🛫 The contrail dilemma: When to avoid contrails and when to ignore them 🛬

✅ Introducing the Contrail Avoidance Decision Matrix

Contrails mostly warm the climate when contrail clouds act like a blanket on Earth, but during the day, contrails sometimes have a cooling effect. If a flight is projected to create a highly warming contrail, it makes sense to spend a little more fuel to fly around the contrail-prone areas and avoid that massive warming 🔥 – even if it costs a little more kerosene and associated CO2 emissions.

But what if the predicted contrail warming is more moderate and the extra fuel burn slightly higher? Is it still a good idea to avoid the contrail warming, or should the contrail forming be ignored? A group of scientists, led by Reed Miller from Yale University, has created the Contrail Avoidance Decision Matrix to help with that dilemma.

The X-axis is an exponential scale showing the additional fuel burn needed to avoid forming contrails. The scale is divided into low, medium, high, and very high extra fuel burn. The Y-axis shows the climate warming the contrails are predicted to create, measured in radiative forcing, and divided into contrail severity groups called cooling, very low, low, medium, and high.

Inside the matrix, the climate trade-off has been calculated by taking the potential contrail warming savings and subtracting the climate impact from the extra fuel burn needed to avoid making contrails. If the fuel impact exceeds the saved contrail warming, the output number is negative, and the contrail formation should be ignored. But if the number is positive, contrail avoidance should be considered.

Targeting the big hits

Looking closer at the numbers, it is no surprise that 💧 cooling contrails should always be left alone—there is no reason to avoid those. But what is perhaps surprising is that, from a climate perspective, the highly warming contrails should always be avoided—even if it costs 50% or 100% more in fuel (which is very unlikely in real life).

This illustrates a good point: flights that create highly warming contrails have a combined climate impact many times higher than their CO2 emissions. Flying around the contrail weather to avoid these big hits can have a significant positive impact on the climate, and it’s a good place to start for any airline wanting to lower its climate footprint effectively in a timely and cost-effective way.

Explore for yourself

The values in the above matrix have been calculated for a large four-engine aircraft using GWP100 as a metric. Similar calculations can be made for other aircraft and/or using the GWP20 metric.

The Contrail Decision Matrix is described on page 55 of RMI’s new contrail report, Understanding Contrail Management: Opportunities, Challenges, and Insights. If you want to play around with the input values, the matrix can be downloaded on GitHub.

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Go to Blue Lines’ educational website to explore contrails in depth.

(As regular readers of the Blue Lines newsletter will know, contrails are the wispy white stripes that airplanes sometimes leave behind in the sky (made from water vapor and engine soot). Some of these condensation trails can spread out and become high-altitude ice clouds (cirrus), which reflect some of the sun’s energy back into space but also trap outgoing energy in the atmosphere, resulting in a net heating of our planet equivalent to 1-2% of human-induced global warming. However, we can relatively easily avoid most warming contrails by flying around the contrail-prone areas in the atmosphere. This climate solution – often called contrail management or contrail avoidance – is what Blue Lines promotes and wants to see spread worldwide.)

See you soon.

Joachim Majholm,

Blue Lines