Scientists say sustainable aviation fuel is greener than we thought

Leaving on a jet plane isn’t the most environmentally friendly thing to do. But while airplanes are huge CO2 emitters, but there’s also a lot of work going on behind the scenes to change that.

The most promising is Sustainable Aviation Fuel (SAF). While airline fuel made from renewable sources is still in its infancy, everyone from European aircraft maker Airbus to Shell are working to make it a reality. But, as Delta Air Lines points out, all the SAF produced in 2020 would only be enough to power Delta’s fleet for a single day — and that’s only one airline.

But two new studies published this week in Communications Earth & Environment show how beneficial SAF could be. The results could help green the dirty, dirty airline industry.

What’s new — Both studies were done by the German Aerospace Center (acronym DLR in German), the German equivalent of NASA. The first showed that SAF-powered planes produce less soot and fewer ice crystals in the atmosphere, which means less warming from artificially produced contrail clouds.

The second showed that scientists may have been overestimating the amount of sunlight being reflected back into space by these artificial cirrus clouds. That means more sunlight is getting through than we thought, and thus there is less cooling potential from the contrails.

Put together, the two studies show that SAF could have a significant impact on the total warming of air travel.

Here’s the background — It seems unlikely that folks are going to stop flying. Even during the slowest flying day at the height of the pandemic lockdown, the Transportation Security Administration still screened more than 87,000 passengers, and that's just in the United States. On Sunday, June 13th, TSA screened more than 2 million passengers in a single day for the first time since before COVID.

But while there are active movements to encourage folks to fly less, like the Greta Thunberg-inspired flygskam or “flight shame” that may have led to drops in air travel in Sweden and elsewhere, there are many millions more jumping on planes for the first time in developing economies around the world.

The ICAO expects passenger and cargo transport via air will double by 2036 so without a lot of flygskam, we’ll need an alternate solution. That’s probably Sustainable Aviation Fuel, and thanks to this new study, it’s even more promising.

Aircraft engines spit out 3.16 kg of carbon dioxide and 1.23 kilograms of water vapor for every kilogram of fuel consumed. Though planes fly high, temperatures are very low and that water turns into ice crystals. With the right conditions, those contrails can remain for hours which can keep warm air trapped in the atmosphere and attract more sunlight to create even more warmth.

How they did it — According to the DLR study, unlike CO2 emissions, contrail cirrus clouds dissipate within a few hours. That means that getting rid of them could have an immediate cooling effect on the climate.

The researchers took an Airbus A320 aircraft and used five different types of fuel mixes to determine the effect of fuel type (including SAF) on the contrails that the plane produced.

• Two traditional Jet A1 aviation fuel mixes were used, as well as three other blends of Jet A1 with synthetic or bio-based alternative jet fuel mixed in.
• The A320 flew at a constant airspeed and altitude in closed research air space in Germany and used a chase plane to measure the results.
• The team showed significant reductions of soot emissions and an increase in ice crystal size, which correspondingly showed smaller total numbers of crystals.

“Initial ice number concentrations in contrails are a key parameter for determining light extinction by contrails, and their climate impact,” the authors write.

What’s next — Contrails with a smaller initial ice number have a shorter atmospheric lifetime, which directly contributes to reduced energy disposition into the atmosphere and reduced warming.

In other words, mixing more sustainable aviation fuel into commercial jet fuel has an immediate regional and global effect on warming from contrails.

This, combined with the findings of the second DLR study that cooling effects of these artificial cirrus clouds from the reflection of sunlight back into space has been overestimated previously, shows how impactful these contrail clouds can be — and if switching to SAF blends can get rid of them faster, we’ll both help the planet and improve our views of beautiful blue sky.

Abstract from study 1: Contrail cirrus account for the major share of aviation’s climate impact. Yet, the links between jet fuel composition, contrail microphysics and climate impact remain unresolved. Here we present unique observations from two DLR-NASA aircraft campaigns that measured exhaust and contrail characteristics of an Airbus A320 burning either standard jet fuels or low aromatic sustainable aviation fuel blends. Our results show that soot particles can regulate the number of contrail cirrus ice crystals for current emission levels. We provide experimental evidence that burning low aromatic sustainable aviation fuel can result in a 50 to 70% reduction in soot and ice number concentrations and an increase in ice crystal size. Reduced contrail ice numbers cause less energy deposition in the atmosphere and less warming. Meaningful reductions in aviation’s climate impact could therefore be obtained from the widespread adoptation of low aromatic fuels, and from regulations to lower the maximum aromatic fuel content.

Abstract from study 2: Fully accounting for the climate impact of aviation requires a process-level understanding of the impact of aircraft soot particle emissions on the formation of ice clouds. Assessing this impact with the help of global climate models remains elusive and direct observations are lacking. Here we use a high-resolution cirrus column model to investigate how aircraft- emitted soot particles, released after ice crystals sublimate at the end of the lifetime of contrails and contrail cirrus, perturb the formation of cirrus. By allying cloud simulations with a measurement-based description of soot-induced ice formation, we find that only a small fraction (<1%) of the soot particles succeeds in forming cloud ice alongside homogeneous freezing of liquid aerosol droplets. Thus, soot-perturbed and homogeneously-formed cirrus fundamentally do not differ in optical depth. Our results imply that climate model estimates of global radiative forcing from interactions between aircraft soot and large-scale cirrus may be overestimates. The improved scientific understanding reported here provides a process- based underpinning for improved climate model parametrizations and targeted field observations.

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