Despite robust research into aviation’s non-CO2 effects – especially contrails – there remain some uncertainties that cloud the path to immediate implementation of mitigation strategies. We hear about these uncertainties in contrail management time and again, but what are we so uncertain about?
The Case for Contrail Mitigation Despite Uncertainties
We have compiled some of the main uncertainties (and the uncertainties within those uncertainties) hindering implementation of contrail mitigation. For each, we also identify the work being done to clarify those uncertainties. We hope you can share this article to assuage the concerns of those in the airline industry who remain reticent to commence contrail management.
First of all, the greatest uncertainty that impedes progress is the weather and the reliability of weather prediction tools and models. Weather forecasting is a challenge as it is, but with contrails it is more complicated, because not only is it weather prediction, but we have to identify ice supersaturated regions (ISSRs) and the relative humidity over ice (RHi) at 30,000+ feet on a given flight trajectory.
Dr. Roger Teoh spoke to the difficulty in doing this on a recent episode of the Aerospace Ambition Podcast. When discussing the various uncertainties surrounding contrail mitigation, he explained that meteorological projections using numerical weather prediction (NWP) and spatial temporal resolution (HRES), need more scale variability as ISSRs can be larger than predicted.Even once the regions are identified, they may shift and that can throw off forecasts.1
Currently, researchers rely on models like the Contrail Cirrus Prediction Model (CoCiP) to assess whether a contrail persists and calculate its optical depth and radiative forcing.2 This tool has improved the precision of contrail prediction, but a model is only as good as its input, so work is being done to refine its predictive accuracy.
Shifting weather patterns can impact the accuracy of these prediction models greatly. As David Lee et. al. point out in the now-infamous paper on the uncertainties in mitigating non-CO2 emissions: “As with contrails, it is not only when and where the emission occurs, but the weather conditions at the time of emission and specifically the subsequent trajectory of the air as it is blown by the winds.”3
These weather conditions subsequently cause uncertainties in flight planning. If a flight is rerouted into an ISSR or if an ISSR moves into the path of a contrail-optimized flight it could still form persistent contrails as well as add the cost of the extra fuel-burn for the rerouting.3 This possibility poses a great challenge to current flight simulators, which can only go so far in the prediction of contrail-free routes.
Flight planning services like FlightKeys and SATAVIA are working diligently to resolve this uncertainty with their software. These flight planners already know how to divert flights to account for changes in weather, so they are now working to do the same for contrail avoidance, and they hope to do so on a fleet-basis instead of just a flight-by-flight basis to account for and ultimately save on extra fuel burn.
Speaking of fuel, the type of fuel used can play a major role in contrail formation/ mitigation. However, uncertainties abound when looking into the details of just how much of an impact these fuels can have. The very shape of the ice crystals can be affected by the fuel, and this ultimately determines the lifetime of the contrails by influencing the short-wave or long-wave cycles, but there is as yet no data from persistent contrails.1
These crystals are formed from nuclei like soot and other particulate matter (PM) that interact with the cold air, but the degree of impact that soot-cirrus has on the radiative forcing of contrails is still uncertain.4 That’s where sustainable aviation fuel (SAF) comes in; it could reduce the lifespan of a contrail, but research, all though promising so far, needs to be continued.
Though SAF, with its lower PM, holds promise for the future of aviation, its effect on contrail forcing remains unresolved.3 There isn’t a large enough supply and there hasn’t been enough testing for SAF as a means of contrail mitigation; but projects like Virgin Atlantic’s Flight100 are being done to investigate the emissions of SAF and whether it will increase or decrease the lifecycle and radiative forcing of contrails.
Finally, once a contrail is formed, its impact on natural cirrus clouds is unknown. It dehydrates the atmosphere around it and competes with natural clouds for the moisture in the air, which could have an impact on the properties of the natural cirrus and the lifetime of future contrails.1 The aerosols emitted by the flight may interfere with the natural formation of clouds and lead to greater radiative forcing.3 Further studies are being done to ascertain just how much of an impact man-made cirrus clouds have on the natural atmosphere.
Those are the major uncertainties that the aviation industry faces regarding contrail mitigation efforts. While they may seem daunting, innovative work is being done to illuminate each of those areas of uncertainty. Rest assured that here at ETL, we will continue to research and report on organizations and their strategies to explore and master these uncertainties.
While further efforts are needed to establish the accuracy and efficacy of contrail interventions in practice, we are certain that contrail avoidance has the potential to achieve massive immediate climate benefits at a lower price than most other climate interventions regardless of the season, and as such, contrail mitigation should become one of aviation’s primary focuses in the coming years.
[1] Aerospace Ambitions Podcast with Dr. Roger Teoh | Aviation Emissions and Contrail Cirrus
[2] Geraedts et. al. Paper on Contrail Formation | A Scalable System to Measure Contrail Formation on a Per-flight Basis
[3] Lee et. al. Paper on Non-CO2 Mitigation Uncertainties | Uncertainties in Mitigating Aviation Non-CO2 Emissions for Climate and Air Quality Using Hydrocarbon Fuels
[4] Righi et. al. Paper on Aviation Soot-cirrus | Exploring the Uncertainties in the Aviation Soot–cirrus Effect
Additional Resources for the Avid Reader
Green Air News Commentary | Navigational Avoidance of Contrails a Good Idea, But Not Yet
Royal Aeronautical Society non-CO2 Conference | Easy Does it for Greener Skies
Aerospace Ambition Podcast with Marc Shapiro | Establishing a Baseline for Contrails
Gierens et. al. Paper on Contrail Prediction | How Well Can Persistent Contrails Be Predicted?