Why flying at lower or higher altitudes might reduce the climate impact of air travel

Only a small fraction of flights would need to adjust their altitudes to achieve the effect, new research suggests.

Air travel is growing around the globe, and so is its contribution to climate change. The climate cost of flying has drawn more public attention in recent years, even leading to a social stigma in some places, especially for local or avoidable flights. In Sweden, for example, this is known as flygskam, or "flight shame."

Commercial flights emitted 918 million metric tons of carbon dioxide in 2018, or about 2.4% of humanity's total for that year, but their fuel use and CO2 emissions could both triple by 2050, according to United Nations projections. Flight shame may not yet be a major disruption for air travel, but it is quickly gaining attention, both among travelers and the airline industry.

And while a decline in air travel would help with climate change, flight shame can also be complemented by other strategies that make air travel more sustainable. That includes switching to cleaner, renewable fuel, but as a new study highlights, there's also another, less obvious option: flying at lower or higher altitudes.

Aircraft would only need to adjust their altitudes by about 2,000 feet (600 meters), the study found, and since some flights have a larger climate impact than others, only a small fraction of flights would need to make any adjustments.

"According to our study, changing the altitude of a small number of flights could significantly reduce the climate effects of aviation contrails," says lead author Marc Stettler, of Imperial College London's Department of Civil and Environmental Engineering, in a statement. "This new method could very quickly reduce the overall climate impact of the aviation industry."

Hot on the contrail

But why would flying lower or higher affect an airplane's climate impact? In addition to CO2, many aircraft leave condensation trails in the sky, commonly known as "contrails" or vapor trails. These form when planes fly through very cold, moist air, where black carbon particles in their exhaust provide a surface on which moisture can condense into ice particles. We see this as fluffy white lines across the sky.

Most contrails last only a few minutes, but some spread and mix with other contrails as well as cirrus clouds, forming "contrail cirrus" clouds that linger for longer. Along with CO2, these also play a big role in the climate impact of air travel, even rivaling the warming effect of all CO2 emissions from aviation. That's because of an effect called "radiative forcing," in which the balance is disrupted between solar energy coming to Earth and heat emitted from Earth's surface into space.

Scientists know contrails can be limited when planes fly at lower altitudes, but since this increases flying time, it also means burning significantly more fuel, and thus emitting more CO2. But can the benefits of curbing contrails outweigh the negative impact of burning more fuel?

Yes, at least in certain circumstances. According to a 2014 study, published in the journal Environmental Research Letters, re-routing flights in strategic ways can allow significant contrail reduction without major extensions to trip length. For instance, avoiding a major contrail on a flight between New York and London would only add about 14 miles (23 km) to the journey, the study found.

"You think that you have to do some really huge distance to avoid these contrails," lead author Emma Irvine told the BBC in 2014. "But because of the way the Earth curves, you can actually have quite small extra distances added onto the flight to avoid some really large contrails."

Of course, the precise adjustments required for flights to avoid generating long contrails will depend on the type of aircraft and the specific conditions present on the day of the flight, but these are easy factors to calculate. "The key things you need to know are the temperature of the air and how moist it is, [and] these are things we forecast at the moment, so the information is already in there," Irvine said.

Changing altitudes and attitudes

In the new study, published in Environmental Science & Technology, researchers used computer simulations to predict how adjusting aircraft altitudes might reduce the number and duration of contrails, thus reducing their warming impact. Because contrails only form and persist in thin layers of humid atmosphere, aircraft can avoid them with fairly small changes in altitude, resulting in fewer contrails.

Using data from airspace above Japan, the researchers found that just 2% of flights were responsible for 80% of radiative forcing in this sample area. "A really small proportion of flights are responsible for the vast majority of contrail climate impact, meaning we can focus our attention on them," Stettler says.

Stettler and his colleagues simulated these flights at either 2,000 feet higher or lower than their actual paths, and found the contrail climate forcing could be cut by nearly 60% if just 1.7% of flights adjusted their altitudes. This caused less than a 0.1% increase in fuel consumption, and the CO2 emitted by burning that extra fuel was more than offset by the reduced contrail formation, the study's authors report.

"We're conscious that any additional CO2 released into the atmosphere will have a climate impact stretching centuries into the future, so we've also calculated that if we only target flights that wouldn't emit extra CO­2, we can still achieve a 20% reduction in contrail forcing," Stettler says.

In addition to changing altitudes, better engine technology could also help curb contrails, the researchers add, since black carbon particles are produced by incomplete fuel combustion. With more efficient engines, aircraft could reportedly reduce their contrail generation by as much as 70%. Combined with slight altitude tweaks for a small fraction of flights, this could help reduce overall contrail problems by 90%, the study suggests.

This is promising, but more research is still needed, and it may be a while before improvements like these take effect at a significant scale. So, while it's good to know air travel can have a smaller impact on the climate, for now the best way to achieve that is often by simply staying on the ground whenever possible.

Editor's note: This article has been updated since it was originally published in June 2014.