Contrails: an underrated climate mitigation opportunity?

By JuanGarcia @ 2024-12-16T19:15 (+17)

Summary

There could be a major, overlooked climate opportunity: avoiding persistent contrails caused by aircraft. These contrails can form cirrus-like clouds trapping as much heat as all historical aviation CO2 emissions. The IPCC and other experts confirm contrails significantly warm the planet, even if the exact figure is uncertain. The good news is that changing flight altitudes by roughly 2,000 feet to dodge high-humidity zones could dramatically reduce contrail-induced warming. Making this change for contrails could cut the climate impact of aviation in half, at very low cost. This strategy is comparable to how airlines already reroute around turbulence.

Studies from Breakthrough Energy and Imperial College suggest that contrail avoidance might cost as little as $1–2 per metric ton of CO2-equivalent— below the $10/ton estimates of avoided climate damage from Clean Air Task Force and the Coalition for Rainforest Nations (source: Founders Pledge). Such a low-cost, high-impact measure may beat almost all other large-scale mitigation efforts. Yet airlines have not embraced it, citing scientific uncertainties. This hesitation recalls fossil fuel companies’ historical stalling. Acting now on contrail avoidance could be a top priority: it’s cheap, scalable, visible, and can reduce near-term warming. Could we deploy a policy campaign to make governments pay airlines to implement these altitude adjustments? Or is there another way to capture this low-hanging climate solution and significantly reduce our collective climate footprint?

Could this be particularly good opportunity?

Disclaimer: this post was prompted by reading the article "Reducing contrails could be a surprisingly effective climate solution". I am not an expert on aviation and therefore cannot advise on this beyond informational purposes.

We can do a simple analysis using the ITN framework:

Importance: yes!

According to Our World in Data, "Although CO₂ gets most of the attention, it accounts for less than half of this warming. Two-thirds come from non-CO₂ forcings. Contrails — water vapor from aircraft exhausts — account for the largest share. This explains why aviation contributes 2.5% of annual CO₂ emissions but more when it comes to its total impact on warming."

Since about half of the temperature increase of the aviation industry is from contrails, avoiding 70% of this would be equivalent to cutting annual CO2 emissions by 1.75%! While this may not sound like much to the uninitiated, those in the climate space will understand what a massive deal this would be.

For those who want more depth, you can check these graphs from relevant science papers (1, 2) showing how paying a negligible increase in the cost of around 2-3% of all flights globally would reduce more than 70% of the total contrail radiative forcing of the aviation industry. The 80/20 rule at work:

Another fund tidbit from this article: preliminary work suggests eliminating contrails could reduce European summer warming by about a third.

Tractability: unsure

Despite the estimated ease and relatively low cost, no major airlines are currently avoiding contrails. Uncertainty around the exact impact of contrails is an oft-cited reason. Overall, the biggest factor underlying delay is that little economic motivation exists at this time for airlines to avoid contrails, and commercial airlines are obligated to try to maximize return to their stockholders. How can this be changed?

The tractability of this depends on exactly the intervention you would like to implement to realize this. As a person working on a different field, I will not venture a guess as to how tractable different interventions might be. Would it be best to mount a policy campaign to influence the US government? Or the EU? Or the UN? Liaise with aviation industry associations? Work out a startup that raises funds as a step in flight purchase to implement a web-based solution financed by flying customers? Something else? I'm curious to hear ideas from those working in the space.

Neglectedness: maybe not?

My first step before deciding to write this was going on the forum to see if someone had discussed this. I only found this great comment by user MatthewDahlhausen, part of which reads:

If I had to pick interventions in the aviation space that are likely underfunded, I'd focus on atmospheric research to better predict and understand the impact of contrails on radiative forcing, and real-time aircraft re-routing / altitude adjustments to reduce contrail formation. One could image an extension of air-traffic control that uses contrail formation risk to direct planes much as they do with real-time weather data for flight risks and to avoid turbulence.

So, at least Matthew who works at NREL^[1] thinks it's underfunded. That might have pushed me on the edge of actually writing this short post here.

Who might already be working on this? Results from a cursory search:

• Breakthrough Energy has a contrails project at least from the modeling side, and they have the weight of the Gates Foundation behind them.
• EUROCONTROL (European Organisation for the Safety of Air Navigation) claims to be "Developing an Implementation Roadmap for Contrail Mitigation"
• RMI’s Contrail Impact Task Force is "at the forefront of advancing solutions for the warming effect of these aviation-induced clouds". They have a great diagram of the phenomenon which I put at the start of the post.
• The Air Transport Action Group (ATAG) is an aviation nonprofit that advocates for "operational trials in several areas before viable mitigation approaches are brought into daily operation across the aviation system"
• The Project “Better Contrail Mitigation” (BeCoM) funded by the EU does research on this.
• I could probably keep going.

So there clearly already is some significant effort being invested into this. With that in mind, what matters is how contingent you think contrail mitigation is, how likely it is to happen without further action. If anyone has reason to think that this will eventually get fully solved by the people already working on this, the value of getting involved is marginal. If on the other hand you think this is unlikely to realize its full potential without additional people pushing on this, that could be a reason to jump in. I'm curious to hear if anyone has thoughts around this.

References

Ian McKay, Ken Caldeira: Reducing contrails could be a surprisingly effective climate solution (2024).

Frias, A. M., Shapiro, M. L., Engberg, Z., Zopp, R., Soler, M., & Stettler, M. E. J. (2024). Feasibility of contrail avoidance in a commercial flight planning system: an operational analysis. Environmental Research: Infrastructure and Sustainability, 4(1), 015013.

Teoh, R., Schumann, U., Majumdar, A., & Stettler, M. E. (2020). Mitigating the climate forcing of aircraft contrails by small-scale diversions and technology adoption. Environmental Science & Technology, 54(5), 2941-2950.

1. ^{^}

   so he very probably knows what he's talking about