, and engaging in the discussion, he said, is still meaningful. “If everyone talks about it,” he added, “that can create political change.”

“Flying is also the domain of the globally privileged, and perhaps the most important example of lifestyle change we’ll all need to accept, sooner or later, as we transition away from fossil fuel,” ” Kalmus writes on the site No Fly Climate Sci. “[T]here’s no feasible replacement for fossil-fueled long-haul aviation at its current scale.”

The authors also propose a target of using 90{6932ee47e64f4ce8eedbbd5224581f6531cba18a35225771c06e4f1b3f0d9667} carbon-neutral fuel mix by 2050 to curb warming. The terms carbon-neutral fuels and sustainable aviation fuels mostly refer to biofuels made of fats, oils and grease from plant products and/or animal products instead of using petroleum-based fossil fuels. This is a booming industry: The U.S. social spending and climate bill passed in 2021 includes $300 million for sustainable fuel research.

However, reliance on sustainable aviation fuels is controversial. Many experts say there aren’t enough resources to create the fuel needed and that the land could be put to much better use.

“[A]ny dedicated use of land for growing bioenergy inherently comes at the cost of not using that land for growing food or animal feed, or for storing carbon,” says the World Resources Institute summary of the “Creating a Sustainable Food Future” paper.

Conventional agriculture is a major source of carbon emissions and presents a host of other environmental issues such as deforestation, pesticide and fertilizer runoff, and soil degradation. That means the whole system of growing or raising biofuels and their impact on the ecosystem would need to be examined to assess whether or not these fuels are truly “carbon neutral.”

Carbon neutral fuels do not address the problems of nitrogen oxides or soot emissions. So, another solution is to clean the exhaust as it leaves the airplane engine, employing post-emissions controls much like a catalytic converter on a car. Doing that, Eastham said, would require “a substantial rethinking of how we build aircrafts.” It’s not cost-effective to convert old planes, so new planes would need to be built with these cleaning machines in place. Governments may need to issue regulations or incentives to push airlines toward buying more efficient planes.

“I think what we’ll be looking at is probably a portfolio of solutions,” Eastham said. Proposed solutions include using flight paths to avoid contrail formation, building more efficient planes with post-emission controls, using a mix of sustainable fuels, and electrifying short-range flights with renewable energy sources.

“The fact is that if we are burning oil to power the aircrafts that we’re flying in,” Eastham, said, “it’s going to mean future climate consequences.”

Citations:

Klöwer, M., Allen, M. R., Lee, D. S., Proud, S. R., Gallagher, L., & Skowron, A. (2021). Quantifying aviation’s contribution to global warming. Environmental Research Letters, 16(10), 104027. doi:10.1088/1748-9326/ac286e

Creutzig, F., Jochem, P., Edelenbosch, O. Y., Mattauch, L., Vuuren, D. P., McCollum, D., & Minx, J. (2015). Transport: A roadblock to climate change mitigation? Science, 350(6263), 911-912. doi:10.1126/science.aac8033

Meijer, V. R., Kulik, L., Eastham, S. D., Allroggen, F., Speth, R. L., Karaman, S., & Barrett, S. R. (2022). Contrail coverage over the United States before and during the COVID-19 pandemic. Environmental Research Letters, 17(3), 034039. doi:10.1088/1748-9326/ac26f0

Yim, S. H., Lee, G. L., Lee, I. H., Allroggen, F., Ashok, A., Caiazzo, F., … Barrett, S. R. (2015). Global, regional and local health impacts of civil aviation emissions. Environmental Research Letters, 10(3), 034001. doi:10.1088/1748-9326/10/3/034001

Banner image of plane by weichen_kh via Flickr (CC BY-NC-ND 2.0).

Liz Kimbrough is a staff writer for Mongabay. Find her on Twitter @lizkimbrough_

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