'Contrails': Do airplane contrails affect global warming or not?

As an airplane soars through the sky, the intense heat from its engines releases water vapour which, upon encountering the frigid temperatures at high altitudes, transforms into icy streaks known as contrails. These are not to be mistaken for the baseless chemtrails conspiracy.

The very same white lines that one might spot from their home, gazing up at the azure expanse, have become a subject of worry for scientists and the aviation industry alike, as they ponder the potential environmental impact of these vapour trails on Earth.

Environmental advocates and various non-profit groups have long been raising concerns that the white contrails produced by aircraft may have a greater impact on global warming than the carbon dioxide emissions from their engines.

The assertions stem from a 2021 research paper authored by David Lee, who leads the aviation working group for the UN’s Intergovernmental Panel on Climate Change.

In his study, Lee suggests that the heat-trapping effect of contrails surpasses the warming impact of carbon dioxide released by aircraft engines. The paper, which has been widely accepted, indicates that contrails account for approximately 1% to 2% of the overall contribution to climate change.

Do contrails impact global warming?

Yes, contrails can contribute to global warming. Contrails are the white lines that sometimes appear behind airplanes. They are formed when water vapour from aircraft engines condenses as ice onto soot particles. Contrails trap heat in the atmosphere, which contributes to global warming.

According to a 2022 IPCC report, contrails account for about 35% of aviation's global warming impact. This is more than half of the impact of jet fuel.  A study that looked at aviation's contribution to climate change between 2000 and 2018 found that contrails create 57% of the sector's warming impact. This is significantly more than the CO2 emissions from burning fuel.

An impact called into question

The previously established social agreement on the detrimental effects of vapor trails has recently encountered a surprising challenge. David Lee, the researcher behind the influential study, has revisited his own findings, shedding light on the numerous scientific uncertainties that persist in this area and urging for a measured response to the issue.

The study in question ultimately determines that the “fundamental premise” advocating for the mitigation of contrails lacks a solid scientific foundation. Furthermore, Lee highlights a critical oversight in the discourse: while the potential greenhouse effect of these trails is a concern, it’s also possible they could be exerting an unanticipated influence—namely, a cooling effect on the planet.

The author posits that the cirrus clouds formed in the wake of certain aircraft, particularly those that evolve into widespread, slender cirrus formations, might aid in the planet’s cooling by reflecting sunlight during daylight hours. While this remains a theoretical consideration, it warrants attention.

Conversely, it’s acknowledged that these elevated, chilly clouds can retain heat from the Earth’s surface, suggesting they could contribute to the “greenhouse effect” as well.

This quandary serves as a testament to the “uncertain” and “complex” nature of studying contrails’ effects on climate, illustrating the intricate challenges researchers face in this field.

Lee has expressed concerns over the misinterpretation of his research. The interactions between aerosols (suspended particles) and clouds are so uncertain—with a multitude of studies yielding vastly divergent outcomes—that the 2021 evaluation refrained from making any estimations on this matter.

More tests

Amidst the prevailing uncertainty, the aviation industry, facing significant media scrutiny from concerned groups, has ramped up its scientific endeavours to unravel the extent of contrails’ impact on climate change.

In a collaborative effort this past October, Boeing and NASA conducted a series of flight tests in Everett, Washington. A NASA DC-8 aircraft trailed an Airbus 737 MAX 10 above the city to collect and examine its exhaust gases and contrails. The objective was to determine whether the innovative sustainable aviation fuel (SAF) could lessen the emission of aerosols by these planes into the atmosphere.

Simultaneously, Google and the Breakthrough Energy Researchers Association, a climate action research group initiated by Bill Gates, embarked on a trial with American Airlines. The experiment aimed to ascertain if commercial pilots could steer clear of atmospheric regions most susceptible to the formation of vapour trails.

In this year’s preliminary test, Google harnessed artificial intelligence to sift through satellite imagery, meteorological data, and flight information to craft contrail prediction maps for 35 American Airlines routes. Pilots were then instructed to take alternate paths: one avoiding areas prone to contrail creation and the other along the usual trajectory. The alternative route achieved a 54% reduction in contrail formation.

While some scientists are advocating for a cautious approach to avoid potentially counterproductive measures, some consider such caution to be an overreaction. Marc Sapiro, the director of the Breakthrough Energy project, is one such individual who has expressed disagreement with Lee’s recent findings in an interview.

Smaller impact

In Europe, the urgency to address the environmental impact of contrails has intensified, leading to regulatory changes. Starting in 2025, European airlines will be obligated to monitor, report, and verify the non-CO2 climate effects of their operations. By 2028, the European Commission is expected to propose measures to mitigate these impacts, with contrails currently at the forefront of concerns.

The push for these regulations is rooted in the belief that contrails could be more harmful to the Earth’s atmosphere than even greenhouse gases—a notion that gained traction from David Lee’s influential 2021 assessment. This analysis, which has been referenced by the IPCC, environmentalists, and aerospace giants like Boeing and NASA, remains a key study on the climate impact of aviation.

However, interpretations of the data are not uniform, prompting Lee to revisit and clarify his conclusions. His 2021 paper provided “best estimates” based on multiple studies, suggesting that contrails are the most significant non-carbon impact of aviation, accounting for 57% of the industry’s total warming effect, compared to 34% from carbon emissions.

Yet, the precision of these estimates is questionable, with a substantial error margin of plus-or-minus 70%. Drew Shindell, a Duke University professor and IPCC climate impact assessor, pointed out that the “enormous uncertainty” stems from the imprecise analysis of cloud formation and its effects.

What’s next?

The latest IPCC figures place the contribution of contrails and aviation-induced cirrus clouds to global warming between 0.7% and 3.7%. Shindell’s interpretation of recent studies leans towards the lower end of this spectrum.

He stated, “The overall assessment is that it’s probably in fact, very small. I don’t personally think that it is something we have to worry about more than the CO2. I don’t think the science supports that.”

Even Shapiro of Breakthrough, whose research focus is on reducing contrails, said that "to be totally frank, our numbers are coming up on the low end of David Lee's <2021> estimates as well."

The strategy of avoiding contrails presents an opportunity for an economical and expandable approach to diminish the environmental effects of aviation.

Ongoing research and development efforts are focused on automating this avoidance, pinpointing contrails with the most significant climate impact, and refining the accuracy of satellite-based confirmation. There is a collective dedication within the aviation sector to harness artificial intelligence in making contrail avoidance an operational practice in the near future.

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