The discussion around climate change is often centered around a single substance: carbon dioxide. But in aviation in particular, there are other elements that contribute. Radiative Forcing (RF) is the term used to describe the difference in the atmoshere’s energy balance caused by human or natural factors, and in aviation a substantial amount of radiative forcing comes from contrails and cirrus clouds that grow out of contrails.
In a 2018 publication in Nature, contrails and the cirrus clouds that can result from it represent more than half of the total RF of aviation. Surprisingly, while contrails are very visible, they do not receive nearly as much attention in the aviation climate change discussion as CO2 emissions. But thankfully, options do exist for cutting down on contrails, and perhaps the opportunity to make a real impact here is a lot easier than reducing CO2 emissions. So what can be done?
Fly somewhere else
You may have noticed from just looking at the sky the not all aircraft leave contrails, and contrails sometimes suddenly just stop, start or show gaps. This is pretty obvious in the photo shown on the right.
Only certain atmospheric conditions are conducive to contrail formation, so there is an obvious thought on how to avoid contrails: don’t fly where they form. This is both horizontal and vertical, avoiding certain areas and certain altitudes. Doing this may cause the flight to tank longer and burn more fuel, but, if this avoids the creation of contrails the overall impact on RF can be very significant.
But what routes to follow and altitudes to use to avoid contrail creation? And how to balance this with fuel burn and keeping the operation on schedule? British company SATAVIA is working to solve this very problem with their DECISIONX software, and claims they can reduce the climate impact of air travel by as much as 60%.
There are a lot of challenges with Sustainable Aviation Fuels (or SAF for short), but they do have an additional advantage over conventional fuels. Research from NASA in partnership with DLR demonstrated that even at a 50 / 50 blend a notable reduction in soot was observed when compared to burning only conventional jet fuel, which in turn reduced the climate impact of contrails by 20% to 30%.
As there are various different types of SAF which won’t all have fully identical chemical composition, these are still promising results. The challenge of actually producing enough SAF in a truly sustainable and economic way still remains to be solved however.
Electric won’t help, hydrogen isn’t clear
Contrails form at higher altitudes, and all current commercial electric aircraft under development are intended to operate at lower altitudes where contrails wouldn’t form to begin with, and will for the most part replace aircraft that do not produce contrails.
For aircraft that may be developed in the future burning hydrogen as their source of fuel the story isn’t as clear. Burning hydrogen results in water as a waste product, so intuitively one might think they produce worse contrails, and the limited experimental flight operated using hydrogen in the past did show contrails being produced. However, a major contributor to contrail formation in conventional aircraft is actually the soot particles emitted, which isn’t an issue with burning hydrogen.
So, while contrails will be produced, their composition will be different and potentially less impactful. At the time of writing, it isn’t well understood what the true impact of switching to hydrogen would be on contrail impact on climate change. To get to a better understanding, Airbus has launched the Blue Condor project which will used modified gliders fitted with small jet engines to compare the contrails produced by conventional fuels versus those produced by burning hydrogen. To be continued.