Curtailing contrails’ climate impact
Aviation’s rather substantial carbon footprint is well known. But CO2 isn’t the only thing that aircraft are emitting. They also produce contrails. What we see as the long, white lines left behind in the sky by high-flying jets are actually ice clouds. Yet despite being composed of tiny ice crystals, contrails mostly create a warming effect, similar to that caused by greenhouse gases. If aviation wants to reduce its climate impact, it must work to reduce both CO2 and contrails. Helping with the latter is the EU-funded BeCoM(opens in new window) project. “By developing new ways to accurately predict the location and timing of persistent contrails, we look to deliver a scientific basis for policy strategies aimed at reducing aviation’s climate impact,” says Feijia Yin, an associate professor of Aerospace Engineering at Delft University of Technology(opens in new window), the project’s coordinating partner.
Addressing the uncertainty in predicting contrails
The challenge is that today’s numerical weather prediction systems struggle to accurately predict ice supersaturation – a key determinant of contrail persistence and radiative forcing. “This uncertainty limits the effectiveness of the flight rerouting strategies used to reduce contrails’ radiative effects,” explains Yin. To address this uncertainty, the BeCoM project is taking a multistage approach, one that combines targeted water vapour observations with improved representations of relevant physical processes in numerical weather prediction models. It also uses advanced artificial intelligence techniques for enhanced weather forecasting and contrail detection in satellite imagery. With this model in hand, the project simulated aircraft trajectories that have the potential to deliver meaningful reductions in contrails at a relatively modest cost. It also created an improved physical representation, as well as data-assimilation methods, for reducing upper-tropospheric humidity biases in the ICON Model(opens in new window), a global numerical weather prediction system.
Incentivising contrail mitigation
BeCoM investigated a pricing concept for non-CO2 climate effects that could serve as an accounting system to incentivise climate-friendly operational measures. “With accurate weather-dependent climate-response models, such as those proposed by BeCoM, pricing aviation’s non-CO2 climate effects becomes conceptually feasible,” notes Yin. In fact, preliminary results from the project indicate that such pricing could deliver meaningful contrail reductions with modest impact on ticket prices. To ensure effective implementation, researchers recommend taking a phased approach. “Gradual integration of non-CO2 effects into the EU Emissions Trading System(opens in new window) (EU ETS) could limit economic impacts, facilitate operational adaptation and give methodologies enough time to mature all while maintaining incentives for contrail mitigation,” adds Yin.
A road map for limiting contrails’ climate impact
Mitigating contrails starts with better ice-supersaturation forecasts – which is exactly what the BeCoM project delivered. “The enhanced measurements, improved predictions and operational trials offer a strong foundation for addressing the remaining uncertainties in ice-supersaturation prediction and mitigating contrails’ climate impact,” concludes Yin. Leveraging this foundation, the project partners(opens in new window) are now working to address remaining uncertainties through expanded observational networks for upper-tropospheric water vapour, numerical weather prediction model intercomparison, satellite data integration, real-flight evaluation, network-scale simulation, and risk assessment of contrail mitigation measures.
