Climate Scientists Take Their Closest Look Yet at the Warming Impact of Aviation Emissions

A new study reaffirms that contrail clouds produce more global warming than carbon dioxide from flights, a finding that could help reduce emissions from air travel.

Sep 18, 2020
Contrails from airplanes cross in the sky on June 2, 2020 in Aylesbury, United Kingdom. Credit: Catherine Ivill/Getty Images

Contrails from airplanes cross in the sky on June 2, 2020 in Aylesbury, United Kingdom. Credit: Catherine Ivill/Getty Images

An international team of prominent scientists has published what they say is the most comprehensive study to date calculating the complex climate impact of aviation emissions, reaffirming that contrail clouds produce more warming than carbon dioxide. 

This kind of comprehensive analysis has only been performed a handful of times, starting with a report from the Intergovernmental Panel on Climate Change (IPCC) in 1999. The new study, looking at data from 2000 to 2018, notes that while the understanding of the impacts of aviation emissions "remains incomplete," a series of new calculations considered "factors not previously applied in a common framework."

"We wanted to produce a very high-quality benchmark assessment," said David Lee, a climate scientist at Manchester Metropolitan University, who led the study and was also part of the IPCC assessment.

The study, which had been in the works since 2015, looked at both carbon dioxide and several types of "non-CO2" emissions in aviation. Carbon dioxide emissions are fairly well understood at this point, Lee said, but the impacts of non-CO2 emissions, which the study found account for about two-thirds of the net warming effect, are considerably harder to calculate.

The primary non-CO2 impact results from the emission of nitrogen oxides, water vapor and soot that can create heat-trapping contrail clouds. They form as emissions of hot gases and soot from aircraft engines activate water particles that freeze, producing the contrails, those straight, wispy white markings of a plane's path through the sky.   

Other non-CO2 emissions involve what the study calls "aviation aerosols"—small particles composed of black and organic carbon known as soot, sulfur and nitrogen compounds. 

The team measured the "radiative forcing" of each item—a measure of how much potential it has to exert a change on the global climate.

Over the last decade the science around these emissions has improved drastically, Lee said, but he added that the better science doesn't always mean the uncertainties will narrow. "We find out other stuff that we didn't know before," he said.

The climate impact of contrails has been a topic of contention in recent years. Contrails form in certain atmospheric conditions when the water vapors from airplane engines condense and freeze in the air—or when soot particles allow water in the atmosphere to condense around them—creating artificial clouds. The trails can become cirrus, or curling, clouds and hang around for hours, trapping atmospheric heat which contributes to global warming. 

Of all the emissions measured, the researchers found these contrails to be the most impactful, but there's a caveat.

"Contrail cirrus forcing is not as powerful as we used to think it was," Lee said. The contrails appeared to be less than half as effective as previous estimates found. Carbon dioxide emissions, meanwhile, were the second most impactful, with roughly 60 percent as much effect on the climate as the vapor trails.

Measuring the effects of nitrogen oxides was tricky because they both eliminate atmospheric methane, which has a cooling effect, but create ozone which can act as a greenhouse gas. Between these two mechanisms, the researchers estimate that nitrogen oxides have a net warming effect that is about 30 percent as powerful as the vapor trails. The other emissions didn't appear to have strong effects.

Even though contrails showed less effect than previous estimates, their impact is still considerable, given that warming from the contrails is greater than from carbon dioxide—and that the carbon dioxide emissions alone from the airline industry equal 2.4 percent of global CO2 emissions, according to a 2019 study by the International Council on Clean Transportation. 

The magnitude of these emissions is also growing quickly. The International Civil Aviation Organization estimated pre-Covid-19 pandemic that demand for air travel would grow 4.3 percent annually for the next two decades.

"The airlines did not dispute that there was an impact of CO2 on the atmosphere," said Annie Petsonk, the international counsel at the Environmental Defense Fund, who was not involved in the study. But until now, she said, they have claimed the science isn't in on non-CO2 airline emissions. 

This paper, in filling that knowledge gap, deprives airlines of excuses to avoid dealing with non-CO2 emissions, said Petsonk. While the European Union has voluntarily adopted stricter standards for aviation emissions, the United Nations body that governs international aviation standards recently adjusted an international agreement after airlines asked for reprieve, citing lost revenue because of Covid-19. 

"The airlines are in the midst of a Covid crisis, which has hit them with a gut punch and they're trying to get back on their feet. If they fail to put the climate crisis central to their rebuilding, then their efforts... will fail," Petsonk said.

She said that getting aviation "on a trajectory for net zero climate" needs to be at the core of the industry's recovery efforts from Covid-19 setbacks.

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