High-flying jets often create long white lines in the sky. Called contrails, these narrow clouds can disappear in minutes or last for days. Like other clouds, long-lived contrails can trap heat in the atmosphere. Scientists have discovered in the last year or two that these contrails can add to the warming of Earth’s atmosphere. But it can be quite simple to reduce that contribution: just make some planes fly higher for much of their route.
A recently studied flights and contrails in a heavily trafficked part of the sky over Japan. Along the way, they found a surprising pattern. About 80 per cent of the contrail heating came from just 2 per cent of the flights.
“The fact that it’s such a small [number] of flights means we think it’s a feasible solution,” says engineer Marc Stettler. He led the study and works at Imperial College London.
Sending just 1.7 per cent of flights to a higher altitude should reduce the global warming impact of contrails by 59 per cent, his team calculates. If those planes fly higher and use more efficient ways to burn fuel, scientists predict the impact of warming could be reduced by more than 90 per cent.
You should not change any flight. “We only looked to divert a flight if the number of planes in the sky was low,” he says. “[With fewer flights] late at night, for example, we have more room to move planes.”
A 2011 study calculated that the heat-trapping potential of contrail cirrus clouds is actually greater than the carbon emissions produced by aircraft, due to the way contrail clouds heat the planet through a process called radiative forcing.
Now one of those researchers, atmospheric physicist Ulrike Burkhardt of the German Aerospace Center, is back with a new study that examines how this phenomenon might play out in the future.
The results are not promising. Burkhardt and co-author Lisa Bock’s new model suggests that radiative forcing effects attributable to aircraft contrails will triple from 2006 levels by 2050, due to increases in air traffic volumes as well as a slight shift in air traffic to higher altitudes.
It’s a disturbing prospect, given that most of the world’s efforts to combat the environmental impacts of aviation are aimed directly at limiting CO2 emissions, such as the CORSIA scheme.
“A lot of people talk about the need to stop the increase in air traffic all the time, but this [cirrus contrail] is not taken that seriously,” Burkhardt told New Scientist.
“That’s a problem if the non-CO2 effects are larger than the CO2 ones.” You can think of contrails as little lines against a vast sky, but under certain conditions, they can be much more. They can stretch tens of miles. The wind can spread them. They can stay for hours. So, as plane after plane travels the same route through the air, new and old contrails mix and accumulate, forming airborne ice cloud wells.
Scientists call them “contrail clouds”—high-altitude clouds that can extend over hundreds of square miles. And they are likely to become a bigger problem: One study found that as air traffic increases, the heat-trapping effect of contrail cirrus clouds in 2050 could be three times greater than in 2006. Clouds trap heat coming from Earth that would otherwise head into space, making them the single most important variable in the planet’s temperature and climate, according to NASA.
New research from the UK has found two silver linings in this confusing conundrum. The first is that only 2.2 per cent of flights generate 80 per cent of contrail-related heating. According to a January article in the journal Environmental Science & Technology, the troublemakers are mostly flights taking off in the late afternoon and early evening, whose contrails live mostly at night, when they still trap some heat but can’t deflect sunlight (which can balance its impact).
The warming effect of contrails was already the largest contributor to aviation’s climate impact in 2005 when aviation accounted for 5 per cent of human impact on climate. That’s less than the overall contribution from cars and other ground vehicles, but aviation’s impact is likely to increase given growing air traffic.
To make their projections about the growing threat posed by contrails, Burkhardt and his colleagues ran several simulations to study the impact of increased air traffic, general climate change, and how increased aircraft engine efficiency combined with alternative fuels might stop the formation of contrails.
Critics of the research warn that the simulations could exaggerate the threat, though they agree that lawmakers should take steps to address the climate impact of contrails regardless of the unknowns, steps that could include changing jet fuel blends or diverting aircraft flight paths.
Many contrails last only a few minutes before the frozen water turns back into steam and the plumes dissipate. Others persist in the atmosphere for more than 10 minutes and some spread out to become cirrus clouds.
These persistent contrails are thought to have a more significant effect on global temperatures.
Another complication is that contrails can have both a heating effect by trapping heat and a separate cooling effect by reflecting sunlight back into space. For example, contrails at night tend to have a warming effect, because they primarily trap heat from the Earth without reflecting incoming sunlight.
- Electricity from nuclear fusion, How it is different from existing nuclear energy?
- What Is a Sploot, Why Do Dogs and Cats Do It?
- Number of wildfires forecast to rise by 14% by 2030