There is now no question that flying is not good for the earth: global air traffic contributes around 3.5 percent to climate change. The most important factor is not the carbon dioxide that the jets blow into the air. The biggest part, this is shown by current studies, rather make up contrails: The fine clouds of ice crystals arise at a height of about ten kilometers, stay there for several hours and prevent terrestrial heat from being released into space. That accelerates global warming.
Researchers at the German Aerospace Center (DLR) and the US aviation authority NASA have now discovered a means that could weaken this effect: biokerosene. The alternative fuel, made from plant-based materials and actually intended to improve the CO₂ balance in aviation, does not produce as many soot particles when burned as fossil kerosene. This results in fewer condensation nuclei on which ice crystals can form. The extent is apparently considerable: the concentration of soot and ice crystals fell by 50 to 70 percent during test flights, writes study director Christiane Voigt from the DLR Institute for Atmospheric Physics in the Nature portfolio– Sheet Communications Earth & Environment. The climate impact of the contrails could thus be reduced by 20 to 30 percent.
Thinner contrails allow more terrestrial heat to be released into space
Science has long suspected a connection between soot and contrails. Attempts to prove it in practice, however, always failed due to the weather: No contrails formed during flight attempts. This time the teams from DLR and NASA were more lucky. At the beginning of 2018 they sent the DLR research plane Atra, an Airbus A320, with various fuel mixtures in the air – with conventional kerosene, but also with up to 50 percent added biofuel, made from animal fats, algae or vegetable oils. A DC-8 NASA followed at least ten kilometers away and examined the contrails that had formed behind the Airbus.
It was found that the biokerosene produced significantly fewer ice crystals. The crystals were also larger. Both of these could have consequences for the climate: fewer crystals – and thus thinner clouds – block less heat. More massive crystals sink faster and melt in the layers of air below. Flight tests are currently underway in which even 100 percent biokerosene is used. In their study, however, the researchers warn that with future fuels with an even lower soot content, other particles and atmospheric effects could come to the fore – with as yet unexplained effects on the strength of the contrails. A targeted design of new types of fuel with a view to the soot emissions is therefore necessary.
Alternative aviation fuels are currently seen as great hope in aviation. After decades of missing out on promoting climate-friendly concepts such as electric flying, hybrid or hydrogen propulsion, the industry is under pressure. A kerosene substitute, marketed as “sustainable aviation fuel”, should now be the solution. It does not need any new and expensive technology, but can be conveniently refueled using conventional aircraft.
The fuels are controversial, however: Airlines don’t like them because they are significantly more expensive than conventional kerosene. And environmentalists are skeptical, since bio-kerosene made from plant-based products could compete with food production, while enormous amounts of energy are required for synthetic kerosene, which is to be produced with renewable energies.