Contrails, the cloud-like streaks formed by aircraft at high altitudes, have a complex relationship with the climate. While these vapour trails often contribute to global warming by trapping heat like a blanket, they can sometimes have a cooling effect during the day. The challenge for airlines is deciding when it’s worth spending extra fuel to avoid forming contrails that significantly warm the climate, and when it’s better to ignore them.
This dilemma has led a team of scientists, led by Reed Miller from Yale University, to develop the Contrail Avoidance Decision Matrix—a tool designed to help airlines make informed choices about contrail management.
The matrix evaluates the trade-offs between additional fuel burn and the potential climate impact of contrails. The X-axis of the matrix represents the extra fuel needed to avoid contrails, ranging from low to very high. The Y-axis measures the predicted climate warming caused by contrails, categorised from cooling to high severity.
Inside the matrix, the climate trade-off is calculated by comparing the warming avoided by steering clear of contrail formation against the climate impact of the additional CO2 emissions from burning more fuel. If the climate cost of the extra fuel outweighs the contrail warming avoided, the recommendation is to ignore the contrail formation. Conversely, if the benefits outweigh the costs, avoiding contrails is advised.
One key insight from the matrix is that cooling contrails should never be avoided—there’s no climate benefit to doing so. However, the matrix also reveals that highly warming contrails should always be avoided, even if it means burning significantly more fuel. This is because the combined climate impact of these contrails can far exceed the impact of the CO2 emissions alone.
For airlines looking to reduce their climate footprint, focusing on avoiding highly warming contrails could be a strategic starting point. The matrix highlights that flights generating these contrails have a climate impact many times greater than their CO2 emissions, suggesting that rerouting to avoid these “big hits” can be both effective and cost-efficient.
The Contrail Avoidance Decision Matrix was developed using data from a large four-engine aircraft, with the Global Warming Potential over 100 years (GWP100) as a metric. The matrix can be adapted for different aircraft types and alternative metrics like GWP20.
For a deeper dive into contrail management, the matrix and related findings are detailed in RMI’s report, *Understanding Contrail Management: Opportunities, Challenges, and Insights*.
